Going gentle on mechanical quantum systems

When thinking about quantum mechanical systems, single photons and well-isolated ion and atoms may come to mind. Massive objects in which mechanical motion is quantized are more exotic in the context of quantum mechanics. In a series of seminal experiments, quintessential quantum-mechanical features have been observed in mechanical systems.

With a view to putting such systems to use in fundamental studies and technological applications, observing quantum properties is a first step. The next step is to master the handling of mechanical quantum objects so that their quantum states can be measured and exploited in device-like structures. The group of Yiwen Chu is in the Departement of Physics at ETH Zurich. They reported the extraction of information from a mechanical quantum system without destroying the precious quantum state. The path to applications such as quantum error correction is paved by this advance.

Massive quantum mechanics.

The physicists use a slab of high-quality sapphire, a little under half a millimeter thick, as their mechanical system. A thin piezoelectrical transducer sits on the top of the structure, which is reflected at the bottom and extends across a well-defined volume inside the slab. The collective motion of a large number of atoms can be quantized in energy units known as phonons and can be subjected to quantum operations in the same way as the quantum states of atoms.

It is possible to interface the mechanical resonator with other quantum systems. One of the leading platforms for building quantum computers are the tiny electronic circuits in which electromagnetic energy states are quantized. The mechanical quantum states of the qubit are enabled by the electrical fields associated with the circuit.

The best of two worlds can be combined in a hybrid qubit. Computational capabilities of qubits can be used in conjunction with the long lifetime of acoustical modes, which can serve as quantum memories or transducers. For such applications, there needs to be more. A simple measurement of the quantum state in the resonator makes it impossible to repeat. The ability to extract information about the mechanical quantum state in a more gentle, well-controlled manner is what is needed.

The path is non-destructive.

Demonstrating a protocol for quantum non-demolition measurements is what Chu's PhD students Uwe von Lüpke, Yu Yang, and Marius Bild, as well as Branco Weiss fellow Matteo Fadel and semester project student Laurent, are doing. There is no direct energy exchange between the qubit and the acoustic resonator in their experiments. The properties of the qubit are made to depend on the number of phonons in the acoustic resonator, with no need to touch the quantum state.

Creating a hybrid system in which the state of the resonator is reflected in the qubit is very challenging. There are strict demands on how long the quantum states can be sustained in the qubit and in the resonator before they fade away. The lifetimes of the qubit and the resonator quantum states were pushed by the team. They succeeded by making a series of improvements, including a careful choice of the type of superconducting qubit used and the encapsulated hybrid device in a superconducting aluminum cavity to ensure tight electromagnetic shielding.

Information on a need-to-know basis.

The team were able to gently extract the phonon-number distribution in their acoustic resonator after exciting it with different amplitudes. They showed a way to determine if the number of phonons in the resonator is even or odd in a single measurement. Obtaining very specific information is important in a number of quantum-technological applications. A change in parity can indicate that an error has affected the quantum state and that correction is needed. The to-be-corrected state must not be destroyed.

The hybrid system needs to be further refined in order to improve the fidelity of the operations. There are other uses for quantum error correction on the horizon. There are many exciting theoretical proposals in the scientific literature for quantum-information protocols as well as for fundamental studies that benefit from the fact that the acoustic quantum states reside in massive objects. There are unique opportunities for exploring the scope of quantum mechanics in the limit of large systems and for exploiting the mechanical quantum systems as a sensor.

More information: Uwe von Lüpke et al, Parity measurement in the strong dispersive regime of circuit quantum acoustodynamics, Nature Physics (2022). DOI: 10.1038/s41567-022-01591-2 Journal information: Nature Physics Citation: Going gentle on mechanical quantum systems (2022, May 13) retrieved 13 May 2022 from https://phys.org/news/2022-05-gentle-mechanical-quantum.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.