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Cavity Optomechanics for Hybrid Quantum Systems


Ren, Hengjiang (2020) Cavity Optomechanics for Hybrid Quantum Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vr67-w986.


Recent advances in optomechanical systems have led to a series of scientific and technical advances. In addition, they have demonstrated macroscopic quantum phenomena, including probabilistic preparation of quantum states, squeezed light, and coherent transduction between photons with different energies. There are advantages in using phonons within a quantum information network. Within the solid state, all optical and electronic phenomena strongly depend on the local distortions of the crystal lattice, i.e. mechanical phonons, hence could connect dissimilar degrees of freedom such as superconducting qubits operating at gigahertz frequencies with atomic/optical states. Also, unlike photons, phonons do not radiate into free space. Energy damping of phonon can occur through radiation into bulk structure which support the mechanical resonator, through impurities and defects in the material, and due to the inherent anharmonic motion of atoms within solid-state materials.

In this thesis, we explore the limits of acoustic damping and coherence of a microwave-frequency acoustic nanocavity with a phononic crystal shield that possesses a wide bandgap for all polarizations of acoustic waves. The nanocavity is formed from an optomechanical crystal (OMC) nanobeam resonator. It supports an acoustic breathing mode at ~ 5 GHz and a co-localized telecom optical resonant mode which allows us to excite and readout mechanical motion using radiation pressure from a pulsed laser source. This minimally invasive pulsed measurement technique avoids a slew of parasitic damping effects - typically associated with electrode materials and mechanical contact, or probe fields for continuous readout - and allows for the sensitive measurement of motion at the single phonon level. The results of acoustic ringdown measurements at millikelvin temperatures show that damping due to radiation is effectively suppressed by the phononic shield, with breathing mode quality factors reaching mechanical quality factor Q = 4.9 x 1010, corresponding to an unprecedented frequency-Q product of f-Q = 2.6 x 1020 and an effective phonon propagation length of several kilometers. Measurement of the frequency jitter of the acoustic resonance is also performed, indicating telegraph-like noise corresponding to a coherence time of ~ 130 µs. The observed breathing mode behavior can be explained by TLS interactions when taking into account the highly modified density of phonon states in the shielded OMC cavity, which are most likely present in the amorphous etch-damaged region of the silicon surface. In particular, we find that damping due to nearly resonant TLS is suppressed due to the bandgap of the phononic shield, and that relaxation damping from non-resonant TLS can explain the magnitude, low temperature dependence of the breathing mode damping, and lack of saturation of the damping with both temperature and acoustic amplitude.

The extremely small motional mass and narrow linewidth of the OMC cavity make it ideal for precision mass sensing and in exploring limits to alternative quantum collapse models.

Our mechanical modes exist in the same frequency range as common superconducting qubits, suggesting a possibility for creating a hybrid quantum architecture consisting of acoustic and superconducting quantum circuits, where the small scale, reduced cross-talk, and ultralong coherence time of quantum acoustic devices may provide significant improvements in connectivity and performance of current quantum hardware. A proposal of mechanical quantum memory based on ultra-high-Q mechanical model and piezo-electrical coupling is also discussed in this work. One remaining roadblock, which significantly compromises the utility of OMCs integration with superconducting circuits, is the very weak, yet non-negligible parasitic optical absorption, which is thought to occur due to surface defect states, and together with inefficient thermalization can yield significant heating of the hypersonic mechanical mode of the device at ultralow temperatures, where microwave systems can be reliably operated as quantum devices. In 1D OMC experiments, the quantum cooperativity (Ceff), which corresponds to the standard photon-phonon cooperativity divided by the Bose factor of the thermal bath and is the most relevant figure-of-merit for operation of optomechanical systems at ultralow temperatures, was lower than unity for all but a microsecond around the time an optical pulse is applied. This limits quantum optomechanical experiments to schemes with short pulses. Increased Ceff can be achieved with improved thermalization, for example, by employing a two-dimensional (2D) OMC cavity.

In this thesis, we demonstrate an improved silicon quasi-2D OMC with an over 50-fold improvement in back-action per photon over previous reports. We are able to measure the dynamics of the internal cavity acoustic modes of both 1D nanobeam and quasi-2D OMCs. Quasi-2D OMC shows much lower bath occupancy compared to 1D structures. Most importantly, quasi-2D OMCs demonstrated a Ceff greater than unity under steady-state optical pumping, a crucial threshold for realizing a variety of optomechanical applications. For example, bi-directional transduction or amplification of continuous quantum signals require the optomechanical device to be operated in a continuous mode. An analysis of piezo-optomechanical bi-directional microwave to optics transducer is also presented in this thesis.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Photonics, Quantum Device, Quantum Measurement, Quantum Acoustics, Optomechanics.
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Electrical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Painter, Oskar J.
Group:Institute for Quantum Information and Matter, Kavli Nanoscience Institute
Thesis Committee:
  • Faraon, Andrei (chair)
  • Marandi, Alireza
  • Painter, Oskar J.
  • Wang, Lihong
Defense Date:16 January 2020
Record Number:CaltechTHESIS:06082020-144243454
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapters 2, 6-7. adapted for Chapters 4-5.
Ren, Hengjiang0000-0002-5612-8287
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13798
Deposited By: Jared Ren
Deposited On:24 Jan 2023 00:48
Last Modified:24 Jan 2023 00:48

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