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Physical Review A

High-intensity pulsed source of space-time and polarization double-entangled photon pairs

Yoon-Ho Kim, S. P. Kulik, Yanhua H. Shih

Published Jun 13, 2000• 70 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.62.011802

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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.
Summary form only given. By far the most efficient source of two-particle entangled state is spontaneous parametric downconversion (SPDC). Especially, cw laser pumped type-II SPDC has been widely treated as a good source of entangled photon pairs, since one can easily prepare a polarization Bell state. However, femtosecond pulse pumped type-II SPDC shows poor quantum interference visibility when a thick crystal is used. To restore the visibility, one has to use narrow-band spectral filters at the expense of available photon flux. We demonstrate a scheme for high-visibility two-photon interference with thick (3.4 mm.) type-II BaB/sub 2/O/sub 4/ (BBO) crystals in femtosecond pulse pumped SPDC. Polarization Bell states can be prepared using this method without spectral and amplitude post-selection.
We report the generation of polarization-entangled photons by femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of two type-I crystals. Highly entangled pulsed states were obtained by introducing a temporal delay between the two orthogonal polarization components of the pump field. They exhibited high-visibility quantum interference and a large concurrence value, without the need of postselection using narrow-bandwidth spectral filters. The results are well explained by the theory which incorporates the space-time dependence of interfering two-photon amplitudes if dispersion and birefringence in the crystals are appropriately taken into account. Such a pulsed entangled photon well localized in time domain is useful for various quantum communication experiments, such as quantum cryptography and quantum teleportation.
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.
In this paper, we report a scheme for the generation of a pulsed polarization entangled-photon pair in a two-crystal geometry. These two crystals are oriented with their optical axes aligned in a perpendicular plane, stacked in the vertical direction. The stack direction and the pass direction of pump laser are orthogonal. This scheme has some advantages compared to other schemes: no time compensation or narrow-band filter is needed, it is insensitive to the thickness of the nonlinear crystal. As a demonstration of its workability, we get more than 86% high visibility of two-photon quantum interference experimentally for the polarization variable, without any time compensation and narrow-band filter. This scheme may be a useful technique for the generation of a pulsed entangled-photon pair.
The two-photon polarized entangled state is generated from spontaneous parametric down conversion due to nonlinear process of type Ⅱ BBO crystal pumped by ultrashot pulsed laser. We discussed interference visibility of the polarized entangled state generated from different BBO crystals through experiments and numerical simulation. The narrow-band interference filters are also considered in our analysis. Our results show that the interference visibility can be increased by using shorter BBO crystal and narrow-band interference filters.