Optical parametric oscillator: Difference between revisions

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== Quantum properties of the generated light beams ==

[[Image:OPO Crystals.jpg‎‎|thumb|300 px|right|[[Potassium titanyl phosphate|KTP]] crystals in an OPO]]

The OPO is the physical system most widely used to generate [[squeezed coherent states]] and [[entangled]] states of light in the continuous variables regime. Many demonstrations of quantum information protocols for continuous variables were realized using OPO's <ref>J. Jing, J. Zhang, Y. Yan, F. Zhao, C. Xie, and K. Peng, Phys. Rev. Lett. '''90''', 167903 (2003).</ref><ref>S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''96''', 060504 (2006).</ref><ref>N. Takei, H. Yonezawa, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''94''', 220502 (2005).</ref><ref>S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''96''', 060504 (2006).</ref>.

The [[Spontaneous parametric down conversion|downconversion]] process really occurs in the single photon regime: each pump photon that is annihilated inside the cavity gives rise to a pair of photons in the signal and idler intracavity modes. This leads to a quantum correlation between the intensities of signal and idler fields, so that there is squeezing in the subtraction of intensities <ref>A. Heidmann, R. J. Horowicz, S. Reynaud, E. Giacobino, C. Fabre, and G. Camy, Phys. Rev. Lett. '''59''', 2555 (1987).</ref>, which motivated the name "twin beams" for the downconverted fields. The highest squeezing level attained was by10.12 +/- 0.15 dB <ref>Schnabel et aliial., "ObservationPhys. ofRev. Squeezed Light with 10-dB Quantum-Noise Reduction" PRLLett. '''100''', 033602 (2008). They achieved squeezing levels of 10.12 +</- 0.15 dBref>.

It turns out that the phases of the twin beams are quantum correlated as well, leading to [[entanglement]], theoretically predicted in 1988 <ref>M. D. Reid and P. D. Drummond, Phys. Rev. Lett. '''60''', 2731 (1988).</ref>. Below threshold, entanglement was measured for the first time in 1992 <ref>Z. Y. Ou, S. F. Pereira, H. J. Kimble, and K. C. Peng, Phys. Rev. Lett. '''68''', 3663 (1992).</ref>, and in 2005 above threshold <ref>A. S. Villar, L. S. Cruz, K. N. Cassemiro, M. Martinelli, and P. Nussenzveig, Phys. Rev. Lett. '''95''', 243603 (2005).</ref>.

Above threshold, the pump beam depletion makes it sensitive to the quantum phenomena happening inside the crystal. The first measurement of squeezing in the pump field after parametric interaction was done in 1997 <ref>K. Kasai, J.G. Gao, and C. Fabre, Europhys. Lett. '''40''', 25 (1997).</ref>. Actually, it has been recently predicted that all three fields (pump, signal and idler) must be entangled <ref>A. S. Villar, M. Martinelli, C Fabre, and P. Nussenzveig, Phys. Rev. Lett. '''97''', 140504 (2006).</ref>.

25 (1997).</ref>. Actually, it has been recently predicted that all three fields (pump, signal and idler) must be entangled<ref>A. S. Villar, M. Martinelli, C Fabre, and P. Nussenzveig, Phys. Rev. Lett. 97, 140504 (2006).</ref>.

Not only intensity and phase of the twin beams share quantum correlations, but also do their spatial modes <ref>M. Martinelli, N. Treps, S. Ducci, S. Gigan, A. Maître, and C. Fabre, Phys. Rev. A '''67''', 023808 (2003).</ref>. This feature could be used to enhance signal to noise ratio in image systems.

The OPO is being employed nowadays as a source of squeezed light tuned to atomic transitions, in order to study how the atoms interact with squeezed light <ref>T. Tanimura, D. Akamatsu, Y. Yokoi, A. Furusawa, M. Kozuma, Opt. Lett. '''31''', 2344 (2006).</ref>.

== References ==