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Interfacing Levitated Optomechanics with Superconducting Qubits

Periodic Reporting for period 1 - ELOTEQ (Interfacing Levitated Optomechanics with Superconducting Qubits)

Reporting period: 2019-05-01 to 2020-04-30

The universal validity of the quantum superposition principle is an open fundamental question with far reaching implications for science and technology. Testing and exploiting quantum physics with objects of ever-increasing mass and complexity requires extremely low motional temperatures and superior environmental isolation to avoid decoherence. One of the most promising platforms for the next generation of quantum superposition tests is provided by levitating nanoscale dielectric particles in ultra-high vacuum and controlling their motion with lasers. However, such levitated optomechanical systems are limited by photon scattering decoherence due to the optical trap. The project ELOTEQ developed novel schemes to perform trapped quantum superposition tests in an all-electrical setup, coherently interfacing a charged nanoparticle with superconducting qubits, via which the rotational and translational quantum state can be controlled, interfered, and monitored.
Within its one year of activity, ELOTEQ has achieved all its scientific objectives and made significant progress beyond its goals. The research performed within ELOTEQ can be grouped in two categories: (1) devising and developing the theory required to perform all-electric quantum interference schemes with nano- to microscale objects, and (2) developing schemes for optically trapped quantum experiments with nanoscale particles.

(1) ELOTEQ showed how highly charged dielectric nanoparticles can be coherently interfaced with superconducting circuitry. I demonstrated that the rotational and translational motion of charged nanorotors suspended in a Paul trap induces a current in the endcap electrodes, which couples to the quantum state of an attached superconducting Cooper pair box. An ultra-fast protocol of qubit rotations and measurements enables the preparation and read-out of nanoparticle quantum superposition states on nanosecond timescales and in the presence of ambient decoherence channels.

ELOTEQ also developed the theory of how arbitrarily charged nanoparticles in a Paul trap interact with homogeneous magnetic fields. The rotational motion of a charged particle induces a magnetic dipole moment, which tends to align the particle rotation axis with the external field. We showed that time-dependent magnetic fields can induce mechanical rotation and we currently work on proposing an experiment to observe and exploit this effect. Moreover, we proposed and analysed an experiment carried out in the group of James Millen (King’s College London), which tests non-equilibrium thermodynamics with charged particles.

(2) In addition, ELOTEQ developed schemes to perform orbital angular momentum interference with Bose-Einstein condensates in torus traps, a protocol to entangle two tweezer-levitated nanoparticles via coherent scattering, and the theoretical framework to observe and exploit quantum persistent tennis-racket oscillations. We also demonstrated how elliptic coherent scattering enables simultaneous rotational and translational groundstate cooling of aspherical nanoparticles, and, I provided theory support for the first experimental observation of Bragg diffraction of molecules.

The work of ELOTEQ has been disseminated in 13 talks at scientific conferences and invited seminars, two public outreach talks, and two public science events at Imperial College London. Two manuscripts have been published recently in peer-reviewed journals, one manuscript has been accepted in Physical Review Letters and selected as Editor’s Suggestion, further three works are currently under review, and several are in preparation.
ELOTEQ significantly advanced the state of the art in the theory of macroscopic quantum physics. Its results open the door for all-electrical quantum technologies with levitated nano- to microscale objects, holding great potential for fundamental experiments and scalable sensing applications. The theoretical framework of how electrically charged nanoparticles of arbitrary shape interact with external electric and magnetic fields will be instrumental for ongoing cutting-edge experiments. The developed schemes for 6D nanoparticle cooling, entanglement generation, observing the tennis-racket flips, and angular momentum BEC interference will pave the way for experiments observing and exploiting novel aspects of quantum physics with macroscopic systems. The results of ELOTEQ initialized novel research avenues which I continue to pursue with international collaborators. In summary, the results of ELOTEQ will significantly contribute to testing quantum physics in an unprecedented mass and complexity regime, with the potential of enabling future quantum technology.
Nanoparticle in Paul trap, interfaced with a Cooper pair box.