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A multimode fiber (MMF) is a minimally invasive imaging probe. The most popular approach of MMF-based microscopy is raster-scan imaging, where the sample is illuminated by foci optimized on the fiber output facet by wavefront shaping (WFS). Imaging quality can be quantified by characteristic parameters of the optimized spots. We investigate the influence of the input light position on WFS through a round-core MMF with partial mode control, a situation often encountered in real life. We further demonstrate a trade-off between the shape and contrast of the foci generated on the output facet: the center input position is beneficial for high-contrast imaging, while the edge input position helps to reduce focus aberrations. These results are important for high field-of-view raster-scan imaging via an MMF.

Quantum mechanics suggests a variety of velocity definitions, and some of them are not fully consistent with each other. For instance, in the issue of including density gradients of the wave function. Here we intend to reveal the significance of density gradients in determining photon velocity. We consider photon motion in a system of waveguide potentials described in our recent theoretical proposal [1]. In such a system, density transfer between two coupled waveguides acts as a clock and allows photon velocities to be determined. We implement a system of waveguides, confining 2D gas of massive photons, on one of the microcavity mirrors using a novel direct laser writing technique for mirror nanostructuring [2]. Such a microcavity platform has full experimental access to the wave function, and allows us to measure the period of density transfer, which is related to photon in-plane velocity. Application of this scheme to tunneling phenomena at a reflective step potential also makes it possible to measure the velocity in the classically forbidden regime and reveals the existence of a classically forbidden long-range photon transport as predicted.

Bose-Einstein condensation (BEC) is the massive occupation of a single quantum state in a gas of bosons at a finite temperature and is one of the few phenomena demonstrating quantum behaviour at the macroscopic scale. BEC of photons can be relised in quantum confined structures such as optical microcavities, where photons can thermalise to the temperature of the optical medium through repeated absorption and emission cycles. The critical particle number for such a system can be as low as 10 photons at room temperature, making photon BEC experimentally easily accessible. In an optical microcavity, photons behave like massive 2-dimensional particles confined in a potential defined by the mirror geometry and the optical medium refractive index. Controlling this potential is essential for exploring the physics of the condensation process. In this thesis, we develop, characterise and employ a number of flexible methods which rely on modified dielectric mirrors that have an additional optically absorptive silicon layer between the substrate and the dielectric stack. This layer allows us to locally heat the mirror using laser light. The first method is the permanent nanostructuring, resulting from pore formation inside the dielectric stack layers. The second method involves thermal expansion. Finally, introducing a thermoresponsive polymer into the optical medium allows us to control the refractive index. We experimentally investigate BEC under non-equilibrium conditions in potentials resembling a Mach-Zehnder interferometer. The switching behaviour of the interferometer in various configurations provides insight into the formation process of the condensate. We show that by adjusting their frequency, BECs naturally try to avoid particle loss and destructive interference in their environment. As a first step towards spin-glass simulators, an optical analogue of a 0,pi Josephson junction is implemented and characterised. Our experiments show a high degree of control over the junction state. Alternative types of condensate couplings and their possible implementations are investigated as well. In particular, we show that couplings can range from dispersive to dissipative, and show that the loss channel in dissipative couplings may be realised by drilling holes using focused ion beam milling.

An alternative approach to solving computationally hard problems is to use a physical system of coupled Bose-Einstein condensates (BEC). The problem of finding its ground state can be mapped in polynomial time to any problem of the NP-Complete class. Photonic BECs (pBECs) can thermalize to the ground state in a fraction of a nanosecond and then be interfered to extract their relative phases. They are then interpreted as a solution to the mapped problem. In our system we use two highly reflective Bragg mirrors as a cavity, a dye as a thermalizing medium, and a green laser as a photon source. By nanostructuring one of the mirrors, transverse confinement of pBEC is achieved. I will present our latest results on coupled condensate system with frustration, such as a negatively coupled triangular structure of pBECs.

Integrated photonics is an essential technology for quantum computing. Universal, phase-stable, reconfigurable quantum photonic processors enable manipulation of quantum states of light and are one of the main components of photonic quantum computers. In this work [1], we report the realization of the largest quantum photonic processor to date. The processor is realized in Si3N4 waveguides, and it enables arbitrary unitary transformations on its 20 input modes with a fidelities above 97%, an average optical loss of 2.9dB per mode, and high-visibility quantum interference VHOM = 98%.

