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.
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.
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.
Owing to their strong dipole moment and long coherence times, superconducting qubits have demonstrated remarkable success in hybrid quantum circuits. However, most qubit architectures are limited to the GHz frequency range, severely constraining the class of systems they can interact with. The fluxonium qubit, on the other hand, can be biased to very low frequency while being manipulated and read out with standard microwave techniques. Here, we design and operate a heavy fluxonium with an unprecedentedly low transition frequency of 1.8 MHz. We demonstrate resolved sideband cooling of the “hot” qubit transition with a final ground state population of 97.7%, corresponding to an effective temperature of 23 μ K . We further demonstrate coherent manipulation with coherence times T 1 = 34 μ s , T 2 * = 39 μ s , and single-shot readout of the qubit state. Importantly, by directly addressing the qubit transition with a capacitively coupled waveguide, we showcase its high sensitivity to a radio-frequency field. Through cyclic qubit preparation and interrogation, we transform this low-frequency fluxonium qubit into a frequency-resolved charge sensor. This method results in a charge sensitivity of 33 μ e / Hz , or an energy sensitivity (in joules per hertz) of 2.8 ℏ . This method rivals state-of-the-art transport-based devices, while maintaining inherent insensitivity to dc-charge noise. The high charge sensitivity combined with large capacitive shunt unlocks new avenues for exploring quantum phenomena in the 1–10 MHz range, such as the strong-coupling regime with a resonant macroscopic mechanical resonator. Published by the American Physical Society 2024
Phonon modes at microwave frequencies can be cooled to their quantum ground state using conventional cryogenic refrigeration, providing a convenient way to study and manipulate quantum states at the single phonon level. Phonons are of particular interest because mechanical deformations can mediate interactions with a wide range of different quantum systems, including solid-state defects, superconducting qubits, and optical photons when using optomechanically active constructs. Phonons, thus, hold promise for quantum-focused applications as diverse as sensing, information processing, and communication. Here, we describe a piezoelectric quantum bulk acoustic resonator (QBAR) with a 4.88 GHz resonant frequency, which, at cryogenic temperatures, displays large electromechanical coupling strength combined with a high intrinsic mechanical quality factor, Qi≈4.3×104. Using a recently developed flip-chip technique, we couple this QBAR resonator to a superconducting qubit on a separate die and demonstrate the quantum control of the mechanics in the coupled system. This approach promises a facile and flexible experimental approach to quantum acoustics and hybrid quantum systems.
Observing quantum phenomena in macroscopic objects, and the potential discovery of a fundamental limit in the applicability of quantum mechanics, has been a central topic of modern experimental physics. Highly coherent and heavy micromechanical oscillators controlled by superconducting circuits are a promising system for this task. Here we focus in particular on the electrostatic coupling of motion to a weakly anharmonic circuit, namely, the transmon qubit. In the case of a megahertz mechanical oscillator coupled to a gigahertz transmon, we explain the difficulties in bridging the large electromechanical frequency gap. To remedy this issue, we explore the requirements to reach phonon-number resolution in the resonant coupling of a megahertz transmon and a mechanical oscillator.
We demonstrate strong dispersive coupling between a fluxonium superconducting qubit and a 690 megahertz mechanical oscillator, extending the reach of circuit quantum acousto-dynamics (cQAD) experiments into a new range of frequencies.We have engineered a qubit-phonon coupling rate of g ≈ 2π × 14 MHz, and achieved a dispersive interaction that exceeds the decoherence rates of both systems while the qubit and mechanics are highly nonresonant ( /g 10).Leveraging this strong coupling, we perform phonon-number-resolved measurements of the mechanical resonator and investigate its dissipation and dephasing properties.Our results demonstrate the potential for fluxonium-based hybrid quantum systems, and a path for developing new quantum sensing and information processing schemes with phonons at frequencies below 700 MHz to significantly expand the toolbox of cQAD.
We present experiments aimed at the creation and study of non-Gaussian states of a mesoscopic mechanical oscillator. On the mechanical front, we have developped, fabricated and operated phononic-crystal SiN membranes, with mechanical quality factors in the 10^8 to 10^9 range. To allow resonant coupling of the membrane (typically in the MHz band) to a two-level system, we have developped a heavy fluxonium superconducting qubit architecture, with a transition frequeny as low as 1.8 MHz.. Design, fabrication and operation of the qubit in a 10 mK environment are discussed, together with a charge sensing experiment The long-term goal of these experiments is to create and explore non-Gaussian states of an electromechanical membrane.