Ultrabright tunable photon-pair source with total-flux polarization-entanglement Marco Fiorentino, Gaetan Messin, Christopher E. Kuklewicz, Franco N. C. Wong, and Jeffrey H. Shapiro Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 Phone: +1-617-253-4027, Fax: +1-617-258-7864, e-mail:
[email protected]
Abstract: We demonstrate a continuous-wave dual-pump tunable source of polarizationentangled photons that requires no spectral or spatial filtering and emits ten times more pairs per mW of pump than pervious sources. c 2003 Optical Society of America ° OCIS codes: (270.0270) Quantum optics; (270.5290) Photon statistics
Polarization-entangled photons have been used in a wide range of experiments from tests of Bell’s inequalities [1] to the demonstration of quantum communication protocols [2]. Ideally, a practical source of polarizationentangled photons emits an intense beam of maximally-entangled and tunable photon pairs. In this paper we describe a novel entanglement source whose flux is, to the best of our knowledge, one order of magnitude larger than the best previously reported results [3], and that can be temperature tuned over several nm. This is accomplished in a dual-pump continuous-wave (cw) parametric downconverter. Applying an analysis similar to that of Ref. [4] it can be demonstrated that all the downconverted photon pairs are polarization entangled thus eliminating the need for spectral or spatial filtering. The downconverter is based on a type-II phase-matched periodically poled KTP crystal that is pumped in both directions (see Fig. 1) by a cw frequency-doubled Ti:sapphire laser at 398.5 nm. The two beams of collinearly propagating signal and idler photons at ∼797 nm are collimated and combined at a polarizing beam splitter (PBS) after the polarization of one of the beams is rotated by 90◦ by a half-wave plate (HWP2). The Hamiltonian of the system can be written as ³ ´ ˆ ∝ a H ˆ†H1 (ωs )ˆ a†V 2 (ωi ) + eiφ a ˆ†V 1 (ωs )ˆ a†H2 (ωi ) + h.c.
(1)
where H and V refer to the horizontal and vertical polarizations, 1 and 2 refer to the two outputs of the PBS, and ωs and ωi are the signal and idler frequencies satisfying ωs + ωi = ωp , and ωp is the pump frequency. The dichroic mirrors (DM1-4) reflect a small amount of the pump into the two PBS output paths. Interference of the pump (detected on the photodiode PD-UV) allows locking of the path-length phase φ, which can be controlled to yield the singlet (φ = π) or triplet (φ = 0) states. The pairs are sent through polarization analyzers and adjustable irises to single photon detectors for analysis and coincidence counting. A summary of our experimental results for φ = π (singlet) is shown in Figs. 2 and 3. Figure 2 shows the interference fringes in the coincidence counts obtained by varying the analyzer angle in arm 2 for a fixed angle in arm 1. We observed a visibility of 99% (91%) when analyzer 1 is set to 0◦ (45◦ ). Following [5] we tested Bell’s inequalities and obtained S = 2.599 ± 0.006, violating the classical limit by 100 σ. Figure 3 shows that the 45◦ -visibility is almost independent of the iris diameter. This allows us to increase the pair flux (left inset, Fig. 3) while preserving the purity of the state. We observed a visibility of 90% and a flux ' 11000 pairs s−1 mW−1 with a 4-mm iris. In the right inset of Fig. 3 we show the 45◦ -visibility as a function of the signal wavelength. The tuning was obtained by changing the PPKTP temperature for a fixed pump wavelength, thus producing non-degenerate polarization-entangled pairs. References 1. J. S. Bell, Physics (Long Island City, NY) 1, 195 (1964). A. Aspect et al.,Phys. Rev. Lett. 47, 460 (1981). 2. T. Jennewein et al., Phys. Rev. Lett. 84, 4729 (2000). D. S. Naik et al., Phys. Rev. Lett. 84, 4735 (2000).
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Fig. 1. Experimental setup. BS: 50/50 beamsplitter. DM: dichroic mirror. PBS: polarizing beamsplitter. HWP: half-wave plate. IF: 3-nm interference filter centered at 797 nm. HWP1 is used to balance the flux of downconverted photons in the two directions. 3. P. G. Kwiat et al., Phys. Rev. A, 60, R773 (1999). C. Kurtsiefer et al., Phys. Rev. A 64, 023802-1 (2001). J. V¨ oltz et al., Appl. Phys. Lett. 79, 869 (2001). M. Barbieri et al. quant-ph/0303018. 4. J. H. Shapiro and F. N. C. Wong, J. Opt. B: Quantum Semiclass. Opt. 4, L1 (2000). 5. A. Aspect, P. Grangier, and G. Roger, Phys. Rev. Lett. 49, 91 (1982).
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Fig. 3. Frequency-degenerate singlet state 45◦ -visibility versus iris diameter. Left inset: pairs flux versus iris aperture. Right inset: 45◦ -visibility versus signal wavelength (no interference filter, iris diameter 2.2 mm) with a measured flux of 3500 pairs s−1 mW−1 .
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