Many real-world problems can be reduced to combinatorial optimization on a graph, where the subset or ordering of vertices that maximize some objective function must be found. With such tasks often NP-hard and analytically intractable, reinforcement learning (RL) has shown promise as a framework with which efficient heuristic methods to tackle these problems can be learned. Previous works construct the solution subset incrementally, adding one element at a time, however, the irreversible nature of this approach prevents the agent from revising its earlier decisions, which may be necessary given the complexity of the optimization task. We instead propose that the agent should seek to continuously improve the solution by learning to explore at test time. Our approach of exploratory combinatorial optimization (ECO-DQN) is, in principle, applicable to any combinatorial problem that can be defined on a graph. Experimentally, we show our method to produce state-of-the-art RL performance on the Maximum Cut problem. Moreover, because ECO-DQN can start from any arbitrary configuration, it can be combined with other search methods to further improve performance, which we demonstrate using a simple random search.
Optics is a promising platform in which to help realize the next generation of fast, parallel, and energy-efficient computation. We demonstrate a reconfigurable free-space optical multiplier that is capable of over 3000 computations in parallel, using spatial light modulators with a pixel resolution of only 340 × 340 . This enables vector–matrix multiplication and parallel vector–vector multiplication with vector size of up to 56. Our design is, to the best of our knowledge, the first to simultaneously support optical implementation of reconfigurable, large-sized, and real-valued linear algebraic operations. Such an optical multiplier can serve as a building block of special-purpose optical processors such as optical neural networks and optical Ising machines.
We propose a practical scheme for end-to-end optical backpropagation in neural networks. Using saturable absorption for the nonlinear units, we find that the backward-propagating gradients required to train the network can be approximated in a surprisingly simple pump-probe scheme that requires only simple passive optical elements. Simulations show that, with readily obtainable optical depths, our approach can achieve equivalent performance to state-of-the-art computational networks on image classification benchmarks, even in deep networks with multiple sequential gradient approximation. With backpropagation through nonlinear units being an outstanding challenge to the field, this work provides a feasible path toward truly all-optical neural networks.
We present the effects of resonator birefringence on the cavity-enhanced interfacing of quantum states of light and matter, including the first observation of single photons with a time-dependent polarisation state that evolves within their coherence time. A theoretical model is introduced and experimentally verified by the modified polarisation of temporally-long single photons emitted from a 87 Rb atom coupled to a high-finesse optical cavity by a vacuum-stimulated Raman adiabatic passage (V-STIRAP) process. Further theoretical investigation shows how a change in cavity birefringence can both impact the atom-cavity coupling and engender starkly different polarisation behaviour in the emitted photons. With polarisation a key resource for encoding quantum states of light and modern micron-scale cavities particularly prone to birefringence, the consideration of these effects is vital to the faithful realisation of efficient and coherent emitter-photon interfaces for distributed quantum networking and communications.Cavity quantum electrodynamics (CQED) allows for the nature of light and matter to be interrogated through the enhanced interaction of an emitter with the resonant modes of a cavity [1][2][3]. This allows these fundamental interactions to be leveraged for quantum technologies [4][5][6][7][8] and, consequently, realising novel regimes in CQED has the potential to impact both foundational research and cutting-edge technological applications. Single photons are fundamental particles, they possess no deeper substructure, therefore it is tempting to consider their properties to be similarly immutable. However, CQED has shown photons to be a far richer resource, with a high degree of control demonstrated over the wavepackets [9], frequency [10], polarisation [11] and phase [12] of temporally-long single photons. Here, we report the first observation of a single-photon with a time-dependent polarisation state that evolves along its wavepacket. Moreover, this effect arises from a system increasingly prevalent in the pursuit of scalable quantum technologies.The coherent interfacing of light and matter qubits lies at the heart of many quantum networking proposals [4][5][6][7][8], and the interaction of atom-like emitters with a single photonic mode of a resonator provides a platform for realising this control. CQED is a vibrant field with single atoms and ions particularly suitable candidates with which to realise network nodes and singlephoton sources due to their inherently homogeneous nature. The a priori deterministic emission of single photons into well-defined quantum states has been realised in both atom-cavity [11, 14-17] and ion-cavity systems [18]. Proof-of-principle quantum networking demonstrations have leveraged this control to, for example, remotely entangle two atoms [19] and perform two-bit quantum gates [20][21][22]. Improving the efficiency and scalability of such systems ultimately requires increasing the strength and reliability of the emitter-cavity coupling, motivating the development of mic...
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