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Physical Review Letters

Dispersion-Independent High-Visibility Quantum Interference in Ultrafast Parametric Down-Conversion

Mete Atatüre, Alexander V. Sergienko, Bahaa E. A. Saleh, Malvin Carl Teich

Published Jan 24, 2000• 14 citations
Publisher

Abstract

We demonstrate the effective removal of intrinsic distinguishability between entangled-photon pairs in femtosecond spontaneous parametric down-conversion. High-visibility quantum interference is recovered (an increase to 96% from 17%) while preserving the high photon-flux density associated with the use of long nonlinear crystals. This new technique is expected to serve as a basic component in the preparation of multiphoton entangled states.

DOI
10.1103/physrevlett.84.618

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We demonstrate the effective removal of intrinsic distinguishability between entangled-photon pairs in femtosecond spontaneous parametric down-conversion. High-visibility quantum interference is recovered (an increase to 96% from 17%) while preserving the high photon-flux density associated with the use of long nonlinear crystals. This new technique is expected to serve as a basic component in the preparation of multiphoton entangled states.
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.
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 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.
We experimentally investigate a double-pass parametric down-conversion scheme for producing pulsed, polarization-entangled photon pairs with high visibility. The amplitudes for creating photon pairs on each pass interfere to compensate for distinguishing characteristics that normally degrade two-photon visibility. The result is a high-flux source of polarization-entangled photon pulses that does not require spectral filtering. We observe quantum interference visibility of over 95% without the use of spectral filters for 200fs200\phantom{\rule{0.3em}{0ex}}\mathrm{fs}200fs pulses, and up to 98.1% with 5nm5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}5nm bandwidth filters.