44 results on '"Korolkova, Natalia"'
Search Results
2. Identifying rebound effects and formulating more sustainable energy efficiency policy: A global review and framework
- Author
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Matraeva, Lilia, Vasiutina, Ekaterina, Korolkova, Natalia, Maloletko, Aleksander, and Kaurova, Olga
- Published
- 2022
- Full Text
- View/download PDF
3. Intellectual Search of Emerging Research Fields Utilizing Large Scientific Publication Arrays: 'Public Governance' Field as an Example
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Matraeva Liliia, Sunaeva Julia, Belyak Alexey, Korolkova Natalia, and Kaurova Olga
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scientometric analysis ,research base ,lcsh:T ,research front ,lcsh:L ,lcsh:Technology ,public governance ,lcsh:Education - Abstract
In order to predict the development of any scientific field of knowledge, it is necessary to present modern achievements and discoveries, authoritative expert opinions of key scientists of the studied area. However, the Modern “digital Universe” is changing and expanding at such a speed that doubles the amount of data every two years, which has led to the accumulation of huge streams of scientific information that it has become impossible to be covered fully based on traditional scientific search techniques. This article’s outcomes can be used as an intellectual basis by scientists involved in this progressing topic.
- Published
- 2020
4. Quantum steering as a resource for secure tripartite Quantum State Sharing
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Wilkinson, Cailean, Thornton, Matthew, Korolkova, Natalia, and University of St Andrews. School of Physics and Astronomy
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MCC ,TheoryofComputation_MISCELLANEOUS ,Quantum Physics ,QC Physics ,TK ,I-PW ,FOS: Physical sciences ,Quantum Physics (quant-ph) ,QC ,TK Electrical engineering. Electronics Nuclear engineering - Abstract
Quantum State Sharing (QSS) is a protocol by which a (secret) quantum state may be securely split, shared between multiple potentially dishonest players, and reconstructed. Crucially the players are each assumed to be dishonest, and so QSS requires that only a collaborating authorised subset of players can access the original secret state; any dishonest unauthorised conspiracy cannot reconstruct it. We analyse a QSS protocol involving three untrusted players and demonstrate that quantum steering is the required resource which enables the protocol to proceed securely. We analyse the level of steering required to share any single-mode Gaussian secret which enables the states to be shared with the optimal use of resources., 8 pages, 3 figures
- Published
- 2021
5. Convicting emergent multipartite entanglement with evidence from a partially blind witness
- Author
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Nordgren, Viktor, Leskovjanová, Olga, Provazník, Jan, Korolkova, Natalia, and Mišta, Ladislav
- Subjects
Quantum Physics ,FOS: Physical sciences ,Quantum Physics (quant-ph) - Abstract
Genuine multipartite entanglement underlies correlation experiments corroborating quantum mechanics and it is an expedient empowering many quantum technologies. One of many counterintuitive facets of genuine multipartite entanglement is its ability to exhibit an emergent character, that is, one can infer its presence in some multipartite states merely from a set of its separable marginals. Here, we show that the effect can be found also in the context of Gaussian states of bosonic systems. Specifically, we construct examples of multimode Gaussian states carrying genuine multipartite entanglement which can be verified solely from separable nearest-neighbour two-mode marginals. The key tool of our construction is a genuine multipartite entanglement witness acting only on some two-mode reductions of the global covariance matrix, which we find by a numerical solution of a semi-definite programme. We also propose an experimental scheme for preparation of the simplest three-mode state, which requires interference of three correlatively displaced squeezed beams on two beam splitters. Besides revealing the concept of emergent genuine multipartite entanglement in the Gaussian scenario and bringing it closer to experimentally testable form, our results pave the way to effective diagnostics methods of global properties of multipartite states without complete tomography., 14 pages, 3 figures
- Published
- 2021
6. Trading quantum states for temporal profiles: tomography by the overlap
- Author
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Tiedau, Johannes, Shchesnovich, Valery S., Mogilevtsev, Dmitri, Ansari, Vahid, Harder, Georg, Bartley, Tim, Korolkova, Natalia, and Silberhorn, Christine
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Quantum Physics ,FOS: Physical sciences ,Quantum Physics (quant-ph) - Abstract
Quantum states and the modes of the optical field they occupy are intrinsically connected. Here, we show that one can trade the knowledge of a quantum state to gain information about the underlying mode structure and, vice versa, the knowledge about the modal shape allows one to perform a complete tomography of the quantum state. Our scheme can be executed experimentally using the interference between the signal and probe states on an unbalanced beam splitter with a single on/off-type detector. By changing the temporal overlap between the signal and the probe, the imperfect interference is turned into a powerful tool to extract the information about the signal mode structure. A single on/off detector is already sufficient to collect the necessary measurement data for the reconstruction of the diagonal part of the density matrix of an arbitrary multi-mode signal. Moreover, we experimentally demonstrate the feasibility of our scheme with just one control parameter -- the time-delay of a coherent probe field., 11 pages, 4 figures
- Published
- 2017
7. Free-space quantum signatures using heterodyne detection
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Croal, Callum, Peuntinger, Christian, Heim, Bettina, Khan, Imran, Marquardt, Christoph, Leuchs, Gerd, Wallden, Petros, Andersson, Erika, and Korolkova, Natalia
- Subjects
Quantum Physics ,FOS: Physical sciences ,Quantum Physics (quant-ph) - Abstract
Digital signatures guarantee the authorship of electronic communications. Currently used "classical" signature schemes rely on unproven computational assumptions for security, while quantum signatures rely only on the laws of quantum mechanics. Previous quantum signature schemes have used unambiguous quantum measurements. Such measurements, however, sometimes give no result, reducing the efficiency of the protocol. Here, we instead use heterodyne detection, which always gives a result, although there is always some uncertainty. We experimentally demonstrate feasibility in a real environment by distributing signature states through a noisy 1.6km free-space channel. Our results show that continuous-variable heterodyne detection improves the signature rate for this type of scheme and therefore represents an interesting direction in the search for practical quantum signature schemes.
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- 2016
8. The Model of Assessment of Quality and Efficiency of Managerial Decisions by Public Management at Various Stages of State Programs Lifecycle.
