Search Results (15 total)

We report on a spectral asymmetry in cw-pumped type-II spontaneous parametric down-conversion. We observe that when the pump beam is focused, the spectra of ordinary and extraordinary down-converted photons broaden unequally. Theoretical analysis indicates that this asymmetry can be attributed to the difference in the angular dispersion (walk-off) of the two kinds of photons, coupled with the well-known correlation between wavelength and emission direction.

We examine how spectral entanglement in polarization-entangled photon states generated from bulk-crystal, spontaneous parametric down-conversion affects the success of entanglement swapping and type-I fusion gates. We quantify the success of the entanglement swapping and fusion gates by calculating the bipartite concurrence and residual tangle, respectively, in terms of the joint spectral probability amplitudes of the initial broad-bandwidth polarization-entangled states. We find that both polarization-entanglement measures depend strongly on the initial spectral entanglement, as well as on the configuration of the independent sources. Specifically, when spectral differences correlate with polarization, the optimal source configuration is different for the two protocols. We conclude that this distinction is founded in how the underlying Bell-state measurement and quantum-erasure techniques respond differently to distinguishing spectral information.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

We use a multimode description of polarization-encoded qubits to analyze the quantum teleportation protocol. Specifically, we investigate how the teleportation fidelity depends on the spectral correlations inherent to polarization-entangled photons generated by type-II spontaneous parametric down conversion. We find that the maximal obtainable fidelity depends on the spectral entanglement carried by the joint probability amplitude, a result which we quantify for the case of a joint spectrum approximated by a correlated Gaussian function. We contrast these results with a similar analysis of the visibility obtained in a polarization-correlation experiment.

It is known that photons produced by spontaneous parametric down-conversion can be coupled into optical fibers more efficiently by focusing the pump field. We find that focusing the pump in type-II down-conversion causes photons of ordinary and extraordinary polarization to acquire very different angular spreads, which amounts to spatial information that distinguishes between the polarization states. Numerical studies predict that the photons collected by a detector or quantum channel will be of predominantly one polarization and that the degree of polarization entanglement will be lessened in some cases.

A linear 50-50 beam splitter, together with a coincidence measurement, has been widely used in quantum optical experiments, such as teleportation, dense coding, etc., for interferometrically distinguishing, measuring, or projecting onto one of the four two-photon polarization Bell states |ψ^(-)〉. In this paper, we demonstrate that the coincidence measurement at the output of a beam splitter cannot be used as an absolute identifier of the input state |ψ^(-)〉 nor as an indication that the input photons have been projected to the |ψ^(-)〉 state.

We report experimental observations of correlated-photon statistics in the single-photon detection rate. The usual quantum interference in a two-photon polarization interferometer always accompanies a dip in the single-detector counting rate, regardless of whether a dip or a peak is seen in the coincidence rate. This effect is explained by taking into account all possible photon number states that reach the detector, rather than considering just the state postselected by the coincidence measurement. We also report an interferometeric scheme in which the interference peak or dip in the coincidence corresponds directly to a peak or a dip in the single-photon detection rate.

We report a Bell-state synthesizer in which an interferometric entanglement concentration scheme is used. An initially mixed polarization state from type-II spontaneous parametric down-conversion becomes entangled after the interferometric entanglement concentration. This Bell-state synthesizer is universal in the sense that the output polarization state is not affected by spectral filtering, crystal thickness, and, most importantly, the choice of pump source. It is also robust against environmental disturbance and a more general state, partially mixed–partially entangled state, can be readily generated as well.

An ultrabright source of polarization-entangled photons has been realized using type-II phase matching in the spontaneous parametric down-conversion process in two cascaded crystals. The optical axes of the crystals are aligned in such a way that the extraordinarily (ordinarily) polarized cone from one crystal overlaps with the ordinarily (extraordinarily) polarized cone from the second crystal. This spatial overlapping removes the association between the polarization and the output angle of the photons that exists in a single type-II down-conversion process. Hence, entanglement of photon pairs originating from any conjugate points on the output cones is possible if a suitable optical delay line is used. This delay line is particularly simple and easy to implement.

Multiphoton states constructed from photon pairs generated in the process of spontaneous parametric down-conversion possess frequency and space-time correlations that may carry undesired distinguishing information. It is shown that these correlations may be eliminated if certain conditions in the source configuration are satisfied. For the cases in which these conditions cannot be satisfied because of experimental constraints, it is shown that the correlations may be reduced through proper choices of crystal length and pump bandwidth. The advantage of such source engineering is that it yields much higher count rates, since no photon pairs are lost by predetection spectral filtering.

It is a well-known and remarkable fact that in certain coincidence photon-counting experiments with cw-pumped parametric down-converters, the effects of group-velocity dispersion arising from media interposed between source and detectors are completely canceled, even if the media physically affect only one of the photons of the pair. Recently Perina et al. [Phys. Rev. A 59, 2359 (1999)] showed that this phenomenon does not occur when certain classical timing information is available about the arrival of individual photons at the detectors, as is the case when the photon pairs are produced via spontaneous parametric down-conversion using an ultrashort pump pulse. In this paper we show that the nonlocal cancellation of dispersion for such a source of entangled photons can be restored in principle by proper engineering of the source properties. In particular, we describe techniques for recovering interference in coincidence-counting experiments by suppressing distinguishing information without the post selection of photons. Moreover, a precise classical timing signal coincident with the photon pair is still available.

We give a detailed account of a recently introduced technique to suppress distinguishing information in the space-time component of the state vector of a pair of photons. The method works by creating interference between two possible ways for each photon to occupy any given space-time mode. Under certain conditions, this technique also allows the preparation of the two photons in a highly entangled space-time state.

We present a novel interferometric technique for suppressing distinguishing information in the space-time component of the state vector of an entangled pair of photons by providing two indistinguishable ways for each photon to occupy any given space-time mode. We demonstrate this method by using spontaneous parametric down-conversion to generate a pair of photons in the state with the least distinguishing information consistent with the set of modes available. The technique also allows the preparation of the two photons in a highly entangled space-time state provided certain criteria are met.

We report a fourth-order interference experiment in which pairs of photons produced in parametric down-conversion pumped by short optical pulses interfere in a Hong-Ou-Mandel interferometer. The visibility of the interference pattern is reduced for larger pump bandwidths. This effect can be understood in terms of the spectral distinguishability of the photon pairs. The interference can be restored by blocking the distinguishing information with a spectral filter.

A model is presented to describe spontaneous type-II parametric down-conversion pumped by a broadband source. This process differs from the familiar cw-pumped down-conversion in that a broader range of pump energies is available for down-conversion. The properties of the nonlinear crystal determine how these energies are distributed into the down-converted photons. Because the two photons are polarized along different crystal axes, they have different spectral characteristics and are no longer exactly anticorrelated. As the pump bandwidth is increased, this effect becomes more pronounced. A fourth-order interference experiment is proposed, illustrating some of the features of broadband pumped down-conversion.