Controlling the flow of light in optical microcavities enables a vast range of interesting physical phenomena. In these systems, varying the local optical path length in the microcavity is effectively equal to changing the potential landscape for the light in the resonator. Previous approaches to achieve this rely either on permanent modification of the resonator structure or reversible tuning using a fluorescent medium with a strong nonlinearity or high thermo-optic coefficient. Here, we introduce a new method based on patterned laser heating that causes a thermal expansion of the dielectric stack of the mirror. We find that our approach reaches similar potential height and width resolution compared to other reversible thermal approaches, while improving tuning speed over tenfold and removing the need for a specific medium inside of the resonator. As a proof of principle, the method is used to define and modify the potential landscape for photon Bose-Einstein condensates.

Open quantum systems can be systematically controlled by making changes to their environment. A well-known example is the spontaneous radiative decay of an electronically excited emitter, such as an atom or a molecule, which is significantly influenced by the feedback from the emitter’s environment, for example, by the presence of reflecting surfaces. A prerequisite for a deliberate control of an open quantum system is to reveal the physical mechanisms that determine its state. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that by adjusting their frequency Bose-Einstein condensates naturally try to avoid particle loss and destructive interference in their environment. In this way our experiments reveal physical mechanisms involved in the formation of a Bose-Einstein condensate, which typically remain hidden when the system is close to thermal equilibrium., Non-equilibrium Bose-Einstein condensates exist in different systems like polaritons, photons. Here the authors demonstrate photonic BECs in an excited or a non-equilibrium state and explore the flow of the photons coupled to the interferometer in order to minimize the loss.

We demonstrate the manipulation of a five-dimensional spatially entangled two-photon state using reprogrammable multi-mode optical circuits implemented within a multi-mode fiber.

We discuss the design and fabrication of deterministic photonic scattering media using direct laser writing. Static measurements and light transport calculations validate that the light scattering parameters match the design model.

In a quantum-photonic experiment with an integrated quantum photonics network, we observe a quantum state locally evolve towards a thermal state. By undoing the evolution with the inverse network, we recover the input pure state.

We experimentally demonstrate an efficient, semi-device-independent photonic indistinguishability witness by certifying three-photon indistinguishability on a programmable integrated quantum photonic processor.

Multimode fibers (MMFs) show great promise as miniature probes for sensing, imaging and spectroscopy applications. Different parameters of the fibers, such as numerical aperture, refractive index profile and length, have been already optimized for better performance. Here we investigate the role of the core shape, in particular for wavefront shaping applications where a focus is formed at the output of the MMF. We demonstrate that in contrast to a conventional round-core MMF, a square-core design doesn't suffer from focus aberrations. Moreover, we find that how the interference pattern behind a square-core fiber decorrelates with the input frequency is largely independent of the input light coupling. Finally, we demonstrate that a square core shape provides an on-average uniform distribution of the output intensity, free from the input-output correlations seen in round fibers, showing great promise for imaging and spectroscopy applications., Comment: 9 pages, 5 figures

We experimentally investigate the relation between thermodynamics and quantum mechanics by demonstrating equilibration of a quantum state towards a thermal state in an integrated quantum photonics platform.

One of the core questions of quantum physics is how to reconcile the unitary evolution of quantum states, which is information-preserving and time-reversible, with the second law of thermodynamics, which is neither. The resolution to this paradox is to recognize that global unitary evolution of a multi-partite quantum state causes the state of local subsystems to evolve towards maximum-entropy states. In this work, we experimentally demonstrate this effect in linear quantum optics by simultaneously showing the convergence of local quantum states to a generalized Gibbs ensemble constituting a maximum-entropy state under precisely controlled conditions, while using a new, efficient certification method to demonstrate that the state retains global purity. Our quantum states are manipulated by a programmable integrated photonic quantum processor, which simulates arbitrary non-interacting Hamiltonians, demonstrating the universality of this phenomenon. Our results show the potential of photonic devices for quantum simulations involving non-Gaussian states.

