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arXiv:1803.01778 · New Journal of Physics

Probing macroscopic quantum superpositions with nanorotors

Benjamin A. Stickler, Birthe Papendell, Stefan Kühn, Björn Schrinski, James Millen, Markus Arndt, Klaus Hornberger

Published Oct 31, 2018• 106 citations
arXivHTMLPDF

Abstract

Whether quantum physics is universally valid is an open question with far-reaching implications. Intense research is therefore invested into testing the quantum superposition principle with ever heavier and more complex objects. Here we propose a radically new, experimentally viable route towards studies at the quantum-to-classical borderline by probing the orientational quantum revivals of a nanoscale rigid rotor. The proposed interference experiment testifies a macroscopic superposition of all possible orientations. It requires no diffraction grating, uses only a single levitated particle, and works with moderate motional temperatures under realistic environmental conditions. The first exploitation of quantum rotations of a massive object opens the door to new tests of quantum physics with submicron particles and to quantum gyroscopic torque sensors, holding the potential to improve state-of-the-art devices by many orders of magnitude.

DOI
10.1088/1367-2630/aaece4

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Whether quantum physics is universally valid is an open question with far-reaching implications. Intense research is therefore invested into testing the quantum superposition principle with ever heavier and more complex objects. Here we propose a radically new, experimentally viable route towards studies at the quantum-to-classical borderline by probing the orientational quantum revivals of a nanoscale rigid rotor. The proposed interference experiment testifies a macroscopic superposition of all possible orientations. It requires no diffraction grating, uses only a single levitated particle, and works with moderate motional temperatures under realistic environmental conditions. The first exploitation of quantum rotations of a massive object opens the door to new tests of quantum physics with submicron particles and to quantum gyroscopic torque sensors, holding the potential to improve state-of-the-art devices by many orders of magnitude.
We propose an experimentally viable scheme to probe orientational quantum revivals with nanoscale particles, an interference effect of the rotational degrees of freedom testifying a superposition of all orientations. Our simulations demonstrate that such revivals are observable with current technology, even in the presence of environmental decoherence, using carbon nanotubes or silicon nanorods with a length of 50 nm. This opens the door to tests of rotational quantum physics with submicron objects and to quantum gyroscopic torque sensors.
Abstract The quantum superposition principle is a fundamental concept of physics 1 and the basis of numerous quantum technologies 2,3 . Yet, it is still often regarded counterintuitive because we do not observe its key features on the macroscopic scales of our daily lives. It is, therefore, interesting to ask how quantum properties persist or change as we increase the size and complexity of objects 4 . A model test for this question can be realized by matter-wave interferometry, in which the motion of individual massive particles becomes delocalized and needs to be described by a wave function that spans regions far larger than the particle itself 5 . Over the years, this has been explored with a series of objects of increasing mass and complexity 6–9 and a growing community aims at pushing this to ever larger limits. Here we present an experimental platform that extends matter-wave interference to large metal clusters, a qualitatively new material class for quantum experiments. We specifically demonstrate quantum interference of sodium nanoparticles, which can each contain more than 7,000 atoms at masses greater than 170,000 Da. They propagate in a Schrödinger cat state with a macroscopicity 10 of μ = 15.5, surpassing previous experiments 5,9,11 by an order of magnitude.
In this thesis I present a universal scheme to assess the macroscopicity of experimentally fabricated quantum superposition states based on the degree to which they probe the boundaries of quantum mechanics. The measure relies on the falsification of the weakest macrorealist modifications of quantum mechanics; it treats all experiments in an equal manner, regardless the actual form and size of the empirical data, using an uninformative approach to Bayesian inference. The versatility of this scheme is illustrated by applying it on state-of-the-art quantum tests where the study of minimal modifications of quantum mechanics often coincides with the examination of conventional decoherence effects. Finally, the environmental impact on orientational degrees of freedom, leading to diffusion, friction, and thermalization, are described analytically. Together with the formulation of minimal modifications of quantum mechanics applied to orientational degrees of freedom this paves the way for a thorough description of experiments with levitated nanorotors and the assessment of the respective macroscopicities.
The intrinsically nonlinear rotation dynamics of rigid bodies offer unprecedented ways to exploit their quantum motion. In this Letter, we devise a rotational analog of Mach-Zehnder interferometry, which allows steering symmetric rotors from fully aligned to completely antialigned. The scheme uses a superposition of four distinct orientations, emerging at the eighth of the quantum revival time, whose interference can be controlled by a weak laser pulse. We develop a semiclassical model of the effect and demonstrate that it persists even in presence of imperfections and decoherence.
The probing of the quantum nature of a macroscopic object is an open problem. Approaches in preparing quantum superposition towards a larger scale, which has been progressively accessible in a variety of experimental implementations, may detect new physics or extend the boundary of quantum theory. Development on a nano-mechanical system in the framework of quantum optics and information theory has opened new research directions. It paves ways to manipulate the wavefunction of a massive particle over a large spatial range and probe its quantum behavior to an unprecedented precision. In this thesis, quantum superposition on a mechanical system is studied theoretically in the light of possible implementations under current techniques in order to test superposition principle and the quantum nature of gravity in the mesoscopic region. The work includes two lines of study. In the first line, the motion of the center of mass (c.m.) of the mechanical system is coupled to its internal spin system magnetically, and a Ramsey scheme is developed based on coherent spin control. The wavepacket of the test object, under a spin-dependent force, may then be delocalized to a macroscopic scale. A gravity induced dynamical phase (accrued solely on the spin state, and measured through a Ramsey scheme) is used to reveal the above spatially delocalized superposition of the spin-nano-object composite system that arises during the scheme. A remarkable immunity to the motional noise in the c.m. (initially in a thermal state with moderate cooling) and also a dynamical decoupling nature of the scheme itself is revealed. A careful examination of the perturbation effect due to setup imperfection and environment-induced decoherence is performed, which shows that the Ramsey fringes have a high tolerance on those unwanted faults under realistic experimental conditions. The scheme also facilitates a gravimetry protocol that potentially could be developed for a novel on-chip gravimeter with a precision of 10−6g/Hz10^{-6}g/\sqrt{\text{Hz}}10−6g/Hz​. In the second line, an opto-mechanical experiment is proposed to entangle the motion of two mechanical oscillators through their mutual gravitational interaction. The feasibility of such an experiment is critically examined within the framework of cavity optomechanics. It is shown that within the decoherence time of mechanical noise and potentially gravitational induced state collapse, entanglement between mirrors could be obtained when the initial state of the cavity field is prepared in some particular states. Strategies that give enhancement to entanglement generation rate are studied in light of quantum estimation theory. It is shown that for a cavity initially in a coherent state, the resulting entanglement generation rate would be enhanced linearly with the amplitude of the coherent state. Lastly, it shows that in the proposed setup Casimir effect would significantly affect the gravity-induced entanglement. A proper shield can eliminate the detrimental effect of Casimir force, which, however, will constrain the closest distance between the mirrors in its implementation.
Quantum theory is incredibly successful, explaining the microscopic world with great accuracy, from the behaviour of subatomic particles to chemical reactions to solid-state electronics. There is not a single experimental finding challenging its predictions, and ever more quantum phenomena are exploited in technology, including interferometric sensing and quantum cryptography. In order to explore novel applications and test the validity of quantum physics at the macroscale, researchers strive to prepare ever heavier and bigger objects in quantum superpositions. Experiments with levitated microscale particles are about to push this quest into uncharted waters.