The ability to access a broad range of quantum states with mechanical oscillators has many applications and is an enduring ambition in the fields of opto- and electromechanics. However, as mechanical oscillators are linear at the quantum scale, arbitrary quantum control over them requires an extrinsic nonlinearity. In this thesis, I aim to provide this extrinsic nonlinearity and establish quantum control over the motion of a suspended aluminum disk by coupling it strongly to a superconducting qubit. To this end, I design and fabricate a device, when operated at its maximum coupling strength, could enable phonon-number-resolved measurements and arbitrary quantum control over the mechanical motion. However, limited by unclear reasons that break the qubit readout, I operate at approximately a quarter of the maximum coupling strength. Nevertheless, at this smaller coupling strength, I demonstrate the preparation of a non-Gaussian nonclassical state of motion.Because of the large coupling achieved in this work, the sideband transitions are phonon-number-sensitive, selectively altering the phonon populations in only a few Fock states. Using these phonon-number-sensitive sideband transitions, I dissipatively stabilize the mechanical oscillator into a highly-energized sub-Poissonian state, where its energy fluctuations are below the classical limit. This result represents a major step toward the long-time ambition of accessing a broad range of nonclassical states with macroscopic mechanical oscillators. Moreover, requiring neither number-resolution nor coherent control of the qubit, the dissipative stabilization technique demonstrated in this work also provides an accessible path toward preparing nonclassical states in other harmonic oscillators coupled to qubits.
We propose and analyze a quantum electromechanical system composed of a monolithic quartz bulk acoustic wave oscillator coupled to a superconducting transmon qubit via an intermediate
LC electrical circuit. Monolithic quartz oscillators offer unprecedentedly high effective masses and quality factors for the investigation of mechanical oscillators in the quantum regime. Ground-state cooling of such mechanical modes via resonant piezoelectric coupling to an
LC circuit, which is itself sideband cooled via coupling to a transmon qubit, is shown to be feasible. The fluorescence spectrum of the qubit, containing motional sideband contributions due to the couplings to the oscillator modes, is obtained and the imprint of the electromechanical steady state on the spectrum is determined. This allows the qubit to function both as a cooling resource for, and transducer of, the mechanical oscillator. The results described are relevant to any hybrid quantum system composed of a qubit coupled to two (coupled or uncoupled) thermal oscillator modes.
We use the strong intrinsic nonlinearity of a microwave superconducting qubit with a 4 GHz transition frequency to directly detect and control the energy of a micromechanical oscillator vibrating at 25 MHz. The qubit and the oscillator are coupled electrostatically at a rate of approximately 2π×22 MHz. In this far off-resonant regime, the qubit frequency is shifted by 0.52 MHz per oscillator phonon, or about 14% of the 3.7 MHz qubit linewidth. The qubit behaves as a vibrational energy detector and from its line shape we extract the phonon number distribution of the oscillator. We manipulate this distribution by driving number state sensitive sideband transitions and creating profoundly nonthermal states. Finally, by driving the lower frequency sideband transition, we cool the oscillator and increase its ground state population up to 0.48±0.13, close to a factor of 8 above its value at thermal equilibrium. These results demonstrate a new class of electromechanics experiments that are a promising strategy for quantum nondemolition measurements and nonclassical state preparation.