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arXiv:1709.03155

Optimal interfacing a GHz-bandwidth heralded single photon source with on-demand, broadband quantum memories

Patrick Michelberger, Michał Karpiński, Ian Walmsley, Joshua Nunn

Published Sep 10, 2017• 0 citations
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Abstract

Photonics offers a route to fast and distributed quantum computing in ambient conditions, provided that photon sources and logic gates can be operated deterministically. Quantum memories, capable of storing and re-emitting photons on demand, enable quasi-deterministic operations by synchronising stochastic events. We recently interfaced a Raman-type quantum memory with a travelling-wave heralded photon source. Here we discuss the trade-space for the spectral characteristics of such sources, and we present measurements of our source, which represents a practical compromise enabling passive stability, high brightness in a single-pass configuration, high purity and good matching to our Cs vapour memory.

DOI
10.48550/arxiv.1709.03155

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Photonics offers a route to fast and distributed quantum computing in ambient conditions, provided that photon sources and logic gates can be operated deterministically. Quantum memories, capable of storing and re-emitting photons on demand, enable quasi-deterministic operations by synchronising stochastic events. We recently interfaced a Raman-type quantum memory with a travelling-wave heralded photon source. Here we discuss the trade-space for the spectral characteristics of such sources, and we present measurements of our source, which represents a practical compromise enabling passive stability, high brightness in a single-pass configuration, high purity and good matching to our Cs vapour memory.
Photonics offers a route to fast and distributed quantum computing in ambient conditions, provided that photon sources and logic gates can be operated deterministically. Quantum memories, capable of storing and re-emitting photons on demand, enable quasi-deterministic operations by synchronizing stochastic events. Interfaced source–memory systems are thus a key building block in photonics-based quantum information processors. We discuss the design of the single-photon source in this type of light–matter interface and present an experimental system based on a Raman-type quantum memory. In addition to the spectral purity of the produced heralded single photons, we find that their temporal distinguishability also becomes important due to the implicit temporal binning derived from photon storage in the memory. When aiming to operate the source–memory system at high repetition rates, a practical compromise between both of these requirements needs to be found. Our implemented photon source system demonstrates such a solution and enables passive stability, high brightness in a single-pass configuration, high purity as well as good mode matching to our Raman memory.
Broadband quantum memories, used as temporal multiplexers, are a key component in photonic quantum information processing, as they make repeat-until-success strategies scalable. We demonstrate a prototype system, operating on-demand, by interfacing a warm vapour, high time-bandwidth-product Raman memory with a travelling wave spontaneous parametric down-conversion source. We store single photons and observe a clear influence of the input photon statistics on the retrieved light, which we find currently to be limited by noise. We develop a theoretical model that identifies four-wave mixing as the sole important noise source and point towards practical solutions for noise-free operation.
Quantum memories are key components in quantum information networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their long storage times and experimental simplicity. Operation with the Raman protocol enables high time-bandwidth products, which allows for multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing. This makes the Raman memory a promising tool, whose broad spectral bandwidth facilitates direct interfacing with other photonic primitives, such as single photon sources. Here, such a light-matter interface is implemented in a warm caesium vapour. Firstly, we study the storage of polarisation-encoded information in the memory. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which is a key technological capability. Prior to storage, single photons are produced in a spontaneous parametric down conversion source, whose bespoke design spectrally tailors the photons to the memory acceptance line. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of 0.3 in the memory output. Nevertheless, a clear influence of the input's quantum nature is observed in the retrieved light by measuring signal's photon statistics. Finally, the memory noise processes are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory.
Photonic quantum information processing has emerged as a powerful platform for realising quantum-enhanced technologies. In order to be scalable, many of these technologies depend on the availability of a suitable quantum memory for the coherent storage and on-demand retrieval of photonic quantum states. In this thesis, I investigate broadband light storage in a room-temperature Raman memory, implemented both in free space and, for the first time, inside a low-finesse optical cavity designed for low-noise operation. The ability of the Raman memory to preserve phase coherence was tested by storing coherent polarisation states in two spatially separate atomic ensembles. Polarisation storage with a fidelity of up to 97 ± 1% was demonstrated by performing full process tomography on the system. The Raman memory was then interfaced for the first time with a spontaneous parametric downconversion (SPDC) source of heralded, GHz-bandwidth single photons. The memory performance was characterised by measuring the second-order autocorrelation of the retrieved fields. While the SPDC input photon statistics showed a clear influence on the statistics of the retrieved field, four-wave mixing (FWM) noise, stimulated by spontaneous Raman scattering, prevented the preservation of non-classical photon statistics during read-out. Suppressing this source of noise represents the last remaining challenge for realising a broadband single-photon Raman memory suitable for quantum information applications. To this end, I demonstrate a novel cavity implementation of the Raman memory which reduces the FWM contribution relative to the signal field by re-distributing the density of states into which the noise photons can be scattered. Cavity-enhanced memory operation was investigated using weak coherent input states, showing a significant improvement of the signal-to-noise ratio compared to the free-space memory implementation. This proof-of-principle demonstration suggests that cavity Raman memories may offer a practical route towards low-noise, high-bandwidth quantum storage at room temperature.
Single photons are a vital resource for optical quantum information processing. Efficient and deterministic single photon sources do not yet exist, however. To date, experimental demonstrations of quantum processing primitives have been implemented using non-deterministic sources combined with heralding and/or postselection. Unfortunately, even for eight photons, the data rates are already so low as to make most experiments impracticable. It is well known that quantum memories, capable of storing photons until they are needed, are a potential solution to this `scaling catastrophe'. Here, we analyze in detail the benefits of quantum memories for producing multiphoton states, showing how the production rates can be enhanced by many orders of magnitude. We identify the quantity etaBeta BetaB as the most important figure of merit in this connection, where etaetaeta and BBB are the efficiency and time-bandwidth product of the memories, respectively. We go on to review our progress in implementing the most broadband memory to date, with B<1000B<1000B<1000, in room-temperature cesium vapour. We consider the noise properties for single photon storage and the integration of the memory using waveguides.