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The interaction between cavity photons and molecular vibrations leads to the formation of vibrational polaritons, which have demonstrated the ability to influence chemical reactivity and change material characteristics. Although ultrafast spectroscopy has been extensively applied to study vibrational polaritons, the nonlinear relationship between signal and quantum state population complicates the analysis of their kinetics. Here, we employ a second-order kinetic model and transform matrix method (TMM) to develop an effective model to capture the nonlinear relationship between the two-dimensional IR (or pump–probe) signal and excited state populations. We test this method on two types of kinetics: a sequential relaxation from the second to the first excited states of dark modes, and a Raman state relaxing into the first excited state. By globally fitting the simulated data, we demonstrate accurate extraction of relaxation rates and the ability to identify intermediate species by comparing the species spectra with theoretical ground truth, validating our method. This study demonstrates the efficacy of a second-order TMM approximation in capturing essential spectral features with up to 10% excited state population, simplifying global analysis and enabling straightforward extraction of kinetic parameters, thus empowering our methodology in understanding excited-state dynamics in polariton systems. [ABSTRACT FROM AUTHOR]

When combined with nanostructured substrates, two-dimensional semiconductors can be engineered with strain to tailor light–matter interactions on the nanoscale. Recently, room-temperature nanoscale exciton localization with controllable wrinkling in 1L-WSe2 was achieved using arrays of gold nanocones. Here, the characterization of quantum dot-like states and single-photon emitters in the 1L-WSe2/nanocone system is reported. The nanocones induce a wide range of strains, and as a result, a diverse ensemble of narrowband, potential single-photon emitters is observed. The distribution of emitter energies reveals that most reside in two spectrally isolated bands, leaving a less populated intermediate band that is spectrally isolated from the ensembles. The spectral isolation is advantageous for high-purity quantum light emitters, and anti-bunched emission from one of these states is confirmed up to 25 K. Although the spatial distribution of strain is expected to influence the orientation of the transition dipoles of the emitters, multimodal emission polarization anisotropy and atomic force microscopy reveal that the macroscopic orientation of the wrinkles is not a good predictor of dipole orientation. Finally, the emission is found to change with thermal cycling from 4 to 290 K and back to 4 K, highlighting the need to control factors such as temperature-induced strain to enhance the robustness of this quantum emitter platform. The initial characterization here shows that controlled nanowrinkles of 1L-WSe2 generate quantum light in addition to uncovering potential challenges that need to be addressed for their adoption into quantum photonic technologies. [ABSTRACT FROM AUTHOR]

When combined with nanostructured substrates, two-dimensional semiconductors can be engineered with strain to tailor light–matter interactions on the nanoscale. Recently, room-temperature nanoscale exciton localization with controllable wrinkling in 1L-WSe2 was achieved using arrays of gold nanocones. Here, the characterization of quantum dot-like states and single-photon emitters in the 1L-WSe2/nanocone system is reported. The nanocones induce a wide range of strains, and as a result, a diverse ensemble of narrowband, potential single-photon emitters is observed. The distribution of emitter energies reveals that most reside in two spectrally isolated bands, leaving a less populated intermediate band that is spectrally isolated from the ensembles. The spectral isolation is advantageous for high-purity quantum light emitters, and anti-bunched emission from one of these states is confirmed up to 25 K. Although the spatial distribution of strain is expected to influence the orientation of the transition dipoles of the emitters, multimodal emission polarization anisotropy and atomic force microscopy reveal that the macroscopic orientation of the wrinkles is not a good predictor of dipole orientation. Finally, the emission is found to change with thermal cycling from 4 to 290 K and back to 4 K, highlighting the need to control factors such as temperature-induced strain to enhance the robustness of this quantum emitter platform. The initial characterization here shows that controlled nanowrinkles of 1L-WSe2 generate quantum light in addition to uncovering potential challenges that need to be addressed for their adoption into quantum photonic technologies. [ABSTRACT FROM AUTHOR]

Oblivious transfer (OT) is a significant two party privacy preserving cryptographic primitive. OT involves a sender having several pieces of information and a receiver having a choice bit. The choice bit represents the piece of information that the receiver wants to obtain as an output of OT. At the end of the protocol, sender remains oblivious about the choice bit and receiver remains oblivious to the contents of the information that were not chosen. It has applications ranging from secure multi-party computation, privacy-preserving protocols to cryptographic protocols for secure communication. Most of the classical OT protocols are based on number theoretic assumptions which are not quantum secure and existing quantum OT protocols are not so efficient and practical. Herein, we present the design and analysis of a simple yet efficient quantum OT protocol, namely qOT. qOT is designed by using the asymmetric key distribution proposed by Gao et al. (Opt Express 20(16):17411–17420, 2012) as a building block. The designed qOT requires only single photons as a source of a quantum state, and the measurements of the states are computed using single particle projective measurement. These make qOT efficient and practical. Our proposed design is secure against quantum attacks. Moreover, qOT also provides long-term security. [ABSTRACT FROM AUTHOR]

