Measurement of the squeezed vacuum state by a bichromatic local oscillator

Heterodyne laser Doppler vibrometer with squeezed light enhancement

An important feature of a heterodyne laser Doppler vibrometer (LDV) is the possibility of measuring an optical path length oscillation at a frequency f at a choosable frequency fhet ± f, at which the photo-electric measurement shows an optical quantum noise that is significantly greater than the detector dark noise. The full-squeezed light enhancement of a heterodyne LDV's signal-to-noise ratio has not been achieved so far. Here we use a sideband spectrum that is squeezed around fhet = 40 MHz and demonstrate the squeezing-enhanced measurement of an optical path length vibration at f = 1 MHz of about 3.5 dB while fully maintaining the signal power. The proof of principle we provide will enable the realization of ultra-precise LDVs over an extended signal bandwidth for probes or environments that require low intensities.

Generating six pairs of bandwidth-expanded entangled sideband modes via time delay compensation

Quantum entanglement is an important pillar of quantum information processing. In addition to the entanglement degree, the bandwidth of entangled states becomes another focus of quantum communication. Here, by virtue of a broadband frequency-dependent beam splitter, we experimentally demonstrate six pairs of independent entangled sideband modes with maximum entanglement degree of 8.1 dB. Utilizing a time delay compensation scheme, the bandwidth of independent entangled sideband modes is expanded to dozens of megahertz. This work provides a valuable resource to implement efficient quantum information processing.

Cavity enhanced parametric homodyne detection of a squeezed quantum comb

A squeezed state with higher-order sidebands is a valuable quantum resource for channel multiplexing quantum communication. However, balanced homodyne detection used in nonclassical light detection has a trade-off performance between the detection bandwidth and clearance, in which the verification of a highly squeezing factor faces a challenge. Here, we construct two optical parametric amplifiers with cavity enhancement; one is for the generation of a -10.5 dB squeezed vacuum state, and the other is for all-optical phase-sensitive parametric homodyne detection. Finally, -6.5 dB squeezing at the carrier with 17 pairs of squeezing sidebands (bandwidth of 156 GHz) is directly and simultaneously observed. In particular, for the cavity-enhanced parametric oscillation and detection processes, we analyze the limiting factors of the detectable bandwidth and measurement deviation from the generated value, which indicates that the length difference and propagation loss between two optical parametric amplifiers should be as small as possible to improve the detection performance. The experimental results confirm our theoretical analysis.

Two-Carrier Scheme: Evading the 3 dB Quantum Penalty of Heterodyne Readout in Gravitational-Wave Detectors

Precision measurements using a traditional heterodyne readout suffer a 3 dB quantum noise penalty compared with a homodyne readout. The extra noise is caused by the quantum fluctuations in the image vacuum. We propose a two-carrier gravitational-wave detector design that evades the 3 dB quantum penalty of the heterodyne readout. We further propose a new way of realizing frequency-dependent squeezing utilizing two-mode squeezing in our scheme. It naturally achieves more precise audio frequency signal measurements with radio frequency squeezing. In addition, the detector is compatible with other quantum nondemolition techniques.

Observation of a comb of squeezed states with a strong squeezing factor by a bichromatic local oscillator

We demonstrate the experimental detection of an optical squeezing covering several higher resonances of the optical parametric amplifier (OPA) by adopting a bichromatic local oscillator (BLO). The BLO is generated from a waveguide electro-optic phase modulator (WGM) and subsequent optical mode cleaner (OMC), without the need of additional power balance and phase control. The WGM is used for generating the frequency-shifted sideband beams with equal power and certain phase difference, and the OMC is used for filtering the unwanted optical modes. Among a measurement frequency range from 0 to 16.64 GHz, the maximum squeezing factors are superior to 10 dB below the shot noise limit for the first three discrete odd-order resonances of the OPA.

Squeezing-enhanced heterodyne detection of 10 Hz atto-Watt optical signals

A phase-sensitive (PS) heterodyne detector is intrinsically resistant to classical noises and useful in measurement of low-frequency signals below the shot noise. Despite the existence of image band vacuum, we show that the quantum-noise power level of this heterodyne detector sensing a coherent signal is exactly one light quantum per measurement time, i.e., twice the vacuum fluctuation power, which can be further reduced by use of squeezed light. We then report on an experiment on a PS heterodyne detector with a 10 Hz 1.0×10^-18  W optical signal (1064 nm wavelength) at its input. The noise floor of the unmodulated coherent light is 2.2(±0.1)×10^-19  W/Hz from 2 Hz to 20 Hz, and the signal-to-noise ratio is about 6.6 dB for the measured signal when the resolution bandwidth is 1 Hz. The quantum noise floor is reduced by 1.6(±0.3)dB when squeezed light is used, and the sub-shot-noise power spectral density is 1.6(±0.1)×10^-19  W/Hz between 2 Hz and 20 Hz. This work should be an important advance towards squeezing-improved precision measurements of low-frequency signals with heterodyne detectors, including audio-band gravitational-wave detection.

Bichromatic homodyne detection of broadband quadrature squeezing

We experimentally study a homodyne detection technique for the characterization of a quadrature squeezed field where the correlated bands, here created by four-wave mixing in a hot atomic vapor, are separated by a large frequency gap of more than 6 GHz. The technique uses a two-frequency local oscillator to detect the fluctuations of the correlated bands at a frequency accessible to the detection electronics. Working at low detection frequency, the method allows for the determination of both the amplitude and the phase of the squeezing spectrum. In particular, we show that the quadrature squeezing created by our four-wave mixing process displays a noise ellipse rotation of π/2 across the squeezing spectrum.

Complex Squeezing and Force Measurement Beyond the Standard Quantum Limit

A continuous quantum field, such as a propagating beam of light, may be characterized by a squeezing spectrum that is inhomogeneous in frequency. We point out that homodyne detectors, which are commonly employed to detect quantum squeezing, are blind to squeezing spectra in which the correlation between amplitude and phase fluctuations is complex. We find theoretically that such complex squeezing is a component of ponderomotive squeezing of light through cavity optomechanics. We propose a detection scheme called synodyne detection, which reveals complex squeezing and allows the accounting of measurement backaction. Even with the optomechanical system subject to continuous measurement, such detection allows the measurement of one component of an external force with sensitivity only limited by the mechanical oscillator's thermal occupation.