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Quantum Information and Measurement (QIM)

Entanglement and Force Sensing with Massive Mechanical Oscillators

Matthew J. Woolley

Published Jan 1, 2019• 0 citations
Publisher

Abstract

There has recently been a great deal of interest in the study of massive mechanical oscillators in the quantum regime. This is motivated by the possibility of universal interfaces in future quantum networks, sensing with unprecedented resolution, and fundamental interest in the limits of quantum mechanics. I will describe the recently reported first demonstrations of the measurement of mechanical motion in a negative-mass reference frame, and stabilisation of the entanglement between massive mechanical oscillators [C. F. Ockeloen-Korppi et al., Nature 556, 478-482 (2018)]. I will describe proposals for extending these demonstrations to multipartite entanglement, and a recent analysis of the force sensing possibilities that emerge from combining these technologies. The latter shall be discussed in the context of recently formulated general limits to quantum sensing.

DOI
10.1364/qim.2019.s3c.3

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There has recently been a great deal of interest in the study of massive mechanical oscillators in the quantum regime. This is motivated by the possibility of universal interfaces in future quantum networks, sensing with unprecedented resolution, and fundamental interest in the limits of quantum mechanics. I will describe the recently reported first demonstrations of the measurement of mechanical motion in a negative-mass reference frame, and stabilisation of the entanglement between massive mechanical oscillators [C. F. Ockeloen-Korppi et al., Nature 556, 478-482 (2018)]. I will describe proposals for extending these demonstrations to multipartite entanglement, and a recent analysis of the force sensing possibilities that emerge from combining these technologies. The latter shall be discussed in the context of recently formulated general limits to quantum sensing.
Recent ideas and technological developments have made possible studies of macroscopic objects deep in the quantum regime. In the talk I will discuss the experiment where an entangled Einstein-Podolsky-Rosen (EPR) state has been generated between distant mechanical and atomic oscillators [1]. The concept of an oscillator with an effective negative mass crucial for those experiments will be presented. Such entanglement allows to measure disturbances in the position and the momentum of an oscillator simultaneously, the trait which may seem counterintuitive. Using this entangled state, the trajectory of motion of the oscillator in phase space can be traced with, in principle, unlimited accuracy. Progress towards application of those ideas to gravitational wave detectors [2] following the proposals [3], [4] will be reported.
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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.
It is well established that quantum back action (QBA) can be reduced (or even completely canceled) in composite systems by engineering so-called quantum-mechanics-free subspaces (QMFSs) of commuting variables. An important ingredient in this approach is the negative-mass oscillator, a phenomenon that arises in, e.g., polarized spin ensembles and two-tone-driven optomechanical systems. Entanglement established between a system of interest and an auxiliary negative-mass reference system allows measurements of motion, fields and forces with, in principle, unlimited precision. To date, these principles have been developed theoretically and demonstrated experimentally for a number of composite systems. However, the utility of the concept has been limited by the dominating requirement of close proximity of the resonance frequencies of the system of interest and the negative-mass reference system, and by the need to embed the subsystems in a narrowband cavity, which could be problematic while at the same time achieving good overcoupling. Here we propose a general approach which overcomes these limitations by employing periodic modulation of the driving fields (e.g., two-tone driving) in combination with coherent or measurement-based anti-noise paths. This approach makes it possible to engineer a QMFS of two systems with vastly different spectra and with arbitrary signs of their masses, while dispensing with the need to embed the subsystems in a sideband-resolving cavity. We discuss the advantages of this novel approach for applications such as QBA evasion in gravitational wave detection, force sensing, and entanglement generation between disparate systems.
Quantum theory challenges the view that individual measurement outcomes are predefined and independent of the measurement context. Yet the quantum state itself -- the catalogue of probabilities for all possible measurements -- is usually assumed to be well defined. We argue that this assumption tacitly relies on measurements being performed relative to ideal, infinitely-resourceful reference frames. We show that, when measurements are made relative to non-ideal quantum reference frames, the probabilities themselves become indefinite: even in the limit of arbitrarily large number of runs, the relative frequencies may remain uncertain. The uncertainty is irreducible in a quantum-mechanical sense, as we show by proving a Bell-type theorem for relative frequencies. We further propose a quantum-optical implementation of these relational measurements based on pulsed homodyne detection. Our findings motivate an extension of the notion of the quantum state to regimes constrained by finite resources. We expect them to be especially relevant at the interface between quantum theory and general relativity, where the resources and information available in a bounded region of spacetime are fundamentally limited.