Quantum Secure Direct Communication (QSDC) is a promising approach for secure information exchange. This paper proposes an efficient and secure four-party QSDC scheme utilizing hyperentangled Bell states in the polarization degree of freedom, the first longitudinal momentum degree of freedom and the second longitudinal momentum degree of freedom. The four participants can perform different unitary operations to independently encode their secret messages onto photons in three degrees of freedom, subsequently transmitting them directly through the quantum channel. In this proposed protocol, each degree of freedom of the photon can effectively carry two bits of information. Each round of transmission by a photon enables the four legitimate participants to obtain six classical bits of information. Notably, when compared to other photons based single-degree-of-freedom QSDC network protocols, the capacity of proposed QSDC protocol is tripled. Therefore, it significantly enhances the information transmission capability. Furthermore, comprehensive security analysis shows that our QSDC network protocol can withstand various attacks from external eavesdroppers. [ABSTRACT FROM AUTHOR]

We carry out a theoretical investigation about quantum noise in coherent mixtures of states with different statistics. On the one hand, we investigate the dependence between the behavior wave-particle and associated noise. On the other hand, we are interested in studying the persistence of chaotic behavior in mixtures of states with quantum and classical behavior. For this, we perform mixtures of single photons with coherent states of different averages. We find that the noise associated with quantum chaos depends on the proportion of the quantum–classical mixture and that quantum wave states are more efficient than classical wave states in terms of their randomness properties. [ABSTRACT FROM AUTHOR]

We consider Markovian open quantum dynamics (MOQD) in the continuum. We show that, up to small-probability tails, the supports of quantum states evolving under such dynamics propagate with finite speed in any finite-energy subspace. More precisely, we prove that if the initial quantum state is localized in space, then any finite-energy part of the solution of the von Neumann–Lindblad equation is approximately localized inside an energy-dependent light cone. We also obtain an explicit upper bound for the slope of this light cone. [ABSTRACT FROM AUTHOR]

Conditional addition of photons represents a crucial tool for optical quantum state engineering and it forms a fundamental building block of advanced quantum photonic devices. Here we report on experimental implementation of the conditional addition of several photons. We demonstrate the addition of one, two, and three photons to input coherent states with various amplitudes. The resulting highly nonclassical photon-added states are completely characterized with time-domain homodyne tomography, and the nonclassicality of the prepared states is witnessed by the negativity of their Wigner functions. We experimentally demonstrate that the conditional addition of photons realizes approximate noiseless quantum amplification of coherent states with sufficiently large amplitude. We also investigate certification of the stellar rank of the generated multiphoton-added coherent states, which quantifies the non-Gaussian resources required for their preparation. Our results pave the way towards the experimental realization of complex optical quantum operations based on combination of multiple photon additions and subtractions. [ABSTRACT FROM AUTHOR]

As one of the most important branches of quantum information science, quantum communication is known for its unconditional security and efficiency. Nevertheless, the practical security of quantum key distribution protocols and quantum secure direct communication protocols is challenged due to the imperfections in experimental devices. Despite significant progress in theoretical and experimental research on the MDI-QSDC Protocol, challenges and unresolved issues remain. For example, further enhancing the scalability and system complexity of the protocol to meet the demands of large-scale quantum networks is necessary. In this paper, we propose a multi-party MDI-QSDC scheme based on multi-degree-of-freedom hyperentangled photons. Compared to the original MDI-QSDC protocol, our protocol allows multiple parties to participate in the information transmission process. For example, for four communicating parties, we can encode the information of three independent degrees of freedom so that each photon of each degree of freedom can transmit 2 bits of information. Moreover, all measurement tasks are performed by the fifth party, which can be untrusted or even completely controlled by eavesdroppers. The protocol is resistant to all possible attacks from imperfect measurement devices. It can eventually be extended to arbitrary degrees of freedom, allowing multiple parties to participate. [ABSTRACT FROM AUTHOR]

Neural networks are becoming ubiquitous in various areas of physics as a successful machine learning (ML) technique for addressing different tasks. Based on ML technique, we propose and experimentally demonstrate an efficient method for state reconstruction of the widely used Sagnac polarization-entangled photon source. By properly modeling the target states, a multi-output fully connected neural network is well trained using only six of the sixteen measurement bases in standard tomography technique, and hence our method reduces the resource consumption without loss of accuracy. We demonstrate the ability of the neural network to predict state parameters with a high precision by using both simulated and experimental data. Explicitly, the mean absolute error for all the parameters is below 0.05 for the simulated data and a mean fidelity of 0.99 is achieved for experimentally generated states. Our method could be generalized to estimate other kinds of states, as well as other quantum information tasks. [ABSTRACT FROM AUTHOR]

Polarization-encoded single-photon qubit and single-photon entanglement are important resources of quantum communication. In practical long-distance quantum communication, photon transmission loss caused by the channel noise is a big obstacle. Noiseless linear amplification (NLA) is a useful method to solve the photon transmission loss problem. In the paper, we propose two efficient NLA protocols for the polarization-encoded single-photon qubit and single-photon entanglement, respectively. Our NLA protocols can increase the fidelity of the target photon state and perfectly preserve the encoded polarization feature. Moreover, local-quadrature squeezing operation is introduced into the NLA protocols to further improve the amplification. Our NLA protocols have important application in future quantum communication field. [ABSTRACT FROM AUTHOR]

