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arXiv:2006.05446 · Physical Review Applied

Sideband Control of a Multimode Quantum Bulk Acoustic System

Mikael Kervinen, Alpo Välimaa, Jhon E. Ramírez-Muñoz, Mika E. T. Sillanpää

Published Nov 11, 2020• 12 citations
arXivHTMLPDF

Abstract

Multimode bulk acoustic systems show promise for use in superconducting quantum computation. They can serve as a medium-term memory storage, with exceptional coherence times being demonstrated, and they exhibit a mode density that is physically highly compact. Here, we experimentally demonstrate accessing individual acoustic modes without being hindered by the uniform frequency spacing of the modes. We use sideband control where a low-frequency modulation is applied to the transmon qubit energy. The amplitude of the modulation defines the qubit-acoustic mode coupling and its frequency detunes the acoustic sidebands, therefore selectively enabling the switching on or off of the interaction, and it allows for full control of the individual modes.

DOI
10.1103/physrevapplied.14.054023

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Multimode bulk acoustic systems show promise for use in superconducting quantum computation. They can serve as a medium-term memory storage, with exceptional coherence times being demonstrated, and they exhibit a mode density that is physically highly compact. Here, we experimentally demonstrate accessing individual acoustic modes without being hindered by the uniform frequency spacing of the modes. We use sideband control where a low-frequency modulation is applied to the transmon qubit energy. The amplitude of the modulation defines the qubit-acoustic mode coupling and its frequency detunes the acoustic sidebands, therefore selectively enabling the switching on or off of the interaction, and it allows for full control of the individual modes.
Hybrid quantum systems in which acoustic resonators couple to superconducting qubits are promising quantum information platforms. High quality factors and small mode volumes make acoustic modes ideal quantum memories, while the qubit-phonon coupling enables the initialization and manipulation of quantum states. We present a scheme for quantum computing with multimode quantum acoustic systems, and based on this scheme, propose a hardware-efficient implementation of a quantum random access memory (QRAM). Quantum information is stored in high-Q phonon modes, and couplings between modes are engineered by applying off-resonant drives to a transmon qubit. In comparison to existing proposals that involve directly exciting the qubit, this scheme can offer a substantial improvement in gate fidelity for long-lived acoustic modes. We show how these engineered phonon-phonon couplings can be used to access data in superposition according to the state of designated address modes-implementing a QRAM on a single chip.
Mechanical objects have important practical applications in the fields of quantum information and metrology as quantum memories or transducers for measuring and connecting different types of quantum systems. The field of electromechanics is in pursuit of a robust and highly coherent device that couples motion to nonlinear quantum objects such as superconducting qubits. Here, we experimentally demonstrate a high-frequency bulk acoustic wave resonator that is strongly coupled to a superconducting qubit using piezoelectric transduction with a cooperativity of 260. We measure qubit and mechanical coherence times on the order of 10 microseconds. Our device requires only simple fabrication methods and provides controllable access to a multitude of phonon modes. We demonstrate quantum control and measurement on gigahertz phonons at the single-quantum level.
Hybrid mechanical-superconducting systems for quantum information processing have attracted significant attention due to their potential applications. In such systems, the weak coupling regime, dominated by dissipation, has been extensively studied. The strong coupling regime, where coherent energy exchange exceeds losses, has also been widely explored. However, the transition-coupling regime, which lies between the above two and exhibits rich, unique physics, remains underexplored. In this study, we fabricate a tunable coupling device to investigate the coupling of a superconducting transmon qubit to a seven-mode surface acoustic-wave resonator (SAWR), with a particular focus on the transition-coupling regime. Through a series of phonon oscillation experiments and studies in the dispersive regime, we systematically characterize the performance of the SAWR. We then explore the complex dynamics of energy exchange between the qubit and the mechanical modes, highlighting the interplay between dissipation and coherence. Finally, we propose a protocol for qubit readout and fast reset with a multimode mechanical cavity using one mode for readout and another mode for reset. We have demonstrated in simulation that the qubit achieves both fast reset and high coherence performance when the qubit is coupled to the reset mode in the transition-coupling regime.
Qubit connectivity is an important property of a quantum processor, with an ideal processor having random access-the ability of arbitrary qubit pairs to interact directly. This a challenge with superconducting circuits, as state-of-the-art architectures rely on only nearest-neighbor coupling. Here, we implement a random access superconducting quantum information processor, demonstrating universal operations on a nine-qubit memory, with a Josephson junction transmon circuit serving as the central processor. The quantum memory uses the eigenmodes of a linear array of coupled superconducting resonators. We selectively stimulate vacuum Rabi oscillations between the transmon and individual eigenmodes through parametric flux modulation of the transmon frequency. Utilizing these oscillations, we perform a universal set of quantum gates on 38 arbitrary pairs of modes and prepare multimode entangled states, all using only two control lines. We thus achieve hardware-efficient random access multi-qubit control in an architecture compatible with long-lived microwave cavity-based quantum memories.
Studies of micromechanical and acoustic modes in the quantum regime have shed light on quantum properties of massive objects. Integrating these systems into superconducting circuits shows great promise for applications as quantum memory elements, bosonic codes, or in frequency conversion. To this end, investigation of nonclassical properties of acoustic degrees of freedom is critical also for applications. Here, we investigate a strongly driven system consisting of a transmon qubit interacting with a high-overtone bulk acoustic resonator. We observe multiphonon transitions, which enable mapping the energy landscape in the coupled system. At a high driving amplitude comparable to the qubit-oscillator coupling, we observe a shift of the multiphonon spectral lines, reminiscent of Stark shift, which is explained using a simple model. Our work thus also provides insight in multiquanta transitions in other qubit-oscillator systems, not limited to acoustics or circuit quantum electrodynamics.