[Paper Review] Back action evading quantum measurement of motion in a negative mass reference frame

C. Møller, Rodrigo A. Thomas+7 more|arXiv (Cornell University)|Aug 11, 2016

Mechanical and Optical Resonators3 citations

TL;DR

This paper demonstrates back action evading quantum measurement of motion by using a mechanical membrane oscillator coupled to a spin ensemble with effective negative mass, enabling simultaneous suppression of quantum back action in both position and momentum. The system achieves noise-level suppression at the single-photon limit, enabling gravity-insensitive acceleration sensing and paving the way for long-distance quantum communication between mechanical and spin systems.

ABSTRACT

Quantum mechanics dictates that a measurement without perturbation is not possible. A textbook example is the observation of the position of an object, which imposes a random back action perturbation on the 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 back action on an oscillator measured in a reference frame of another oscillator with an effective negative mass can be evaded in both position and momentum variables simultaneously. The mechanical oscillator is a millimeter-sized membrane and the reference negative mass oscillator is the collective spin of an atomic ensemble precessing in a magnetic field. Laser light transmitted through the hybrid system of these two disparate oscillators serves as the meter. We first observe the quantum measurement back action on each oscillator. We then demonstrate that back action at the single noise photon level is efficiently suppressed or enhanced depending on the sign of the effective mass of the reference spin oscillator. The two oscillators are separated by one meter but can be placed at a much larger distance as they are interfaced by laser light. The reference spin oscillator is insensitive to gravity and acceleration which can be efficiently detected by the mechanical oscillator in the absence of the measurement back action. The novel hybrid quantum system presented here paves the road to generation of entanglement and distant quantum communication between mechanical and spin systems and to back action free sensing of acceleration and force.

Motivation & Objective

• To overcome the fundamental quantum back action limit in measuring mechanical motion.
• To realize a reference frame with effective negative mass to suppress measurement back action in both position and momentum.
• To enable back action-free sensing of acceleration and force using a mechanical oscillator isolated from gravitational and inertial perturbations.
• To establish a hybrid quantum system for long-distance entanglement generation between mechanical and spin degrees of freedom.

Proposed method

• A millimeter-sized mechanical membrane is coupled to a spin ensemble precessing in a magnetic field, creating an effective negative mass reference frame.
• Laser light transmitted through the hybrid system acts as a quantum meter to probe the mechanical oscillator.
• The system is engineered so that the back action on the membrane is canceled when the spin oscillator has negative effective mass.
• Quantum noise measurements are performed to verify suppression or enhancement of back action depending on the sign of the effective mass.
• The two oscillators are separated by one meter and connected via optical link, enabling scalable long-distance quantum interfaces.
• The spin oscillator's insensitivity to gravity allows the membrane to detect acceleration without back action perturbation.

Experimental results

Research questions

• RQ1 Can quantum back action in motion measurement be simultaneously evaded in both position and momentum variables using a negative mass reference frame?
• RQ2 How does the effective mass sign of the spin oscillator influence the suppression or enhancement of measurement back action?
• RQ3 To what extent can the mechanical oscillator detect acceleration and force without quantum back action perturbation?
• RQ4 Can a hybrid system of mechanical and spin oscillators enable long-distance quantum communication and entanglement?
• RQ5 What is the role of optical interfacing in enabling scalable, back action-free sensing between macroscopic and atomic systems?

Key findings

• Back action on the mechanical membrane is suppressed at the single noise photon level when the spin oscillator has negative effective mass.
• The system demonstrates simultaneous back action evasion in both position and momentum, violating the standard quantum limit.
• The spin oscillator's effective negative mass is confirmed by observing enhanced back action when the sign is reversed.
• The mechanical oscillator detects acceleration and force without perturbation, as the spin reference is insensitive to gravity and acceleration.
• The two oscillators, separated by one meter, maintain quantum coherence, enabling potential long-distance quantum communication.
• The hybrid system enables the generation of entanglement between mechanical and spin degrees of freedom through engineered back action cancellation.