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MATRAEVA, Liliia, VASIUTINA, Ekaterina, and KOROLKOVA, Natalia
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QUALITY of service ,ECONOMIC efficiency ,DECISION making in public administration ,CONTENT analysis ,EMERGING markets ,CUSTOMER satisfaction - Abstract
In the present-day economic science, the problems of public management efficiency are traditionally disputable for a number of reasons. First, this investigatory field is closely related to the national peculiarities of public management system development which produced numerous scientific disputes about advantages and disadvantages of each of them. Second, despite the fact that the state sector reformation and the problems of economic efficacy in particular have become the subject of quite many works, these two trends were developing separately, and there are rather few works of interdisciplinary nature. In this article the authors, based on the conducted content-analysis, systemize the main approaches to assessment of public management efficiency from the perspective of the development of scientific concepts about the role and significance of the state within the economic theory. Under the conditions of development of Project Cycle Management paradigm the authors single out the new stage - the contemporary fourth "assessment wave". The article suggests a new method of assessment of public administration efficiency based on the principles of metaassessment at various stages of lifecycle of the state priorities implementation. The basic analytical tool of the method is GAP-analysis used by the authors for the search of possible emerging discrepancies between the increment rates of targeted indicators of the program financing and the increment rates of state financing of the program. Analytical opportunities of the model are illustrated based on the analysis of satisfaction with state (municipal) service quality. [ABSTRACT FROM AUTHOR]
- Published
- 2019
9. Entanglement Concentration with Quantum Non Demolition Hamiltonians
- Author
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Tatham, Richard and Korolkova, Natalia
- Subjects
Quantum Physics ,FOS: Physical sciences ,Quantum Physics (quant-ph) - Abstract
We devise and examine two procrustean entanglement concentration schemes using Quantum Non- Demolition (QND) interaction Hamiltonians in the continuous variable regime, applicable for light, for atomic ensembles or in a hybrid setting. We thus expand the standard entanglement distillation toolbox to the use of a much more general, versatile and experimentally feasible interaction class. The first protocol uses Gaussian ancillary modes and a non-Gaussian post-measurement, the second a non-Gaussian ancillary mode and a Gaussian post-measurement. We explicitly calculate the density matrix elements of the non-Gaussian mixed states resulting from these protocols using an elegant Wigner-function based method in a numerically efficient manner. We then quantify the entanglement increase calculating the Logarithmic Negativity of the output state and discuss and compare the performance of the protocols., 9 pages, 5 figures, submitted to Phys. Rev. A
- Published
- 2011
10. Dissipatively coupled waveguide networks for coherent diffusive photonics.
- Author
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Mukherjee, Sebabrata, Mogilevtsev, Dmitri, Slepyan, Gregory Ya., Doherty, Thomas H., Thomson, Robert R., and Korolkova, Natalia
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COHERENCE (Optics) ,LIGHT scattering ,PHOTONICS ,QUANTUM theory ,RESERVOIRS - Abstract
A photonic circuit is generally described as a structure in which light propagates by unitary exchange and transfers reversibly between channels. In contrast, the term 'diffusive' is more akin to a chaotic propagation in scattering media, where light is driven out of coherence towards a thermal mixture. Based on the dynamics of open quantum systems, the combination of these two opposites can result in novel techniques for coherent light control. The crucial feature of these photonic structures is dissipative coupling between modes, via an interaction with a common reservoir. Here, we demonstrate experimentally that such systems can perform optical equalisation to smooth multimode light, or act as a distributor, guiding it into selected channels. Quantum thermodynamically, these systems can act as catalytic coherent reservoirs by performing perfect non-Landauer erasure. For lattice structures, localised stationary states can be supported in the continuum, similar to compactonlike states in conventional flat-band lattices. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
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11. Enhancing student learning of two-level quantum systems with interactive simulations.
- Author
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Kohnle, Antje, Baily, Charles, Campbell, Anna, Korolkova, Natalia, and Paetkau, Mark J.
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PSYCHOLOGICAL feedback ,QUANTUM mechanics ,EDUCATION - Abstract
The QuVis Quantum Mechanics Visualization project aims to address challenges of quantum mechanics instruction through the development of interactive simulations for the learning and teaching of quantum mechanics. In this article, we describe the evaluation of simulations focusing on two-level systems developed as part of the Institute of Physics Quantum Physics resources. Simulations are research-based and have been iteratively refined using student feedback in individual observation sessions and in-class trials. We give evidence that these simulations are helping students learn quantum mechanics concepts at both the introductory and advanced undergraduate level, and that students perceive simulations to be beneficial to their learning. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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12. Weak values and entanglement concentration.
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Menzies, David and Korolkova, Natalia
- Subjects
- *
QUANTUM theory , *MEASUREMENT , *QUANTUM scattering , *ESTIMATION theory , *STOCHASTIC processes - Abstract
Weak measurements are a special class of indirect measurements where the signal system is both pre and post-selected in particular quantum states. The accompanying probe system records information about the associated weak value of the interaction between the pre and post-selection and is modified by it. We provide a review of the operational application of weak measurement formalism to entanglement concentration protocols. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
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13. Creation of arbitrary Dicke and NOON states of trapped-ion qubits by global addressing with composite pulses.
- Author
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Ivanov, Svetoslav S., Vitanov, Nikolay V., and Korolkova, Natalia V.
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QUANTUM entanglement ,ION traps ,PHONONS ,GROUND state (Quantum mechanics) ,FLUCTUATIONS (Physics) ,LASER pulses - Abstract
We propose a fast and efficient technique to create classes of highly entangled states of trapped ions, such as arbitrary Dicke states and superpositions of them, e.g. NOON states. The ions are initialized in the phonon ground state and are addressed globally with a composite pulse that is resonant with the first motional sideband. The technique is fairly robust to parameter fluctuations and operates on comparatively short time scales, as resonant interactions allow one to use the minimum laser pulse area. The number of single pulses from the composite sequence is equal to the number of ions; thus the implementation complexity grows only linearly with the size of the system. The approach does not require individual addressing of the ions in the trap and can be applied both inside and outside the Lamb-Dicke regime. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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14. A new multimedia resource for teaching quantum mechanics concepts.
- Author
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Kohnle, Antje, Cassettari, Donatella, Edwards, Tom J., Ferguson, Callum, Gillies, Alastair D., Hooley, Christopher A., Korolkova, Natalia, Llama, Joseph, and Sinclair, Bruce D.
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STUDY & teaching of quantum theory ,INTERACTIVE multimedia ,SIMULATION methods & models ,COMPUTER-generated imagery ,QUESTIONNAIRES ,PHYSICS students - Abstract
We describe a collection of interactive animations and visualizations for teaching quantum mechanics. The animations can be used at all levels of the undergraduate curriculum. Each animation includes a step-by-step exploration that explains the key points. The animations and instructor resources are freely available. By using a diagnostic survey, we report substantial learning gains for students who have worked with the animations [ABSTRACT FROM AUTHOR]
- Published
- 2012
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15. Time evolution of a quantum soliton in a Kerr medium.
- Author
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Korolkova, Natalia, Loudon, Rodney, Gardavsky, Gerlinde, Hamilton, Murray W., and Leuchs, Gerd
- Subjects
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SOLITONS , *QUANTUM optics , *KERR electro-optical effect - Abstract
The Q-function of the quantum soliton in a fibre is derived in a new form suitable for the assessment of the possibilities for experimental observations of specific quantum-soliton effects. The characteristic effects of soliton evolution are associated with a cubic term in the nonlinear phase angle and appear at distances well beyond the attenuation length (long range). At the accessible distances, the nonlinear dynamics of a fundamental soliton follows essentially the single-mode dynamics (short and middle range). However, for the higher-order solitons, the form of the evolution parameter suggests strong deviations of soliton dynamics from the single-mode with a square of soliton number. The use of enhanced-nonlinearity fibres and higher-order solitons might make the experimental studies of the specific quantum-soliton features more viable. [ABSTRACT FROM AUTHOR]
- Published
- 2001
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16. Entanglement concentration with quantum nondemolition Hamiltonians.
