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arXiv:2408.08375 · Physical review. E

Producing entangled photon pairs and quantum squeezed states in plasmas

Kenan Qu, N. J. Fisch

Published Dec 20, 2024• 2 citations
arXivHTMLPDF

Abstract

Plasma is capable of mediating the conversion of two pump photons into two different photons through a relativistic four-wave mixing nonlinearity. Spontaneously created photon pairs are emitted at symmetric angles with respect to the colinear pump direction, and the emission rate is largest if they have identical frequency. Thus, two orthogonally polarized pumps can produce polarization-entangled photon pairs through a millimeter-long homogeneous plasma. The noise from Raman scattering can be avoided if the pump detuning differs from twice the plasma frequency. However, pump detuning exactly equal to twice the plasma frequency can significantly enhance the interaction rate, which allows for the production of strong two-mode squeezed states. Remarkably, the amplified noise from Raman scattering are correlated and hence can be suppressed in one of the output quadratures, thereby maintaining the squeezing magnitude.

DOI
10.1103/physreve.110.065211

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Plasma is capable of mediating the conversion of two pump photons into two different photons through a relativistic four-wave mixing nonlinearity. Spontaneously created photon pairs are emitted at symmetric angles with respect to the colinear pump direction, and the emission rate is largest if they have identical frequency. Thus, two orthogonally polarized pumps can produce polarization-entangled photon pairs through a millimeter-long homogeneous plasma. The noise from Raman scattering can be avoided if the pump detuning differs from twice the plasma frequency. However, pump detuning exactly equal to twice the plasma frequency can significantly enhance the interaction rate, which allows for the production of strong two-mode squeezed states. Remarkably, the amplified noise from Raman scattering are correlated and hence can be suppressed in one of the output quadratures, thereby maintaining the squeezing magnitude.
We present a general quantum theory capable of describing photon statistics under the combined effects of four-wave mixing and Raman scattering inside optical fibers. Our theory is vectorial in nature and includes all polarization effects. Our analysis shows that spontaneous Raman scattering degrades the pair correlation in all cases but the extent of degradation depends on the pumping configuration employed. In a single-pump configuration, photon pairs can be created with polarization either parallel or orthogonal to the pump. Our results show that the orthogonal configuration can improve the extent of quantum correlation considerably over a broad bandwidth. In the case of a dual-pump configuration, we show that imbalance of two pump powers can be used to improve the quality of photon pairs. We show that orthogonally polarized pumps can generate photon pairs automatically in a polarization-entangled state. In particular, orthogonal pumping with circular polarizations produces such an entangled state with relatively high quality. We also quantify the quality of polarization entanglement as well as energy-time entanglement constructed using correlated photon pairs.
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Quantum entanglement is one of the most important resources for quantum communication, information and computing. Since active polarization stabilization methods could alleviate the polarization-drift problem over optical fibre, telecom-band polarization-entangled photon-pairs can be well distributed and shared. Spontaneous four-wave mixing in a dispersion-shifted fibre (DSF) can be used to realize a source of telecom-band polarization-entangled photon pairs, but DSF is often cooled to suppress noise photons generated by spontaneous Raman-scattering. Second-order nonlinear crystals or waveguides such as periodically-poled lithium niobate (PPLN) waveguides are also available and generation has been reported on the conditions of careful selection of two nearly identical nonlinear crystals or accurate temperature-control of waveguides.
Quantum squeezed states enable precision measurements beyond the standard quantum limit, but conventional solid-state media fundamentally limit pump intensities to the ionization threshold. We demonstrate that plasma waves can mediate ultra-strong two-mode squeezing through stimulated Raman scattering, achieving up to ultrastrong squeezing using 1016Wcm−210^{16}{Wcm^{-2}}1016Wcm−2 pump lasers. Employing two copropagating pump beams with frequency difference matching twice the plasma frequency, we generate quantum-correlated photon pairs through phonon-mediated four-wave mixing. The process exhibits remarkable thermal noise tolerance, allowing strong squeezing even with large thermal phonon numbers. This plasma-based approach produces squeezed states with ultrahigh photon numbers, opening new possibilities for strong-field applications across optical to X-ray wavelengths.
The four-wave-mixing interactions produced by pumping at two well-separated frequencies can couple four different radiation modes to generate a new kind of squeezed state of light. A dual-frequency homodyne detection scheme is described and is used to observe this type of nonclassical correlation. Initial experimental results in an optical fiber show a noise level 20% below the vacuum noise level for the output of the dual-frequency detector, even though the noise at each individual detector remains above the vacuum level.
Using a Sagnac fiber loop functions as a deterministic splitter of photon pairs produced by the frequency degenerate four wave mixing, we show that the background noise of the degenerate photon pairs is contributed by both Raman scattering and frequency non-degenerate four wave mixing. To improve the purity of photon pairs in the high gain regime, in addition to suppressing the noise photons by cooling the nonlinear fiber and by optimizing the detuning between the frequencies of the pump and photon pairs, the walk-off effect of the two pulsed pump fields should be mitigated by managing the dispersion of the fiber. Our investigation is not only the first step towards the generation of multi-mode squeezed vacuum in fiber optical parametric amplifiers pumped with pulsed lights, but also contributes to improving the purity of the fiber sources of degenerate photon pairs.