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Conference on Lasers and Electro-Optics

QLad: A Noise-Free Quantum Memory for Broadband Light at Room Temperature

K. T. Kaczmarek, Patrick M. Ledingham, Benjamin Brecht, Amir Feizpour, Guillaume Thekkadath, Sarah E. Thomas, J. H. D. Munns, D. J. Saunders, Ian A. Walmsley, J. Nunn

Published Jan 1, 2017• 2 citations
Publisher

Abstract

We implement a low-noise, broadband quantum memory for light via off-resonant two-photon absorption in warm atomic vapour. We store heralded single photons and verify that the retrieved fields are anti-bunched.
DOI
10.1364/cleo_qels.2017.fm2e.2

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Quantum optical memories are devices that store quantum states of light, which can allow for the active synchronization of probabilistic events within large-scale quantum networks. Recent work on quantum memories have seen impressive quantum operation, albeit still suffering from noise on the output mode of the device. Here we demonstrate a noise-free quantum memory for light based on the off-resonant cascaded absorption of photons in a warm vapour of caesium atoms. The memory is characterized by measuring a noise floor of 8 × 10−6 photons per pulse. We demonstrate genuine quantum operation by storing and recalling on-demand heralded single photons with a heralded second-order autocorrelation function of g(2) = 0.028 ± 0.009.
Optical quantum memories are devices that store and recall quantum light and are vital to the realization of future photonic quantum networks. To date, much effort has been put into improving storage times and efficiencies of such devices to enable long-distance communications. However, less attention has been devoted to building quantum memories which add zero noise to the output. Even small additional noise can render the memory classical by destroying the fragile quantum signatures of the stored light. Therefore, noise performance is a critical parameter for all quantum memories. Here we introduce an intrinsically noise-free quantum memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We demonstrate successful storage of GHz-bandwidth heralded single photons in a warm atomic vapor with no added noise, confirmed by the unaltered photon-number statistics upon recall. Our ORCA memory meets the stringent noise requirements for quantum memories while combining high-speed and room-temperature operation with technical simplicity, and therefore is immediately applicable to low-latency quantum networks.
Light is an ideal information carrier for quantum networks: its quantum properties are not degraded by noise in ambient conditions and it has a large information capacity owing to its high bandwidth. However, quantum technologies based on photonic networks have been hampered by the exponentially poor scaling of the underlying probabilistic quantum operations. Quantum optical memories, devices that store, manipulate, and release on- demand quantum light, have been identified as an indispensable network component, because they facilitate scalability. Noise-free operation of the memory is critical, since even small additional noise can render the memory classical by destroying the quantum character of the light. Here I introduce a new broadband quantum memory protocol - the off-resonant cas- caded absorption (ORCA) memory - that is inherently noise-free and operates in ambient conditions. I model ORCA theoretically, after which I characterise the classical perfor- mance of a proof-of-concept implementation in warm caesium vapour, using weak near- infrared coherent states. In order to verify quantum operation of the ORCA memory, I interface it with a broadband heralded single-photon source. I measure and compare the quantum statistics of the stored and retrieved light, observing for the first time their full preservation in a room-temperature atomic quantum memory. Finally, I make headway into integrating the ORCA vapour memory by investigating an alkali-filled hollow-core fibre platform. I use light-induced atomic desorption (LIAD) to demonstrate record- breaking alkali vapour densities in fibre, a prerequisite for efficient memory operation. Because of its technical simplicity and integrability, its high bandwidth and its low noise, ORCA provides a viable route towards next generation, photonic quantum network technologies.
Summary form only given. A quantum optical memory (QM) is a device that can store and release quantum states of light on demand. Such a device is capable of synchronising probabilistic events, for example, locally synchronising non-deterministic photon sources for the generation of multi-photon states, or successful quantum gate operations within a quantum computational architecture [1], as well as for globally synchronising the generation of entanglement over long distances within the context of a quantum repeater [2]. Desirable attributes for a QM to be useful for these computational and communicational tasks include high end-to-end transmission (including storage and retrieval efficiency), large storage-time-bandwidth product, room temperature operation for scalability and, of utmost importance, noise free performance for true quantum operation.Here we present a new QM protocol that addresses the above issues, the quantum ladder memory (QLAD). This protocol is based on a two-photon `ladder' transition between the 6S1/2 ground state and 6D5/2 excited state of a caesium (Cs) ensemble at room temperature (see Fig. 1a). We characterise the memory with weak coherent states, storing GHz-band pulses with η=22% storage and retrieval efficiency and a characteristic storage time of τ=5.3ns. We measure the noise at the output to be 8×10-6 photons per pulse, giving rise to a noise-to-efficiency ratio of 3.6×10-5, three orders of magnitude better than any memory to date. We generate GHz-band heralded single photons from a spontaneous parametric down conversion source that are matched to the QLAD, coincidence traces seen in Fig. 1b. A heralded second-order autocorrelation function of g(2)=0.02+0.005 is measured for the input photon, and true quantum operation is confirmed by storing and retrieving this photon with g(2)=0.03+0.01 for a 3.5ns storage time. This result is the lowest g(2) on the output of an on-demand memory to date, and represents a significant step toward large-scale quantum technologies.
This thesis is about the experimental implementation of a high-speed and robust quantum memory for light. A novel far off-resonant Raman approach to ensemble-based quantum memories in a room-temperature environment is developed and demonstrated. Storage and retrieval of sub-nanosecond, weak coherent light pulses at the single-photon-level with total efficiencies exceeding 30% and storage times of up to 4 μs are achieved. The coherence of the memory is shown by directly interfering a copy of the incident signal with the retrieved signal from the memory. The unconditional noise floor of the memory is found to be low enough to operate the memory in the quantum regime at room temperature. Multiple readout of a single stored excitation is demonstrated, suggesting that 100% readout is possible in different temporal modes. Furthermore, first results regarding the storage and retrieval of polarisation encoded qubits are obtained. This and the memory’s ability to operate in the quantum regime at room temperature with a low unconditional noise floor illustrate its potential usefulness for real world applications.