Pushing Purcell-enhancement beyond its limits

Purcell-enhanced emission from a coupled emitter-cavity system is a fundamental manifestation of cavity quantum electrodynamics. Starting from a theoretical description we derive a scheme for photon emission from an emitter coupled to a birefringent cavity that exceeds hitherto anticipated limitations. Based on a recent study and experimental investigation of the intra-cavity coupling of orthogonal polarisation modes in birefringent cavities, we now decouple the emitter and the photon prior to emission from the cavity mode. Effectively, this is "hiding" the emitter from the photon in the cavity to suppress re-excitation, increasing the overall emission through the cavity mirrors. In doing so we show that tailored cavity birefringence can offer significant advantages and that these are practically achievable within the bounds of present-day technology. It is found that birefringence can mitigate the tradeoff between stronger emitter-cavity coupling and efficient photon extraction. This allows for longer cavities to be constructed without a loss of performance -- a significant result for applications where dielectric mirrors interfere with any trapping fields confining the emitter. We then generalise our model to consider a variety of equivalent schemes. For instance, detuning a pair of ground states in a three-level emitter coupled to a cavity in a Lambda-system is shown to provide the same enhancement, and it can be combined with a birefringent cavity to further increase performance. Additionally, it is found that when directly connecting multiple ground states of the emitter to form a chain of coupled states, the extraction efficiency approaches its fundamental upper limit. The principles proposed in this work can be applied in multiple ways to any emitter-cavity system, paving the way to surpassing the traditional limits of such systems with technologies that exist today.Comment: 8 pages, 8 figures plus 3 page appendi

Fault tolerant quantum computation with very high threshold for loss errors

Many proposals for fault tolerant quantum computation (FTQC) suffer detectable loss processes. Here we show that topological FTQC schemes, which are known to have high error thresholds, are also extremely robust against losses. We demonstrate that these schemes tolerate loss rates up to 24.9%, determined by bond percolation on a cubic lattice. Our numerical results show that these schemes retain good performance when loss and computational errors are simultaneously present.Comment: 4 pages, comments still very welcome. v2 is a reasonable approximation to the published versio

Pipes and Connections

This document describes the low-level Pipe and ConnectionManager objects of the Mesh- Router system. The overall MeshRouter framework provides a general scheme for interest- limited communications among a number of client processes. This generality is achieved by a carefully factorized, object-oriented software implementation. Within this framework, the Pipe and ConnectionManager (base) classes dened in this note specify the interfaces for i) ac- tual `bits on the wire' communications and ii) dynamic client insertions during overall system execution. Two specic implementations of the Pipe class are described in detail: a `Memo- ryPipe' linking objects instanced on a single processor and a more general 'rtisPipe' providing inter-processor communications built entirely from the standard RTI-s library used in current JSAF applications. Initialization procedures within the overall MeshRouter system are dis- cussed, with particular attention given to dynamic management of inter-processor connections. Prototype RTI-s router processes are discussed, and simple extensions of the standard system conguration data les are presented

Nonlinear Zeeman Effects in the Cavity-Enhanced Emission of Polarised Photons

We theoretically and experimentally investigate nonlinear Zeeman effects within a polarised single-photon source that uses a single 87Rb atom strongly coupled to a high finesse optical cavity. The breakdown of the atomic hyperfine structure in the D2 transition manifold for intermediate strength magnetic fields is shown to result in asymmetric and, ultimately, inhibited operation of the polarised atom-photon interface. The coherence of the system is considered using Hong-Ou-Mandel interference of the emitted photons. This informs the next steps to be taken and the modelling of future implementations, based on feasible cavity designs operated in regimes minimising nonlinear Zeeman effects, is presented and shown to provide improved performance.Comment: 12 pages, 8 figure

Polarisation oscillations in birefringent emitter-cavity systems

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.Comment: 9 pages, 5 figures including Supplemental Materia

Fully fault tolerant quantum computation with non-deterministic gates

In certain approaches to quantum computing the operations between qubits are non-deterministic and likely to fail. For example, a distributed quantum processor would achieve scalability by networking together many small components; operations between components should assumed to be failure prone. In the logical limit of this architecture each component contains only one qubit. Here we derive thresholds for fault tolerant quantum computation under such extreme paradigms. We find that computation is supported for remarkably high failure rates (exceeding 90%) providing that failures are heralded, meanwhile the rate of unknown errors should not exceed 2 in 10^4 operations.Comment: 5 pages, 3 fig

Pushing Purcell enhancement beyond its limits

Purcell-enhanced photon emission into a cavity is at the heart of many schemes for interfacing quantum states of light and matter. We show that the intra-cavity coupling of orthogonal polarisation modes in a birefringent cavity allows for the emitter and photon to be decoupled prior to emission from the cavity mode, enabling photon extraction efficiencies that exceed the, previously considered fundamental, limits of Purcell enhancement. Tailored cavity birefringence is seen to mitigate the tradeoff between stronger emitter-cavity coupling and efficient photon extraction, providing significant advantages over single-mode cavities. We then generalise this approach to show that engineered coupling between states of the emitter can equivalently 'hide' the emitter from the photon, ultimately allowing the extraction efficiency to approach its fundamental upper limit. The principles proposed in this work can be applied in multiple ways to any emitter-cavity system, paving the way to surpassing the traditional limitations with technologies that exist today

Population-Based Reinforcement Learning for Combinatorial Optimization

Applying reinforcement learning (RL) to combinatorial optimization problems is attractive as it removes the need for expert knowledge or pre-solved instances. However, it is unrealistic to expect an agent to solve these (often NP-)hard problems in a single shot at inference due to their inherent complexity. Thus, leading approaches often implement additional search strategies, from stochastic sampling and beam-search to explicit fine-tuning. In this paper, we argue for the benefits of learning a population of complementary policies, which can be simultaneously rolled out at inference. To this end, we introduce Poppy, a simple theoretically grounded training procedure for populations. Instead of relying on a predefined or hand-crafted notion of diversity, Poppy induces an unsupervised specialization targeted solely at maximizing the performance of the population. We show that Poppy produces a set of complementary policies, and obtains state-of-the-art RL results on three popular NP-hard problems: the traveling salesman (TSP), the capacitated vehicle routing (CVRP), and 0-1 knapsack (KP) problems. On TSP specifically, Poppy outperforms the previous state-of-the-art, dividing the optimality gap by 5 while reducing the inference time by more than an order of magnitude