We investigate the utilization of advanced single photons produced by quantum dots (QDs) in a microcavity for quantum metrology. Through the integration of lateral excitation and the Purcell effect in an Fabry–Perot microcavity, we realized single-photon emission with an extraction efficiency of 46.39%, high purity of 96.91%, and high indistinguishability of 98.32%. Our QD-generated single photons enabled the creation of high-quality NOON states (N = 2) for phase measurement, yielding an interference contrast of 79.79% and surpassing the standard quantum limit (SQL) with phase super-sensitivity. Our results underscore the immense potential of QD-derived single photons for propelling quantum metrology forward, facilitating enhanced precision measurements across diverse applications. [ABSTRACT FROM AUTHOR]

Fractional quantum Hall (FQH) states are known for their robust topological order and possess properties that are appealing for applications in fault-tolerant quantum computing. An engineered quantum platform would provide opportunities to operate FQH states without an external magnetic field and enhance local and coherent manipulation of these exotic states. We demonstrate a lattice version of photon FQH states using a programmable on-chip platform based on photon blockade and engineering gauge fields on a two-dimensional circuit quantum electrodynamics system. We observe the effective photon Lorentz force and butterfly spectrum in the artificial gauge field, a prerequisite for FQH states. After adiabatic assembly of Laughlin FQH wave function of 1/2 filling factor from localized photons, we observe strong density correlation and chiral topological flow among the FQH photons. We then verify the unique features of FQH states in response to external fields, including the incompressibility of generating quasiparticles and the smoking-gun signature of fractional quantum Hall conductivity. Our work illustrates a route to the creation and manipulation of novel strongly correlated topological quantum matter composed of photons and opens up possibilities for fault-tolerant quantum information devices. [ABSTRACT FROM AUTHOR]

In this paper, we design the first semiquantum private comparison (SQPC) protocol which is realized via cavity quantum electrodynamics (QED) by making use of the evolution law of atom. With the help of a semi-honest third party (TP), the proposed protocol can compare the equality of private inputs from two semiquantum parties who only have limited quantum capabilities. The proposed protocol uses product states as initial quantum resource and employs none of unitary operations, quantum entanglement swapping operation or delay lines. Security proof turns out that it can defeat both the external attack and the internal attack. [ABSTRACT FROM AUTHOR]

Based on the four-dimensional layered quantum states and the delegating quantum computation (DQC) model, a new quantum private comparison (QPC) protocol is designed to compare private information between two participants with the help of a semi-honest third party (TP). The TP may misbehave on his own, though he enables to perform the protocol loyally. Benefiting from the properties of 4D layered quantum states in quantum secure communication, the QPC protocol performs the comparison of secrets better. The proposed QPC protocol based on a DQC scheme can be realized by the participants with insufficient capacity. The security of the proposed protocol is ensured by the usage of decoy photons and location obfuscation technique of sequences. The correctness of the proposed protocol is validated by simulations on the IBM's qiskit. The performance of the protocol against the external and internal attacks is also analyzed. [ABSTRACT FROM AUTHOR]

The preparation of Fock states is important for quantum information storage and processing. Herein, a two‐step protocol with topological protection for multi‐photon Fock states synthesizing in a circuit quantum electrodynamics (QED) architecture is presented. Due to photon number‐dependent vacuum Rabi splitting, the dynamics of the Jaynes–Cummings ladder in the resonant regime can be equivalent to a spin‐1/2 chain while a multi‐frequency microwave pulse is applied. Based on the topologically assisted quantum state transfer (QST)technique, multi‐photon Fock states can be conditionally generated. This method significantly reduces the complexity of experimental manipulation compared with previous multi‐step methods and is more robust as a result of topological protection. Furthermore, the protocol indicates that the circuit QED architecture could potentially provide a platform to explore the exotic phase in condensed matter. [ABSTRACT FROM AUTHOR]

The design and parameters of scintillation detectors of Compton polarimeters for measuring photons formed during the annihilation of electron-positron pairs at rest are considered. The discussed setup makes it possible to measure and compare the polarization correlations of scattered annihilation photons in these two quantum states. The amplitude parameters of scatterers and scattered photon detectors obtained from preliminary tests of these detectors using radioactive gamma sources, as well as in the process of collecting experimental data, are given. The possibility of measuring and comparing the polarization correlations of scattered annihilation photons is shown. [ABSTRACT FROM AUTHOR]

Kinetic models are essential for describing how molecules interact in a variety of biochemical processes. The estimation of a model's kinetic parameters by experiment enables researchers to understand how pathogens, such as viruses, interact with other entities like antibodies and trial drugs. In this work, we report a simple proof-of-principle experiment that uses quantum sensing techniques to give a more precise estimation of kinetic parameters than is possible with a classical approach. The interaction we study is that of bovine serum albumin (BSA) binding to gold via an electrostatic mechanism. BSA is an important protein in biochemical research as it can be conjugated with other proteins and peptides to create sensors with a wide range of specificity. We use single photons generated via parametric down-conversion to probe the BSA–gold interaction in a plasmonic resonance sensor. We find that sub-shot-noise-level fluctuations in the sensor signal allow us to achieve an improvement in the precision of up to 31.8% for the values of the kinetic parameters. This enhancement can, in principle, be further increased in the setup. Our work highlights the potential use of quantum states of light for sensing in biochemical research. [ABSTRACT FROM AUTHOR]

