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arXiv:2203.06151 · Physical Review A

Optimization and readout-noise analysis of a warm-vapor electromagnetically-induced-transparency memory on the Cs D1 line

Luisa Esguerra, Leon Meßner, Elizabeth Robertson, Norman Vincenz Ewald, Mustafa Gündoğan, Janik Wolters

Published Apr 12, 2023• 12 citations
arXivHTMLPDF

Abstract

Quantum memories promise to enable global quantum repeater networks. For field applications, alkali-metal vapors constitute an exceptional storage platform, as neither cryogenics, nor strong magnetic fields are required. We demonstrate a technologically simple, in principle satellite-suited quantum memory based on electromagnetically induced transparency on the cesium $D1$ line, and focus on the tradeoff between end-to-end efficiency and signal-to-noise ratio, both being key parameters in applications. For coherent pulses containing one photon on average, we achieve storage and retrieval with end-to-end efficiencies of ${\ensuremath{\eta}}_{\text{e2e}}=13(2)%$, which correspond to internal memory efficiencies of ${\ensuremath{\eta}}_{\text{mem}}=33(1)%$. Simultaneously, we achieve a noise level corresponding to ${\ensuremath{\mu}}_{1}=0.07(2)$ signal photons. This noise is dominated by spontaneous Raman scattering, with contributions from fluorescence. Four-wave mixing noise is negligible, allowing for further minimization of the total noise level.

DOI
10.1103/physreva.107.042607

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Quantum memories promise to enable global quantum repeater networks. For field applications, alkali-metal vapors constitute an exceptional storage platform, as neither cryogenics, nor strong magnetic fields are required. We demonstrate a technologically simple, in principle satellite-suited quantum memory based on electromagnetically induced transparency on the cesium D1D1D1 line, and focus on the tradeoff between end-to-end efficiency and signal-to-noise ratio, both being key parameters in applications. For coherent pulses containing one photon on average, we achieve storage and retrieval with end-to-end efficiencies of \ensuremathηe2e=13(2){\ensuremath{\eta}}_{\text{e2e}}=13(2)%\ensuremathηe2e​=13(2), which correspond to internal memory efficiencies of \ensuremathηmem=33(1){\ensuremath{\eta}}_{\text{mem}}=33(1)%\ensuremathηmem​=33(1). Simultaneously, we achieve a noise level corresponding to \ensuremathμ1=0.07(2){\ensuremath{\mu}}_{1}=0.07(2)\ensuremathμ1​=0.07(2) signal photons. This noise is dominated by spontaneous Raman scattering, with contributions from fluorescence. Four-wave mixing noise is negligible, allowing for further minimization of the total noise level.
Quantum memories are a key element for the realisation of quantum repeaters, essential for long-distance quantum communication. Especially for satellite-based quantum networks, alkali metal vapours constitute an excellent storage platform, as neither cryogenics, nor strong magnetic fields are required. We have realised a technologically simple, in principle satellite-suited quantum memory in Caesium vapor, based on electromagnetically induced transparency (EIT) on the Cs D1 line. We focus on the simultaneous optimization of end-to-end efficiency and signal-to-noise level in the memory, which will make our system suitable for many different applications. We have achieved light storage at the single-photon level with end-to-end efficiencies up to 12%, which correspond to internal memory efficiencies of up to 30%. Simultaneously we achieve a minimal noise level corresponding to µ_1=0.029 signal photons. Furthermore, we have determined the limiting noise source at this level to be four-wave mixing noise in the Lambda-system and present solutions to minimise this read-out noise.
The ability to store large amounts of photonic quantum states is regarded as substantial for future optical quantum computation and communication technologies. However, research for multiplexed quantum memories has been focused on systems that show good performance only after an elaborate preparation of the storage media. This makes it generally more difficult to apply outside a laboratory environment. In this work, we demonstrate a multiplexed random-access memory to store up to four optical pulses using electromagnetically induced transparency in warm cesium vapor. Using a Λ-System on the hyperfine transitions of the Cs D1 line, we achieve a mean internal storage efficiency of 36% and a 1/e lifetime of 3.2 µs. In combination with future improvements, this work facilitates the implementation of multiplexed memories in future quantum communication and computation infrastructures.
Quantum memories are key components in quantum information networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their long storage times and experimental simplicity. Operation with the Raman protocol enables high time-bandwidth products, which allows for multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing. This makes the Raman memory a promising tool, whose broad spectral bandwidth facilitates direct interfacing with other photonic primitives, such as single photon sources. Here, such a light-matter interface is implemented in a warm caesium vapour. Firstly, we study the storage of polarisation-encoded information in the memory. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which is a key technological capability. Prior to storage, single photons are produced in a spontaneous parametric down conversion source, whose bespoke design spectrally tailors the photons to the memory acceptance line. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of 0.3 in the memory output. Nevertheless, a clear influence of the input's quantum nature is observed in the retrieved light by measuring signal's photon statistics. Finally, the memory noise processes are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory.
Quantum memories matched to single photon sources will form an important cornerstone of future quantum network technology. We demonstrate such a memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency. With an acceptance bandwidth of δf=0.66 GHz, the memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity, and atomic collisions have negligible influence on the optical coherences. Operation of the memory is demonstrated using attenuated laser pulses on the single photon level. For a 50 ns storage time, we measure η_{e2e}^{50 ns}=3.4(3)% end-to-end efficiency of the fiber-coupled memory, with a total intrinsic efficiency η_{int}=17(3)%. Straightforward technological improvements can boost the end-to-end-efficiency to η_{e2e}≈35%; beyond that, increasing the optical depth and exploiting the Zeeman substructure of the atoms will allow such a memory to approach near unity efficiency. In the present memory, the unconditional read-out noise level of 9×10^{-3} photons is dominated by atomic fluorescence, and for input pulses containing on average μ_{1}=0.27(4) photons, the signal to noise level would be unity.
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)%.
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.