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Journal of Modern Optics

Experimental measuring of the coherence length of a single photon generated via a degenerated optical parametric oscillator far below threshold

Fuyuan Wang, Bao‐Sen Shi, Chang Zhai, Guang‐Can Guo

Published Feb 20, 2010• 7 citations
Publisher

Abstract

In this work, we report on experimental measuring of the coherence length of a single photon, which is generated via a degenerated optical parametric oscillator (OPO) operated far below threshold. Although the bandwidth of the OPO cavity is about 7.8 MHz, the measured coherence length of the single photon is only about 90 µm via a Michelson interferometer, due to the multimode property of the photon generated in the experiment. Therefore, further reducing the bandwidth of the photon is necessary for its application in some fields, such as transferring the information between the photon and an atomic system.

DOI
10.1080/09500341003624727

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In this work, we report on experimental measuring of the coherence length of a single photon, which is generated via a degenerated optical parametric oscillator (OPO) operated far below threshold. Although the bandwidth of the OPO cavity is about 7.8 MHz, the measured coherence length of the single photon is only about 90 µm via a Michelson interferometer, due to the multimode property of the photon generated in the experiment. Therefore, further reducing the bandwidth of the photon is necessary for its application in some fields, such as transferring the information between the photon and an atomic system.
We present a single-resonant optical parametric oscillator pumped far below threshold as a source for narrow-band single photons. Spontaneous generation of single photons via parametric downconversion is modified and follows the cavity transfer function. Cross-correlation measurements between signal and idler beams show a cavity bandwidth of 62MHz. As the main improvement to prior realizations, our cavity is locked to the pump beam via the Hänsch-Couillaud method and therefore allows the continuous generation of heralded single photons with long-term stability.
This paper presents an experiment on single-photon spectrum generation using an optical parametric oscillator device with cavity-enhanced parametric down-conversion. A PPKTP crystal inside a cavity far below threshold is pumped in order to create signal-idler photon pairs. Triple-resonance of pump, signal, and idler fields are achieved using a compensating KTP crystal. Long-term stability, ultra-high brightness (14000 biphotons/s per mW), and MHz signal bandwidth behind a single-mode fiber on the cesium D1 line (894.3 nm) are attained by locking the cavity length of the pump via the Hansch-Couillaud method. The spectral width of the parametric fluorescence is also derived using the signal-to-idler cross-correlation measurements, and has been found to be < 3 MHz.
The theory of the optical parametric oscillator is examined and compared to the experiment in the regime of far below threshold. It is found that the output state has no difference from spontaneous parametric down-conversion except that the bandwidth of down-conversion is reduced to that of the resonator and the conversion rate is enhanced by cavity resonance. The reduction of the bandwidth of the down-converted fields makes it possible for a direct measurement of the time interval distribution between two down-converted photons. The observed distributions are well explained by the theory. Such a narrow-band two-photon source will find wide applications in quantum information processing.
In this paper, we present a narrow bandwidth source at the telecom wavelength based on type II PPKTP crystal inside an optical cavity. Simultaneous resonances of the pump and the two down-converted fields are achieved by special design of the cavity. This is a triple resonances optical parametric oscillator (OPO) far below threshold, which generates photon pairs at around 1560 nm with a bandwidth of 8 MHz. A coherence time of 27.7 ns is estimate by a time correlation measurement, and the estimated photon pairs generating rate is 134 s-1MHz-1mW-1. As entangled photon pairs in the telecom regime is suitable for long distance transmission and manipulation, this source is desirable for the demands of quantum communication.
We report on the generation of a type-I multimode two-photon state on a rubidium D(2) line (780 nm) using periodically poled KTiOPO(4) crystals. With a degenerate optical parametric oscillator far below threshold, we observe an oscillatory correlation function; the cross correlation between two photons shows a cavity bandwidth of about 7.8 MHz. We also use a Fabry-Perot etalon to filter its most longitudinal modes and observe its time correlation function. The experimental data are well fitted to theoretical curves. This system could be utilized for demonstrating storage and retrieval of narrow-band photons in Rb atomic ensembles, which is important for long-distance quantum communication.
We report a 65 MHz-bandwidth triangular-shaped optical parametric oscillator (OPO) for squeezed vacuum generation at 860 nm. The triangle structure of our OPO enables the round-trip length to reach 45 mm as a ring cavity, which provides a counter circulating optical path available for introducing a probe beam or generating another squeezed vacuum. Hence our OPO is suitable for the applications in high-speed quantum information processing where two or more squeezed vacua form a complicated interferometer, like continuous-variable quantum teleportation. With a homemade, broadband and low-loss homodyne detector, a direct measurement shows 8.4 dB of squeezing at 3 MHz and also 2.4 dB of squeezing at 100 MHz.