Quantum photonic platforms have proven to be essential in realizing fundamentally secure quantum transfer of information, with commercially ready systems already deployed in municipal and terrestrial links. The drive toward higher bit rates and robustness to eavesdropping and noisy channels has focused attention on moving from the present two-dimensional quantum states of polarization, to harnessing all of light's degrees of freedom for multi-dimensional quantum coding with structured photons. In this Perspective, we outline the present state-of-the-art in achieving this control with spatial modes of light, both as single photon and entangled states, highlight the open challenges that remain, and consider the roadmap that might see its full potential realized. [ABSTRACT FROM AUTHOR]

To examine the second-order coherence of light propagation of quantum states in arbitrary directions through dispersive non-Hermitian optical media, we considered two sets of non-Hermitian periodic structures that consist of gain/loss unit cells. We show that each batch can satisfy the parity-time symmetry conditions at a distinct frequency. We then varied the gain/loss strength in the stable electromagnetic regime to evaluate the transmittance of N-photon number states through each structure. The results show both sets preserve their antibunching characteristics under specific incident light conditions. Furthermore, s(p)-polarized light exhibits higher (lower) second-order coherence at larger incident angles. In addition, the antibunching features of the transmitted states degrade with an increase in the number of unit cells in multilayered structures for both polarizations. [ABSTRACT FROM AUTHOR]

Optimal preparation of quantum entanglement is of significance to information processing and state engineering. In this paper, an efficient scheme is proposed to implement error-insensitive generation of entangled states between a Josephson qubit and microwave photons by the technique of invariant-based shortcuts to adiabaticity. A superconducting qubit is dispersively coupled to a quantized cavity field of one-dimensional transmission line resonator. Within a considered subspace spanned by three composite states, we deal with an effective interaction of the composite system with two classical drivings. A maximally entangled qubit-photon state can be deterministically induced using a splitting-like quantum state transfer. To nullify the deviation errors of Rabi coupling and frequency detuning, we optimize the driving parameters and then make the entanglement creation insusceptible to these control imperfections. Thanks to the mitigation of deviation effects, robustness against the residual noisy environment could be obtained numerically. The proposed strategy could provide a promising avenue towards fast and robust information processing with superconducting circuit quantum electrodynamics. [ABSTRACT FROM AUTHOR]

Quantum key distribution (QKD), that is, exchanging cryptographic keys encoded in quantum particles exploiting the laws of quantum physics, is already a reality in our society. Current implementations are based on attenuated laser technique, a practical replacement of single photons which requires a random phase for each quantum state in order to achieve the highest level of security. In particular, the time‐bin and phase encoding techniques are mainly exploiting laser in gain‐switching modes combined with asymmetric interferometers or multiple laser sources in a master–slave configuration, which present limitations in terms of stability and scalability. In this work, a novel scheme for implementing a reconfigurable and scalable QKD transmitter based on the time‐bin encoding protocol with a decoy‐state method employing phase‐randomized weak coherent states is proposed and demonstrated. The scheme is tested and validated up to 26 dB‐attenuation channel using standard single‐photon detectors working in the telecom wavelength range. [ABSTRACT FROM AUTHOR]

To harness the potential of a quantum computer, quantum information must be protected against error by encoding it into a logical state that is suitable for quantum error correction. The Gottesman-KitaevPreskill (GKP) qubit is a promising candidate because the required multiqubit operations are readily available at optical frequency. To date, however, GKP qubits have been demonstrated only at mechanical and microwave frequencies. We realized a GKP state in propagating light at telecommunication wavelength and verified it through homodyne measurements without loss corrections. The generation is based on interference of cat states, followed by homodyne measurements. Our final states exhibit nonclassicality and non-Gaussianity, including the trident shape of faint instances of GKP states. Improvements toward brighter, multipeaked GKP qubits will be the basis for quantum computation with light. [ABSTRACT FROM AUTHOR]

Quantum secure direct communication (QSDC) can guarantee security using the characteristics of quantum mechanics even when a message is directly transmitted through a quantum channel without using a secret key. However, the transmission rate of the QSDC is limited by the dead time of a single photon detector (SPD) as well as channel loss over the distance. To overcome this limited transmission rate, we propose a high-dimensional single photon-based QSDC protocol that applies two optical degrees of freedom: time and phase state. First, an N-dimensional time and phase state generation method that considers the dead time is proposed to minimize the measurement loss of a transmitted message. Second, among the two types of quantum states, the phase state with relatively low measurement efficiency is used only for eavesdropping detection, and the time state is used for sending messages with differential delay time bin-based encoding techniques. Lastly, we propose an efficient method for measuring N-dimensional time and phase-based quantum states and recovering classical bit information. This study performs security analysis against various attacks, and verifies the transmission rate improvement effect through simulation. The result indicates that our proposal can guarantee higher security and transmission rates compared to the conventional DL04 QSDC. [ABSTRACT FROM AUTHOR]

