Parametric Amplification and Back-Action Noise Squeezing by a Qubit-Coupled Nanoresonator

We demonstrate the parametric amplification and noise squeezing of nanomechanical motion utilizing dispersive coupling to a Cooper-pair box qubit. By modulating the qubit bias and resulting mechanical resonance shift, we achieve gain of 30 dB and noise squeezing of 4 dB. This qubit-mediated effect is 3000 times more effective than that resulting from the weak nonlinearity of capacitance to a nearby electrode. This technique may be used to prepare nanomechanical squeezed states

The Radio-Frequency Single-Electron Transistor Displacement Detector

For more than two decades, the standard quantum limit (SQL) has served as a benchmark for researchers involved in ultra-sensitive force and displacement detection. In this thesis, I discuss a novel displacement detection technique which we have implemented that has allowed us to come within a factor of 4.3 from the limit, closer than any previous effort. Additionally, I show that we were able to use this nearly quantum-limited scheme to observe the thermal motion of a 19.7 MHz in-plane mode of a nanomechanical resonator down to a temperature of 56 mK. At this temperature, the corresponding thermal occupation number of the mode was <nth> ~ 60. This is the lowest thermal occupation number ever demonstrated for a nanomechanical (or larger) device. We believe that the combination of these two results has important and promising implications for the future study of nanoelectromechanical systems (NEMS) at the quantum limit. The detection scheme that we used was based upon the single-electron transistor (SET). The SET has been demonstrated to be the world's most sensitive electrometer and is considered to be a near-ideal linear amplifier. We used standard lithographic techniques for the on-chip integration of the SET with both a microwave-matching network and nanomechanical resonator. The SET served as a transducer of the resonator's motion: fluctuations in the resonator's position modulated the SET impedance. The microwave-matching circuit allowed us to read-out the modulation of the SET's impedance with ~ 75 MHz bandwidth. The combination of microwave-matching circuit and SET is known as the radio-frequency single-electron transistor (RFSET). Including the nanomechanical resonator, the configuration is called the radio-frequency single-electron transistor displacement detector. In this thesis, I discuss the basics of quantum-limited measurement and some of the subtleties of observing mechanical quantum phenomena. I then discuss the basics of the RFSET displacement detector, its ultimate limits, its engineering and operation, the first generation results, and finally what improvements could be made to future generation devices

Modular tunable coupler for superconducting qubits

The development of modular and versatile quantum interconnect hardware is a key next step in the scaling of quantum information platforms to larger size and greater functionality. For superconducting quantum systems, fast and well-controlled tunable circuit couplers will be paramount for achieving high fidelity and resource efficient connectivity, whether for performing two-qubit gate operations, encoding or decoding a quantum data bus, or interfacing across modalities. Here we propose a versatile and internally-tunable double-transmon coupler (DTC) architecture that implements tunable coupling via flux-controlled interference in a three-junction dcSQUID. Crucially, the DTC possesses an internally defined zero-coupling state that is independent of the coupled data qubits or circuit resonators. This makes it particular attractive as a modular and versatile design element for realizing fast and robust linear coupling in several applications such as high-fidelity two-qubit gate operations, qubit readout, and quantum bus interfacing

Principles for Optimizing Quantum Transduction in Piezo-Optomechanical Systems

Two-way microwave-optical quantum transduction is an essential capability to connect distant superconducting qubits via optical fiber, and to enable quantum networking at a large scale. In Bl\'esin, Tian, Bhave, and Kippenberg's article, ``Quantum coherent microwave-optical transduction using high overtone bulk acoustic resonances" (Phys. Rev. A, 104, 052601 (2021)), they lay out a quantum transduction system that accomplishes this by combining a piezoelectric interaction to convert microwave photons to GHz-scale phonons, and an optomechanical interaction to up-convert those phonons into telecom-band photons using a pump laser set to an adjacent telecom-band tone. In this work, we discuss these coupling interactions from first principles in order to discover what device parameters matter most in determining the transduction efficiency of this new platform, and to discuss strategies toward system optimization for near-unity transduction efficiency, as well as how noise impacts the transduction process. In addition, we address the post-transduction challenge of separating single photons of the transduced light from a classically bright pump only a few GHz away in frequency by proposing a novel optomechanical coupling mechanism using phonon-photon four-wave mixing via stress-induced optical nonlinearity and its thermodynamic connection to higher-orders of electrostriction. Where this process drives transduction by consuming pairs instead of individual pump photons, it will allow a clean separation of the transduced light from the classically bright pump driving the transduction process.Comment: 30 pages (23 pages main + 7 pages appendices), 9 figures, 4 tables (additions include: empirical formula for second-order photoelasticity for side-by-side comparison with ordinary optomechanical coupling; a discussion on concatenated filter transmission spectra; and a tangent on torsion waves in optomechanic

Towards strong multi-mode coupling between a transmon and a metamaterial resonator

A high density of modes can be produced using metamaterial resonant structures made from arrays of lumped circuit elements, to which a flux-tunable transmon qubit can be coupled. For such a system, we have measured the coupling strength of the qubit to multiple modes by tuning the flux and observing the splitting in the transmission of each mode. In these initial measurements, the coupling strengths were larger than the individual mode linewidths, but did not exceed the inter-mode spacing. We discuss approaches to decrease the mode spacing in this system and simulate the spectrum numerically. In addition, we discuss techniques for increasing the coupling strength between these modes and the transmon qubit. For appropriate parameters, we show that it will be to possible to reach the regime where the qubit can be coupled to multiple modes simultaneously, which will have applications in analog quantum simulations and multi-mode cQED

Digital Quantum Simulations of Spin Models on Hybrid Platform and Near-Term Quantum Processors

We review a recent theoretical proposal for a universal quantum computing platform based on tunable nonlinear electromechanical nano-oscillators, in which qubits are encoded in the anharmonic vibrational modes of mechanical resonators coupled to a superconducting circuitry. The digital quantum simulation of spin-type model Hamiltonians, such as the Ising model in a transverse field, could be performed with very high fidelities on such a prospective platform. Here we challenge our proposed simulator with the actual IBM-Q quantum processor available on cloud. We show that such state-of-art implementation of a quantum computer, based on transmon qubits and superconducting technology, is able to perform digital quantum simulations. However, encoding the qubits in mechanical degrees of freedom would allow to outperform the current implementations in terms of fidelity and scalability of the quantum simulation.Comment: Proceedings of the 2018 Italian Quantum Information Science (IQIS18) conference, to appear in Proceedings MDP