Laser Power Stabilization beyond the Shot Noise Limit Using Squeezed Light

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.

One-thousandth-level laser power stabilization based on optical feedback from a well-designed high-split-ratio and nonpolarized beam splitter

Laser power stabilization plays a significant role in atomic and molecular physics, quantum precision measurement, and optical sensing and measurement. In the classical method of using a feedback control loop to stabilize the laser power, the beam splitter is the conjunction element to connect the feedback beam inside the loop and the output beam outside the loop. The stability of its split ratio will directly affect the result of power stabilization, especially in demand of high split ratios for high-efficiency output. For the compatibility of a high split ratio and high stability in a power-stabilized system, we designed and manufactured a high-split-ratio nonpolarized plate beam splitter, whose split ratio was insensitive to variations of beam intensity, polarization, and ambient temperature. Based on the optical feedback of the designed beam splitter, the light intensity was closed-loop controlled by an acousto-optic modulator; finally, the power outside the loop was stabilized as well. The output power was stabilized at 537 mW and a 6 h long-term test was performed. The relative stability of laser power outside the loop in terms of root mean square and peak to peak was 2.72×10^-4 and 1.60×10^-3, respectively. The relative Allan standard deviation reached 2.78×10^-5 at an average time of 200 s. These results will greatly benefit many practical fields that require laser power stabilization with high split ratios and one-thousandth-level stability.

Laser power stabilization via radiation pressure

This Letter reports the experimental realization of a novel, to the best of our knowledge, active power stabilization scheme in which laser power fluctuations are sensed via the radiation pressure driven motion they induce on a movable mirror. The mirror position and its fluctuations were determined by means of a weak auxiliary laser beam and a Michelson interferometer, which formed the in-loop sensor of the power stabilization feedback control system. This sensing technique exploits a nondemolition measurement, which can result in higher sensitivity for power fluctuations than direct, and hence destructive, detection. Here we used this new scheme in a proof-of-concept experiment to demonstrate power stabilization in the frequency range from 1 Hz to 10 kHz, limited at low frequencies by the thermal noise of the movable mirror at room temperature.

Probe noise characteristics of the spin-exchange relaxation-free (SERF) magnetometer

In the spin-exchange relaxation-free (SERF) magnetometer, the probe noise is a consequential factor affecting the gradiometric measurement sensitivities. In this paper, we proposed a new characteristics model of the probe noise based on noise separation. Different from noise analysis on single noise source, we considered most of the noise sources influencing the probe system and realized noise sources level measurement experimentally. The results demonstrate that the major noise type changes with the signal frequency. Below 10 Hz, the probe noise mainly comes from the sources independent of light intensity such as the vibration, which accounts for more than 50%; while at 30 Hz, the photon shot noise and the magnetic noise are the main origins, with proportion about 43% and 32%, respectively. Moreover, the results indicate that the optimal probe light intensity with highest sensitivity appears when the response of the magnetic noise is equal to the sum of the electronic noise and half of the shot noise. The optimal intensity gets larger with higher signal frequency. The noise characteristics model could be applied in modulating or differential optical systems and helps sensitivity improvement in SERF magnetometer.

High-precision cavity spectroscopy using high-frequency squeezed light

In this article, we present a novel spectroscopy technique that improves the signal-to-shot-noise ratio without the need to increase the laser power. Detrimental effects by technical noise sources are avoided by frequency-modulation techniques (frequency up-shifting). Superimposing the signal on non-classical states of light leads to a reduced quantum noise floor. Our method reveals in a proof-of-concept experiment small signals at Hz to kHz frequencies even below the shot noise limit. Our theoretical calculations fully support our experimental findings. The proposed technique is interesting for applications such as high-precision cavity spectroscopy, e.g., for explosive trace gas detection where the specific gas might set an upper limit for the laser power employed.

