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arXiv:quant-ph/0503186 · Physical Review A

Quantum theory for generation of nonclassical photon pairs by a medium with collective atomic memory

Narek Sisakyan, Yuri P. Malakyan

Published Oct 12, 2005• 5 citations

Abstract

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.

DOI: 10.1103/physreva.72.043806

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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.

The time dependence of nonclassical correlations is investigated for two fields

(1,2)

generated by an ensemble of cold cesium atoms via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. The correlation function

R({t}_{1},{t}_{2})

for the ratio of cross to autocorrelations for the

(1,2)

fields at times

({t}_{1},{t}_{2})

is found to have a maximum value ${R}^{\mathrm{max}}=292\ifmmode\pm\else\textpm\fi{}57$, which significantly violates the Cauchy-Schwarz inequality $R\ensuremath{\le}1$ for classical fields. Decoherence of quantum correlations is observed over ${\ensuremath{\tau}}_{d}\ensuremath{\simeq}175\text{ }\text{ }\mathrm{n}\mathrm{s}$, and is described by our model, as is a new scheme to mitigate this effect.

We report experimental generation of nonclassically correlated photon pairs from collective emission in a room temperature atomic vapor cell. The nonclassical feature of the emission is demonstrated by observing a violation of the Cauchy-Schwarz inequality. Each pair of correlated photons are separated by a controllable time delay up to 2 microseconds. This experiment demonstrates an important step towards the realization of the Duan-Lukin-Cirac-Zoller scheme for scalable long-distance quantum communication.

We report experimental generation of nonclassically correlated photon pairs from collective emission in a room-temperature atomic vapor cell. The nonclassical feature of the emission is demonstrated by observing a violation of the Cauchy-Schwarz inequality. Each pair of correlated photons is separated by a controllable time delay up to $2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. This experiment demonstrates an important step towards the realization of the Duan-Lukin-Cirac-Zoller scheme for scalable long-distance quantum communication.

The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to ~77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.

The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to ~77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.