This paper proposes two efficient block transmission and two efficient two-step transmission quantum dialogue (QD) protocols that are robust against collective-dephasing and collective-rotation noises, respectively. To counter collective noise, the carriers used to carry the message must correspond to the decoherence-free states under this collective noise. In addition to carrying messages, these quantum states and their combinations are used to ensure the security of transmission and prevent message distortion. In quantum communications, decoy photons are often used to detect eavesdroppers and always account for half of the total number of qubits, which is a burden on scarce quantum resources. Due to quantum state disclosure, the decoy photons used for inspection can no longer be utilized for transmission. Therefore, a dictionary is employed as an encoding mechanism to achieve self-checking without revealing these states. This approach enables photons to detect eavesdroppers while concurrently carrying keys. The proposed QD protocols can eliminate the consumption of decoy photons, significantly improve the qubit efficiency, and conduct security analysis to ensure that there is no information leakage. Two transmission modes with the same efficiency can be selected according to the noisy environment and time windows. [ABSTRACT FROM AUTHOR]

Two-photon absorption (TPA) and other nonlinear interactions of molecules with time–frequency-entangled photon pairs have been predicted to display a variety of fascinating effects. Therefore, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This Tutorial presents a detailed theoretical study of one- and two-photon absorption by molecules, focusing on how to treat the quantum nature of light. We review some basic quantum optics theory and then we review the density-matrix (Liouville) derivation of molecular optical response, emphasizing how to incorporate quantum states of light into the treatment. For illustration, we treat in detail the TPA of photon pairs created by spontaneous parametric down conversion, with an emphasis on how quantum light TPA differs from that with classical light. In particular, we treat the question of how much enhancement of the TPA rate can be achieved using entangled states. This Tutorial includes a review of known theoretical methods and results as well as some extensions, especially the comparison of TPA processes that occur via far-off-resonant intermediate states only and those that involve off-resonant intermediate states by virtue of dephasing processes. A brief discussion of the main challenges facing experimental studies of entangled two-photon absorption is also given. [ABSTRACT FROM AUTHOR]

A novel method for controlling and manipulating quantum states of both matter and field has been developed. The approach has been applied to govern the population in rotational levels of weakly aligned molecules. This technique entails the use of an adiabatically varying electric field to interact with dipole molecules. The interaction of weakly aligned molecules with a microwave field within a high finesse cavity indicates the feasibility of achieving population inversion, rendering them well suited for maser operation even at room temperature. In addition to this, the research has shown that the absorption of molecules can be controlled too making it exceptionally well suited for gas sensing with high sensitivity and selectivity. These sensors hold promise across diverse domains, including technology, the sciences, environmental monitoring, biology, and medicine. Furthermore, the technique has the potential for employing control techniques to vacuum fields that holds promise for the generation of strongly correlated, entangled photons. This development could have far-reaching implications in quantum technology, and various applications in quantum information science and beyond. [ABSTRACT FROM AUTHOR]

Postselective transformations of quantum states is a broader class of operations than deterministic quantum channels. Here, we describe the possibility of increasing the success probability of postselective operations by using additional information, which has a form of pure quantum states and should not be changed in case of success. We describe the conditions under which assistance becomes useful, and provide application of our method which improves the efficiency of photon number splitting attack for a variant of SARG04 quantum key distribution protocol. In our attack scenario, one extra photon, which is unchanged, plays the role of assistance. [ABSTRACT FROM AUTHOR]

By using the six-particle maximally entangled cluster state as a quantum channel, three new theoretical protocols are put forward to synchronously transmit different quantum information from one transmitter to multiple receivers. The first protocol is a three-party quantum multicast protocol, which realizes the synchronous transmission of two different known states of arbitrary single-particle to two receivers with a 100% success probability. The other two protocols are the five-party protocol of multi-output quantum state sharing and five-party protocol of multi-output quantum operation sharing, respectively. In each protocol, four receivers are evenly divided into two groups, both groups are allowed to definitively and synchronously share different single-particle unknown states or different partially unknown single-particle Quantum operations, respectively. In addition, the above three protocols can not only be modified respectively into quantum broadcast protocol, multi-output controlled teleportation protocol and multi-output controlled quantum remote control protocol, but also can be extended to the situation of six-particle non-maximally entangled channel. [ABSTRACT FROM AUTHOR]

By utilizing a generalized version of the Madelung quantum hydrodynamic framework that incorporates noise, we derive a solution using the path integral method to investigate how a quantum superposition of states evolves over time. This exploration seeks to comprehend the process through which a stable quantum state emerges when fluctuations induced by the noisy gravitational background are present. The model defines the conditions that give rise to a limited range of interactions for the quantum potential, allowing for the existence of coarse-grained classical descriptions at a macroscopic level. The theory uncovers the smallest attainable level of uncertainty in an open quantum system and examines its consistency with the localized behavior observed in large-scale classical systems. The research delves into connections and similarities alongside other theories such as decoherence and the Copenhagen foundation of quantum mechanics. Additionally, it assesses the potential consequences of wave function decay on the measurement of photon entanglement. To validate the proposed theory, an experiment involving entangled photons transmitted between detectors on the moon and Mars is discussed. Finally, the findings of the theory are applied to the creation of larger Q-bit systems at room temperatures. [ABSTRACT FROM AUTHOR]

