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arXiv:1601.00157 · Physical Review A

Theory of noise suppression in Λ -type quantum memories by means of a cavity

Joshua Nunn, J. H. D. Munns, Sarah E. Thomas, K. T. Kaczmarek, C. Qiu, Amir Feizpour, Eilon Poem, Benjamin Brecht, D. J. Saunders, Patrick M. Ledingham, Dileep V. Reddy, Michael G. Raymer, Ian A. Walmsley

Published Jul 31, 2017• 41 citations
arXivHTMLPDF

Abstract

Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of $\mathrm{\ensuremath{\Lambda}}$-type atoms. However, at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here, we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.

DOI
10.1103/physreva.96.012338

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Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of \ensuremathΛ\mathrm{\ensuremath{\Lambda}}\ensuremathΛ-type atoms. However, at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here, we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.
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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.
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.
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.
Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali-vapor Raman memories combine high-bandwidth storage, on-demand readout, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and have suffered from four-wave-mixing noise. Here, we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering antiresonance for the anti-Stokes field, we also suppress the four-wave-mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapor memory, (15±2)×10^{-3} photons per pulse, with a total efficiency of (9.5±0.5)%.
Optical quantum memories are vital for the scalability of future quantum technologies, enabling long-distance secure communication and local synchronization of quantum components. We demonstrate a THz-bandwidth memory for light using the optical phonon modes of a room temperature diamond. This large bandwidth makes the memory compatible with down-conversion-type photon sources. We demonstrate that four-wave mixing noise in this system is suppressed by material dispersion. The resulting noise floor is just 7×10(-3) photons per pulse, which establishes that the memory is capable of storing single quanta. We investigate the principle sources of noise in this system and demonstrate that high material dispersion can be used to suppress four-wave mixing noise in Λ-type systems.