Shortcuts to Adiabaticity for the Quantum Rabi Model: Efficient Generation of Giant Entangled Cat States via Parametric Amplification

We propose a method for the fast generation of nonclassical ground states of the Rabi model in the ultrastrong and deep-strong coupling regimes via the shortcuts-to-adiabatic (STA) dynamics. The time-dependent quantum Rabi model is simulated by applying parametric amplification to the Jaynes-Cummings model. Using experimentally feasible parametric drive, this STA protocol can generate large-size Schrödinger cat states, through a process that is ∼10 times faster compared to adiabatic protocols. Such fast evolution increases the robustness of our protocol against dissipation. Our method enables one to freely design the parametric drive, so that the target state can be generated in the lab frame. A largely detuned light-matter coupling makes the protocol robust against imperfections of the operation times in experiments.

Hybrid two-mode squeezing of microwave and optical fields using optically pumped graphene layers

A measurable quadrature of a squeezed quantum state manifests a small uncertainty below the Heisenberg limit. This phenomenon has the potential to enable several extraordinary applications in quantum information, metrology and sensing, and other fields. Several techniques have been implemented to realize squeezed electromagnetic states, including microwave fields and optical fields. However, hybrid squeezed modes (that incorporate both microwave and optical fields) have not yet been proposed despite their vital functionality to combine the two worlds of quantum superconducting systems and photonics systems. In this work, for the first time, we propose a novel approach to achieve two-mode squeezing of microwave and optical fields using graphene based structure. The proposed scheme includes a graphene layered structure that is driven by a quantum microwave voltage and subjected to two optical fields of distinct frequencies. By setting the optical frequency spacing equal to the microwave frequency, an interaction occurs between the optical and microwave fields through electrical modulation of the graphene conductivity. We show that significant hybrid two-mode squeezing, that includes one microwave field and one optical field, can be achieved. Furthermore, the microwave frequency can be tuned over a vast range by modifying the operation parameters.

Fundamental limits of laser power stabilization via a radiation pressure transfer scheme

Traditional active laser power stabilization schemes are fundamentally limited by quantum shot noise on the in-loop photodetector. One way to overcome this limitation is to implement a nondemolition sensing scheme where laser power fluctuations are transferred to motion of a micro-oscillator, which can be sensed with a high signal-to-noise ratio. In this Letter, we analyze the power stability achievable in a nondemolition scheme limited by quantum and thermal noise. Under the assumption of realistic experimental parameters, we show that generation of a strong bright squeezed quantum state of light should be possible.

Laser Intensity Noise Suppression for Preparing Audio-Frequency Squeezed Vacuum State of Light

Laser intensity noise suppression has essential effects on preparation and characterization of the audio-frequency squeezed vacuum state of light based on a sub-threshold optical parametric oscillator (OPO). We have implemented two feedback loops by using relevant acousto-optical modulators (AOM) to stabilize the intensity of 795-nm near infrared (NIR) fundamental laser and 397.5-nm ultraviolet (UV) laser generated by cavity-enhanced frequency doubling. Typical peak-to-peak laser intensity fluctuation with a bandwidth of ~10 kHz in a half hour has been improved from ±7.45% to ±0.06% for 795-nm NIR laser beam, and from ±9.04% to ±0.05% for 397.5-nm UV laser beam, respectively. The squeezing level of the squeezed vacuum state at 795 nm prepared by the sub-threshold OPO with a PPKTP crystal has been improved from −3.3 to −4.0 dB around 3~9 kHz of analysis frequency range.

Sideband amplitude modulation absorption spectroscopy of CH4 at 1170 nm

We report on the method of the sideband amplitude modulation (SAM) to achieve high-sensitivity spectroscopy with a fiber electro-optic modulator (fiber-EOM). This method increases the signal to noise ratio (SNR) by a factor of forty, comparing with conventional absorption spectroscopy. It is a temporal balanced detection to eliminate the intensity noise of the light source, and capable of preserving an undistorted Doppler profile for further quantitative analysis. Taking advantage of the newly developed fiber-EOM, SAM is applicable for various spectroscopies with a simple experimental setup. We performed SAM on CH₄3ν₃ overtone band at 1170 nm using an external cavity Quantum dot laser. We demonstrated that one of the absorption lines buried in the other ten times stronger nearby lines was clearly extracted. SAM shows great potential on the molecular spectroscopy, where the spectrum is complicated and quantitative analysis is required.