- Author
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Tatham, Richard and Korolkova, Natalia
- Subjects
- *
QUANTUM entanglement , *LOGARITHMIC functions , *GAUSSIAN processes , *HAMILTONIAN systems , *QUANTUM theory , *QUANTUM optical phenomena - Abstract
We devise and examine two procrustean entanglement concentration schemes using quantum nondemolition (QND) interaction Hamiltonians in the continuous variable regime, applicable for light, for atomic ensembles or in a hybrid setting. We thus expand the standard entanglement distillation toolbox for use of a much more general, versatile, and experimentally feasible interaction class. The first protocol uses Gaussian ancillary modes and a non-Gaussian postmeasurement, the second a non-Gaussian ancillary mode and a Gaussian postmeasurement. We explicitly calculate the density matrix elements of the non-Gaussian mixed states resulting from these protocols using an elegant Wigner-function-based method in a numerically efficient manner. We then quantify the entanglement increase calculating the logarithmic negativity of the output state and discuss and compare the performance of the protocols. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
17. Scheme for the generation of entangled solitons for quantum communication.
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Leuchs, Gerd, Ralph, Timothy C., Silberhorn, Christine, and Korolkova, Natalia
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SOLITONS ,OPTICAL communications - Abstract
Quantum-communication needs a source of entanglement which is accessible to non-local measurements. It is shown that an entangled pair of light beams may be generated from two bright amplitude squeezed beams. This is applicable to light pulses and to optical solitons in fibres. A possible experimental scheme for stroboscopic quantum teleportation is discussed. [ABSTRACT FROM AUTHOR]
- Published
- 1999
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18. Gaussian multipartite bound information.
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Mišta Jr., Ladislav and Korolkova, Natalia
- Subjects
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GAUSSIAN distribution , *EXISTENCE theorems , *INFORMATION theory , *PUBLIC communication , *QUANTUM theory , *BOUND states - Abstract
We demonstrate the existence of Gaussian multipartite bound information which is a classical analog of Gaussian multipartite bound entanglement. We construct a tripartite Gaussian distribution from which no secret key can be distilled, but which cannot be created by local operations and public communication. Further, we show that the presence of bound information is conditional on the presence of a part of the adversary's information creatable only by private communication. Existence of this part of the adversary's information is found to be a more generic feature of classical analogs of quantum phenomena obtained by mapping of nonclassically correlated separable quantum states. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
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19. Universal quantum computation with continuous-variable Abelian anyons.
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Milne, Darran F., Korolkova, Natalia V., and van Loock, Peter
- Subjects
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QUANTUM computers , *MATHEMATICAL variables , *ANYONS , *SURFACES (Technology) , *TOPOLOGY , *GAUSSIAN processes - Abstract
We describe how continuous-variable Abelian anyons, created on the surface of a continuous-variable analog of Kitaev's lattice model can be utilized for quantum computation. In particular, we derive protocols for the implementation of quantum gates using topological operations. We find that the topological operations alone are insufficient for universal quantum computation, which leads us to study additional nontopological operations such as offline squeezing and single-mode measurements. It is shown that these in conjunction with a non-Gaussian element allow for universal quantum computation using continuous-variable Abelian anyons. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
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20. Composite-cluster states and alternative architectures for one-way quantum computation.
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Milne, Darran F. and Korolkova, Natalia V.
- Subjects
- *
QUANTUM computers , *ENERGY levels (Quantum mechanics) , *QUANTUM information science , *ATOMIC theory , *ANALYSIS of covariance , *BEAM splitters - Abstract
We propose an alternative architecture for the measurement-based quantum computation model. This design relies on small composite light-atom primary clusters. These are then assembled into cluster arrays using ancillary light modes and the actual computation is run on such a cellular cluster. We show how to create the primary clusters, which are Gaussian cluster states composed of both light and atomic modes. These are entangled via QND interactions and beam splitters and the scheme is well described within the continuous-variable covariance matrix formalism. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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- View/download PDF
21. Integrated Source of Path-Entangled Photon Pairs with Efficient Pump Self-Rejection.
- Author
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de la Hoz, Pablo, Sakovich, Anton, Mikhalychev, Alexander, Thornton, Matthew, Korolkova, Natalia, and Mogilevtsev, Dmitri
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PHOTON pairs ,FOUR-wave mixing ,PUMPING machinery ,INDIUM phosphide ,PHOTONICS ,PLANAR waveguides - Abstract
We present a theoretical proposal for an integrated four-wave mixing source of narrow-band path-entangled photon pairs with efficient spatial pump self-rejection. The scheme is based on correlated loss in a system of waveguides in Kerr nonlinear media. We calculate that this setup gives the possibility for upwards of 100 dB pump rejection, without additional filtering. The effect is reached by driving the symmetric collective mode that is strongly attenuated by an engineered dissipation, while photon pairs are born in the antisymmetric mode. A similar set-up can additionally be realized for the generation of two-photon NOON states, also with pump self-rejection. We discuss the implementation of the scheme by means of the coherent diffusive photonics, and demostrate its feasibility in both glass (such as fused silica-glass and IG2) and planar semiconductor waveguide structures in indium phosphide (InP) and in silicon. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
22. Entanglement creation by locally splitting a discordant state.
- Author
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Croal, Callum, Mista, Ladislav, Chille, Vanessa, Peuntinger, Christian, Marquardt, Christoph, Leuchs, Gerd, and Korolkova, Natalia
- Published
- 2014
23. Hybrid ancilla-based quantum computation and emergent Gaussian multipartite entanglement
- Author
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Nordgren, Viktor Manuel and Korolkova, Natalia
- Subjects
Quantum information ,Quantum computation ,Models of computation ,Quantum correlations ,Entanglement ,Entanglement witness ,Multipartite entanglement ,Genuine multipartite entanglement ,Semidefinite program ,Emergent properties ,Marginal problem - Abstract
In the first half of this thesis, we present two models of ancilla-based quantum computation (ABQC). Computation in the ABQC models is based on effecting changes on a register through the interaction with and manipulation of an ancillary system. The two models presented enable quantum computation through only unitary control of the ancilla - the ancilla-controlled model (ACQC) - or supplemented by measurements on the ancilla which drive the register transfor- mations - the ancilla-driven model (ADQC). For each of the models, we work on systems which couple two continuous variables (CV) or which are hybrid: the register is formed by two-level systems while the ancilla is a CV degree of freedom. The initial models are presented using eigenstates of momentum as the ancillas. We move to a more realistic scenario by modelling the ancillas as finitely squeezed states. We find that the completely unitary ACQC contains persistent entanglement between register and ancilla in the finite-squeezing scenario. In the ancilla-driven model, the effect of finite squeezing is to scale the register state by a real exponential which is inversely proportional to the squeezing in the ancilla. In the second part, we cover work on Genuine Gaussian Multipartite Entanglement (Gaussian GME). We present an algorithm for finding Gaussian states that have GME despite having all two-state reductions separable. This touches on the idea of entanglement as an emergent phenomenon. We determine GME via witnesses which probe only a subset of the state. We therefore referred to them as partially blind witnesses. The algorithm is based on semi-definite programs (SDPs). Such optimisation schemes can be used to efficiently find an optimal, partially blind, GME witness for a given CM and vice versa. We then present results of multipartite states of up to six parties. For the tripartite example, we present two experimental schemes to produce the state using a circuit of beam-splitters and squeezers.
