Cavity-Enhanced Room-Temperature Broadband Raman Memory

D. J. Saunders, J. H. D. Munns, T. F. M. Champion, C. Qiu, K. T. Kaczmarek, E. Poem, P. M. Ledingham, I. A. Walmsley, and J. Nunn

Phys. Rev. Lett. 116, 090501 – Published 3 March 2016

Abstract

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, $(15 \pm 2) \times 10^{- 3}$ photons per pulse, with a total efficiency of $(9.5 \pm 0.5) %$ .

Received 15 October 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.090501

Physics Subject Headings (PhySH)

Cooling & trapping Light-matter interaction Quantum communication Quantum information processing

Atomic, Molecular & OpticalQuantum Information, Science & Technology

Authors & Affiliations

D. J. Saunders^1,*, J. H. D. Munns^1,2, T. F. M. Champion¹, C. Qiu^1,3, K. T. Kaczmarek¹, E. Poem¹, P. M. Ledingham¹, I. A. Walmsley¹, and J. Nunn¹

¹Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
²QOLS, Blackett Laboratory, Imperial College London, London SW7 2BW, United Kingdom
³Department of Physics, Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, People’s Republic of China

^*dylan.saunders@physics.ox.ac.uk

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Vol. 116, Iss. 9 — 4 March 2016

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Images

Figure 1
(a) Energy level diagrams showing the $D 2$ $Λ$ level scheme in Cs. There are two major interactions: one, the Stokes scattering—involving the signal field $\hat{s}$ (the green arrow), the control field (the red arrow), and spin wave $\hat{b}$ (the red loop), this is the desired memory interaction; two, anti-Stokes scattering—involving the control field, anti-Stokes mode $\hat{a}$ (the blue arrow), and spin wave $\hat{b}$ . The third diagram is the effective noise-process, anti-Stokes scattering followed by Stokes scattering, a four-wave-mixing interaction. (b) Measured frequency response of the atom-filled cavity system. The top is for the signal and anti-Stokes polarization, and the bottom is for the control polarization. The dashed lines are measured responses (see the Supplemental Material [31]) and the solid lines are our theory model using measured values of the cavity parameters [33]. The grey solid areas represent the absorption in the cavity, and they are an indication of our spin polarization, with $\sim 80 %$ in the ideal $F = 4$ state. The green pulse shows the location of the signal and the blue the anti-Stokes, while the red is the control. (c) Scale diagram of the cavity Raman quantum memory. We operate the ring cavity in transmission, where incoupling and outcoupling mirrors have the amplitude reflectivities $r_{1}$ and $r_{2}$ , respectively. The curved mirror has $R = 40 mm$ , and the small green rectangle represents the $200 μ m$ -thick ${LiNbO}_{3}$ birefringent crystal, which we tune with temperature to reach the desired cavity conditions. (d) Experimental schematic; for more details see the main text and the Supplemental Material [31] (SMF, single-mode fiber; MMF, multimode fiber; APD, avalanche photodiode; EOM, electro-optic modulator).
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Figure 2
(a) Arrival time histograms for the signal field emerging from the cavity with the control pulses blocked (signal in), unblocked (memory on) and with the input signal field blocked (noise). This is measured for a coherent state amplitude of $| α^{2} | = 0.7 \pm 0.1$ interacting with the memory. (b) Measured (points) and predicted (line) memory efficiency as a function of the intracavity control pulse energy.
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Figure 3
Measurement of $μ_{1}$ . To measure $μ_{1}$ , we operate the memory with a control pulse energy of $\sim 1.4 nJ$ , corresponding to our optimal SNR condition. We then measure the signal-to-noise ratio for various input coherent state amplitudes. Where the fits cross the horizontal-black dashed line ( $SNR = 1$ ) is the value of $μ_{1}$ , with a smaller $μ_{1}$ desirable. The blue circles are our measured values; error bars are smaller than the marker dimension. The lower-red dashed line represents the state-of-the-art using cavity-free Raman memory experiments [21], with a ${μ_{1}}^{'} \approx 0.5$ . The upper-blue dashed line is the expected performance from our theoretical model, with $μ_{1}^{Ideal} = 0.005$ . We do not reach this ideal noise floor due to experimental imperfections not included in the model (see the Supplemental Material [31]). After fitting our data with $y = x / μ_{1}$ , we extract $μ_{1} = 0.17 \pm 0.02$ . The error is calculated using a Monte Carlo simulation.
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