Mechanically detecting and avoiding the quantum fluctuations of a microwave field

Abstract

During the theoretical investigation of the ultimate sensitivity of gravitational wave detectors through the 1970's and '80's, it was debated whether quantum fluctuations of the light field used for detection, also known as photon shot noise, would ultimately produce a force noise which would disturb the detector and limit the sensitivity. Carlton Caves famously answered this question with "They do." [1] With this understanding came ideas how to avoid this limitation by giving up complete knowledge of the detector's motion [2][3][4]. In these back-action evading (BAE) or quantum non-demolition (QND) schemes, one manipulates the required quantum measurement back-action by placing it into a component of the motion which is unobserved and dynamically isolated. Using a superconducting, electro-mechanical device, we realize a sensitive measurement of a single motional quadrature with imprecision below the zero-point fluctuations of motion, detect both the classical and quantum measurement back-action, and demonstrate BAE avoiding the quantum back-action from the microwave photons by 9 dB. Further improvements of these techniques are expected to provide a practical route to manipulate and prepare a squeezed state of motion with mechanical fluctuations below the quantum zero-point level, which is of interest both fundamentally [5] and for the detection of very weak forces [6].Since the discovery of Shor's Algorithm [7] almost 20 years ago, a major theme in physics has been about the untapped power and benefits of quantum phenomena, largely stemming from the resource of quantum entanglement. However much earlier, it was understood how quantum physics places limits on our knowledge [8,9]. This limitation can be useful, as in the case of quantum cryptography schemes where the required quantum measurement back-action of an eavesdropper leaves its trace on the transmitted information, providing proof of their snooping. For measurements of position, this limitation, called the Standard Quantum Limit (SQL) [9] is not beneficial: back-action due to the quantum nature of the measurement field, ultimately obscures our vision for a sufficiently sensitive measurement.Quantum limitations on the detection of position are no longer academic issues; in recent years, the detection of motion has now advanced to the point that quantum back-action engineering is now required to improve the sensitivity. Detections of motion have been realized with imprecision below that at SQL [10,11]. Back-action forces from the quantum noise of the detection field have been demonstrated to drive the motion of mechanical oscillators, first with electrons in an electro-mechanical structure [12] and then with photons in opto-mechanical systems [13,14]. In this work, we demonstrate the backaction forces due to the shot noise of microwave photons, which are 10 4 times lower in energy than optical photons.Strategies to manipulate the quantum measurement back-action have included modifying the quantum fluctuations of the measurement field [15,16], and modulati...

Quantum Backaction Evading Measurement of Collective Mechanical Modes

“…Classical analogs of BAE measurements were demonstrated a long time ago [5][6][7]. In cavity optomechanical systems, where a mechanical oscillator is dispersively coupled to a driven optical or microwave cavity, BAE measurements that evade the quantum backaction [8][9][10] have recently been demonstrated [11]. In the context of generating squeezed states of mechanical motion [12][13][14] they have been used for detection [12,14], and they have also been demonstrated in atomic spin systems [15].…”

Overwhelming Thermomechanical Motion with Microwave Radiation Pressure Shot Noise

“…While many experiments are now able to engineer low enough P SQL to be achieved in the laboratory, the quantum backaction from the radiation pressure shot noise is often obscured by residual thermal motion of the oscillator [4][5][6]. It is only recently that cavity optomechanical experiments observe backaction noise on par with the thermal occupancy [7][8][9][10]. While clever cross-correlation techniques can separate the thermal motion from the effects of the measurement [7,11,12], one would ideally like to operate at much larger powers where the quantum backaction dominates all other sources of noise including the thermal motion, S ba x S th x .…”

Cavity-mediated coupling of mechanical oscillators limited by quantum back-action

“…We observe the quantum back-action noise imparted by the optical coupling resulting in correlated mechanical fluctuations of the two oscillators. Our results illustrate challenges and opportunities of coupling quantum objects with light for applications of quantum cavity optomechanics [8][9][10][11][12][13][14] .Cavity optomechanical systems comprised of a single mechanical oscillator interacting with a single electromagnetic cavity mode 15 serve useful quantum-mechanical functions, such as generating squeezed light [16][17][18] , detecting forces with quantum-limited sensitivity 19 or through back-action-evading measurement 20 , and both entangling and amplifying mechanical and optical modes 21 . Systems containing several mechanical elements offer additional capabilities.…”

“…Cavity optomechanical systems comprised of a single mechanical oscillator interacting with a single electromagnetic cavity mode 15 serve useful quantum-mechanical functions, such as generating squeezed light [16][17][18] , detecting forces with quantum-limited sensitivity 19 or through back-action-evading measurement 20 , and both entangling and amplifying mechanical and optical modes 21 . Systems containing several mechanical elements offer additional capabilities.…”