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arXiv:2212.04263 · Communications Physics

Fast, noise-free atomic optical memory with 35-percent end-to-end efficiency

Omri Davidson, Ohad Yogev, Eilon Poem, Ofer Firstenberg

Published Jun 6, 2023• 26 citations
arXivHTMLPDF

Abstract

Abstract Coherent optical memories will likely play an important role in future quantum communication networks. Among the different platforms, memories based on ladder-type orbital transitions in atomic gasses offer high bandwidth (>100 MHz), continuous (on-demand) readout, and low-noise operation. Here we report on an upgraded setup of our previously-reported fast ladder memory, with improved efficiency and lifetime, and reduced noise. The upgrade employs a stronger control field, wider signal beam, reduced atomic density, higher optical depth, annular optical-pumping beam, and weak dressing of an auxiliary orbital to counteract residual Doppler-broadening. For a 2 ns-long pulse, we demonstrate 53% internal efficiency, 35% end-to-end efficiency, 3 × 10−5 noise photons per pulse, and a 1/e lifetime of 108 ns. This combination of performances is a record for continuous-readout memories.

DOI
10.1038/s42005-023-01247-4

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Abstract Coherent optical memories will likely play an important role in future quantum communication networks. Among the different platforms, memories based on ladder-type orbital transitions in atomic gasses offer high bandwidth (>100 MHz), continuous (on-demand) readout, and low-noise operation. Here we report on an upgraded setup of our previously-reported fast ladder memory, with improved efficiency and lifetime, and reduced noise. The upgrade employs a stronger control field, wider signal beam, reduced atomic density, higher optical depth, annular optical-pumping beam, and weak dressing of an auxiliary orbital to counteract residual Doppler-broadening. For a 2 ns-long pulse, we demonstrate 53% internal efficiency, 35% end-to-end efficiency, 3 × 10−5 noise photons per pulse, and a 1/e lifetime of 108 ns. This combination of performances is a record for continuous-readout memories.
Future quantum photonic networks require coherent optical memories, preferably operating at room temperature, for synchronizing quantum sources and gates of probabilistic nature. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise. Here we demonstrate a fast ladder memory (FLAME) mapping the optical field onto the superposition between electronic orbitals of rubidium vapor. Employing a ladder level-system of orbital transitions with nearly degenerate frequencies simultaneously enables high bandwidth, low noise, and long memory lifetime. We store and retrieve 1.7-ns-long pulses, containing 0.5 photons on average, and observe short-time external efficiency of 25%, memory lifetime (1/e) of 86 ns, and below 10−4 added noise photons. Consequently, coupling this memory to a probabilistic source would enhance the on-demand photon generation probability by a factor of 12, the highest number yet reported for a noise-free, room-temperature memory. This paves the way towards the controlled production of large quantum states of light from probabilistic photon sources.
Optical quantum memory describes the process of on-demand storage and retrieval of single photon-level quantum states, and is a critical enabling technology for many quantum applications. Memory bandwidth plays an important role in these applications, as it determines the pulse durations compatible with the memory and places an upper bound on the clock rate and processing speed of a quantum device. Here we present experimental results of an atomic barium quantum memory that enables storage and retrieval of ultra-broadband(>800(> 800(>800GHz) signal photons with high storage efficiency [95.6(3)%] and low noise [3.8(6)×10−5\times 10^{-5}×10−5noise photons]. Experimental Results.- The quantum memory operation is based on the ground(6s2 1S0)(6s^{2}\ {}^{1}S_{0})(6s2 1S0​), excited(6s6p 1P1)(6s6p \ {}^{1}P_{1})(6s6p 1P1​), and metastable(6s5d 1D2)(6s5d \ {}^{1}D_{2})(6s5d 1D2​)orbital states of atomic barium in aΛ\LambdaΛ-type configuration. The barium vapor is created in an 800–900 °C barium heat pipe oven with 0–1000 torr tunable argon buffer gas pressure. The ground-excited transition at 553.5 nm features large and tunable homogeneous collisional broadening due to the argon buffer gas and a peak optical depth ofd=50d=50d=50. The memory operates in the so-called absorb-then-transfer (ATT) regime, in which the signal field is linearly absorbed along the ground-excited transition, whose collisionally broadened lineshape enables efficient absorption of ultra-broadband photons; the resulting atomic polarization is transferred into a so-called spin wave by application of a strong[O(10 uJ),100 fs][O(10 \ \text{uJ}),100\ \text{fs}][O(10 uJ),100 fs]control pulse along the excited-metastable transition at 1500 nm. The storage state has a 0(0.1) second coherence lifetime in the bare atom [1], but this is reduced to theO(ns)O(\text{ns})O(ns)level due to motional dephasing [0.49(1) ns measured memory lifetime]. The memory experiment is repeated at a repetition rate of 1 kHz. The total end-to-end efficiency of the memory at 900°C is 31(1)%, which is limited by available control field power.
We demonstrate quantum memory of single-photon-level coherent pulses of 880 GHz bandwidth with 95.6(3)% storage efficiency in collisionally broadened barium vapor. We measure 26(1)% total efficiency, limited by control field power; 0.49(1) ns memory lifetime, limited by motional dephasing; and a signal-to-noise ratio of O(103){O}(10^{3})O(103), limited by two-photon control field scattering. To the best of the authors’ knowledge, this represents the best efficiency, lifetime, and noise performance of atomic quantum memories in the ultrabroadband regime (>100(\gt100(>100 GHz bandwidth) to date.
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.
On-demand storage and retrieval of quantum information in coherent light-matter interfaces is a key requirement for future quantum networking and quantum communication applications. Alkali vapor memories offer scalable and robust high-bandwidth storage at high repetition rates which makes them a natural fit to interface with solid-state single-photon sources. Here, we experimentally realize a room-temperature ladder-type atomic vapor memory that operates on the Cs D1 line. We provide a detailed experimental characterization and demonstration of on-demand storage and retrieval of weak coherent laser pulses with 0.06 photons per pulse at a high signal-to-noise ratio of SNR=830(80)=830(80)=830(80). The memory achieves a maximum internal storage efficiency of ηint=15(1)%η_{\text{int}}=15(1)\%ηint​=15(1)% and an estimated 1/e1/e1/e-storage time of τs≈32 τ_{\mathrm{s}}\approx32\,τs​≈32ns. Benchmark properties for the storage of single photons from inhomogeneously broadened state-of-the-art solid-state emitters are estimated from the performance of the memory. Together with the immediate availability of high-quality InGaAs quantum dots emitting at 895\,nm, these results provide clear prospects for the development of a heterogeneous on-demand quantum light interface.
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.