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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Squeezed Vacuum from a Photonic Crystal Fibre Parametric Oscillator

Alex O. C. Davis, Alex I. Flint, Will A. M. Smith, Peter J. Mosley, Rex H. S. Bannerman, Kerrianne Harrington, Charlotte Parry, Peter G. R. Smith

Published Jun 26, 2023• 1 citation
Publisher

Abstract

Squeezed light sources are crucial components of emerging quantum technologies for computing, communications, and measurement. The direct generation of squeezed light inside optical fibre is of particular interest due to minimal material loss, desirable mode properties and compatibility with long-distance optical networks. Squeezing of bright coherent-state laser pulses in fibre is regularly achieved via the Kerr effect, but this is not readily extensible to the continuous-wave (CW) and null-mean field regimes, and the maximum measurable squeezing is degraded by various noise processes around the common pump/signal wavelength [1].

DOI
10.1109/cleo/europe-eqec57999.2023.10232601

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This thesis presents a comprehensive numerical investigation of polarisation squeezing in photonic crystal fibre. Squeezing is the reduction of quantum noise in one variable beyond the Heisenberg limit at the expense of increased noise in the conjugate. The greater confinement of light that is possible in photonic crystal fibres leads to a greater effective nonlinearity, which promises to yield greater quantum squeezing than is possible in standard optical fibre. There are two dominant noise sources that limit the degree of polarisation squeezing. Raman noise is the interaction of photons with phonons in the fibre medium the degree of which is characteristic of the fibre medium and scales inversely with pulse width. And guided acoustic wave Brillouin scattering is due to thermally excited mechanical eigenmodes which scales with fibre length. The larger nonlinearity of photonic crystal fibres allows shorter fibre lengths with larger pulse widths providing a means to significantly reduce the dominant noise sources and obtain polarisation squeezing beyond that of ordinary silica fibre. This thesis presents an extensive quantum treatment using the truncated Wigner method that includes loss, a non-instantaneous Raman response, excess phase-noise, second- and third- order dispersion and self-steepening to identify the physical factors that limit current photonic crystal fibre squeezing experiments and demonstrate the optimal parameters to obtain the best squeezing.
A continuous-wave squeezed vacuum at a telecommunication wavelength was reported. Using an Er-doped fiber amplifier seeded by a single-frequency laser diode as the pump laser source, via a frequency doubler and an optical parametric oscillator based on periodically poled LiNbO3, the squeezed vacuum of 2.4±0.1dB below the shot noise limit at 1560nm was observed (the inferred initial squeezing is about 6.4±0.5dB). This system is compatible with standard telecommunication optical fiber, and will be useful for long-distance quantum communication and distributed quantum computing.
The level of quantum noise in measurements is bounded from below by the Heisenberg uncertainty principle, but it can be unequally distributed between two non-commuting observables — it can be “squeezed”. Since 2019, all gravitational-wave observatories have been using squeezed light for increasing the astronomical reach. Squeezed laser light is efficiently produced by degenerate parametric down-conversion in a nonlinear crystal located inside an optical resonator. A spontaneously generated initial pair of indistinguishable photons is amplified to a squeezed vacuum state. Overlapped with bright coherent light, the photo electric measurement shows a sub-Poissonian photon statistics. Squeezed states have ample applications in nonlocal quantum sensing, device-independent quantum key distribution, and quantum computing. Here, we present our continuous-wave 1550 nm ‘squeeze laser’ with a footprint of 80 cm × 80 cm. The well-defined output beam has an interference contrast of≳99%\gtrsim 99\%≳99%with an overlapped 10 mW beam being in an almost perfect TEM00 mode. The interference result shows 13 dB squeezing of the photon shot noise in balanced detection.
Fiber optical parametric oscillators are being busily investigated as sources of classical light, but are comparatively neglected as potential quantum light sources. Squeezed light is a crucial resource for emerging quantum technologies for computing, communications, and measurement. The direct generation of squeezed light inside optical fiber is of particular interest due to minimal material loss, desirable mode properties and compatibility with long-distance optical networks. Here, we describe a novel all-fiber continuous-wave optical parametric oscillator developed with the aim of generating squeezed light. Our device is based on a 10 cm Fabry-Perot cavity formed by two high-reflectivity fiber Bragg gratings written directly into a single section of photonic crystal fiber. This enables low-loss optical feedback and strongly resonance-enhanced four-wave mixing, with a cavity finesse of up to 400. We observe resonance-enhanced four-wave mixing in these devices, demonstrating a step towards their application as quantum light sources.