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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Generating and storing nonclassical correlations in a warm Rb vapor cell with buffer gas

Mark Bashkansky, Fredrik K. Fatemi, I. Vurgaftman

Published May 1, 2012• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.918306

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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.
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.
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.
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 (${F}^{\ensuremath{'}}=I\ensuremath{-}\frac{1}{2}$ 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.
Abstract Quantum memory capable of stopping flying photons and storing their quantum coherence is essential for scalable quantum technologies. A room-temperature broadband quantum memory will enable the implementation of large-scale quantum systems for real-life applications. Due to either intrinsic high noises or short lifetime, it is still challenging to find a room-temperature broadband quantum memory beyond conceptual demonstration. Here, we present a far off-resonance Duan–Lukin–Cirac–Zoller (FORD) protocol and demonstrate the broadband quantum memory in room-temperature atoms. We observe a low unconditional noise level of 10−4 and a cross-correlation up to 28. A strong violation of Cauchy–Schwarz inequality indicates high-fidelity generation and preservation of non-classical correlation. Furthermore, the achieved cross-correlation in room-temperature atoms exceeds the key boundary of 6, above which quantum correlation is able to violate Bell’s inequality. Our results open up the door to an entirely new realm of memory-enabled quantum applications at ambient conditions.
The generation and distribution of quantum correlations between photonic qubits is a key resource in quantum information science. For applications in quantum networks and quantum repeaters, it is required that these quantum correlations be stored in a quantum memory. In 2001, Duan, Lukin, Cirac, and Zoller (DLCZ) proposed a scheme combining a correlated photon-pair source and a quantum memory in atomic gases, which has enabled fast progress towards elementary quantum networks. In this Letter, we demonstrate a solid-state source of correlated photon pairs with embedded spin-wave quantum memory, using a rare-earth-ion-doped crystal. We show strong quantum correlations between the photons, high enough for performing quantum communication. Unlike the original DLCZ proposal, our scheme is inherently multimode thanks to a built-in rephasing mechanism, allowing us to demonstrate storage of 11 temporal modes. These results represent an important step towards the realization of complex quantum networks architectures using solid-state resources.
We present the quantum theory for creation of collective atomic memory and generation of nonclassically correlated photon pairs from an ensemble via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. The temporal evolution of photon numbers, photon statistics, and cross-correlation between the Stokes and anti-Stokes fields is found by solving the equation of motion for atomic spin-wave excitations. We consider a low-finesse cavity model with collectively enhanced signal-to-noise ratio, which remains still considerably large in the free-space limit. Our results describe analytically the dependence of quantum correlations on spin decoherence time and time delay between the write and read lasers and reproduce the observed data very well including the generated pulse shapes, strong violation of Cauchy-Schwarz inequality and conditional generation of anti-Stokes single-photon pulse. The approach we developed may be used also for quantum description of storage and retrieval of quantum information, especially when the statistical properties of nonclassical pulses are studied.