Aaron Weinstein

Quantum squeezing of motion in a mechanical resonator

Manipulation of a quantum squeeze The uncertainty principle of quantum mechanics dictates that even when a system is cooled to its ground state, there are still fluctuations. This zero-point motion is unavoidable but can be manipulated. Wollman et al. demonstrate such manipulation with the motion of a micrometer-sized mechanical system. By driving up the fluctuations in one of the variables of the system, they are able to squeeze the other related variable below the expected zero-point limit. Quantum squeezing will be important for realizing ultrasensitive sensors and detectors. Science, this issue p. 952 The fluctuating motion of a mechanical system can be squeezed below the zero-point limit. According to quantum mechanics, a harmonic oscillator can never be completely at rest. Even in the ground state, its position will always have fluctuations, called the zero-point motion. Although the zero-point fluctuations are unavoidable, they can be manipulated. Using microwave frequency radiation pressure, we have manipulated the thermal fluctuations of a micrometer-scale mechanical resonator to produce a stationary quadrature-squeezed state with a minimum variance of 0.80 times that of the ground state. We also performed phase-sensitive, back-action evading measurements of a thermal state squeezed to 1.09 times the zero-point level. Our results are relevant to the quantum engineering of states of matter at large length scales, the study of decoherence of large quantum systems, and for the realization of ultrasensitive sensing of force and motion.

Mechanically detecting and avoiding the quantum fluctuations of a microwave field

Avoiding back-action in quantum measurements The very process of measuring a quantum system has an influence on the system through the process of back-action. Suh et al. used a back-action evasion scheme to monitor the motion of a miniature oscillator without influencing its motion (see the Perspective by Bouwmeester). The scheme should help in the understanding of the fundamental limits associated with measurement and will have practical implications in providing a low-temperature thermometer and a probe of extremely weak forces. Science, this issue p. 1262 The measurement-induced back-action effects on a quantum system can be avoided. [Also see Perspective by Bouwmeester] Quantum fluctuations of the light field used for continuous position detection produce stochastic back-action forces and ultimately limit the sensitivity. To overcome this limit, the back-action forces can be avoided by giving up complete knowledge of the motion, and these types of measurements are called “back-action evading” or “quantum nondemolition” detection. We present continuous two-tone back-action evading measurements with a superconducting electromechanical device, realizing three long-standing goals: detection of back-action forces due to the quantum noise of a microwave field, reduction of this quantum back-action noise by 8.5 ± 0.4 decibels (dB), and measurement imprecision of a single quadrature of motion 2.4 ± 0.7 dB below the mechanical zero-point fluctuations. Measurements of this type will find utility in ultrasensitive measurements of weak forces and nonclassical states of motion.

Thermally Induced Parametric Instability in a Back-Action Evading Measurement of a Micromechanical Quadrature near the Zero-Point Level

We report the results of back-action evading experiments utilizing a tightly coupled electro-mechanical system formed by a radio frequency micromechanical resonator parametrically coupled to a NbTiN superconducting microwave resonator. Due to excess dissipation in the microwave resonator, we observe a parametric instability induced by a thermal shift of the mechanical resonance frequency. In light of these measurements, we discuss the constraints on microwave dissipation needed to perform BAE measurements far below the zero-point level.

Optomechanical effects of two-level systems in a back-action evading measurement of micro-mechanical motion

We show that the two-level systems (TLS) in lithographic superconducting circuits act as a power-dependent dielectric leading to non-linear responses in a parametrically coupled electromechanical system. Driven TLS shift the microwave resonance frequency and modulate the mechanical resonance through the optical spring effect. By pumping with two tones in a back-action evading measurement, these effects produce a mechanical parametric instability which limits single quadrature imprecision to 1.4 x_(zp). The microwave resonator noise is also consistent to a TLS-noise model. These observations suggest design strategies for minimizing TLS effects to improve ground-state cooling and quantum non-demolition measurements of motion.

Optomechanical effects of two-level systems

Two-level systems are observed to affect quantum measurements with superconducting electromechanical systems via Kerr-like nonlinearity and excess phase noise. We propose a magnetic electromechanical coupling scheme as its solution.