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Applied Optics

Generation of entangled photon pairs using small-coherence-time continuous wave pump lasers

S. Cialdi, F. Castelli, I. Boscolo, Matteo G. A. Paris

Published Apr 8, 2008• 18 citations
Publisher

Abstract

We address the generation of entangled photon pairs by parametric downconversion from solid state cw lasers with small coherence time. We consider a compact and low-cost setup based on a two-crystal scheme with type-I phase matching. We reconstruct the full density matrix by quantum tomography and analyze in detail the entanglement properties of the generated state as a function of the crystal's length and the coherence time of the pump. We verify the possibility to improve the visibility using a purification protocol based on a compensation crystal.

DOI
10.1364/ao.47.001832

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Parametric down conversion permits the generation of entangled photon pairs. However, the production rate is unfortunately very low, typically with nine to ten orders of magnitude between input and output power. A combination of approaches is considered to significantly enhance the overall prodcuiton rate. The use of large crystals simply improves the production rate by increasing the interaction length, as does the use of beam-folding optics. Since the produced entangled photon pairs have twice the wavelength of the pump beam, the use of 'hot' and 'cold' mirrors can be used to redirect unused pump power back into the crystal. Analysis of these approaches is used to indicate a potential improvement of six orders of magnitude. Practical design limitations such as the rejection of waste heat, currently available crystal dimensions, and differential walkoff of correlated photons due to birefringence are considered. Application to type I and type II parametric down conversion and walkoff compensation techniques are detailed. Application to an entangled optical communication link and its use over astronomical distances are outlined.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Dec 1, 2002•0 citations
We address the generation of entangled photon pairs by parametric downconversion from solid state cw lasers with small coherence time. We consider a compact and low-cost setup based on a two-crystal scheme with type-I phase matching. We reconstruct the full density matrix by quantum tomography and analyze in detail the entanglement properties of the generated state as a function of the crystal's length and the coherence time of the pump. We verify the possibility to improve the visibility using a purification protocol based on a compensation crystal.
The generation of entangled photon pairs by parametric down-conversion from solid state CW lasers with small coherence time is theoretically and experimentally analyzed. We consider a compact and low-cost setup based on a two-crystal scheme with Type-I phase matching. We study the effect of the pump coherence time over the entangled state visibility and over the violation of Bell's inequality, as a function of the crystal length. The full density matrix is reconstructed by quantum tomography. The proposed theoretical model is verified using a purification protocol based on a compensation crystal.
Quantum computation and quantum communication require sources of entangled photons, especially from low-cost diode lasers. This study considered a polarization entangled photons pulse source produced via a spontaneous parametric down-conversion process in the type-I phase-matching of two nonlinear crystals, incorporating a continuous wave laser diode operating in pulse mode as a pump. The generated photons were measured based on visibility as a function of the pump frequency and collection apertures. A polarization correlation between photon pairs of approximately 93% and a Bell parameter S value of 2.56 ± 0.05 were obtained using the source described here.
Summary form only given. We have experimentally demonstrated that the two-photon probability amplitudes of photon pairs generated using spontaneous parametric down conversion from separate nonlinear crystals, under the conditions outlined above, remain independent and cannot be used to engineer more complex quantum states. The spontaneous nature of the down-conversion process prevents pairs of photons generated in unconnected regions of interaction from having a stable phase relationship between components of their probability amplitudes. The experiment has been performed using two BaB/sub 2/O/sub 4/ nonlinear crystals, of lengths 0.5 mm and 1.5 mm respectively. They have been cut to generate phase-matched type-II collinear down conversion.
The generation of interband photon pairs with wavelengths near 0.5 and 1.6 μm in a photonic-crystal fiber under low-power cw optical pumping by a diode laser with a central wavelength of 0.8 μm has been experimentally demonstrated. It has been found that the generation rate of entangled photons under cw pumping is comparable with values obtained with pulsed pumping by a femtosecond Ti:sapphire laser if the average cw pump power is an order of magnitude higher than the average pulsed pump power. The reached rates of photon generation are ensured by the used photonic-crystal fiber with a small effective mode area and a special dispersion profile. The reached low noise in the output signal is ensured by the separation of carrier frequencies of generated photons into different spectral bands.
Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.
Parametric down conversion permits the generation of entangled photon pairs. However, the production rate is unfortunately very low, typically with nine to ten orders of magnitude between input and output power. A combination of approaches is considered to significantly enhance the overall prodcuiton rate. The use of large crystals simply improves the production rate by increasing the interaction length, as does the use of beam-folding optics. Since the produced entangled photon pairs have twice the wavelength of the pump beam, the use of 'hot' and 'cold' mirrors can be used to redirect unused pump power back into the crystal. Analysis of these approaches is used to indicate a potential improvement of six orders of magnitude. Practical design limitations such as the rejection of waste heat, currently available crystal dimensions, and differential walkoff of correlated photons due to birefringence are considered. Application to type I and type II parametric down conversion and walkoff compensation techniques are detailed. Application to an entangled optical communication link and its use over astronomical distances are outlined.