The rate of absorption of entangled photon pairs is linear in the photon-flux density. We demonstrate that the two-photon absorption cross section is a nonmonotonic function of the entanglement time; it vanishes for certain energy-level configurations and values of the entanglement time. This entanglement-induced two-photon transparency arises from the coherent summation of transition-amplitude contributions over the finite entanglement time. As an example, the entangled two-photon cross section for the 1S-2S electronic transition in atomic hydrogen is obtained.
A new form of absorption spectroscopy is introduced in which the energy levels and matrix elements of virtual-state transitions in a medium are determined by the use of continuous-wave degenerate entangled photons without varying their wavelengths. Extractable spectroscopic information is embedded in the entangled-photon absorption cross section when measured over a range of entanglement and interbeam-delay times. This technique exploits the fundamental quantum interference that arises during the interaction of an entangled quantum state with a quantum system. [S0031-9007(98)05928-6]
A single-photon-sensitive intensified charge-coupled-device (ICCD) camera has been used to simultaneously detect, over a broad area, degenerate and nondegenerate photon pairs generated by the quantum-optical process of spontaneous parametric down-conversion. We have developed a new method for determining the quantum fourth- order correlations in spatially extended detection systems such as this one. Our technique reveals the expected phase-matching-induced spa- tial correlations in a 2-f Fourier-transform system.
We theoretically investigate the quantum interference of entangled two-photon states generated in a nonlinear crystal pumped by femtosecond optical pulses. Interference patterns generated by the polarization analog of the Hong-Ou-Mandel interferometer are studied. Attention is devoted to the effects of the pump-pulse profile (pulse duration and chirp) and the second-order dispersion in both the nonlinear crystal and the interferometer's optical elements. Dispersion causes the interference pattern to have an asymmetric shape. Dispersion cancellation occurs in some cases.
Quantum interference is observed using femtosecond parametric down-conversion in the absence of spectral filtering of the down-converted light. The well-known symmetric dip observed in cw polarization-interferometry experiments becomes less pronounced and substantially asymmetric under such pulsed excitation. We show that this asymmetry arises from the partial distinguishability of contributions from space-time segments within the down-conversion crystal. We develop a theory that achieves agreement with experiment by coherently adding intrasegment contributions while incoherently adding intersegment contributions. PACS numbers: 42.50.Dv, 42.65.Re, 42.65.Ky Entangled photon pairs (biphotons) generated via the nonlinear optical process of spontaneous parametric downconversion (SPDC) [1,2] have enabled us to carry out many valuable quantum-optics experiments, particularly during the past few years. These range from fundamental physics experiments that have shed light on the validity of quantum mechanics [3,4] to useful applications such as the determination of detector absolute quantum efficiency [5]. Entangled photon pairs have recently also found a place in unusual but powerful techniques such as quantum cryptography [6] and quantum teleportation [7,8].Almost all of these experiments have made use of stationary streams of biphotons generated by illuminating an anisotropic second-order nonlinear crystal with a continuous-wave (cw) laser pump. The unique possibilities offered by such light sources reside in their entanglement property, which is mathematically described in terms of a composite quantum state that cannot be factored into a produce of single-particle states (entangled states thereby have no classical analogs) [9]. A great variety of quantuminterference experiments have been carried out to examine the unusual second-and fourth-order coherence properties [10,11] of partially entangled biphoton beams.Since ultrafast (sub-ps) pump sources in the ultraviolet and near-ultraviolet have come into their own, it is important to determine the entanglement properties imparted to biphoton beams by very brief pulses of pump light impinging on a nonlinear parametric down-conversion crystal. The reason is simple: entangled photon pairs generated by mode-locked pump lasers emitting femtosecond pulses are expected to be crucial for carrying out important experiments in quantum teleportation [12], entanglement swapping [13], and the production of three-photon entangled [Greenberger-Horne-Zeilinger (GHZ)] states [14]. Several experiments along these lines have already been conducted. Some of these have made use of crystals whose lengths are short relative to the length of the coherent nonlinear interaction region [15]. Others have employed narrow-band interference filters placed immediately before the detectors [12,13,15,16]. The filters serve to lengthen the coherence time of the photons and thereby to increase the fringe visibility in interference experiments. Although the results of both of these types of experiments...
Recently, a ‘‘shooting’’ method has been used to obtain exact expressions for eigenvalues and eigensolutions of the two-dimensional hydrogen atom. This paper shows that the shooting method is easy for undergraduate students to understand and implement numerically. The highly accurate approximations for both eigenvalues and eigensolutions are then used to contrast the two-dimensional and three-dimensional hydrogen atoms. Finally, previous methods for solving the two-dimensional hydrogen atom are compared with the shooting method.
All-optical signal-to-noise ratio improvements by stochastic resonance have been obtained by use of the intensity bistability of a unidirectional photorefractive ring resonator. A signal-to-noise ratio gain of 10.5 dB has been obtained with a near-unity signal-to-noise ratio input signal at 6 mHz.
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