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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Towards scalable photonics via quantum storage

Joshua Nunn, Nathan K. Langford, W. S. Kolthammer, T. F. M. Champion, Michael Sprague, Patrick Michelberger, Xian‐Min Jin, Duncan G. England, Ian A. Walmsley

Published Mar 6, 2013• 2 citations
Publisher

Abstract

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 $eta B$ as the most important figure of merit in this connection, where $eta$ and $B$ 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 $B<1000$, in room-temperature cesium vapour. We consider the noise properties for single photon storage and the integration of the memory using waveguides.

DOI
10.1117/12.2012487

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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.
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 nondeterministic 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 ηB as the most important figure of merit in this connection, where η and B are the efficiency and time-bandwidth product of the memories, respectively.
This thesis is divided into five parts, each with an accompanying publication. In Chapter 1, I motivate the broad scientific pursuit that underlies this thesis: the quest to build universal, fault-tolerant technology that operates on the principles of quantum mechanics and quantum information theory. I motivate the use of single photons as the quantum particles of choice for this task, and I describe the open challenges facing the field of researchers embarking on this quest. This motivation pervades each subsequent chapter of this thesis, which focus in turn on overcoming four distinct technological and scientific hurdles related to these open challenges. The first of these open challenges is to demonstrate experimentally a useful single-photon source. This source of single photons ought to release just one photon at a time, in an on-demand fashion, with high-rate, low-latency, and high quantum-state purity and indistinguishability, in a single spatial mode, at the right wavelengths for a given application, and ought to satisfy certain practical constraints on size, weight, power, and sensitivity to environmental noise and perturbation. This is a long list of performance metrics, some of which are orthogonal, some not, and non-trivial tradeoffs often exist between metrics. While in this thesis we do not present a solution to this general open problem, we modestly expand scientific knowledge of one of the fundamental mechanisms behind single-photon generation. In particular, we present a new formalism for describing the quantum properties of spontaneous Stokes scattering in the Raman interaction, and we probe its effects on the quantum state purity of resulting Stokes photons both experimentally and theoretically. This is the focus of Chapter 2. Another open challenge to universal, fault-tolerant photonic quantum processing is the experimental demonstration of a useful photonic quantum memory—a device that is capable of storing and releasing single photons in a quantum-information-conserving manner, and which possesses sufficiently high performance across, again, a large set of non-orthogonal metrics. On this challenge, we have made several substantial contributions. Chapter 3 describes a novel approach to optimizing the efficiency of photonic quantum memories in atomic ensembles possessing a three-level Lambda-type energy level structure, in an experimentally friendly fashion. Chapter 4 discusses the practical side of implementing robust quantum memory in these atomic systems, where we carefully consider the theoretical sensitivity of these systems to experimental noise and perturbation. These two Chapters lead up to the experimental work described in Chapter 5, where we experimentally demonstrate a state-of-the-art and best-in-class photonic quantum memory in neutral barium vapor that possesses simultaneously high efficiency, broad bandwidth, and low noise. Our experimental demonstration features >95% storage efficiency, at roughly 3 orders of magnitude larger bandwidth (higher speed) than the previous state of the art, as well as record-low noise for Lambda-type atomic memories. The noise is so low, and the bandwidth so large, that we are able to perform a novel characterization experiment to extract the temporal amplitude and phase of the photons retrieved from our memory, based on spectral interferometry with a known reference. This is the most demanding possible characterization of one of these memories, and has direct implications for using this type of memory in quantum processing tasks that require temporally indistinguishable photons. In Chapter 6 I do the requisite summing up of the thesis, and discuss ongoing and future work on the memory project. In the various and sundry appendices I include information that will chiefly be useful for my successors in the lab, but may be of passing interest to other readers.