Structured photons are a crucial resource in both classical and quantum technologies, particularly in spin–orbit hybrid photons, enabling various practical applications ranging from ultra-sensitive metrology techniques to quantum-enhanced information processing tasks. However, the two-photon interference of spin–orbit hybrid photons, which combines polarization modes and complex transverse spatial structures across the beam profile, remains unexplored. Here, we present an experimental observation of Hong–Ou–Mandel (HOM) interference of spin–orbit hybrid photons. The tunable q-plates that work as spin–orbit coupler devices are used to prepare various forms of spin–orbit hybrid entangled photons. By harnessing the match degree in the temporal domain, the coalescence and anti-coalescence effects resulting from the symmetric and anti-symmetric properties of the incident quantum states are observed. Moreover, we demonstrated the feasibility of quantum-enhanced photon polarization gears through HOM interference and theoretically analyze the noise-resilient advantages based on coherent HOM measurements. These results provide an alternative route toward quantum experiments with structured photons that allows for controlling their quantum interference in a compact, stable, and efficient way. [ABSTRACT FROM AUTHOR]

The storage of quantum states and information is essential for enabling large quantum networks. The direct implementation of storage in magnonic systems, which are emerging as crucial components in quantum networks, has also garnered attention. In this study, we present experimental investigations of magnomechanical microwave storage for the first time. By reducing the ambient temperature to 8 K, we can achieve a mechanical mode with a narrow linewidth as low as 6.4 Hz, resulting in an energy decay time of 24.8 ms. Furthermore, we employ Ramsey interferometry to investigate the coherence of the magnomechanical memory. The mechanical interference can be utilized to evaluate the decoherence lifetime of 19.5 ms. Our proposed scheme provides the potential to utilize magnomechanical systems as quantum memory for photonic quantum information. [ABSTRACT FROM AUTHOR]

High‐dimensional quantum systems offer many advantages over low‐dimensional quantum systems. Meanwhile, unitary transformations on quantum states are important parts in various quantum information tasks, whereas they become technically infeasible as the dimensionality increases. The photonic orbital angular momentum (OAM), which is inherent in the transverse spatial mode of photons, offers a natural carrier to encode information in high‐dimensional spaces. However, it is even more challenging to realize arbitrary unitary transformations on the photonic OAM states. Here, by combining the path and OAM degrees of freedom of a single photon, an efficient scheme to realize arbitrary unitary transformations on the path‐OAM coupled quantum states is proposed. The proposal reduces the number of required interferometers by approximately one quarter compared with previous works, while maintaining the symmetric structure. It is shown that by using OAM‐to‐path interfaces, this scheme can be utilized to realize arbitrary unitary transformations on the OAM states of photons. This work facilitates the development of high‐dimension quantum state transformations and opens a new door to the manipulation of the photonic OAM states. [ABSTRACT FROM AUTHOR]

A Greenberger–Horne–Zeilinger (GHZ) entangled state with a phase is crucial for realizing desired multipartite quantum states for practical applications. Here, it is reported that the violations of the general Bell inequality (GBI) introduced in using the four‐photon polarization‐entangled phase‐GHZ state realized via intrinsic polarization correlation and collective two‐photon coherence in the 5S1/2–5P3/2–5D5/2 transition of 87Rb atoms. The phase‐GHZ state can be achieved by the unitary transformation of only one local phase of the four photons. Theoretically, the GHZ state with the π/4 phase affords maximal violation of the GBI of 2√2 at the local measurement settings of the Pauli operators σx and σy. Strong violations of the GBI of the phase‐GHZ state by 47 standard deviations are experimentally demonstrated. In addition to the entanglement witness for the phase‐GHZ state, the results represent a genuine four‐photon entanglement of the phase‐GHZ state, thereby providing a novel resource for realizing photonic quantum computation, magic‐state distillation from entangled states, and quantum networks based on atom–photon interactions. [ABSTRACT FROM AUTHOR]

We propose a novel general approximation to transform and simplify the description of a complex fully quantized system describing the interacting light and matter. The method has some similarities to the time-dependent Born–Oppenheimer approach: we consider a quantum description of light rather than of nuclei and follow a similar separation procedure. Our approximation allows us to obtain a decoupled system for the light-excited matter and "dressed" light connected parametrically. With these equations at hand, we study how intense light as a quantum state is affected due to the back-action of the interacting matter. We discuss and demonstrate the possibility of the light-mode entanglement and nonclassical light generation during the interaction. [ABSTRACT FROM AUTHOR]

