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Scheme for efficient extraction of low-frequency signal beyond the quantum limit by frequency-shift detection

Rongguo Yang, J. Zhang, Z. H. Zhai, Shenqiang Zhai, K. Liu, Jiangrui Gao

Published Aug 6, 2015• 6 citations
Publisher

Abstract

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.

DOI
10.1364/oe.23.021323

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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.
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.
Generation of squeezed state at telecommunication wavelength has been recently a very interesting issue due to the lowest optical power attenuation of light at a wavelength of 1550 nm in a standard telecommunication fiber. The low-frequency vacuum squeezed state at 1550 nm in combination with fiber based interferometer offers the possibility to implement quantum precision measurement beyond standard quantum limit (SQL). In this paper, we experimentally realize a quantum-enhanced fiber Mach-Zehnder interferometer (FMZI) for measuring the low-frequency phase modulation signal by using low-frequency vacuum squeezing at 1550 nm. Firstly, the low-frequency vacuum squeezed state at the telecommunication wavelength of 1550 nm is generated by using a degenerate optical parametric oscillator (DOPO). The DOPO is a semi-monolithic construction based on a type I periodically poled KTiOPO4 (PPKTP) crystal and a concave mirror. The pump threshold of DOPO is 270 mW. When the pump power is 120 mW that is below the pump threshold of DOPO and the temperature of PPKTP is controlled at 34.8℃, a vacuum squeezing of 3 dB is generated at an analysis frequency range from 10 kHz to 500 kHz. The quadrature phase vacuum squeezing is obtained by locking the squeezed quadrature angle through using a coherent control scheme, in which two acousto-optic modulators are used to shift the frequency and produce the auxiliary beam acting as a coherent control field. Based on the constructed FMZI, a quantum-enhanced FMZI is realized by injecting the generated low-frequency vacuum squeezed state at 1550 nm into the vacuum channel of FMZI. The relative phase between two injected light fields is locked at π by using the Pound-Drever-Hall (PDH) locking technology, and the relative phase between light fields of its arms in FMZI is also locked at π/2 by using the PDH locking technology. When a phase modulation signal at the frequency of 500 kHz is loaded in the signal arm of FMZI, the noise power spectrum of the output from FMZI is measured by a balance homodyne detect system. A 2 dB quantum improvement beyond shot-noise-level at the frequency of 500 kHz is obtained experimentally by using the quantum-enhanced FMZI. The experimental results demonstrate a potential application in quantum precision measurement beyond the SQL based on fiber sensor technique.
We propose and experimentally demonstrate a quantum-enhanced fiber Mach-Zehnder interferometer for low-frequency phase measurement beyond the shot-noise limit using a high-frequency squeezing technique. The local oscillator field is amplitude-modulated in the MHz range and is then demodulated to avoid the technical noise that occurs at low frequencies. After measurement of the phase noise at a frequency of tens of kHz, an improvement of ∼2 dB relative to the shot-noise level is achieved. Additionally, the amplitude modulation depth has no significant effect on the phase noise improvement of the interferometer when deployed in our experimental configuration. The current scheme introduces a quantum technique into the fiber-based measurements, particularly for the low frequency range, and this scheme has potential applications in the high-precision fiber sensing of temperature, strain, and various other parameters.
Two-mode orthogonal squeezed vacuum states are an important quantum resource for quantum communication, quantum computing, quantum simulation, quantum precision measurement and sensing. It is essential to obtain stable two-mode orthogonal squeezed vacuum states in a low frequency range and compact configurations for practical applications, especially in quantum precision measurement and sensing. Two-mode orthogonal squeezed vacuum states are commonly produced via a subthreshold nondegenerate optical parametric amplifier (NOPA) in a continuous variable system. However, it is a difficult problem that the subthreshold NOPA cavity is phase sensitive manipulated to obtain stable squeezed vacuum states. Previous signal light injecting scheme relies on an injection of a weak light field into the subthreshold NOPA for phase sensitive manipulation. The injected signal light has the same frequency as the generated squeezed vacuum state. Thereby even the weakest injected signal light can introduce large amounts of excessive noise at low frequencies and the squeezing degree of two-mode squeezed vacuum states will be reduced or squeezing cannot be achieved. In this paper, a single sideband frequency shifted light injecting scheme is proposed for phase sensitive manipulation of NOPA. The comparison between the single sideband frequency shifted light injecting scheme and the signal light injecting scheme for realization of phase sensitive manipulation of NOPA is conducted. The effects of the two schemes on the generation of the low-frequency two-mode orthogonal squeezed vacuum state light field are investigated experimentally . The experimental results show that in the signal light injecting scheme for phase sensitive manipulation, the squeezing degree of the two-mode orthogonal squeezed vacuum state continuously decreases until it disappears as the power of injected signal light increases. In the process of phase sensitive manipulation of NOPA by using the single sideband frequency shifted light injecting scheme, the squeezing degree of the two-mode orthogonal squeezed vacuum state does not change with the power of the injected frequency shifted light increasing. Stable phase sensitive manipulation is realized by injecting single sideband frequency shifted light into NOPA. The NOPA is operated in a phase sensitive amplification state for 30 min. Stable low-frequency two-mode orthogonal squeezed vacuum states are obtained. The (4.1 ± 0.1) dB amplitude orthogonal squeezed vacuum states and (4.0 ± 0.2) dB phase orthogonal squeezed vacuum states at a frequency of 200 kHz are generated stably, in a compact NOPA configuration.
In this paper, we propose a scheme to produce a multiplexed entanglement frequency comb in a type II phase-matching nondegenerate optical parametric amplifier (NOPA) operating below threshold. The entanglement of the signal and idler frequency combs in the wide frequency range, which is limited by the phase-matching bandwidth of the NOPA, is investigated by the inseparability criterion. Furthermore, N Einstein–Podolsky–Rosen pairs can be created from the two combs without any reduction in the correlations by using frequency-dependent beam splitters.
Stable low-frequency squeezed vacuum states at a wavelength of 1550 nm were generated. By controlling the squeezing angle of the squeezed vacuum states, two types of low-frequency quadrature-phase squeezed vacuum states and quadrature-amplitude squeezed vacuum states were obtained using one setup respectively. A quantum-enhanced fiber Mach–Zehnder interferometer (FMZI) was demonstrated for low-frequency phase measurement using the generated quadrature-phase squeezed vacuum states that were injected. When phase modulation was measured with the quantum-enhanced FMZI, there were above 3 dB quantum improvements beyond the shot-noise limit (SNL) from 40 kHz to 200 kHz, and 2.3 dB quantum improvement beyond the SNL at 20 kHz was obtained. The generated quadrature-amplitude squeezed vacuum state was applied to perform low-frequency amplitude modulation measurement for sensitivity beyond the SNL based on optical fiber construction. There were about 2 dB quantum improvements beyond the SNL from 60 kHz to 200 kHz. The current scheme proves that quantum-enhanced fiber-based sensors are feasible and have potential applications in high-precision measurements based on fiber, particularly in the low-frequency range.