The synchronization of coherent states of light has long been an important subject of basic research and technology. Recently, a new concept for analog computers has emerged where this synchronization process can be exploited to solve computationally hard problems - potentially faster and more energy-efficient than what can be achieved with conventional computer technology today. The unit cell of such systems consists of two coherent centers that are coupled to one another in a controlled manner. Here, we experimentally characterize and analyze the synchronization process of two photon Bose-Einstein condensates, which are coupled to one another, either dispersively or dissipatively. We show that both types of coupling are robust against a detuning of the condensate frequencies and show similar time constants in establishing mutual coherence. Significant differences between these couplings arise in the behaviour of the condensate populations under imbalanced optical pumping. The combination of these two types of coupling extends the class of physical models that can be investigated using analog simulations.

This thesis focuses on optical quantum information processing in two parts. The first part focuses on photonic hardware. Large-scale photonic experiments require the generation of many indistinguishable single photons simultaneously. Traditionally, such photons are generated by spontaneous parametric down-conversion (SPDC). However, the suitability of such sources to generate many photons. We have identified the maximum limits of how many photons SPDC sources can be made. A second requirement of large-scale quantum photonics is having a network with many optical modes while maintaining low optical loss. Such networks are currently lacking. Therefore we introduce the possibility to use a rough surface scatterer for a quantum advantage experiment. Although such a network is far from universal, it has many modes and low loss. Another approach is to use integrated quantum photonics, which is fully reconfigurable and is a promising way of scaling up to large systems in an industrial setting. We have characterized a programmable low-loss 12-mode silicon nitride (SiN). In the second part of the thesis, various proof-of-principle quantum experiments were performed on a 12 mode SiN integrated photonic chip. Firstly, it was shown that these networks can be used to simulate wave propagation through disordered networks. This unconventional approach allows for full experimental access and control to the physical process, which is essential for such experiments. Secondly, we introduced a semi-device independent witness protocol that can in-situ verify the indistinguishability of photons on the chip, which is an essential tool to have when the system size increases and can no longer be efficiently simulated classically. The last experiment is a quantum simulation where we show how a pure quantum state locally thermalizes in a closed system.

Efficient and reliable measurement of photonic indistinguishability is crucial for any claim of a quantum advantage in photonics. Using an integrated photonic processor, we introduce and demonstrate a semi-device-independent witness of photonic indistinguishability.

We propose an integrated network of ring resonators on a silicon-nitride chip for use as an all-optical single-spatial-mode physical unclonable key, enabling secret-free optical authentication with standard communication channels such as telecom fibres.

We report the realization of the largest reconfigurable quantum photonic processor enabling arbitrary unitary transformations on its 20 input & output modes with an average fibre-to-fibre loss of 2.9 dB/channel. High-fidelity operation and high-visibility quantum interference is demonstrated.

Universal, phase-stable, reconfigurable quantum photonic processors enable manipulation of photonic quantum states and are one of the main components of photonic quantu7m computers in various architectures. In this paper, we report the realization of the largest quantum photonic processor to date.

Quantum tunneling is one of the most fundamental quantum phenomena, playing a significant role in quantum computing, modern electronics, and quantum biology. However, it is still not fully understood, in particular in the context of tunneling speed. Here, we suggest a new way to measure the speed of photons in classically forbidden and allowed regions. For this purpose, we use the photon Bose-Einstein Condensation (pBEC) obtained in a microcavity system. Using direct laser writing technique for mirror nanostructuring, we implement a system of waveguides, forming a 2D photon gas. In such a system we can directly measure the tunneling wavefunction, part of which is transmitted through the mirrors, which is not easily achievable in other systems.

Quantum simulations are becoming an essential tool for studying complex phenomena, e.g. quantum topology, quantum information transfer, and relativistic wave equations, beyond the limitations of analytical computations and experimental observations. To date, the primary resources used in proof-of-principle experiments are collections of qubits, coherent states or multiple single-particle Fock states. Here we show the first quantum simulation performed using genuine higher-order Fock states, with two or more indistinguishable particles occupying the same bosonic mode. This was implemented by interfering pairs of Fock states with up to five photons on an interferometer, and measuring the output states with photon-number-resolving detectors. Already this resource-efficient demonstration reveals new topological matter, simulates non-linear systems and elucidates a perfect quantum transfer mechanism which can be used to transport Majorana fermions., Comment: New text and results. 19 pages, 13 figures, supplementary material

