Demonstration of Near-Optimal Discrimination of Optical Coherent States

The optimal discrimination of nonorthogonal quantum states with minimum error probability is a fundamental task in quantum measurement theory as well as an important primitive in optical communication. In this work, we propose and experimentally realize a new and simple quantum measurement strategy capable of discriminating two coherent states with smaller error probabilities than can be obtained using the standard measurement devices: the Kennedy receiver and the homodyne receiver

Experimental observation of three-color optical quantum correlations

Quantum correlations between bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information of pump fluctuations. This is the first observation of three-color optical quantum correlations.Comment: 3 pages, 3 figure

Accessing the purity of a single photon by the width of the Hong-Ou-Mandel interference

We demonstrate a method to determine the spectral purity of single photons. The technique is based on the Hong-Ou-Mandel (HOM) interference between a single photon state and a suitably prepared coherent field. We show that the temporal width of the HOM dip is not only related to reciprocal of the spectral width but also to the underlying quantum coherence. Therefore, by measuring the width of both the HOM dip and the spectrum one can directly quantify the degree of spectral purity. The distinct advantage of our proposal is that it obviates the need for perfect mode matching, since it does not rely on the visibility of the interference. Our method is particularly useful for characterizing the purity of heralded single photon states.Comment: Extended version, 16 pages, 9 figure

Entanglement in the above-threshold optical parametric oscillator

We investigate entanglement in the above-threshold Optical Parametric Oscillator, both theoretically and experimentally, and discuss its potential applications to quantum information. The fluctuations measured in the subtraction of signal and idler amplitude quadratures are $\Delta^2 \hat p_-=0.50(1)$, or $-3.01(9)$ dB, and in the sum of phase quadratures are \Delta^2 \hatq_+=0.73(1), or $-1.37(6)$ dB. A detailed experimental study of the noise behavior as a function of pump power is presented, and discrepancies with theory are discussed.Comment: 9 pages, 6 figs. Important reference for readers of quant-ph/0610197. J. Opt. Soc. Am. B, Feature Issue on Optical Quantum-Information Science, doc. ID 70938 (posted 5 September 2006, in press

Accessing the purity of a single photon by the width of the Hong– Ou–Mandel interference

Abstract. We demonstrate a method for determining the spectral purity of single photons. The technique is based on the Hong-Ou-Mandel (HOM) interference between a single-photon state and a suitably prepared coherent field. We show that the temporal width of the HOM dip is related not only to the reciprocal of the spectral width but also to the underlying quantum coherence. Therefore, by measuring the width of both the HOM dip and the spectrum, one can directly quantify the degree of spectral purity. The distinct advantage of our proposal is that it obviates the need for perfect mode matching, since it does not rely on the visibility of the interference. Our method is particularly useful for characterizing the purity of heralded single-photon states

ID 76384)

Quantum correlations among bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information on pump fluctuations. To our knowledge this is the first observation of three-color optical quantum correlations. © 2007 Optical Society of America OCIS codes: 270.0270, 270.6570, 190.4970. Quantum correlations are a signature of nonclassical light generation. The optical parametric oscillator (OPO) is the best known and most widely used source of such correlations. Squeezing in the intensity difference of the twin beams that the OPO produces above threshold 1 (signal and idler) yielded the record value for squeezing of −9.7 dB. 2 Bipartite continuous variable entanglement in this system, which requires the observation of phase anticorrelations as well, although predicted in 1988, 3 was demonstrated only very recently. Beyond the demonstration of nonclassical light features, one should notice that all three fields have different frequencies. Interest in quantum frequency conversion developed in the early 1990s 9,10 and opens perspectives for the interaction of light with physical systems having different resonance frequencies. The above-threshold OPO produces, in general, nondegenerate twin beams, and the pump beam has approximately twice their frequencies. Three-color quantum correlations increase the number of physical systems that can be simultaneously investigated. Correlations with the pump can also be used to enhance the bipartite entanglement between the twin beams, as we show below. Quantum correlations should exist between the phase quadrature of the pump field and the sum of phase quadratures of the signal and idler fields as a direct consequence of phase matching, k ជ 0 = k ជ 1 + k ជ 2 , and of energy conservation, 0 = 1 + 2 . These conditions imply that phase fluctuations are related following ␦ 0 = ␦ 1 + ␦ 2 . This qualitative argument is confirmed by detailed theoretical predictions. 3,8 Indices j ͕0,1,2͖ refer to pump, signal, and idler fields, respectively. The quadratures are defined through the field annihilation operators â j = exp͑i j ͒͑p j + iq j ͒, where j is chosen so that ͗q j ͘ = 0. When the OPO is detuned from exact triple resonance, this phasephase correlation, C q 0 q + = ͗␦q 0 ␦q + ͘, is partially transferred to an amplitude-phase correlation, C p 0 q + = ͗␦p 0 ␦q + ͘, owing to phase noise to amplitude noise conversion 11 inside the OPO cavity ͓q + ϵ͑q 1 + q 2 ͒ / ͱ 2͔. Our experiment is designed to measure joint fluctuations of a combination of q + and p 0 and compare them with the shot-noise level, which defines the standard quantum limit (SQL). This will enable us to improve the bipartite entanglement of the twin beams. Twin beam entanglement is proved by violation of an inequality derived by Duan et al. 1