Quantum back action evading measurement of motion in a negative mass reference frame (1608.03613v2)

Abstract: Quantum mechanics dictates that a continuous measurement of the position of an object imposes a random back action perturbation on its momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. Here we demonstrate that the quantum back action on a macroscopic mechanical oscillator measured in the reference frame of an atomic spin oscillator can be evaded. The collective quantum measurement on this novel hybrid system of two distant and disparate oscillators is performed with light. The mechanical oscillator is a drum mode of a millimeter size dielectric membrane and the spin oscillator is an atomic ensemble in a magnetic field. The spin oriented along the field corresponds to an energetically inverted spin population and realizes an effective negative mass oscillator, while the opposite orientation corresponds to a positive mass oscillator. The quantum back action is evaded in the negative mass setting and is enhanced in the positive mass case. The hybrid quantum system presented here paves the road to entanglement generation and distant quantum communication between mechanical and spin systems and to sensing of force, motion and gravity beyond the standard quantum limit.

Quantum Back Action Evading Measurement of Motion in a Negative Mass Reference Frame

The paper "Quantum back action evading measurement of motion in a negative mass reference frame" explores a novel methodology in quantum measurements, focusing on evading the quantum back action (QBA) that typically limits the precision of continuous position measurements of macroscopic mechanical oscillators. In this paper, the authors report a significant advancement in suppressing QBA in a hybrid system, which includes a mechanical oscillator and a spin oscillator. By leveraging a negative mass reference frame, the researchers successfully demonstrate back action evading measurements (BAE) on a system of macroscopic dimensions.

Overview of the Hybrid Quantum System

The hybrid quantum system comprises two principal components: a mechanical oscillator, embodied by the drum mode of a silicon nitride membrane, and a spin oscillator, which is an ensemble of cesium atoms. The mechanical aspect of the system is resonated via traditional optomechanical interfaces, while the spin oscillator operates under a magnetic field inducing different effective mass scenarios—both positive and negative—as dictated by the orientation of the atomic spin relative to the field. This capability enables unique interactions that are pivotal for testing quantum measurement theories in unprecedented scales.

The distinctive feature of this arrangement is the ability to treat the spin system as a negative mass oscillator. Negative mass in this context does not imply actual negative mass but describes conditions where the spin population is aligned such that energy is described oppositely to conventional expectations. This configuration is instrumental to circumvent the QBA of measurements, as it allows the system to utilize only the commuting variables, and hence permits certain dynamics classically forbidden due to standard quantum limits.

Experimental Framework and Findings

The researchers constructed a sophisticated experimental setup where light facilitates the interaction between the mechanical and spin systems. Key to the experimental process is the tuning of atomic resonance frequencies to match those of mechanical oscillators, enabling efficient QBA suppression. The paper presents results where the hybrid system shows destructive interference patterns in the negative effective mass setups, leading to appreciable QBA cancellation.

Significantly, the paper highlights that for optimal suppression of QBA, it is not necessary for both oscillators to exhibit complete resonance at equivalent frequencies. The researchers document scenarios with slightly detuned frequencies, resulting in enhanced evasion capabilities when the quadrature phases of light are optimally rotated. The observed QBA reduction for the negative mass configuration is on the order of 1.7 dB, a substantial improvement when extrapolated to potential applications in precise measurement systems.

Implications and Future Directions

The implications of these findings are profound for both theoretical and practical domains. The theoretical impact is the validation of hypotheses concerning quantum measurement theories—broadening perspectives on how negative mass can be effectively exploited to circumvent standard quantum limits. Practically, the ability to bridge optical communication with hybrid quantum systems presents spheres of applicability in quantum sensing technologies, leading to advancements in fields such as gravitational wave detection and quantum information science.

Future research directions could further accentuate enhanced control over hybrid systems, exploring higher quantum cooperative regimes with reduced environmental noise. This advancement could involve implementing the knowledge gained into more robust quantum communication protocols or refining entanglement-based sensing mechanisms. The foundation laid by this research suggests a promising trajectory for developing quantum technologies that could ultimately redefine the limits of measurement accuracy and precision.