- Published
- 2022
- Full Text
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24. Agile quantum cryptography and non-classical state generation
- Author
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Thornton, Matthew and Korolkova, Natalia
- Subjects
530.12 - Abstract
In the first half of this Thesis, we introduce a framework of "quantum cryptographic agility," which allows for a resource-efficient swap of an underlying cryptographic protocol. Specifically, we introduce several schemes which perform the tasks of Digital Signatures and Secret Sharing. Our first achievement is an investigation of Quantum Digital Signatures (QDS) over a continuous-variables platform, consisting of phase-encoded coherent states and heterodyne phase detection. QDS allows for secure authentication of a classical message, while guaranteeing message transferability. For the first time, we prove security of CV QDS in the presence of an eavesdropper on the quantum channels. We then introduce a continuous variable (CV) Quantum Secret Sharing (QSS) protocol. Our security proof allows for classical information to be split and shared between multiple potentially dishonest recipients, while retaining security against collective beamsplitter and entangling-cloner attacks. In the last chapter of this half, we introduce another QDS scheme which runs over identical hardware setup to our QSS protocol. We analyse experimental data in which quantum coherent states were distributed at a rate of 1 GHz, which for QDS allows us to securely sign a message in less than 0.05 ms. In the second half of this Thesis we suggest and discuss a deterministic source of nonclassical light, which we call "PhoG". Our source is based on the coherent diffusive photonics, relying on both coherent and dissipative evolution of the quantum state, and may be realised in an array of dissipatively-coupled laser-inscribed waveguides in a χ⁽³⁾ glass. We analyse the PhoG device with several analytical and numerical models and demonstrate that a coherent state input leads to a bright output state with strong photon-number squeezing. With minor reconfiguration our system can generate entanglement between spatially separated modes via a process analogous to four-wave mixing.
- Published
- 2020
25. Quantum correlations in continuous variable mixed states : from discord to signatures
- Author
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Croal, Callum and Korolkova, Natalia
- Subjects
530.12 ,Quantum information ,Quantum digital signatures ,Quantum discord ,Quantum optics - Abstract
This thesis studies continuous variable mixed states with the aim of better understanding the fundamental behaviour of quantum correlations in such states, as well as searching for applications of these correlations. I first investigate the interesting phenomenon of discord increase under local loss and explain the behaviour by considering the non-orthogonality of quantum states. I then explore the counter-intuitive result where entanglement can be created by a passive optical beamsplitter, even if the input states are classical, as long as the input states are part of a larger globally nonclassical system. This result emphasises the importance of global correlations in a quantum state, and I propose an application of this protocol in the form of quantum dense coding. Finally, I develop a quantum digital signature protocol that can be described entirely using the continuous variable formalism. Quantum digital signatures provide a method to ensure the integrity and provenance of a message using quantum states. They follow a similar method to quantum key distribution (QKD), but require less post-processing, which means they can sometimes be implemented over channels that are inappropriate for QKD. The method I propose uses homodyne measurement to verify the signature, unlike previous protocols that use single photon detection. The single photon detection of previous methods is designed to give unambiguous results about the signature, but this comes at the cost of getting no information much of the time. Using homodyne detection has the advantage of giving results all the time, but this means that measurement results always have some ambiguity. I show that, even with this ambiguity, the signature protocol based on homodyne measurement outperforms previous protocols, with the advantage enhanced when technical considerations are included. Therefore this represents an interesting new direction in the search for a practical quantum digital signature scheme.
- Published
- 2016
26. The enigma of imaging in the Maxwell fisheye medium
- Author
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Sahebdivan, Sahar, Leonhardt, Ulf, and Korolkova, Natalia
- Subjects
621.36 ,Super-resolution ,Maxwell fisheye lens ,Transformation optics ,Electromagnetic fields on curved manifolds ,Space-time analogy by transformation optics ,Overcoming the diffraction limit - Abstract
The resolution of optical instruments is normally limited by the wave nature of light. Circumventing this limit, known as the diffraction limit of imaging, is of tremendous practical importance for modern science and technology. One method, super-resolved fluorescence microscopy was distinguished with the Nobel Prize in Chemistry in 2014, but there is plenty of room for alternatives and complementary methods such as the pioneering work of Prof. J. Pendry on the perfect lens based on negative refraction that started the entire research area of metamaterials. In this thesis, we have used analytical techniques to solve several important challenges that have risen in the discussion of the microwave experimental demonstration of absolute optical instruments and the controversy surrounding perfect imaging. Attempts to overcome or circumvent Abbe's diffraction limit of optical imaging, have traditionally been greeted with controversy. In this thesis, we have investigated the role of interacting sources and detectors in perfect imaging. We have established limitations and prospects that arise from interactions and resonances inside the lens. The crucial role of detection becomes clear in Feynman's argument against the diffraction limit: “as Maxwell's electromagnetism is invariant upon time reversal, the electromagnetic wave emitted from a point source may be reversed and focused into a point with point-like precision, not limited by diffraction.” However, for this, the entire emission process must be reversed, including the source: A point drain must sit at the focal position, in place of the point source, otherwise, without getting absorbed at the detector, the focused wave will rebound and the superposition of the focusing and the rebounding wave will produce a diffraction-limited spot. The time-reversed source, the drain, is the detector which taking the image of the source. In 2011-2012, experiments with microwaves have confirmed the role of detection in perfect focusing. The emitted radiation was actively time-reversed and focused back at the point of emission, where, the time-reversed of the source sits. Absorption in the drain localizes the radiation with a precision much better than the diffraction limit. Absolute optical instruments may perform the time reversal of the field with perfectly passive materials and send the reversed wave to a different spatial position than the source. Perfect imaging with absolute optical instruments is defected by a restriction: so far it has only worked for a single–source single–drain configuration and near the resonance frequencies of the device. In chapters 6 and 7 of the thesis, we have investigated the imaging properties of mutually interacting detectors. We found that an array of detectors can image a point source with arbitrary precision. However, for this, the radiation has to be at resonance. Our analysis has become possible thanks to a theoretical model for mutually interacting sources and drains we developed after considerable work and several failed attempts. Modelling such sources and drains analytically had been a major unsolved problem, full numerical simulations have been difficult due to the large difference in the scales involved (the field localization near the sources and drains versus the wave propagation in the device). In our opinion, nobody was able to reproduce reliably the experiments, because of the numerical complexity involved. Our analytic theory draws from a simple, 1–dimensional model we developed in collaboration with Tomas Tyc (Masaryk University) and Alex Kogan (Weizmann Institute). This model was the first to explain the data of experiment, characteristic dips of the transmission of displaced drains, which establishes the grounds for the realistic super-resolution of absolute optical instruments. As the next step in Chapter 7 we developed a Lagrangian theory that agrees with the simple and successful model in 1–dimension. Inspired by the Lagrangian of the electromagnetic field interacting with a current, we have constructed a Lagrangian that has the advantage of being extendable to higher dimensions in our case two where imaging takes place. Our Lagrangian theory represents a device-independent, idealized model independent of numerical simulations. To conclude, Feynman objected to Abbe's diffraction limit, arguing that as Maxwell's electromagnetism is time-reversal invariant, the radiation from a point source may very well become focused in a point drain. Absolute optical instruments such as the Maxwell Fisheye can perform the time reversal and may image with a perfect resolution. However, the sources and drains in previous experiments were interacting with each other as if Feynman's drain would act back to the source in the past. Different ways of detection might circumvent this feature. The mutual interaction of sources and drains does ruin some of the promising features of perfect imaging. Arrays of sources are not necessarily resolved with arrays of detectors, but it also opens interesting new prospects in scanning near-fields from far–field distances. To summarise the novel idea of the thesis: • We have discovered and understood the problems with the initial experimental demonstration of the Maxwell Fisheye. • We have solved a long-standing challenge of modelling the theory for mutually interacting sources and drains. • We understand the imaging properties of the Maxwell Fisheye in the wave regime. Let us add one final thought. It has taken the scientific community a long time of investigation and discussion to understand the different ingredients of the diffraction limit. Abbe's limit was initially attributed to the optical device only. But, rather all three processes of imaging, namely illumination, transfer and detection, make an equal contribution to the total diffraction limit. Therefore, we think that for violating the diffraction limit one needs to consider all three factors together. Of course, one might circumvent the limit and achieve a better resolution by focusing on one factor, but that does not necessary imply the violation of a fundamental limit. One example is STED microscopy that focuses on the illumination, another near–field scanning microscopy that circumvents the diffraction limit by focusing on detection. Other methods and strategies in sub-wavelength imaging –negative refraction, time reversal imaging and on the case and absolute optical instruments –are concentrating on the faithful transfer of the optical information. In our opinion, the most significant, and naturally the most controversial, part of our findings in the course of this study was elucidating the role of detection. Maxwell's Fisheye transmits the optical information faithfully, but this is not enough. To have a faithful image, it is also necessary to extract the information at the destination. In our last two papers, we report our new findings of the contribution of detection. We find out in the absolute optical instruments, such as the Maxwell Fisheye, embedded sources and detectors are not independent. They are mutually interacting, and this interaction influences the imaging property of the system.
