June 2020

Abstracts of the QSIT Lunch Seminar, Thursday, June 4, 2020

Scheduled Zoom meeting

A high-cooperativity, silicon nitride transducer for room temperature quantum optomechanics

Mohammad Bereyhi – Laboratory of Photonics and Quantum Measurements (Kippenberg group), EPF Lausanne

Mechanical resonators are a canonical system for the exploration of quantum measurements. Milestones such as cooling to the ground state, generation of squeezed light and remote entanglement of mechanical resonators have been demonstrated, but so far typically at cryogenic temperature. Room temperature operation would allow these effects to be observed in simplified experimental setups and potentially enable new applications. However, at room temperature, the thermal noise of a mechanical resonator typically dominates the quantum backaction (QBA) of its position measurement and prohibits entering the quantum regime of optomechanics [2], [3]. Therefore, reduced mechanical dissipation is required to isolate the system from the environment [4].

Integrated optomechanical systems where the mechanical resonator is suspended in the near-field enable large coupling due to the strong optical field gradient of the evanescent field. Simultaneously, new designs of silicon nitride nanobeam mechanical resonators have attained exceptionally low mechanical dissipation at room temperature via elastic strain engineering and clamp-tapering [5], [6]. However, these designs require high aspect ratios, complicating their near-field integration, where the mechanical resonator must be suspended only hundreds of nanometers away from the optical microcavity. Here, we present a platform that solves this problem: a nano-optomechanical transducer using high stress silicon nitride that features a one-dimensional Fabry-Pérot optical cavity integrated with a high aspect ratio nanobeam mechanical resonator. Our approach allows individual optimization of the optical and the mechanical resonator, while maintaining a high optomechanical coupling rate due to large optomechanical mode overlap. This system provides a platform for observation of room temperature quantum backaction on macroscopic mechanical resonators owing to its high single photon cooperativity (C0 = 23).

[1] M. Aspelmeyer, T. J. Kippenberg, and F. Marquardt, “Cavity optomechanics,” Rev. Mod. Phys., vol. 86, no. 4, pp. 1391–1452, Dec. 2014, doi: 10.1103/RevModPhys.86.1391.
[2] T. P. Purdy, R. W. Peterson, and C. A. Regal, “Observation of Radiation Pressure Shot Noise on a Macroscopic Object,” Science, vol. 339, no. 6121, pp. 801–804, Feb. 2013, doi: 10.1126/science.1231282.
[3] V. Sudhir, R. Schilling, S. A. Fedorov, H. Schütz, D. J. Wilson, and T. J. Kippenberg, “Quantum Correlations of Light from a Room-Temperature Mechanical Oscillator,” Phys. Rev. X, vol. 7, no. 3, p. 031055, Sep. 2017, doi: 10.1103/PhysRevX.7.031055.
[4] D. J. Wilson, V. Sudhir, N. Piro, R. Schilling, A. Ghadimi, and T. J. Kippenberg, “Measurement-based control of a mechanical oscillator at its thermal decoherence rate,” Nature, vol. 524, pp. 325--329, Aug. 2015, doi: 10.1038/nature14672.
[5] A. H. Ghadimi et al., “Elastic strain engineering for ultralow mechanical dissipation,” Science, p. eaar6939, Apr. 2018, doi: 10.1126/science.aar6939.
[6] Mohammad. J. Bereyhi et al., “Clamp-Tapering Increases the Quality Factor of Stressed Nanobeams,” Nano Lett., vol. 19, no. 4, pp. 2329–2333, Apr. 2019, doi: 10.1021/acs.nanolett.8b04942.
 

Critical current for an insulating regime of an underdamped current-​biased topological Josephson junction as a signature of Majorana fermions

Aleksandr Svetogorov – Condensed Matter Theory and Quantum Computing (Klinovaja group), University of Basel

We study analytically an underdamped current-biased topological Josephson junction. A trivial Josephson junction, shunted by a huge impedance is in effectively insulating state, when the bias current is smaller than some critical current I_c. If the junction is topological, two Majorana fermions on the junction form an effctive fermionic state, which allows a single-quasiparticle tunneling through the junction. This results in suppression of 2pi phase slips, which significantly decreases I_c. However, if the system is not sufficiently long, Majorana fermions on the junction interact with the Majorana fermions on the outer edges of the system, which could restore 2pi phase slips and, therefore, signifficantly increase I_c. The hybridisation delta oscillates around zero as a function of the applied magnetic field. Therefore, experimentally it should be possible to establish the appearance of Majorana fermions on the junction by measuring I_c at different values of the applied magnetic field. The higly non-monotonic dependence should indicate the topological regime.