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arXiv:1101.2736 · Optics Express

Low-frequency phase measurement with high-frequency squeezing

Zehui Zhai, Jiangrui Gao

Published Jul 23, 2012• 15 citations
arXivHTMLPDF

Abstract

Squeezed-state enhanced audio-frequency signal measurement is crucial for some special applications, such as gravitational wave detection. But generation of squeezed state of light at such frequency is more difficult than that at megahertz-frequency. In this paper we propose an experimental scheme to measure low-frequency phase signal with high-frequency squeezing. To utilize the high-frequency sidebands of the squeezed light, a two-frequency intense laser is applied in the interferometry instead of a single-frequency laser as usual. This technique is in the reach of modern quantum optics technology.

DOI
10.1364/oe.20.018173

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Squeezed-state enhanced audio-frequency signal measurement is crucial for some special applications, such as gravitational wave detection. But generation of squeezed state of light at such frequency is more difficult than that at megahertz-frequency. In this paper we propose an experimental scheme to measure low-frequency phase signal with high-frequency squeezing. To utilize the high-frequency sidebands of the squeezed light, a two-frequency intense laser is applied in the interferometry instead of a single-frequency laser as usual. This technique is in the reach of modern quantum optics technology.
Measurement of audio signal plays a significant role in many applications, such as gravitational wave detection, bio-particle imaging and magnetometer. In this paper, low-frequency squeezed light is generated by a non-degenerate optical parametric amplifier. In order to avoid the effect of injected light on low-frequency squeezing, an auxiliary laser is used to lock the length of non-degenerate optical parametric amplifier and a method of locking quantum noise is employed to lock the phase between the local light and the squeezed light. By isolating the vibration noises at low-frequency and reducing back action of parasitic interference, the squeezing of (7.1±0.1) dB takes place at 19 kHz. Then the squeezed light is injected into the Mach-Zehnder interferometer to measure an audio signal which drives a piezoelectric transducer to generate a small phase variation between two arms of Mach-Zehnder interferometer. According to the low-frequency squeezing, we realize experimentally the measurement of phase signal at audio frequency which exceeds the shot-noise limit of (3.0±0.4) dB. The experiment provides technical supports for the generation of low-frequency squeezed light and the measurement of audio signal. Furthermore it can be extended to other quantum measurements, such as high-precision magnetometer and measurement of small-displacement.
We experimentally study a protocol of using the broadband high-frequency squeezed vacuum to detect the low-frequency signal. In this scheme, the lower sideband field of the squeezed light carries the low-frequency modulation signal, and the two strong coherent light fields are applied as the bichromatic local oscillator in the homodyne detection to measure the quantum entanglement of the upper and lower sideband for the broadband squeezed light. The power of one of the local oscillators for detecting the upper sideband can be adjusted to optimize the conditional variance in the low-frequency regime by subtracting the photocurrent of the upper sideband field of the squeezed light from that of the lower sideband field. By means of the quantum correlation of the upper and lower sideband for the broadband squeezed light, the low-frequency signal beyond the standard quantum limit is measured. This scheme is appropriate for enhancing the sensitivity of the low-frequency signal by the aid of the broad squeezed light, such as gravitational waves detection, and does not need to directly produce the low-frequency squeezing in an optical parametric process.
Low-frequency (Hz~kHz) squeezing is very important in many schemes of quantum precision measurement. But it is more difficult than that at megahertz-frequency because of the introduction of laser low-frequency technical noise. In this paper, we propose a scheme to obtain a low-frequency signal beyond the quantum limit from the frequency comb in a non-degenerate frequency and degenerate polarization optical parametric amplifier (NOPA) operating below threshold with type I phase matching by frequency-shift detection. Low-frequency squeezing immune to laser technical noise is obtained by a detection system with a local beam of two-frequency intense laser. Furthermore, the low-frequency squeezing can be used for phase measurement in Mach-Zehnder interferometer, and the signal-to-noise ratio (SNR) can be enhanced greatly.
Quantum noise limited interferometry for subhertz phase measurement is of essence in precision metrology applications, such as gravitational wave detection. However, suppression of subhertz classical noises below the shot-noise level is very challenging in practice. Two-frequency interferometry has been previously proposed to avoid low-frequency noises and may be useful for squeezing-enhanced high-precision phase measurement. Here we experimentally demonstrate a subhertz interferometer at the standard quantum noise (shot-noise) limit, by use of dual-frequency coherent probe light at the input and phase-sensitive heterodyne detection to measure the output interfered light. When the interferometer is locked at a dark fringe, the noise floor of the phase variance in the interferometer reaches the shot-noise level down to Fourier frequencies below 1 Hz. This work should pave a way for a realization of subhertz interferometry beyond the shot-noise limit.
Continuous variable (CV) audio-band frequency squeezed states at the fiber telecommunication wavelength is an important quantum resource for the practical applications based on optical fiber. As is well known, the optical power attenuation and phase diffusion effect of light at 1.3 μm in standard telecommunication fibres are low and small, respectively. The audio-band frequency squeezed light at 1.34 μm can be utilized to realize quantum precision measurement, such as quantum-enhanced sensing in the low-frequency range, laser interferometer for gravitational wave detection. In this paper, CV audio-band frequency vacuum squeezed states at 1.3 μm are experimentally generated by using a type-I degenerate optical parametric oscillator (DOPO) below the threshold. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1.34 μm) Nd:YVO4/LBO laser is used as a pump source for DOPO based on a type-I quasi-phase-matched periodically poled KTiOPO4 (PPKTP) crystal. Mode cleaners with a finesse of 400 and linewidth of 0.75 MHz are used to filter the noise of lasers at 671 nm and 1.34 μm, respectively. The intensity noises of the two lasers reach a shot noise level for analysis frequencies higher than 1.0 MHz and their phase noises reach shot noise level for analysis frequencies higher than 1.3 MHz, respectively. The low noise single-frequency 671 nm laser is utilized as a pump of the DOPO. The threshold power of the DOPO is 450 mW. In order to detect the audio-band frequency vacuum squeezed states, the power of local oscillator of a homodyne detector system is optimized to 60 μupW. Furthermore, the effect of common mode rejection ratio (CMRR) of detectors is discussed in detecting the audio-band frequency vacuum squeezed states. Improvement of CMRR of detectors is a good way to detect the audio-band frequency vacuum squeezed states effectively. When the phase matching temperature of PPKTP crystal is controlled at 53℃ by using a home-made temperature controller and the pump power is 95 mW, the vacuum squeezed states are generated at analysis frequency ranging from 8-100 kHz. A maximum measured squeeze of 5.0 dB is obtained at analysis frequency of 36 kHz. A 3.0 dB squeezed light is obtained at an audio-band frequency of 8 kHz.
We report the experimental realization of squeezed quantum states of light, tailored for new applications in quantum communication and metrology. Squeezed states in a broad Fourier frequency band down to 1 Hz have been observed for the first time. Nonclassical properties of light in such a low frequency band are required for high efficiency quantum information storage in electromagnetically induced transparency ( EIT) media. The states observed also cover the frequency band of ultra-high precision laser interferometers for gravitational wave detection and can be used to reach the regime of quantum non-demolition interferometry. Furthermore, they cover the frequencies of motion of heavy macroscopic objects and might therefore support attempts to observe entanglement in our macroscopic world.