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IEEE Transactions on Quantum Engineering

The “Squeeze Laser”

Roman Schnabel, Axel Schönbeck

Published Jan 1, 2022• 7 citations
Publisher

Abstract

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$\gtrsim 99\%$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.

DOI
10.1109/tqe.2022.3224686

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During the recent years an international network of interferometric gravitational wave (GW) detectors has been commissioned and searches for various classes of GWsignals are ongoing.Although the observatories are beginning to reach astrophysically interesting sensitivity levels, extensive upgrades are aimed for the next decade bringing us into the era of gravitational wave astronomy.One approach to enhance the detector sensitivity is the injection of squeezed vacuum states of light replacing the vacuum fluctuations entering the detector through its antisymmetric port.In this thesis the unprecedented generation of squeezed vacuum states with a noise power 6.5 dB below vacuum noise within the entire detection bandwidth of ground-based GW-detectors (10 Hz -10 kHz) was experimentally demonstrated at a wavelength of 1064 nm.For the first time a coherent control scheme was employed for comprehensive phase control of the experimental setup.This scheme is directly applicable to future GW-detector topologies.In addition, parasitic interference induced by scattered light was found to be a dominant noise source degrading squeezed states in the audio band.The compatibility of squeezed field injection into a detuned operated dual-recycled Michelson interferometer was demonstrated in a table top experiment.A detected non-classical sensitivity improvement of up to 3.0 dB within the entire detection bandwidth (5 -15 MHz) was accomplished by optimal preparation of the injected squeezing by means of a detuned locked filter cavity.In a further experiment strongly squeezed vacuum states were generated in the MHz-regime.With a maximum measured quantum noise reduction of 11.5 dB below the shot noise limit a new benchmark was set.This was achieved by suppression of phase noise and a remarkable reduction of optical loss in the experimental setup.Based on the squeezed light sources developed in this thesis and the experimental results achieved, a conceptual design of a compact squeezed light source was devised, allowing for a high duty cycle sensitivity improvement of GW-detectors.

