Broadening the bandwidth of entangled photons: a step towards the generation of extremely short biphotons

We demonstrate a technique that allows to fully control the bandwidth of entangled photons independently of the frequency band of interest and of the nonlinear crystal. We show that this technique allows to generate nearly transform-limited biphotons with almost one octave of bandwidth (hundreds of THz) which corresponds to correlation times of just a few femtoseconds. The presented method becomes an enabling tool for attosecond entangled-photons quantum optics. The technique can also be used to generate paired photons with a very high degree of entanglement.Comment: 4 page

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

Quantum identification system

A secure quantum identification system combining a classical identification procedure and quantum key distribution is proposed. Each identification sequence is always used just once and new sequences are ``refuelled'' from a shared provably secret key transferred through the quantum channel. Two identification protocols are devised. The first protocol can be applied when legitimate users have an unjammable public channel at their disposal. The deception probability is derived for the case of a noisy quantum channel. The second protocol employs unconditionally secure authentication of information sent over the public channel, and thus it can be applied even in the case when an adversary is allowed to modify public communications. An experimental realization of a quantum identification system is described.Comment: RevTeX, 4 postscript figures, 9 pages, submitted to Physical Review

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

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

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

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