Tunable control of the bandwidth and frequency correlations of entangled photons

We demonstrate experimentally a new technique to control the bandwidth and the type of frequency correlations (correlation, anticorrelation, and even uncorrelation) of entangled photons generated by spontaneous parametric downconversion. The method is based on the control of the group velocities of the interacting waves. This technique can be applied in any nonlinear medium and frequency band of interest. It is also demonstrated that this technique helps enhance the quality of polarization entanglement even when femtosecond pulses are used as a pump.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Shape of the spatial mode function of photons generated in noncollinear spontaneous parametric downconversion

We show experimentally how noncollinear geometries in spontaneous parametric downconversion induce ellipticity of the shape of the spatial mode function. The degree of ellipticity depends on the pump beam width, especially for highly focused beams. We also discuss the ellipticity induced by the spectrum of the pump beam

Entanglement of coherent states and decoherence

A possibility to produce entangled superpositions of strong coherent states is discussed. A recent proposal by Howell and Yazell [Phys. Rev. A 62, 012102 (2000)] of a device which entangles two strong coherent coherent states is critically examined. A serious flaw in their design is found. New modified scheme is proposed and it is shown that it really can generate non-classical states that can violate Bell inequality. Moreover, a profound analysis of the effect of losses and decoherence on the degree of entanglement is accomplished. It reveals the high sensitivity of the device to any disturbances and the fragility of generated states

Generation of polarization-entangled photon pairs in a Bragg reflection waveguide

We demonstrate experimentally that spontaneous parametric down-conversion in an AlGaAs semiconductor Bragg reflection waveguide can make for paired photons highly entangled in the polarization degree of freedom at the telecommunication wavelength of 1550 nm. The pairs of photons show visibility higher than 90% in several polarization bases and violate a Clauser-Horne-Shimony-Holt Bell-like inequality by more than 3 standard deviations. This represents a significant step toward the realization of efficient and versatile self pumped sources of entangled photon pairs on-chip.Comment: 9 pages, 4 figures, published versio

Tunable control of the frequency correlations of entangled photons

We experimentally demonstrate a new technique to control the type of frequency correlations of entangled photon pairs generated by spontaneous parametric downconversion. Frequency-correlated and frequencyanticorrelated photons are produced when a broadband pulse is used as a pump. The method is based on the control of the group velocities of the interacting waves and can be applied in any nonlinear medium and frequency band of interest. © 2007 Optical Society of America OCIS codes: 270.5290, 270.0270, 190.4410. One of the goals of quantum optics is to implement new sources of quantum light that allow tunable control of the relevant photonic properties. The most appropriate type of frequency correlations between paired photons depends on the specific quantum information application under consideration. Some protocols for quantum enhanced clock synchronization and position measurement rely on the use of frequency-correlated photons It has been shown that noncollinear SPDC allows the generation of frequency-correlated and uncorrelated photons by controlling the pump-beam width and the angle of emission of the downconverted photons In this Letter we experimentally demonstrate a new technique to tailor the frequency correlations of entangled photons Let us consider a collinear SPDC configuration with type-II ͑oee͒ phase matching. The input pump beam with a central angular frequency p passes through a medium with angular dispersion, such as a diffraction grating oriented along the transverse x direction. The transformation of the pump beam that is due to the grating can be written as where ⍀ p is the angular frequency deviation, p = ͑p x , p y ͒ is the transverse wave vector, ␣ = −cos 0 / cos ␤ 0 , 0 is the angle of incidence at the grating, ␤ 0 is the output diffraction angle, and c is the speed of light. The resulting beam acquires a pulse-front tilt such that its peak intensity is located at a different time for each value of x. The tilt angle is tan =− p ⑀, where ⑀ = m / ͑d cos ␤ 0 ͒ is the angular dispersion with d being the groove spacing of the grating, m the diffraction order, and p =2c / p . At the output face of the nonlinear crystal, a second grating is used to recollimate the beam by compensating for the angular dispersion introduced by the first grating. The quantum state of the downconverted photons is ͉⌿͘ = ͐d⍀ s d⍀ i ⌽͑⍀ s

Experimental estimation of the dimension of classical and quantum systems

An overwhelming majority of experiments in classical and quantum physics make a priori assumptions about the dimension of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from the results of measurements performed on it, without any extra assumption? The concept of a dimension witness answers this question, as it allows one to bound the dimension of an unknown classical or quantum system in a device-independent manner, that is, only from the statistics of measurements performed on it. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario. We use pairs of photons entangled in both polarization and orbital angular momentum to generate ensembles of classical and quantum states of dimensions up to 4. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Our results open new avenues for the device-independent estimation of unknown quantum systems and for applications in quantum information science.Comment: See also similar, independent and jointly submitted work of J. Ahrens et al., quant-ph/1111.127