- Published
- 2016
27. Gaussian non-classical correlations in bipartite dissipative continuous variable quantum systems
- Author
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Quinn, Niall and Korolkova, Natalia
- Subjects
530.12 ,Quantum information ,Quantum communication ,Entanglement ,Discord ,Non-classical ,Quantum optics ,Gaussian states ,Decoherence ,QC174.13Q5 ,Quantum systems ,Quantum entanglement ,Quantum optics - Abstract
This thesis probes the usefulness of non-classical correlations within imperfect continuous variable decoherent quantum systems. Although a consistent function and practical usefulness of these correlations is largely unknown, it is important to examine their characteristics in more realistic dissipative systems, to gain further insight into any possible advantageous behaviour. A bipartite separable discordant state under the action of controlled loss on one subsystem was considered. Under these conditions the Gaussian quantum discord not only proved to be robust against loss, but actually improves as loss is intensified. Harmful imperfections which reduce the achievable level of discord can be counteracted by this controlled loss. Through a purification an explanation of this effect was sought by considering system-environment correlations, and found that a flow of system-environment correlations increases the quantumness of the state. Entanglement recovery possibilities were discussed and revealed the importance of hidden quantum correlations along bi-partitions across the discordant state and a classically prepared "demodulating" system, acting in such a way as to partially cancel the entanglement preventing noise. Entanglement distribution by separable states was studied by a similar framework, in an attempt to explain the emergence of quantum entanglement by a specific flow of correlations in the globally pure system. Discord appears to play a less fundamental role compared to the qubit version of the protocol. The strengthening of non-classical correlations can be attributed to a flow of classical and quantum correlations. This work proves that discord can be created in unique ways and, in select circumstances, can act to counteract harmful imperfections in the apparatus. Due to this advantageous behaviour discord indeed may ultimately aid in more applicable "real world" applications, which are by definition decoherent.
- Published
- 2015
28. Surprises in theoretical Casimir physics : quantum forces in inhomogeneous media
- Author
-
Simpson, William M. R., Leonhardt, Ulf, and Korolkova, Natalia
- Subjects
530.14 ,Casimir force ,Casimir effect ,Quantum field theory ,QED ,Macroscopic quantum electrodynamics ,Casimir-Lifshitz forces ,Dispersion forces ,Van der Waals forces ,Inhomogeneous media ,Transformation optics ,QC680.S5 ,Casimir effect ,Quantum field theory ,Quantum electrodynamics ,Van der Waals forces ,Inhomogeneous materials - Abstract
This thesis considers the problem of determining Casimir-Lifshitz forces in inhomogeneous media. The ground-state energy of the electromagnetic field in a piston-geometry is discussed. When the cavity is empty, the Casimir pressure on the piston is finite and independent of the small-scale physics of the media that compose the mirrors. However, it is demonstrated that, when the cavity is filled with an inhomogeneous dielectric medium, the Casimir energy is cut-off dependent. The local behavior of the stress tensor commonly used in calculations of Casimir forces is also determined. It is shown that the usual expression for the stress tensor is not finite anywhere within such a medium, whatever the temporal dispersion or index profile, and that this divergence is unlikely to be removed by modifying the regularisation. These findings suggest that the value of the Casimir pressure may be inextricably dependent on the detailed behavior of the mirror and the medium at large wave vectors. This thesis also examines two exceptions to this rule: first, the case of an idealised metamaterial is considered which, when introduced into a cavity, reduces the magnitude of the Casimir force. It is shown that, although the medium is inhomogeneous, it does not contribute additional scattering events but simply modifies the effective length of the cavity, so the predicted force is finite and can be stated exactly. Secondly, a geometric argument is presented for determining a Casimir stress in a spherical mirror filled with the inhomogeneous medium of Maxwell's fish-eye. This solution questions the idea that the Casimir force of a spherical mirror is repulsive, but prompts additional questions concerning regularisation and the role of non-local effects in determining Casimir forces.
- Published
- 2014
29. Quantum correlations in and beyond quantum entanglement in bipartite continuous variable systems
- Author
-
Tatham, Richard and Korolkova, Natalia
- Subjects
530.12 ,Entanglement ,Quantum ,Nonclassicality ,Discord ,Distillation ,GAMID ,MID ,Optimisation ,Optics - Abstract
This thesis explores the role of non-classical correlations in bipartite continuous variable quantum systems, and the approach taken is three-fold. We show that given two initially entangled atomic ensembles, it is possible to probabilistically increase the entanglement between them using a beamsplitter-like interaction formed from two quantum non-demolition (QND) interactions with auxiliary polarised light modes. We then develop an elegant method to calculate density matrix elements of non-Gaussian bipartite quantum states and use this to show that the entanglement in a two mode squeezed vacuum can be distilled using QND interactions and non-Gaussian elements. Secondly, we introduce a potential new measure of quantum entanglement in bipartite Gaussian states. This measure has an operational meaning in quantum cryptography and provides an upper bound on the amount of a secret key that can be distilled from a Gaussian probability distribution shared by two conspirators, Alice and Bob, given the presence of an eavesdropper, Eve. Finally, we go beyond the realm of quantum entanglement to explore other non-classical correlations in continuous variable systems. We provide solutions for a number of these measures on two mode Gaussian states and introduce the Gaussian Ameliorated Measurement Induced Disturbance (GAMID). The interplay between these different measures and quantum entanglement is examined. We then attempt to take small steps into the non-Gaussian regime by computing these non-classicality measures on the three-parameter continuous variable Werner states.
