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

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. Femtosecond pulse pumped spontaneous parametric down-conversion ~SPDC! is very useful for the realization of certain types of experiments in quantum optics, such as generation of multiphoton entangled states, quantum teleportation, quantum communications, etc @1#. Since the 1990s type-II SPDC has been used extensively as a source of twophoton entangled states for space-time, polarization, and space-time2polarization double entanglement @2#. However, type-II SPDC has its limitation for femtosecond applications. The degree of entanglement of the two-photon state generated in type-II SPDC pumped by femtosecond pulses strongly depends on the thickness of the nonlinear crystal @3‐5#. To obtain high-visibility quantum interference in pulse pumped type-II SPDC, one can only do the following: ~i! use a thin BBO crystal (’100 mm) or ~ii! accomplish spectral postselection by using narrow-band filters @6,7#. Both methods severely limit the available entangled photon flux reaching the detectors. In this paper, we experimentally demonstrate two methods for generating a pulsed source of space-time and polarization double-entangled photon pairs that exhibit highvisibility quantum interference. The visibility is shown to be insensitive to the thickness of the crystals and the bandwidth of the filters. Therefore, high-intensity pulsed entangled photon pairs can be easily generated by simply using thicker nonlinear crystals, which has a great advantage over pulse pumped type-II SPDC. It has recently been shown by Kwiat et al. and by Burlakov et al. that SPDC created from two spatially separate type-I nonlinear crystals pumped by cw laser beams exhibit high-visibility quantum interference @8,9#. In both cases, temporal compensation was not an important issue. We shall demonstrate in this paper that pulse pumped type-I SPDC in the two-crystal scheme requires great attention to the overlapping of the two-photon amplitudes temporally, unlike cw pumped two-crystal cases. Consider the experimental setup shown in Fig. 1. A frequency doubled radiation of a mode-locked Ti:sapphire laser is used to pump two type-I BBO crystals. The pump has a pulse width of ’80 fsec and a central wavelength of 400 nm. The repetition rate of the pump pulse is 82 MHz. The pump is polarized at 45°. A BBO crystal is placed in each arm of a balanced Mach-Zehnder interferometer ~MZI!. The thickness of both BBO crystals is 3.4 mm. The pump beam is then blocked by mirrors M 3 and M 4 while transmitting 800-nm collinear degenerate SPDC. The optic axes of the two BBO crystals are orthogonal to each other: the optic axis of the BBO (BBO1) in the arm that contains M 3 is oriented in the horizontal ( () plane and the other BBO (BBO2) is oriented vertically ( l). Due to type-I phase matching in the BBO’s, the pair of SPDC photons created from BBO1 are vertically ~V! polarized and the SPDC from BBO2 is polarized horizontally ( H). At each output port of the nonpolarizing beam splitter ~NPBS!, a detector package consisting of a GlanThompson analyzer (A1 , A2), an interference filter (F1 , F2), and a single-photon detector ~EG&G SPCM-AQ-142! are placed. Interference filters are mainly used for alignment purposes and to suppress background noise from the pump. The simplified version of the quantum state after NPBS can be written as ~we only consider coincidence contributing terms!