An optical quantum memory, a device which can store and retrieve on demand an arbitrary quantum state at the single photon level, has been identified as a significant cornerstone of photonic quantum technologies. The development of such a memory would allow the creation and development of a large number of quantum technologies. These would range from the ability to synchronise processing steps in an optical quantum computer, to the construction of a quantum repeater which would allow faithful transmission of quantum states over arbitrarily long distances. In this thesis I investigate broadband light storage in a room-temperature Raman memory for purposes of quantum information processing with photons. During the course of this thesis, a novel scheme to suppress four-wave mixing noise is simulated, based on equations derived to describe the physics of the memory interaction. The experimental arrangement to test the suppression method is described and an original method using photon statistics is demonstrated. The results of these experiments prove the ability of the memory to retrieve and maintain the non-classicality of the state, given a single photon input. Following the affirmation that, using this noise suppression technique, the memory can function at the single-photon level, I go on to consider how the temporal-spectral properties of a pulsed mode of light may be manipulated using the memory. There are two primary reasons for this investigation: 1) Increasing the efficiency of the read-in process by carving the optimum mode of the memory 2) Using the memory as a device operating in a high dimensional quantum information basis. The restrictions of such applications are considered and a method to test the efficacy of transformations in this basis is devised and experimentally tested. I also perform demonstrations of light storage using conventional temporal modes using a novel method of velocity selection within warm vapour. Finally I detail further experiments that are achievable with the Raman quantum memory given the progress that has been achieved during my DPhil research. The results show that the Raman memory is a viable candidate for a practical route towards low-noise quantum storage and for manipulation of temporal modes of light.
Abstract Non-classical photon sources are a crucial resource for distributed quantum networks. Photons generated from matter systems with memory capability are particularly promising, as they can be integrated into a network where each source is used on-demand. Among all kinds of solid state and atomic quantum memories, room-temperature atomic vapours are especially attractive due to their robustness and potential scalability. To-date room-temperature photon sources have been limited either in their memory time or the purity of the photonic state. Here we demonstrate a single-photon source based on room-temperature memory. Following heralded loading of the memory, a single photon is retrieved from it after a variable storage time. The single-photon character of the retrieved field is validated by the strong suppression of the two-photon component with antibunching as low as gRR∣W=1(2)=0.20±0.07{g}_{{\rm{RR| W = 1}}}^{(2)}=0.20\pm 0.07gRR∣W=1(2)​=0.20±0.07 g RR∣W=1 ( 2 ) = 0.20 ± 0.07 . Non-classical correlations between the heralding and the retrieved photons are maintained for up to τNCR=(0.68±0.08) ms{\tau }_{{\rm{NC}}}^{{\mathcal{R}}}=(0.68\pm 0.08)\ {\rm{ms}}τNCR​=(0.68±0.08) ms τ NC R = ( 0.68 ± 0.08 ) ms , more than two orders of magnitude longer than previously demonstrated with other room-temperature systems. Correlations sufficient for violating Bell inequalities exist for up to τBI = (0.15 ± 0.03) ms.
Efficient and long-lived interfaces between light and matter are crucial for the development of quantum information technologies. Integrated photonic solutions for quantum storage devices offer improved performances due to light confinement and enable more complex and scalable designs. We demonstrate a novel platform for quantum light storage based on laser written waveguides. The new adopted writing regime allows us to attain waveguides with improved confining capabilities compared to previous demonstrations. We report the first demonstration of single-photon storage in laser written waveguides. While we achieve storage efficiencies comparable to those observed in massive samples, the power involved for the memory preparation is strongly reduced, by a factor 100, due to an enhancement of the light–matter interaction of almost one order of magnitude. Moreover, we demonstrate excited-state storage times 100 times longer than previous realizations with single photons in integrated quantum memories. Our system promises to effectively fulfill the requirements for efficient and scalable integrated quantum storage devices.
Quantum memories capable of storing single photons are essential building blocks for quantum information processing, enabling the storage and transfer of quantum information over long distances. Devices operating at room temperature can be deployed on a large scale and integrated into existing photonic networks, but so far warm quantum memories have been susceptible to noise at the single photon level. Using a fundamentally different approach to quantum memories, i.e. the recently developed far off-resonant Raman memory scheme, the authors present a highly efficient room-temperature memory that is able to operate with a low unconditional noise floor in the quantum regime, something that no other room-temperature memory has been able to demonstrate before.