The tantalizing promise of quantum computational speedup in solving certain problems has been strongly supported by recent experimental evidence from a high-fidelity 53-qubit superconducting processor1 and Gaussian boson sampling (GBS) with up to 76 detected photons. Analogous to the increasingly sophisticated Bell tests that continued to refute local hidden variable theories, quantum computational advantage tests are expected to provide increasingly compelling experimental evidence against the Extended Church-Turing thesis. In this direction, continued competition between upgraded quantum hardware and improved classical simulations is required. Here, we report a new GBS experiment that produces up to 113 detection events out of a 144-mode photonic circuit. We develop a new high-brightness and scalable quantum light source, exploring the idea of stimulated squeezed photons, which has simultaneously near-unity purity and efficiency. This GBS is programmable by tuning the phase of the input squeezed states. We demonstrate a new method to efficiently validate the samples by inferring from computationally friendly subsystems, which rules out hypotheses including distinguishable photons and thermal states. We show that our noisy GBS experiment passes the nonclassicality test using an inequality, and we reveal non-trivial genuine high-order correlation in the GBS samples, which are evidence of robustness against possible classical simulation schemes. The photonic quantum computer, Jiuzhang 2.0, yields a Hilbert space dimension up to $10^{43}$, and a sampling rate $10^{24}$ faster than using brute-force simulation on supercomputers., 23 pages, 6 figures. Comments are welcome

Multiphoton quantum interference is a one of the two technology platforms in which a quantum advantage has been demonstrated [1]. In these systems, a large-scale quantum state forms due to repeated effective interactions of photons (through the generalized Hong-Ou-Mandel effect) at the nodes of a photonic processor, resulting in a state that intrinsically possesses quantum complexity. A key question is how to validate the formation of a large-scale quantum state. One aspect of this is to measure whether the output quantum state is entangled – a necessary but not sufficient criterion for complexity. In this work, we derive and demonstrate an entanglement witness for large-scale optical quantum interference, and experimentally use this to demonstrate the presence of entanglement in our optical processor [2]. [1] Zhong et al., Science 370, 1460 – 1463 (2020) [2] Taballione et al., ArXiv 2012.05673 (2021)

Recently, quantum advantage over classical computation has been claimed using photons [1] . To control photonic quantum computations such as Boson sampling, a non-universal approach on quantum computation, large-scale quantum processors are needed [2] , [3] . To realize scalable and robust photonic quantum processors integrated photonics is a key technology. In our approach we use the integrated waveguide platform based on silicon nitride [4] , due to its low intrinsic losses and technological maturity. The low losses result in a high fidelity of the quantum photonics processor which is mandatory for reliable quantum computation while maturity of the platform allows for scaling up of the dimensions of the processor. Here, we present a universal 12-mode quantum photonics processor which is the largest of its kind to date.

Photonic processors are pivotal for both quantum and classical information processing tasks using light. In particular, linear optical quantum information processing requires both large-scale and low-loss programmable photonic processors. In this paper, we report the demonstration of the largest universal quantum photonic processor to date: a low-loss 12-mode fully tunable linear interferometer with all-to-all mode coupling based on stoichiometric silicon nitride waveguides.

Josephson junctions are the basis for the most sensitive magnetic flux detectors, the definition of the unit volt by the Josephson voltage standard, and superconducting digital and quantum computing. They result from the coupling of two coherent quantum states, as they occur in superconductors, superfluids, atomic Bose-Einstein condensates, and exciton-polariton condensates. In their ground state, Josephson junctions are characterised by an intrinsic phase jump. Controlling this phase jump is fundamental for applications in computing. Here, we experimentally demonstrate controllable phase relations between photon Bose-Einstein condensates resulting from particle exchange in a thermo-optically tunable potential landscape. Our experiment realises an optical analogue of a controllable 0,$\pi$-Josephson junction. By connecting several junctions, we can study a reconfigurable 4-condensate system demonstrating the potential of our approach for analog spin glass simulation. More generally, the combination of static and dynamic nanostructuring techniques introduced in our work offers a powerful platform for the implementation of adaptive optical systems for paraxial light in and outside of thermal equilibrium., Comment: 21 pages, 5 figures