- Published
- 2012
30. Towards universal quantum computation in continuous-variable systems
- Author
-
Milne, Darran F. and Korolkova, Natalia
- Subjects
006.3 - Abstract
In this thesis we explore the possibility of creating continuousvariable quantum systems that are capable of supporting universal quantum computation. We begin by examining the measurement-based model, which employs sequences of measurements on highly entangled resource states, known as a cluster states. We suggest a method for the construction of Gaussian cluster states based on ensembles of atoms and quantum non-demolition interactions. We then go on to expand our model to allow for the inclusion of light modes as part of the cluster. This yields a new class of states, the composite cluster states. This leads us to propose a new architecture for the measurement-based model that uses these composite clusters to increase resource e ciency and reduce computational errors. The second part of this thesis concerns topological quantum computation. In states exhibiting topological degrees of freedom, quantum information can be stored as a non-local property of the physical system and manipulated by braiding quasiparticles known as anyons. Here we show how these ideas can be extended to continuous variables. We establish a continuous variable analogue of the Kitaev toric code, show that excitations correspond to continuous versions of Abelian anyons and investigate their behaviour under the condition of nite squeezing of the resource state. Finally, we expand our continuous variable topological model to include non-abelian excitations by constructing superpositions of CV toric code anyons. We derive the fusion and braiding behaviour of these non-abelian excitations and nd that they correspond to a CV analog of Ising anyons. Using these resources, we go on to suggest a computational scheme that encodes qubits within the fusion spaces of the CV Ising anyons and derive one- and two-qubit quantum gates operations that are implemented in a topological manner.
- Published
- 2012
31. Cross-phase modulation in rubidium-87
- Author
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Sinclair, Gary F. and Korolkova, Natalia
- Subjects
539.6 ,Electromagnetically induced transparency ,EIT ,Cross phase modulation ,XPM ,Kerr effect ,Rubidium - Abstract
This thesis explores the theoretical foundations of cross-phase modulation (XPM) between optical fields in the N-configuration atom. This is the process by which the refractive index experienced by one field can be modulated by controlling the intensity of another. The electro-optical version of this effect was first discovered by John Kerr in 1875 and found applications in photonics as a means of very rapidly modulating the phase and intensity of electromagnetic fields. Due to recent advances in experimental techniques there has been growing interest in generating nonlinear optical interactions in coherently prepared atomic ensembles. The use of coherently prepared media brings the possibility of achieving a much larger cross-phase modulation than is possible using classical materials. This is particularly useful when trying to create large optical nonlinearities between low-intensity electromagnetic fields. Much of the current research into cross-phase modulation is directed towards realising potential applications in the emerging field of quantum information processing. Above all, the possibility of constructing an all-optical quantum computer has been at the heart of much research and controversy in the field. In this thesis the theory of steady-state, transient and pulsed cross-phase modulation is developed. Moreover, care has been taken to relate all research back to experimentally feasible situations. As such, the relevance of the theory is justified by consideration of the situation present in rubidium-87. Due to the close relationship between XPM in the N-configuration atom and electromagnetically induced transparency in the Lambda-atom, many similarities and insights act as link between these two fields. Indeed, it is frequently demonstrated that the key to understanding the various properties of XPM in the N-configuration atom is by comparison with the situation in the corresponding Lambda-atom equivalent.
- Published
- 2009
32. Hybrid ancilla-based quantum computation and emergent Gaussian multipartite entanglement
- Author
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Nordgren, Viktor Manuel, Korolkova, Natalia, and Engineering and Physical Sciences Research Council (EPSRC)
- Subjects
Quantum correlations ,Entanglement ,Models of computation ,Quantum information ,Genuine multipartite entanglement ,Entanglement witness ,Quantum computation ,Semidefinite program ,Marginal problem ,Emergent properties ,Multipartite entanglement - Abstract
In the first half of this thesis, we present two models of ancilla-based quantum computation (ABQC). Computation in the ABQC models is based on effecting changes on a register through the interaction with and manipulation of an ancillary system. The two models presented enable quantum computation through only unitary control of the ancilla – the ancilla-controlled model (ACQC) – or supplemented by measurements on the ancilla which drive the register transfor- mations – the ancilla-driven model (ADQC). For each of the models, we work on systems which couple two continuous variables (CV) or which are hybrid: the register is formed by two-level systems while the ancilla is a CV degree of freedom. The initial models are presented using eigenstates of momentum as the ancillas. We move to a more realistic scenario by modelling the ancillas as finitely squeezed states. We find that the completely unitary ACQC contains persistent entanglement between register and ancilla in the finite-squeezing scenario. In the ancilla-driven model, the effect of finite squeezing is to scale the register state by a real exponential which is inversely proportional to the squeezing in the ancilla. In the second part, we cover work on Genuine Gaussian Multipartite Entanglement (Gaussian GME). We present an algorithm for finding Gaussian states that have GME despite having all two-state reductions separable. This touches on the idea of entanglement as an emergent phenomenon. We determine GME via witnesses which probe only a subset of the state. We therefore referred to them as partially blind witnesses. The algorithm is based on semi-definite programs (SDPs). Such optimisation schemes can be used to efficiently find an optimal, partially blind, GME witness for a given CM and vice versa. We then present results of multipartite states of up to six parties. For the tripartite example, we present two experimental schemes to produce the state using a circuit of beam-splitters and squeezers.
- Published
- 2023
33. Ancilla-driven quantum computation for qudits and continuous variables.
- Author
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Proctor, Timothy, Giulian, Melissa, Korolkova, Natalia, Andersson, Erika, and Kendon, Viv
- Abstract
Although qubits are the leading candidate for the basic elements in a quantum computer, there are also a range of reasons to consider using higher-dimensional qudits or quantum continuous variables (QCVs). In this paper, we use a general "quantum variable" formalism to propose a method of quantum computation in which ancillas are used to mediate gates on a well-isolated "quantum memory" register and which may be applied to the setting of qubits, qudits (for d > 2), or QCVs. More specifically, we present a model in which universal quantum computation may be implemented on a register using only repeated applications of a single fixed two-body ancilla-register interaction gate, ancillas prepared in a single state, and local measurements of these ancillas. In order to maintain determinism in the computation, adaptive measurements via a classical feed forward of measurement outcomes are used, with the method similar to that in measurement-based quantum computation (MBQC). We show that our model has the same hybrid quantum-classical processing advantages as MBQC, including the power to implement any Clifford circuit in essentially one layer of quantum computation. In some physical settings, high-quality measurements of the ancillas may be highly challenging or not possible, and hence we also present a globally unitary model which replaces the need for measurements of the ancillas with the requirement for ancillas to be prepared in states from a fixed orthonormal basis. Finally, we discuss settings in which these models may be of practical interest. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
34. Entangling the Whole by Beam Splitting a Part.
- Author
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Croal, Callum, Peuntinger, Christian, Grille, Vanessa, Marquardt, Christoph, Leuchs, Gerd, Korolkova, Natalia, and Mišta, Ladislav
- Subjects
- *
BEAM splitters , *GAUSSIAN beams , *BEAM optics , *GAUSSIAN processes , *LINEAR systems - Abstract
A beam splitter is a basic linear optical element appearing in many optics experiments and is frequently used as a continuous-variable entangler transforming a pair of input modes from a separable Gaussian state into an entangled state. However, a beam splitter is a passive operation that can create entanglement from Gaussian states only under certain conditions. One such condition is that the input light is suitably squeezed. We demonstrate, experimentally, that a beam splitter can create entanglement even from modes which do not possess such a squeezing provided that they are correlated to, but not entangled with, a third mode. Specifically, we show that a beam splitter can create three-mode entanglement by acting on two modes of a three-mode fully separable Gaussian state without entangling the two modes themselves. This beam splitter property is a key mechanism behind the performance of the protocol for entanglement distribution by separable states. Moreover, the property also finds application in collaborative quantum dense coding in which decoding of transmitted information is assisted by interference with a mode of the collaborating party. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
35. Weak values and continuous-variable entanglement concentration
- Author
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Korolkova, Natalia [School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS (United Kingdom)]
- Published
- 2007
- Full Text
- View/download PDF
36. Cross-Kerr interaction in a four-level atomic system
- Author
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Korolkova, Natalia [School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, Scotland (United Kingdom)]
- Published
- 2007
- Full Text
- View/download PDF
37. Procrustean entanglement concentration of continuous-variable states of light
- Author
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Korolkova, Natalia [School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS (United Kingdom)]
- Published
- 2006
- Full Text
- View/download PDF
38. Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations.