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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.
The generation of strongly squeezed vacuum states of light is a key technology for future ground-based gravitational wave detectors (GWDs) to reach sensitivities beyond their quantum noise limit. For some proposed observatory designs, an operating laser wavelength of 1550 nm or around 2 μm is required to enable the use of cryogenically cooled silicon test masses for thermal noise reduction. Here, we present for the first time the direct measurement of up to 11.5 dB squeezing at 1550 nm over the complete detection bandwidth of future ground-based GWDs ranging from 10 kHz down to below 1 Hz. Furthermore, we directly observe a quantum shot-noise reduction of up to (13.5±0.1) dB at megahertz frequencies. This allows us to derive a precise constraint on the absolute quantum efficiency of the photodiode used for balanced homodyne detection. These results hold important insight regarding the quantum noise reduction efficiency in future GWDs, as well as for quantum information and cryptography, where low decoherence of nonclassical states of light is also of high relevance.
Squeezed states of light were observed for the first time in 1985.Since then they have developed into an important nonclassical tool in the field of high-precision metrology, for instance in gravitational wave detection, as well as in the field of quantum information, e.g. for teleportation and quantum cryptography.For all these applications a strong squeezing factor is beneficial.This thesis presents a new benchmark value for squeezed light from non-monolithic squeezed-light sources.By means of parametric down-conversion, squeezed states at a wavelength of 1064 nm were generated and a nonclassical noise suppression of 12.3 dB was observed.In contrast to a monolithic resonator, the resonance frequency of this squeezed-light source can be adjusted to a downstream experiment.In an additional experiment this squeezed-light source was utilized to determine the quantum efficiency of photo diodes.The applied method permits high precision since strong squeezing is sensitive to even low values of optical loss.In view of the application of squeezed light in future-generation gravitational wave detectors and quantum information science, a dedicated squeezed-light source was developed and at a wavelength of 1550 nm squeezing in the continuous-wave regime was observed for the first time.These squeezed states were employed to quantumenhance the sensitivity of a fiber-based Sagnac interferometer.Thus the compatibility of squeezing at 1550 nm with fiber-based quantum communication was demonstrated.Furthermore, upon thorough optimization of the squeezed-light generation and detection at 1550 nm, a quantum noise suppression of 12.3 dB was measured.Moreover, squeezing in the Fourier audio-band was observed for the first time.The findings presented in this thesis constitute key research towards the squeezed-light technology at 1550 nm as it is required for future gravitational wave detectors such as the recently proposed Einstein Telescope.
Albert Einstein's General Theory of Relativity predicts the existence of ripples in space- time, the so-called gravitational waves (GWs). In the past two decades, an international array of ground-based, kilometer-scale Michelson interferometers has been set up aiming at a rst observation. These Earth-bound GW detectors target signals at audio frequen- cies in a band of ca. 10 Hz { 10 kHz. At Fourier frequencies above 1 kHz, their sensitivity is limited by quantum noise. This shot-noise arises from the zero-point uctuations of the electromagnetic eld, the so-called vacuum uctuations , which enter the interferometer through its anti-symmetric signal port. Future generations of GW observatories will even be limited by quantum noise almost over their entire detection band [ ET ]. A `classical' solution to further improve the signal-to-shot-noise ratio (SNR) of the observatories consists in an increase of the circulating light power. A higher optical load inside the interferometer will, however, lead to thermally-induced beam distortion as well as to an increased radiation pressure noise level. This will eectively result in an upper limit for the optical light power employable in future GW observatory generations. An alternative approach to improve the SNR is based on the injection of so-called squeezed states of light. Such a light eld has a characteristic non-classical noise distribution in the eld quadratures. Injected from the signal port, the squeezed state replaces the vacuum state, thereby reducing the interferometer's quantum noise. The rst part of this thesis discusses the setup of a squeezed-light laser for the German- British GW observatory GEO 600. During characterization of the device, a noise reduc- tion of up to 9.6 dB below the vacuum noise level was observed in the frequency band of 10 Hz {10 kHz, setting a new benchmark for squeezing observed at audio frequencies. Corrected for detection losses, this value furthermore corresponds to more than 11 dB of squeezing available for injection into the dark port of GEO 600. Situated on a 1 : 15 m 1 : 35 m optical breadboard, the device features transportability and can be operated independently of the GW interferometer. A key feature of the squeezed-light laser is its unprecedented long-term stability. The longest characterization measurements lasted for 20 h with an overall duty cycle of more than 99 %. In the second part of this thesis, the implementation of squeezed light into GEO 600 is presented along with the results achieved. The observatory shot noise was reduced by up to 3.4 dB, corresponding to an improvement in sensitivity by a factor of up to 1.5. For isotropically distributed GW sources, this corresponds to an increase in detection probability by 1.5 3 3.4. This makes GEO 600 the rst large-scale GW interferometer with a non-classically enhanced sensitivity. Based on the performance of the squeezed- light laser, a sensitivity improvement by a factor of two, corresponding to an equivalent optical power increase by a factor of four, seems feasible in the near future.
The engineering of strongly squeezed vacuum states of light is a key technology for the reduction of quantum noise in gravitational wave detectors. We report on the observation of up to 12.0 dB squeezed vacuum states of light at the wavelength of 1064 nm in the frequency band from 10 Hz to 100 kHz. This is the strongest squeezing reported to date within this detection band. The squeezed states were generated in a half-monolithic, standing-wave cavity optical parametric amplifier, which was resonant for the fundamental and harmonic light fields. We chose appropriate reflectivities to obtain a significant reduction of the required pump power, which was 8.6 mW only. Our analysis revealed that the residual measurement phase noise was smaller than 3.5 mrad rms and that the squeezed light source provided up to 14 dB of squeezing for a downstream application. The experiment was electronically stabilized in all relevant degrees of freedom, demonstrating the applicability of the linear, doubly resonant cavity topology for current and future gravitational wave detectors.
We present the generation and detection of squeezed light in the 2 μm wavelength region. This experiment is a crucial step in realizing the quantum noise reduction techniques that will be required for future generations of gravitational-wave detectors. Squeezed vacuum is generated via degenerate optical parametric oscillation from a periodically poled potassium titanyl phosphate crystal, in a dual resonant cavity. The experiment uses a frequency stabilized 1984 nm thulium fiber laser, and squeezing is detected using balanced homodyne detection with extended InGaAs photodiodes. We have measured 4.0±0.1 dB of squeezing and 10.5±0.5 dB of antisqueezing relative to the shot noise level in the audio frequency band, limited by photodiode quantum efficiency. The inferred squeezing level directly after the optical parametric oscillator, after accounting for known losses and phase noise, is 10.7 dB.
During the recent years an international network of interferometric gravitational wave (GW) detectors has been commissioned and searches for various classes of GWsignals are ongoing.Although the observatories are beginning to reach astrophysically interesting sensitivity levels, extensive upgrades are aimed for the next decade bringing us into the era of gravitational wave astronomy.One approach to enhance the detector sensitivity is the injection of squeezed vacuum states of light replacing the vacuum fluctuations entering the detector through its antisymmetric port.In this thesis the unprecedented generation of squeezed vacuum states with a noise power 6.5 dB below vacuum noise within the entire detection bandwidth of ground-based GW-detectors (10 Hz -10 kHz) was experimentally demonstrated at a wavelength of 1064 nm.For the first time a coherent control scheme was employed for comprehensive phase control of the experimental setup.This scheme is directly applicable to future GW-detector topologies.In addition, parasitic interference induced by scattered light was found to be a dominant noise source degrading squeezed states in the audio band.The compatibility of squeezed field injection into a detuned operated dual-recycled Michelson interferometer was demonstrated in a table top experiment.A detected non-classical sensitivity improvement of up to 3.0 dB within the entire detection bandwidth (5 -15 MHz) was accomplished by optimal preparation of the injected squeezing by means of a detuned locked filter cavity.In a further experiment strongly squeezed vacuum states were generated in the MHz-regime.With a maximum measured quantum noise reduction of 11.5 dB below the shot noise limit a new benchmark was set.This was achieved by suppression of phase noise and a remarkable reduction of optical loss in the experimental setup.Based on the squeezed light sources developed in this thesis and the experimental results achieved, a conceptual design of a compact squeezed light source was devised, allowing for a high duty cycle sensitivity improvement of GW-detectors.