Quantum acoustics with superconducting qubits (1703.00342v1)

Abstract: The ability to engineer and manipulate different types of quantum mechanical objects allows us to take advantage of their unique properties and create useful hybrid technologies. Thus far, complex quantum states and exquisite quantum control have been demonstrated in systems ranging from trapped ions to superconducting resonators. Recently, there have been many efforts to extend these demonstrations to the motion of complex, macroscopic objects. These mechanical objects have important applications as quantum memories or transducers for measuring and connecting different types of quantum systems. In particular, there have been a few experiments that couple motion to nonlinear quantum objects such as superconducting qubits. This opens up the possibility of creating, storing, and manipulating non-Gaussian quantum states in mechanical degrees of freedom. However, before sophisticated quantum control of mechanical motion can be achieved, we must realize systems with long coherence times while maintaining a sufficient interaction strength. These systems should be implemented in a simple and robust manner that allows for increasing complexity and scalability in the future. Here we experimentally demonstrate a high frequency bulk acoustic wave resonator that is strongly coupled to a superconducting qubit using piezoelectric transduction. In contrast to previous experiments with qubit-mechanical systems, our device requires only simple fabrication methods, extends coherence times to many microseconds, and provides controllable access to a multitude of phonon modes. We use this system to demonstrate basic quantum operations on the coupled qubit-phonon system. Straightforward improvements to the current device will allow for advanced protocols analogous to what has been shown in optical and microwave resonators, resulting in a novel resource for implementing hybrid quantum technologies.

• The paper demonstrates strong coherent interactions between transmon qubits and bulk acoustic resonators, evidenced by vacuum Rabi oscillations and a high cooperativity of 260.
• The study uses piezoelectric transduction with an AlN disk and sapphire substrate to achieve coherence times exceeding 10 μs and nearly 98% phononic ground state fidelity.
• The findings pave the way for integrating mechanical quantum systems into quantum computing architectures by enabling scalable phonon-based memory and processing.

Quantum Acoustics with Superconducting Qubits

The paper "Quantum Acoustics with Superconducting Qubits" presents a paper on the interface between phononic and electronic quantum systems, specifically exploring coupling between superconducting qubits and a high-overtone bulk acoustic wave resonator (HBAR). Utilizing piezoelectric transduction, this work demonstrates coherent interaction of qubits with phonons, potentially paving the way for new forms of quantum information processing and metrology.

Experimental Demonstration and Results

The authors experimentally achieve strong coupling between a transmon qubit and a bulk acoustic wave resonator's phonon modes. This is facilitated by employing a piezoelectric disk of AlN, coupled with a transmon fabricated on a sapphire substrate. The paper is significant in that it succeeds in observing coherence times exceeding 10 μs, asserting superiority in interactions when compared to prior qubit-mechanical systems where coherence time constraints were a notable limitation. The cooperativity metric reported is 260, far exceeding that of earlier systems by an order of magnitude, indicating efficient coupling with reduced decoherence.

Several key demonstrations are included:

• Controlling qubit states by tuning interactions with distinct phononic modes, as evident in vacuum Rabi oscillations.
• Quantifying the phononic ground state and substantiating the quantum nature of the system where nearly 98% ground state population was observed post-interaction.
• Evaluating the phonon's $T_1$ and $T_2$ coherence times, registering values of 17 μs and 27 μs respectively in situ.

Theoretical and Practical Implications

The achievement opens new avenues in the domain of quantum electromechanical systems. One of the principal theoretical leaps made here is demonstrating the potential of HBAR as a resource for quantum information storage, with the localization of phonons in a material that is more compact than traditional electromagnetic resonators. Moreover, the work suggests that coherent phonon states, including non-Gaussian states, can be created and manipulated, mirroring achievements in optical and microwave resonators.

The practical implications of this paper lie in its ability to act as a potential blueprint for integrating mechanical quantum devices into existing quantum computing architectures. The transducers can be enhanced making use of curved geometries for better confinement, or replacing materials to minimize dielectric losses, thus broadening the scope of real-world applications.

Future Directions

Several steps are outlined for advancing this research. Increasing the qubit coherence to 100 μs or higher, optimizing the system to enhance coupling to a single phonon mode, and experimenting with different geometries for confinement are critical advancements expected to emerge. Given that phonons can interact with myriad quantum objects, including photons and solid defects, this research also opens up potential developmental pathways for hybrid systems that can perform multi-modal quantum information processing, or act as robust transducers across different frequency regimes.

In conclusion, the paper lays foundational work for utilizing quantum-level mechanical systems within the broader quantum information sciences. It highlights the feasibility and advantages of phonon-based systems for quantum memory and information processing, providing clear avenues for future exploration and development in the field.