In this work, we present a method for characterizing the transmission matrices of complex scattering media using a physics-informed, multi-plane neural network (MPNN) without the requirement of a known optical reference field. In contrast to previous techniques, our method is able to measure complete information about the transmission matrix, which is necessary for coherent control of light through a complex medium. Here, we design a neural network that describes the exact physical apparatus consisting of a trainable layer describing the unknown transmission matrix. We then employ randomized measurements to train the neural network which accurately recovers the transmission matrix of a commercial multi-mode fiber. We demonstrate how our method is significantly more accurate, and noise-robust than the standard method of phase-stepping holography and show how it can be generalized to characterize a cascade of transmission matrices. This work presents an essential tool for accurate light control through complex media, with applications ranging from classical optical networks, biomedical imaging, to quantum information processing.
Programmable optical circuits form a key part of quantum technologies today. As the size of such circuits is increased, maintaining precise control over every individual component becomes challenging. Here we show how embedding an optical circuit in the higher-dimensional space of a large mode-mixer allows us to forgo control over individual elements, while retaining a high degree of programmability over the circuit. Using this approach, we implement high-dimensional linear optical circuits within a commercial multi-mode fibre placed between controllable phase planes. We employ these circuits to manipulate high-dimensional entanglement in up to seven dimensions, demonstrating their application as fully programmable quantum gates. Furthermore, we show how these circuits turn the multi-mode fibre itself into a generalized multi-outcome measurement device, allowing us to both transport and certify entanglement. Finally, we show how a high circuit fidelity can be achieved with a low circuit depth by harnessing the resource of a high-dimensional mode-mixer. Our work serves as an alternative yet powerful approach for realizing precise control over high-dimensional quantum states of light.
The development of high-quality photonic sources of high-dimensional entanglement and techniques for their manipulation is crucial for the advancement of quantum technologies. We present an efficient technique for generating and characterizing high-dimensional spatially entangled two-photon states with a record quality, dimensionality, and measurement speed. We demonstrate how to precisely manipulate such states by tailoring reprogrammable optical circuits in complex scattering media consisting of off-the-shelf multimode fiber and spatial light modulation. Our techniques open a clear pathway for the adoption of high-dimensional quantum states of light in the high-capacity, noise-robust quantum networks of tomorrow.
Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. Here, we present a four-user QCKA protocol using photonic GHZ states distributed over fibre with combined lengths up to 50 km. Furthermore, we investigate a constrained network consisting of a 6-qubit photonic graph state which we apply network coding routines to distil suitable resource states.
High-dimensional entanglement can give rise to stronger forms of nonlocal correlations compared to qubit systems. Beyond being of fundamental interest, this offers significant advantages for quantum information processing. The problem of certifying these stronger correlations, however, remains an important challenge. Here we theoretically formalise and experimentally demonstrate a notion of genuine high-dimensional quantum steering. We show that high-dimensional entanglement combined with judiciously chosen local measurements can lead to a stronger form of steering, provably impossible to obtain via entanglement in lower dimensions. Exploiting the connection between steering and incompatibility of quantum measurements, we derive two-setting inequalities for certifying the presence of genuine high-dimensional steering. We report the experimental violation of these inequalities using macro-pixel photon-pair entanglement certifying genuine high-dimensional steering in dimensions up to 15.
High-dimensional entanglement of structured light offers the potential for noise-robust, high-capacity quantum communication protocols. However, the generation, measurement, and transport of high-quality entanglement presents some unique challenges. We demonstrate the generation and measurement of two-photon macro-pixel entanglement with a record dimensionality, quality, and measurement speed. We then discuss an experiment where we unscramble high-dimensional pixel entanglement through a commercial multimode fibre. In contrast with classical techniques, entanglement is also used to measure the transmission matrix of the fibre. Interestingly, we are able to regain entanglement without manipulating the fibre or the photon that entered it.
The desire to increase the amount of information that can be encoded onto a single photon has driven research into many areas of optics. One such area is optical orbital angular momentum (OAM) [1]. These beams have helical phasefronts and carry an orbital angular momentum of mbar per photon, where the integer m is unbounded, giving a large state space in which to encode information.
We recently developed a telescope system comprising two bespoke refractive optical elements to transform OAM states into transverse momentum states [2]. This is achieved by mapping the azimuthal position of the input plane to the lateral position in the output [3]. A mapping of this type transforms a set of concentric rings at the input plane into a set of parallel lines in the output plane. A lens can then separate the resulting transverse momentum states into specified lateral positions, allowing for the efficient measurement of multiple OAM states simultaneously.
Separating OAM states in this way presents an opportunity for this larger alphabet to improve the data capacity of a free space link and has potential application in both the classical and quantum regimes.
We will present our latest design, increasing the bandwidth of measurable states to over 50 OAM modes. In such a system we study the crosstalk introduced by a thin phase turbulence, showing that turbulence similarly degrades the purity of all the modes within this range.
We describe a procedure to construct a free-space quantum key distribution system that can carry many bits of
information per photon. We also describe the current status of our laboratory implementation of these plans.
We review recent research in the field of quantum imaging. Quantum imaging deals with the formation of images
that possess higher resolution or better signal-to-noise characteristics than conventional images by making use
of the coherence properties of quantum light fields. Quantum imaging also deals with indirect imaging methods
such as ghost imaging, in which image information is conveyed not by a single light field but by the correlations
between two separate light fields. In this contribution we concentrate primarily on recent results in the area of
ghost imaging.
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