Glauber's g(2)-function provides a common measure of quantum field statistics through two-photon coincidence counting in Hanbury Brown–Twiss measurements. Here, we propose to use nonlinear optical signals as a tool for the characterization of quantum light. In particular, we show that Raman measurements provide an alternative direct probe for a different component of the four-point correlation function underlying the g(2)-function. We illustrate this capacity for a specific quantum state obtained from a frequency conversion process. Our work points out how the analysis of controlled optical nonlinear processes can provide an alternative window toward the analysis of quantum light sources. [ABSTRACT FROM AUTHOR]

In this paper, a modified four-state continuous variable quantum key distribution protocol is proposed by adding a hybrid linear amplifier to Bob's output. The heralded hybrid linear amplifier is composed by an ideal deterministic linear amplifier and a probabilistic noiseless linear amplifier. The degradation of signal-to-noise ratio caused by deterministic linear amplifier during the amplification of quantum light states can be surmounted by the probabilistic amplification process. The hybrid linear amplifier allows us to tune between the regimes of high gain or high noise reduction and control the trade-off between the amplification coefficient and a finite heralding probability flexibly. We examine how the new protocol is affected by the hybrid amplifier's G and g parameters. By adjusting the characteristics of the hybrid linear amplifier, it is possible to optimize the key at a specific distance. In order to improve the simulation's reality value, we take into account the role of finite-size while examining the key rate and simulation. The application for the hybrid linear amplifier is increased by its use in the four-state protocol. [ABSTRACT FROM AUTHOR]

Mechanical resonators, due to their capability to host ultralong-lived phonon modes, are particularly attractive for quantum state storage and as memory elements in conjunction with quantum computing and communication networks. Here we demonstrate absorptive-type coherent memory based on long-lived mechanical excitations. The itinerant coherent microwave field is captured, stored, and retrieved from a mechanical memory oscillator which is pre-cooled to the ground state. The phase space distribution allows us to distinguish between coherent and thermal components and study their evolution as a function of storage time. Our device exhibits attractive functions with an energy decay time of T1 = 15.9 s, a thermal decoherence rate of Γth = 2.85 Hz, and acquires less than one quantum noise during the τcoh = 55.7 ms storage period. We demonstrate that both the amplitude and phase information of microwave coherent states can be recovered, indicating the coherence of our memory device. These results suggest that high-Q mechanical resonators and long coherence time phonons could be ideal candidates for the construction of long-lived and on-demand microwave quantum memories. [ABSTRACT FROM AUTHOR]

Multi-party quantum private comparison (MQPC) assumes responsibility for overseeing the flow of data and communication among diverse entities, wherein it boasts powerful security capabilities that have garnered substantial attention. Most current MQPC protocols rely on difficult-to-prepare quantum states and are inefficient in their use of resources. In this paper, we propose a novel MQPC protocol without entanglement swapping, thereby building upon the assumption of an ideal channel. This protocol is based on Bell states, which simplifies implementation and addresses the challenges associated with using complex quantum states; it also enables the comparison of secret information by having a trusted party prepare and transmit encoded quantum sequences to participants, thereby facilitating efficient equality comparison among all parties. Our MQPC protocol showcased remarkable efficiency in comparison to existing protocols for quantum private comparison. Furthermore, the incorporation of decoy photon and shared key technologies made external and internal attacks ineffective, thereby ensuring the utmost security and integrity of the protocol. [ABSTRACT FROM AUTHOR]

Spectral- and time-multiplexing are currently explored to generate large multipartite quantum states of light for quantum technologies. In the continuous variable approach, the deterministic generation of scalable entangled states requires the generation of a scalable number of squeezed modes. Here, we demonstrate the simultaneous generation of 21 squeezed spectral modes at the repetition rate of our laser, i.e., 156 MHz. We exploit the full repetition rate and the pulse shaping of a femtosecond light source to combine, for the first time, frequency- and time-multiplexing in multimode squeezing. This paves the way for the implementation of multipartite entangled states that are both scalable and fully reconfigurable. [ABSTRACT FROM AUTHOR]

Multiparty quantum private comparison (MQPC) protocol enables the comparison of the private information of n parties through the combination of quantum mechanics with classical private comparison. In order to improve efficiency and practicality, this paper proposes an efficient MQPC protocol based on single photons and rotation encryption with the assistance of a semi-honest third party (TP). TP participates in preparing the initial single photon sequence, which is encrypted and sent to the participants one by one. The private inputs of participants are encoded into particular rotation angles that are used to convert the received single photon sequence to an unknown quantum state sequence, which is returned to TP in a closed-loop transfer mode. By implementing rotation encryption and multiplexing of single photons, the qubit efficiency and practicality of the protocol can be significantly improved, without the need for complex quantum technologies, such as entanglement swapping of high-dimensional quantum states, maximal entanglement states with n-particle and entanglement correlation between various particles in a quantum state. Moreover, the proposed protocol is simulated on IBM Quantum Experience to show its feasibility. Security analysis demonstrates that the proposed protocol is resistant to different types of attacks. [ABSTRACT FROM AUTHOR]

