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Physical Review A

Suppressing four-wave mixing in warm-atomic-vapor quantum memory

I. Vurgaftman, Mark Bashkansky

Published Jun 21, 2013• 27 citations
Publisher

Abstract

Warm-atomic-vapor cells may be employed as quantum-memory components in an experimentally convenient implementation of the Duan-Lukin-Cirac-Zoller protocol. Previous studies have shown the performance of these cells is limited by the combination of collisional fluorescence during the writing process and four-wave mixing during the reading process and have proposed to overcome this by a combination of optimized detuning and prepumping with circularly polarized write and read beams. Here we show that the Raman matrix elements involving the excited $P$ (${F}^{\ensuremath{'}}=I\ensuremath{-}\frac{1}{2}$ and ${F}^{\ensuremath{'}}=I+\frac{1}{2}$) levels of all alkali atoms are always equal in magnitude and opposite in sign when the write and the anti-Stokes (Stokes) photons have the opposite helicity, and the Raman transitions via the two levels interfere destructively. The existence of an optimal detuning is demonstrated for a given dark-count rate of the single-photon detector. The predicted behavior is observed experimentally in a warm Rb cell with buffer gas.

DOI
10.1103/physreva.87.063836

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Warm-atomic-vapor cells may be employed as quantum-memory components in an experimentally convenient implementation of the Duan-Lukin-Cirac-Zoller protocol. Previous studies have shown the performance of these cells is limited by the combination of collisional fluorescence during the writing process and four-wave mixing during the reading process and have proposed to overcome this by a combination of optimized detuning and prepumping with circularly polarized write and read beams. Here we show that the Raman matrix elements involving the excited PPP (F\ensuremath′=I\ensuremath−12{F}^{\ensuremath{'}}=I\ensuremath{-}\frac{1}{2}F\ensuremath′=I\ensuremath−21​ and F\ensuremath′=I+12{F}^{\ensuremath{'}}=I+\frac{1}{2}F\ensuremath′=I+21​) levels of all alkali atoms are always equal in magnitude and opposite in sign when the write and the anti-Stokes (Stokes) photons have the opposite helicity, and the Raman transitions via the two levels interfere destructively. The existence of an optimal detuning is demonstrated for a given dark-count rate of the single-photon detector. The predicted behavior is observed experimentally in a warm Rb cell with buffer gas.
Atomic vapor cells with buffer gas have a number of advantages when employed as quantum memory blocks based on the DLCZ (Duan-Lukin-Cirac-Zoller) protocol: operation slightly above room temperature, ease of handling, as well as commercial availability. Nevertheless, the signal-to-noise ratio in the current implementations is severely limited by the simultaneous presence of collisional fluorescence and the four-wave mixing noise. In our previous work, we have shown how to minimize the influence of the former on the writing process and provided an unambiguous demonstration of quantum memory lasting for 4 μs. An elegant approach to suppress the four-wave-mixing noise by pre-pumping to the state with the hyperfine sublevel with the maximum value was proposed by Walther et al., Int. J. Quantum Inform. 5, 51 (2007). Here we show that this approach is fundamentally limited by the cancellation of the Raman matrix elements involving the Fˊ= 1 and Fˊ= 2 levels, which occurs for all experimental conditions in the S → P transitions of all alkali atoms. A detuning that maximizes the signal-to-noise ratio is shown to exist for a given detector dark-count rate.
Quantum memory is considered to be one of the key elements in the fields of quantum computing and quantum communication. Warm atomic vapor cells for quantum memory in DLCZ (for Duan, Lukin, Cirac, and Zoller) protocol are appealing due to the perceived reduction in experimental complexity and commercial availability. However, reported results on quantum memory using warm vapor cells were done under widely different experimental conditions and produced ambiguous results. In order for the memory to exhibit non-classical behavior, to a high degree of certainty, the cross-correlation value between the Stokes and anti-Stokes photons needs to be greater than two. In this work we demonstrate quantum memory with cross-correlation value between the Stokes and anti-Stokes photons greater than two lasting for 4 μs using warm Rb vapor with buffer gas for nearly co-propagating write and read beams.
Quantum memory is regarded as one of the essential components in the fields of quantum computing and quantum communication. Warm atomic vapor cells for quantum memory, as originally described in DLCZ (for Duan, Lukin, Cirac, and Zoller) protocol, are appealing due to the perceived reduction in experimental complexity and commercial availability. However, published studies on quantum memory using warm vapor cells were performed under widely dissimilar experimental conditions and reported ambiguous results. In order for the memory to exhibit non-classical behavior to a high degree of certainty, the cross-correlation value between the Stokes and anti-Stokes photons needs to be greater than two. In this work we demonstrate quantum memory with cross-correlation value between the Stokes and anti-Stokes photons greater than two lasting for 4 μs using warm Rb vapor with buffer gas for nearly co-propagating write and read beams.
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)%.
The realization of quantum memory using warm atomic vapor cells is appealing because of their commercial availability and the perceived reduction in experimental complexity. In spite of the ambiguous results reported in the literature, we demonstrate that quantum memory can be implemented in a single cell with buffer gas using the geometry where the write and read beams are nearly copropagating. The emitted Stokes and anti-Stokes photons display cross-correlation values greater than 2, characteristic of quantum states, for delay times up to 4 μs.
We present an experimental demonstration of the Hamiltonian manipulation in light-atom interface in Raman-type warm rubidium-87 vapor atomic memory. By adjusting the detuning of the driving beam we varied the relative contributions of the Stokes and anti-Stokes scattering to the process of four-wave mixing which reads out a spatially multimode state of atomic memory. We measured the temporal evolution of the readout fields and the spatial intensity correlations between write-in and readout as a function of detuning with the use of an intensified camera. The correlation maps enabled us to resolve between the anti-Stokes and the Stokes scattering and to quantify their contributions. Our experimental results agree quantitatively with a simple, plane-wave theoretical model we provide. They allow for a simple interpretation of the coaction of the anti-Stokes and the Stokes scattering at the readout stage. The Stokes contribution yields additional, adjustable gain at the readout stage, albeit with inevitable extra noise. Here we provide a simple and useful framework to trace it and the results can be utilized in the existing atomic memories setups. Furthermore, the shown Hamiltonian manipulation offers a broad range of atom-light interfaces readily applicable in current and future quantum protocols with atomic ensembles.