Heterodyne detectors as phase-insensitive (PI) devices have found important applications in precision measurements such as space-based gravitational-wave (GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio (SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal. We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.
Heterodyne detectors as phase-insensitive (PI) devices have found important applications in precision measurements such as space-based gravitational-wave (GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio (SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal. We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.
Cross correlation heterodyne detectors exhibit the potential for suppression of the detection quantum noise below shot noise without use of optical squeezing for capturing weak optical signals in low frequency bands. To understand the underlying mechanism, we develop a quantum theory to describe the noise performance of cross correlation heterodyne detectors. By calculating the cross spectral density (CSD) of the photocurrent fluctuations from a cross correlation heterodyne detector, we prove that its noise performance can break the shot noise limit and exceed that of a regular heterodyne detector for detection of coherent light. When the detected light signal is in a squeezed state, we show that the corresponding CSD value is negative and discuss how a negative CSD may be explored to improve the output signal-to-noise ratio of the detector contaminated by classical noises through tuning the parameter of the degree of squeezing. This work may find itself useful in space-based gravitational wave searching and a variety of other scientific research activities, such as observation of vacuum magnetic birefringence and telecommunications.
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.
Squeezed states of light can be detected for precision measurements with a heterodyne detector by use of a bichromatic field as the local oscillator, due to the phase-sensitive nature of the device. However, divergence consists of the theoretical description of the quantum noise performance of this detector. Two existing theoretical models are briefly reviewed, with one model predicting a 3 dB quantum noise floor change in detection of two-mode squeezed light if the local oscillator is replaced by a monochromatic one, whereas the other model foretells the same noise floor no matter which local oscillator is used. An experiment on heterodyne detection of two-mode squeezed light is carried out to put the two models under test. No significant difference in the noise floor level is observed between the two detectors, showing the noiseless property of both detectors. This work should be of importance for the understanding of noise origin in optical detection and of great interest in practical application for squeezing-enhanced audio band gravitational-wave signal searching.
Although the quantum behaviors of optical detectors have been considered to be fully understood, a fundamental dilemma inevitably arises regarding a phase-sensitive heterodyne detector. On one hand, the detector should suffer a 3 dB noise penalty caused by an image band vacuum; on the other hand, as a phase-sensitive device, it should be noise free at the quantum level. We report on an experiment on a phase-sensitive heterodyne detector, where the quantum noise of the image band vacuum is absent in the observation. The underlying mechanism behind the observation should be closely related to the origin of the quantum noise in optical detection.
It is nontrivial to describe in theory the quantum behavior of a phase-sensitive heterodyne detector with a bichromatic local oscillator. On the one hand, a 3 dB noise penalty is expected for the detector due to the existence of an image band vacuum, which was assumed in a previous theoretical study. On the other hand, as a phase-sensitive device, the detector should be free of the 3 dB extra quantum noise, as verified by a recent experiment. However, the mechanism for the absence of the 3 dB noise penalty in the experimental observation remains unknown. To understand the relevant physics, we develop a comprehensive quantum theory, in agreement with experiment, for phase-sensitive heterodyne detection. This work should be important for us to understand the origin of the quantum noise in heterodyne detection.
A traditional heterodyne detector, as a phase-insensitive device, suffers the well-known 3 dB noise penalty caused by image sideband vacuum. In contrast, a heterodyne detector with a bichromatic local oscillator, as a phase-sensitive device, should be exempted from the 3 dB noise penalty, in spite of the existence of the image sideband vacuum. Assuming coherent light at the input, we develop in this work a theory to describe the quantum nature of the phase-sensitive heterodyne detector, in a good agreement with experiment. The absence of the quantum noise of the image vacuum modes in the heterodyne detector may be explained by that the studied detector senses only a single field of light, i.e., the signal field, according to the theory developed.