Waveguided beam splitters were microfabricated by using a commercial two-photon lithography system (Nanoscribe), Ip-Dip as the waveguides and fused silica as the substrate, and they were covered with Loctite. The gap between the waveguides in the coupler was used to determine the transmission and reflection coefficients, and our results were compared with simulation results (using OptiFDTD software). The input and output ports of the beam splitters were spliced with multimode optical fibers in a robust system that can easily be handled. Then, they were tested by leading single photons (from an SPDC) to the beam splitters to produce different quantum statistics that were rated using the Fano factor. [ABSTRACT FROM AUTHOR]

Quantum computation attaches importance to high-precision quantum manipulation, where the quantum state transfer with high fidelity is necessary. Here, we propose a new scheme to implement the quantum state transfer of high fidelity and long distance, by adding on-site potential into the qubit chain and enlarging the proportion of the coupling strength between the two ends and the chain. In the numerical simulation, without decoherence, the transfer fidelities of 9 and 11 qubit chain are 0.999 and 0.997, respectively. Moreover, we give a detailed physical realization scheme of the quantum state transfer in superconducting circuits, and discuss the tolerance of our proposal against decoherence. Therefore, our scheme will shed light on quantum computation with long chain and high-fidelity quantum state transfer. [ABSTRACT FROM AUTHOR]

The quantum nature of light enables potentially revolutionary communication technologies. A key to advancing this area of research is a clear understanding of the concepts of states, modes, fields, and photons. The concept of field modes carries over from classical optics, while the concept of state has to be considered carefully when treating light quantum mechanically. The term “photon” is an overloaded identifier in the sense that it is often used to refer to either a quantum particle or the state of a field. This overloading, often used without placing it in context, has the potential to obfuscate the physical processes that describe the reality we measure. We review the uses and relationships between these concepts using modern quantum optics theory, including the concept of a photon wave function, the modern history of which was moved forward in a groundbreaking paper in this journal by Iwo BiałynickiBirula, to whom this article is dedicated. [ABSTRACT FROM AUTHOR]

High‐dimensional frequency entanglement is an enabling resource in quantum technology due to its high information capacity and error resilience. A concise yet efficient method for precisely quantifying its dimensionality remains an open challenge, owing to the difficulties for performing required superposition measurements in energy‐time domains, and the complexity associated with full quantum state tomography that scales unfavorably with dimensions. With the assistance of Hong–Ou–Mandel experiment that performs a Fourier transform between the entangled photons in terms of joint spectral intensities and the quantum interference in terms of biphoton temporal coincidences, the concept of Shannon dimensionality as a fast quantifier of bipartite continuous frequency entanglement is unlocked. This quantitative technique reveals the complete distribution of frequency entanglement but without suffering from any limitation of modal capacity of the detection geometry. These results may significantly facilitate the use of quantum interference for characterizing the high‐dimensional entanglement nature by avoiding some stringent conditions. [ABSTRACT FROM AUTHOR]

The promise of universal quantum computing requires scalable single‐ and inter‐qubit control interactions. Currently, three of the leading candidate platforms for quantum computing are based on superconducting circuits, trapped ions, and neutral atom arrays. However, these systems have strong interaction with environmental and control noises that introduce decoherence of qubit states and gate operations. Alternatively, photons are well decoupled from the environment and have advantages of speed and timing for quantum computing. Photonic systems have already demonstrated capability for solving specific intractable problems like Boson sampling, but face challenges for practically scalable universal quantum computing solutions because it is extremely difficult for a single photon to "talk" to another deterministically. Here, a universal distributed quantum computing scheme based on photons and atomic‐ensemble‐based quantum memories is proposed. Taking the established photonic advantages, two‐qubit nonlinear interaction is mediated by converting photonic qubits into quantum memory states and employing Rydberg blockade for the controlled gate operation. Spatial and temporal scalability of this scheme is demonstrated further. These results show photon‐atom network hybrid approach can be a potential solution to universal distributed quantum computing. [ABSTRACT FROM AUTHOR]

Spontaneous vibrational Raman scattering is a ubiquitous form of light–matter interaction whose description necessitates quantization of the electromagnetic field. It is usually considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming field. When probing an ensemble of molecules, the question therefore arises: What quantum state should be used to describe the molecular ensemble following spontaneous Stokes scattering? We experimentally address this question by measuring time-resolved Stokes–anti-Stokes two-photon coincidences on a molecular liquid consisting of several sub-ensembles with slightly different vibrational frequencies. When spontaneously scattered Stokes photons and subsequent anti-Stokes photons are detected into a single spatiotemporal mode, the observed dynamics is inconsistent with a statistical mixture of individually excited molecules. Instead, we show that the data are reproduced if Stokes–anti-Stokes correlations are mediated by a collective vibrational quantum, i.e. a coherent superposition of all molecules interacting with light. Our results demonstrate that the degree of coherence in the vibrational state of the liquid is not an intrinsic property of the material system, but rather depends on the optical excitation and detection geometry. Spontaneous Raman scattering is classically understood as an incoherent process. Here, the authors demonstrate that macroscopic quantum coherence among billions of vibrating molecules in a liquid is generated when single photon detection and single spatio-temporal mode excitation are implemented. [ABSTRACT FROM AUTHOR]

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