Verification of a quantum advantage in the presence of noise is a key open problem in the study of near-term quantum devices. In this work, we show how to assess the quality of photonic interference in a linear optical quantum device (boson sampler) by using a maximum likelihood method to measure the strength at which various noise sources are present in the experiment. This allows us to use a sparse set of samples to test whether a given boson-sampling experiment meets known upper bounds on the level of noise permissible to demonstrate a quantum advantage. Furthermore, this method allows us to monitor the evolution of noise in real time, creating a valuable diagnostic tool. Finally, we observe that sources of noise in the experiment compound, meaning that the observed value of the mutual photon indistinguishability, which is the main imperfection in our study, is an effective value taking into account all sources of error in the experiment.

We shape fs optical pulses and deliver them in a single spatial mode to the input of a multimode fiber. The pulse is shaped in time such that at the output of the multimode fiber an ultrashort pulse appears at a predefined focus. Our result shows how to raster scan an ultrashort pulse at the output of a stiff piece of square-core step-index multimode fiber and in this way the potential for making a nonlinear fluorescent image of the scene behind the fiber, while the connection to the multimode fiber can be established via a thin and flexible single-mode fiber. The experimental results match our numerical simulation well., Comment: V2:29 pages including appendices, 9 figures (1 new), several updated, many improvements throughout

We study the programming fidelity of a 12-mode quantum photonic processor, the largest universal quantum photonic processor to date. The processor is a fully reconfigurable linear interferometer using silicon nitride waveguide technology.

An optical fiber is a promising tool for remote imaging [1]. A multimode fiber (MMF) offers multiple advantages: compactness, flexibility as well as the ability to transmit a large amount of information via multiple spatial modes. There are several methods to do optical imaging through a MMF. One can use wavefront shaping (WFS) to compensate for the modal dispersion and mode mixing so that the incident light can converge to a desired pattern at the fiber output [2]. By using WFS, we can sequentially generate focal spots at the distal fiber facet to scan the sample. Another imaging method through a MMF is compressive imaging (CI). The interfering fiber modes create a speckle pattern, which varies with wavelength and input light position, which. can be used as a basis for CI [3]. CI allows to reconstruct the image with a resolution beating the Abbe limit with fewer measurements than the number of pixels [4]. Finally, the decorrelation of the speckle patterns with wavelength encodes the spectral information of input light, which enables MMF spectroscopy applications [5].

Multimode fibers (MMFs) show great promise in imaging applications where space is limited, due to their small diameter, yet high NA [1] , [2]. One such application is endoscopy, where a multimode fiber can be used as a thin and flexible probe. Unfortunately, the perturbation sensitive mode mixing in a multimode fiber makes it difficult to reconstruct an image that is transmitted through the fiber [3]. Methods to overcome this difficulty, such as spatial wavefront shaping, still need re-optimalization after significant fiber perturbations. Such methods are also usually based on linear imaging. However, nonlinear imaging can provide increased resolution, reduced background and the ability for 3D imaging [4]. Combining ultrashort pulses with MMFs is challenging because of the complex spatiotemporal response of an MMF. So far, only methods based on spatial domain wavefront shaping have been used to selectively focus an ultrashort pulse through an MMF [5] , which are still perturbation-sensitive.

Quantum gases of light, as photons or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from e.g. cavity loss. This makes them a tool to study dissipative phases, an emerging subject in quantum manybody physics. Here we experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a new dissipative phase, characterized by a biexponential decay of the condensate's second-order coherence. The phase transition occurs due to the emergence of an exceptional point in the quantum gas. While Bose-Einstein condensation is usually connected to ordinary lasing by a smooth crossover, the observed phase transition separates the novel, biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our findings pave the way for studies of a wide class of dissipative quantum phases, for instance in topological or lattice systems., 10 pages, 4 figures, (additional 22 pages for supplementary information)

Open quantum systems can be systematically controlled by making changes to their environment. A prerequisite for a deliberate control of an open quantum system is a sufficient understanding of its underlying physical mechanisms. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback realized by a potential landscape that effectively acts as a Mach-Zehnder interferometer. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that the condensation process becomes an interplay between minimizing the energy of the condensate, minimizing the particle losses and maximizing the constructive feedback from the environment. [1] M. Vretenar et al., arXiv:2105.10708 [cond-mat.quant-gas]