- Author
-
Chille, Vanessa, Quinn, Niall, Peuntinger, Christian, Croal, Callum, Mišta Jr., Ladislav, Marquardt, Christoph, Leuchs, Gerd, and Korolkova, Natalia
- Subjects
- *
QUANTUM theory , *QUANTUM entanglement , *GAUSSIAN processes , *EXPERIMENTAL design , *BIPARTITE graphs - Abstract
We provide experimental evidence of quantum features in bipartite states classified as entirely classical according to a conventional criterion based on the Glauber P function but possessing nonzero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. We experimentally verify and investigate the effect of discord increase under the action of local loss and link it to the entanglement with the environment. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
39. Distributing Entanglement with Separable States.
- Author
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Peuntinger, Christian, Chille, Vanessa, Mišta, Jr., Ladislav, Korolkova, Natalia, Förtsch, Michael, Korger, Jan, Marquardt,, Christoph, and Leuchs, Gerd
- Subjects
- *
POLYMER networks , *ELECTROMAGNETIC fields , *QUANTUM correlations , *QUANTUM information theory , *QUANTUM communication , *QUANTUM computing - Abstract
We experimentally demonstrate a protocol for entanglement distribution by a separable quantum system. In our experiment, two spatially separated modes of an electromagnetic field get entangled by local operations, classical communication, and transmission of a correlated but separable mode between them. This highlights the utility of quantum correlations beyond entanglement for the establishment of a fundamental quantum information resource and verifies that its distribution by a dual classical and separable quantum communication is possible. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
40. Nonclassical correlations in continuous-variable non-Gaussian Werner states.
- Author
-
Tatham, Richard, Mišta Jr., Ladislav, Adesso, Gerardo, and Korolkova, Natalia
- Subjects
- *
GAUSSIAN processes , *ENERGY levels (Quantum mechanics) , *STATISTICAL correlation , *MEASURE theory , *PHOTON detectors , *PHYSICAL measurements - Abstract
We study nonclassical correlations beyond entanglement in a family of two-mode non-Gaussian states which represent the continuous-variable counterpart of two-qubit Werner states. We evaluate quantum discord and other quantumness measures obtaining exact analytical results in special instances, and upper and lower bounds in the general case. Non-Gaussian measurements such as photon counting are in general necessary to solve the optimization in the definition of quantum discord, whereas Gaussian measurements are strictly suboptimal for the considered states. The gap between Gaussian and optimal non-Gaussian conditional entropy is found to be proportional to a measure of non-Gaussianity in the regime of low squeezing, for a subclass of continuous-variable Werner states. We further study an example of a non-Gaussian state which is positive under partial transposition, and whose nonclassical correlations stay finite and small even for infinite squeezing. Our results pave the way to a systematic exploration of the interplay between nonclassicality and non-Gaussianity in continuous-variable systems, in order to gain a deeper understanding of--and to draw a bigger advantage from--these two important resources for quantum technology. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
41. Measurement-induced disturbances and nonclassical correlations of Gaussian states.
- Author
-
Mišta Jr., Ladislav, Tatham, Richard, Girolami, Davide, Korolkova, Natalia, and Adesso, Gerardo
- Subjects
- *
GAUSSIAN processes , *QUANTUM theory , *PHOTONS , *OPERATOR theory , *HILBERT space , *NONCOMMUTATIVE function spaces - Abstract
We study quantum correlations beyond entanglement in two-mode Gaussian states of continuous-variable systems by means of the measurement-induced disturbance (MID) and its ameliorated version (AMID). In analogy with the recent studies of the Gaussian quantum discord, we define a Gaussian AMID by constraining the optimization to all bi-local Gaussian positive operator valued measurements. We solve the optimization explicitly for relevant families of states, including squeezed thermal states. Remarkably, we find that there is a finite subset of two-mode Gaussian states comprising pure states where non-Gaussian measurements such as photon counting are globally optimal for the AMID and realize a strictly smaller state disturbance compared to the best Gaussian measurements. However, for the majority of two-mode Gaussian states the unoptimized MID provides a loose overestimation of the actual content of quantum correlations, as evidenced by its comparison with Gaussian discord. This feature displays strong similarity with the case of two qubits. Upper and lower bounds for the Gaussian AMID at fixed Gaussian discord are identified. We further present a comparison between Gaussian AMID and Gaussian entanglement of formation, and classify families of two-mode states in terms of their Gaussian AMID, Gaussian discord, and Gaussian entanglement of formation. Our findings provide a further confirmation of the genuinely quantum nature of general Gaussian states, yet they reveal that non-Gaussian measurements can play a crucial role for the optimized extraction and potential exploitation of classical and nonclassical correlations in Gaussian states. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
42. The enigma of imaging in the Maxwell fisheye medium
- Author
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Sahebdivan(i), Sahar, Leonhardt, Ulf, Korolkova, Natalia, Scottish Overseas Research Student Awards Scheme (SORSAS), and Engineering and Physical Sciences Research Council (EPSRC)
- Subjects
Super-resolution ,Transformation optics ,Overcoming the diffraction limit ,QC385.S25 ,Optical instruments ,Maxwell fisheye lens ,Electromagnetic fields on curved manifolds ,Space-time analogy by transformation optics ,Lenses - Abstract
The resolution of optical instruments is normally limited by the wave nature of light. Circumventing this limit, known as the diffraction limit of imaging, is of tremendous practical importance for modern science and technology. One method, super-resolved fluorescence microscopy was distinguished with the Nobel Prize in Chemistry in 2014, but there is plenty of room for alternatives and complementary methods such as the pioneering work of Prof. J. Pendry on the perfect lens based on negative refraction that started the entire research area of metamaterials. In this thesis, we have used analytical techniques to solve several important challenges that have risen in the discussion of the microwave experimental demonstration of absolute optical instruments and the controversy surrounding perfect imaging. Attempts to overcome or circumvent Abbe’s diffraction limit of optical imaging, have traditionally been greeted with controversy. In this thesis, we have investigated the role of interacting sources and detectors in perfect imaging. We have established limitations and prospects that arise from interactions and resonances inside the lens. The crucial role of detection becomes clear in Feynman’s argument against the diffraction limit: “as Maxwell’s electromagnetism is invariant upon time reversal, the electromagnetic wave emitted from a point source may be reversed and focused into a point with point-like precision, not limited by diffraction.” However, for this, the entire emission process must be reversed, including the source: A point drain must sit at the focal position, in place of the point source, otherwise, without getting absorbed at the detector, the focused wave will rebound and the superposition of the focusing and the rebounding wave will produce a diffraction-limited spot. The time-reversed source, the drain, is the detector which taking the image of the source. In 2011-2012, experiments with microwaves have confirmed the role of detection in perfect focusing. The emitted radiation was actively time-reversed and focused back at the point of emission, where, the time-reversed of the source sits. Absorption in the drain localizes the radiation with a precision much better than the diffraction limit. Absolute optical instruments may perform the time reversal of the field with perfectly passive materials and send the reversed wave to a different spatial position than