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Physical Review A

Temporal indistinguishability and quantum interference

Yoon-Ho Kim, V. Berardi, Maria V. Chekhova, Augusto Garuccio, Yanhua H. Shih

Published Sep 19, 2000• 19 citations
Publisher

Abstract

A ${\ensuremath{\chi}}^{(2)}$ nonlinear optical crystal is pumped by two temporally well-distinguishable femtosecond laser pulses to generate entangled photon pairs in the process of spontaneous parametric down-conversion. We have observed first- and second-order interference between amplitudes generated from the first and the second pump pulse as a function of the time delay between the two pump pulses. The criteria for first- and second-order interference are found to be very different, which reflect the quantum entanglement nature of the state of spontaneous parametric down-conversion.

DOI
10.1103/physreva.62.043820

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A \ensuremathχ(2){\ensuremath{\chi}}^{(2)}\ensuremathχ(2) nonlinear optical crystal is pumped by two temporally well-distinguishable femtosecond laser pulses to generate entangled photon pairs in the process of spontaneous parametric down-conversion. We have observed first- and second-order interference between amplitudes generated from the first and the second pump pulse as a function of the time delay between the two pump pulses. The criteria for first- and second-order interference are found to be very different, which reflect the quantum entanglement nature of the state of spontaneous parametric down-conversion.
We report a two-photon interference effect, in which the entangled photon pairs are generated from two laser pulses well separated in time. In a single pump pulse case, interference effects did not occur in our experimental scheme. However, by introducing a second pump pulse delayed in time, quantum interference was then observed. The visibility of the interference fringes shows a dependence on the delay time between two laser pulses. The results are explained in terms of indistinguishability of biphoton amplitudes that originates from two temporally separated laser pulses.
Summary form only given. Pairs of entangled photons display a complex quantum state that cannot be represented through a product of single-particle states. The spontaneous parametric down-conversion (SPDC) process, that takes place when a laser pulse pumps a second-order nonlinear crystal has become recently a convenient source of such nonclassical states of light in quantum optics experiments. Nonstationary effects are mainly observed when the pump pulse temporal duration is significantly shorter than the characteristic interaction time in the nonlinear crystal. We introduce a nonstationary theory of SPDC to describe the results of fourth-order quantum-interference experiments when no spectral filtering is applied to the down-converted fields. Experiments involving nonlinear SPDC crystals with three different lengths were performed. The ultraviolet laser pulses were obtained from a Ti:sapphire femtosecond laser with a second-harmonic generator. These pulses were used to pump nonlinear crystals to initiate the process of nonstationary spdc and generate orthogonally-polarized photon pairs. These pairs then went into a polarization interferometer, and the coincidence counts of the detectors at the two output ports of the interferometer were recorded.
The generation of entangled states of three or more photons through the nonlinear process of spontaneous parametric down-conversion requires the use of a femtosecond pump to provide an indistinguishability of photon pairs generated from different nonlinear crystals. We demonstrate experimentally that the quantum interference between two photons generated in each of the nonlinear crystals will degrade significantly as the duration of the femtosecond pump pulse becomes shorter than the coherence time of the signal and idler photons. Spectral postselection using narrow-band filters compensates this effect at the expense of the photon flux available for detection.
We report a two-photon interference effect, in which the entangled photon pairs are generated from two laser pulses well-separated in time. In a single pump pulse case, interference effects did not occur in our experimental scheme. However, by introducing a second pump pulse delayed in time, quantum interference was then observed. The visibility of the interference fringes shows a dependence on the delay time between two laser pulses. The results are explained in terms of indistinguishability of biphoton amplitudes which originates from two temporally separated laser pulses.
We report a two-photon interference effect, in which the entangled photon pairs are generated from two laser pulses well-separated in time. In a single pump pulse case, interference effects did not occur in our experimental scheme. However, by introducing a second pump pulse delayed in time, quantum interference was then observed. The visibility of the interference fringes shows a dependence on the delay time between two laser pulses. The results are explained in terms of indistinguishability of biphoton amplitudes which originates from two temporally separated laser pulses.
Two spatially separated type-I nonlinear crystals are pumped by femtosecond laser pulses to create entangled photon pairs in the process of spontaneous parametric down-conversion. The two-photon entangled state exhibits high-visibility quantum interference for both polarization and space-time variables without the need of stringent spectral postselection by using narrow-band filters. The visibility is insensitive to the thickness of the crystals, unlike in the case of pulse pumped type-II parametric down-conversion; therefore the intensity can be easily increased by using thick nonlinear crystals. This method will be indispensable in experiments that require a pulsed source of entangled photon pairs, such as generation of multiphoton entangled states, quantum teleportation, and quantum communications.