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Physical Review Applied

Single-Photon-Compatible Telecommunications-Band Quantum Memory in a Hot Atomic Gas

Sarah E. Thomas, Steven Sagona-Stophel, Zakary Schofield, Ian A. Walmsley, Patrick M. Ledingham

Published Mar 16, 2023• 17 citations
Publisher

Abstract

Tomorrow's quantum Internet will be powered by light and will work over today's telecommunication infrastructure, so we need low-noise, high-bandwidth, telecom-band quantum optical memory to enable scaling in the presence of loss and quantum operations. The authors have built such a device, using coherent two-photon absorption in warm rubidium vapor. This quantum memory stores gigahertz-bandwidth telecom-band light pulses with mean photon number less than one, and retrieves them with a signal-to-noise ratio exceeding 10${}^{4}$. This makes possible ultrahigh-fidelity storage of single-photon qubits and is compatible with quantum-dot light sources, for hybridized quantum photonic networking.

DOI
10.1103/physrevapplied.19.l031005

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Tomorrow's quantum Internet will be powered by light and will work over today's telecommunication infrastructure, so we need low-noise, high-bandwidth, telecom-band quantum optical memory to enable scaling in the presence of loss and quantum operations. The authors have built such a device, using coherent two-photon absorption in warm rubidium vapor. This quantum memory stores gigahertz-bandwidth telecom-band light pulses with mean photon number less than one, and retrieves them with a signal-to-noise ratio exceeding 104{}^{4}4. This makes possible ultrahigh-fidelity storage of single-photon qubits and is compatible with quantum-dot light sources, for hybridized quantum photonic networking.
The efficient storage and on-demand retrieval of quantum optical states that are compatible with the telecommunications C-band is a requirement for future terrestrial-based quantum optical networking. Spectrum in the C-band minimises optical fiber-propagation losses, and broad optical bandwidth facilitates high-speed networking protocols. Here we report on a telecommunication wavelength and bandwidth compatible quantum memory. Using the Off-Resonant Cascaded Absorption protocol in hot 87^{87}87Rb vapour, we demonstrate a total memory efficiency of 20.90(1) %20.90(1)\,\%20.90(1)% with a Doppler-limited storage time of 1.10(2) 1.10(2)\,1.10(2)ns. We characterise the memory performance with weak coherent states, demonstrating signal-to-noise ratios greater than unity for mean photon number inputs above 4.5(6)×10−64.5(6)\times10^{-6}4.5(6)×10−6 per pulse.
A hybrid interface of solid state single-photon sources and atomic quantum memories is a long sought-after goal in photonic quantum technologies. Here we demonstrate deterministic storage and retrieval of photons from a semiconductor quantum dot in an atomic ensemble quantum memory at telecommunications wavelengths. We store single photons from an InAs quantum dot in a high-bandwidth rubidium vapour based quantum memory, with a total internal memory efficiency of (12.9±0.4)%(12.9 \pm 0.4) \%(12.9±0.4)%. The signal-to-noise ratio of the retrieved photons is 18.2±0.618.2\pm 0.618.2±0.6, limited only by detector dark counts. This demonstration paves the way to quantum technologies that rely on distributed entanglement, and is especially suited for photonic quantum networks.
A hybrid interface of solid-state single-photon sources and atomic quantum memories is a long sought-after goal in photonic quantum technologies. Here, we demonstrate deterministic storage and retrieval of light from a semiconductor quantum dot in an atomic ensemble quantum memory at telecommunications wavelengths. We store single photons from an indium arsenide quantum dot in a high-bandwidth rubidium vapor-based quantum memory, with a total internal memory efficiency of (12.9 ± 0.4)%. The signal-to-noise ratio of the retrieved light field is 18.2 ± 0.6, limited only by detector dark counts.
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.
Photonic links between disparate quantum technologies−-−such as photon sources, memories, processors, clocks, and sensors−-−are key to scaling quantum networks and realizing a versatile quantum internet for secure quantum communication, distributed quantum computing, and entanglement-enhanced metrology. In practice, each technology is most suitably implemented on a different quantum platform; the substantial spectral mismatch between them, along with scarce native telecom interfaces, thus poses a major bottleneck to achieving efficient interconnections over long distances. Here we demonstrate the first deployed two-node hybrid network that operates entirely in the telecom C-band. Our approach uses no quantum frequency conversion or external filtering; instead, we develop a neutral atom single photon source and a solid-state rare-earth quantum memory that both operate in previously unexplored telecom regimes with state-of-the-art performance. The source achieves a high single-photon purity at 46 kcps, and the memory a storage efficiency of 10.6% with high multimode capacity. We leverage the intrinsic tunability of both systems to optimize their spectral overlap and demonstrate microsecond-level storage and retrieval with a large time-bandwidth product. Moreover, we showcase real-world networking competencies such as support for multiplexing across 37 temporal modes and preservation of non-classicality over fibers of 10.6 km (metropolitan) and 49.2 km (laboratory). Our work establishes a backbone for telecom-native quantum repeater links and unlocks a path towards high-bandwidth, large-scale quantum networking.
Quantum memories can substantially increase the efficiency of long-distance communications by synchronizing entanglement swapping operations in quantum repeater nodes. To build a quantum memory, electromagnetically induced transparency (EIT) in atomic vapors can be exploited to coherently store light pulses even at room temperatures. As a quantum source of light, semiconductor quantum dots (QD) offer bright on-demand single photons with high purity.4 Interfacing QDs with atomic vapors has been shown by “slow light” but a quantum memory for QDs is yet to be demonstrated. In this work, we develop an EIT quantum memory hosted in warm cesium vapor. Storage of faint coherent light pulses on the single photon level shows high storage efficiency. A measured bandwidth in the order of 200 MHz makes the memory compatible with the Fourier-limited emission of QDs embedded in micropillar cavities. We show the first attempts to interface the emission from a QD-micropillar with our quantum memory by finetuning the emission wavelength of the emitters to one of the hyperfine transitions in Cs, where the EIT memory takes place. This work sets the base for a hybrid quantum memory based on atomic ensembles for an on-demand semiconductor single-photon source.