Scientists from Russia and Britain have exhibited an artificial quantum system where a quantum bit interacts with an acoustic resonator in the quantum regime. This enables the well-known effects of quantum optics to be investigated on acoustic waves and facilitates an alternative strategy to quantum computer design, which is based on acoustics and could render quantum computers highly compact and more stable. The paper describing the outcomes of the study has been published in the Physical Review Letters journal.
We are the first to demonstrate an interaction between a qubit and a surface acoustic wave resonator in the quantum regime. Previously, resonators of this kind were studied, but without a qubit. Likewise, qubits with surface acoustic waves were studied, but those were running waves, without a resonator. The quantum regime was demonstrated on bulk resonators, but this didn’t go far, perhaps due to difficulties in fabrication. We used a planar structure fabricated with existing technologies.
Aleksey Bolgar, Researcher, Artificial Quantum Systems Lab, MIPT
The scientists investigated the interaction of a transmon, which is a superconducting qubit, with surface acoustic waves in a resonator. The transmon acts like an artificial atom — that is, it has multiple energy levels and experiences transitions between them. The traditional microwave strategy involves making one chip to hold both the qubit and a microwave resonator that supports and amplifies the wave. In this system, the qubit is free to interact with the resonator by grasping a photon from it and moving into an excited state or by liberating a photon into it and returning to the ground state, with the condition that the photon frequency should be in agreement with the transition frequency of the qubit. The resonator’s resonant frequency differs in itself based on the state of the qubit. Hence, information can be read from the qubit by modifying the resonator characteristics.
An alternative strategy has emerged recently. Rather than using microwave radiation (photons), the new approach involves using mechanical excitations, or phonons, in the form of acoustic waves. This quantum-acoustic process has been devised to a much lesser extent, than its microwave equivalent, yet it has a lot of benefits.
Due to the fact that the speed of propagation of acoustic waves is 100,000 times slower when compared to light, their wavelength is consequently shorter. A resonator’s size should “fit” the wavelength used. In the case of a microwave quantum system, the wavelength is around 1 cm at best. This suggests that the size of the resonator has to be quite large; however, a larger resonator has the more number of defects as they unavoidably exist on the chip’s surface. Such defects shorten the lifetime of a qubit state, thereby hindering large-scale quantum calculations and making the development of quantum computers more complicated. Currently, the world record for the longest lifetime is about 100 μs, or one-ten-thousandth of 1 second. In the acoustic approach, the wavelength equals approximately 1 μm, hence high-quality resonators with a size of just 300 μm can be fit on the chip.
Another drawback with microwaves is that the long wavelengths render it difficult to put two qubits into one resonator to facilitate interaction at various frequencies. Consequently, a separate resonator is required for each qubit. Under the acoustic approach, a number of qubits can be accommodated within one mechanical resonator with somewhat different transition frequencies. This suggests that a quantum chip developed based on sound waves would be considerably smaller compared to the existing ones. Furthermore, the problem of quantum system sensitivity to electromagnetic noise can be overcome with acoustodynamics.
A resonator was used for surface acoustic waves by the authors of the paper. These are fairly similar to sea waves but propagate in solids. Deposition of an aluminum circuit on a piezoelectric substrate made of quartz is performed. The circuit comprises of a resonator, a transmon, and two interdigital transducers. The transducers function as a transmitter and a receiver. In between them is a piezoelectric layer developed from a material that transforms mechanical stress into electricity and vice versa. A surface acoustic wave produced on the piezoelectric material is absorbed between the two Bragg gratings of the resonator. The qubit, or transmon, in the resonator has two energy levels, and the qubit capacitance is implemented as interdigital transducers. The aim of the research was to demonstrate that the qubit had the ability to interact with the resonator, getting excited and relaxed the same way a quantum system would. The measurements were carried out in a cryostat under temperatures in the tens of milliKelvins.
An attributive feature of the quantum regime is the purported block in the crossing of energy levels. An external magnetic field can be applied to tune the transition frequency of the qubitâ€Œ — to achieve this, a SQUID magnetometer is fitted in the transmon. In case the resonator’s frequency corresponds with the qubit transition frequency, energy splitting is observed in the qubit’s energy spectrum — that is, one magnetic flux value corresponds to two attributive transition frequencies. The scientists noticed this phenomenon in their chip and demonstrated that the transmon and the acoustic resonator interact in the quantum regime.
The basic aim of this study is to show that the phenomena and impacts of quantum optics are also applicable to acoustics. Moreover, it provides a substitutive path to develop a quantum computer. Although microwave-based interfaces realize a remarkable 50-qubit count, which suggests that quantum acoustics still has a long path to tread, the latter technique has a number of advantages that could prove very useful in the future.
Besides the staff of MIPT’s Artificial Quantum Systems Lab, scientists from MISIS National University for Science and Technology, Moscow State Pedagogical University, and Royal Holloway, University of London, took part in the study.
This study was performed using the technological equipment of MIPT and was supported by the Russian Science Foundation and the Ministry of Education and Science of Russia.