the source. Perfect imaging with absolute optical instruments is defected by a restriction: so far it has only worked for a single–source single–drain configuration and near the resonance frequencies of the device. In chapters 6 and 7 of the thesis, we have investigated the imaging properties of mutually interacting detectors. We found that an array of detectors can image a point source with arbitrary precision. However, for this, the radiation has to be at resonance. Our analysis has become possible thanks to a theoretical model for mutually interacting sources and drains we developed after considerable work and several failed attempts. Modelling such sources and drains analytically had been a major unsolved problem, full numerical simulations have been difficult due to the large difference in the scales involved (the field localization near the sources and drains versus the wave propagation in the device). In our opinion, nobody was able to reproduce reliably the experiments, because of the numerical complexity involved. Our analytic theory draws from a simple, 1–dimensional model we developed in collaboration with Tomas Tyc (Masaryk University) and Alex Kogan (Weizmann Institute). This model was the first to explain the data of experiment, characteristic dips of the transmission of displaced drains, which establishes the grounds for the realistic super-resolution of absolute optical instruments. As the next step in Chapter 7 we developed a Lagrangian theory that agrees with the simple and successful model in 1–dimension. Inspired by the Lagrangian of the electromagnetic field interacting with a current, we have constructed a Lagrangian that has the advantage of being extendable to higher dimensions in our case two where imaging takes place. Our Lagrangian theory represents a device-independent, idealized model independent of numerical simulations. To conclude, Feynman objected to Abbe’s diffraction limit, arguing that as Maxwell’s electromagnetism is time-reversal invariant, the radiation from a point source may very well become focused in a point drain. Absolute optical instruments such as the Maxwell Fisheye can perform the time reversal and may image with a perfect resolution. However, the sources and drains in previous experiments were interacting with each other as if Feynman’s drain would act back to the source in the past. Different ways of detection might circumvent this feature. The mutual interaction of sources and drains does ruin some of the promising features of perfect imaging. Arrays of sources are not necessarily resolved with arrays of detectors, but it also opens interesting new prospects in scanning near-fields from far–field distances. To summarise the novel idea of the thesis: • We have discovered and understood the problems with the initial experimental demonstration of the Maxwell Fisheye. • We have solved a long-standing challenge of modelling the theory for mutually interacting sources and drains. • We understand the imaging properties of the Maxwell Fisheye in the wave regime. Let us add one final thought. It has taken the scientific community a long time of investigation and discussion to understand the different ingredients of the diffraction limit. Abbe’s limit was initially attributed to the optical device only. But, rather all three processes of imaging, namely illumination, transfer and detection, make an equal contribution to the total diffraction limit. Therefore, we think that for violating the diffraction limit one needs to consider all three factors together. Of course, one might circumvent the limit and achieve a better resolution by focusing on one factor, but that does not necessary imply the violation of a fundamental limit. One example is STED microscopy that focuses on the illumination, another near–field scanning microscopy that circumvents the diffraction limit by focusing on detection. Other methods and strategies in sub-wavelength imaging –negative refraction, time reversal imaging and on the case and absolute optical instruments –are concentrating on the faithful transfer of the optical information. In our opinion, the most significant, and naturally the most controversial, part of our findings in the course of this study was elucidating the role of detection. Maxwell’s Fisheye transmits the optical information faithfully, but this is not enough. To have a faithful image, it is also necessary to extract the information at the destination. In our last two papers, we report our new findings of the contribution of detection. We find out in the absolute optical instruments, such as the Maxwell Fisheye, embedded sources and detectors are not independent. They are mutually interacting, and this interaction influences the imaging property of the system. EPSRC grant for the QUEST project (The Quest for Ultimate Electromagnetic using Spatial Transformations )
- Published
- 2016
43. Procrustean entanglement concentration, weak measurements and optimized state preparation for continuous-variable quantum optics
- Author
-
Menzies, David, Korolkova, Natalia, and Engineering and Physical Sciences Research Council (EPSRC)
- Subjects
Entanglement ,Quantum optics ,Quantum information ,Quantum theory ,QC446.2M46 ,Quantum communication - Abstract
In this thesis, we are concerned with continuous-variable quantum optical state engineering protocols. Such protocols are designed to repair or enhance the nonclassical features of a given state. In particular, we build a weak measurement model of Gaussian entanglement concentration of the two mode squeezed vacuum state. This model allows the simultaneous description of all possible ancilla system variations. In addition, it provides an explanation of the Gaussian-preserving property of these protocols while providing a success criterion which links all of the degrees of freedom on the ancilla. Following this, we demonstrate the wider application of weak measurements to quantum optical state engineering by showing that they allow probabilistic noiseless amplifi cation of photon number. We then establish a connection between weak measurements and entanglement concentration as a fundamental result of weak measurements on entangled probes. After this, we explore the trade-off between Gaussian and non-Gaussian operations in the preparation of non-Gaussian pure states. In particular, we suggest that an operational cost for an arbitrary non-Gaussian pure state is the largest Fock state required for its approximate preparation. We consider the extent to which this non-Gaussian operational cost can be reduced by applying unitary Gaussian operations. This method relies on the identification of a minimal core state for any target non-Gaussian pure state.
- Published
- 2009
44. Free-Space Quantum Signatures Using Heterodyne Measurements.
- Author
-
Croal, Callum, Peuntinger, Christian, Heim, Bettina, Khan, Imran, Marquardt, Christoph, Leuchs, Gerd, Wallden, Petros, Andersson, Erika, and Korolkova, Natalia
- Subjects
- *
DIGITAL signatures , *TELECOMMUNICATION , *QUANTUM mechanics - Abstract
Digital signatures guarantee the authorship of electronic communications. Currently used "classical" signature schemes rely on unproven computational assumptions for security, while quantum signatures rely only on the laws of quantum mechanics to sign a classical message. Previous quantum signature schemes have used unambiguous quantum measurements. Such measurements, however, sometimes give no result, reducing the efficiency of the protocol. Here, we instead use heterodyne detection, which always gives a result, although there is always some uncertainty. We experimentally demonstrate feasibility in a real environment by distributing signature states through a noisy 1.6 km free-space channel. Our results show that continuous-variable heterodyne detection improves the signature rate for this type of scheme and therefore represents an interesting direction in the search for practical quantum signature schemes. For transmission values ranging from 100% to 10%, but otherwise assuming an ideal implementation with no other imperfections, the signature length is shorter by a factor of 2 to 10. As compared with previous relevant experimental realizations, the signature length in this implementation is several orders of magnitude shorter. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
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