Going gentle on mechanical quantum systems

Nature Physics (2022). DOI: 10.1038/s41567-​022-01591-2.” width=”800″ height=”398″/>

Optical microscope image of the acoustic resonator seen from above (two larger disks, the innermost one being the piezoelectric transducer) and of the antenna connected to the superconducting qubit (white structure). Credit: Adapted from von Lüpke et al, Nature physics (2022). DOI: 10.1038/s41567-​022-01591-2.

When you think of quantum mechanical systems, you might think of single photons and well-insulated ions and atoms, or electrons dispersing through a crystal. More exotic in the context of quantum mechanics are truly mechanical quantum systems; that is, solid objects in which mechanical motion such as vibrations are quantized. In a series of groundbreaking experiments, typical quantum mechanical features have been observed in mechanical systems, including energy quantization and entanglement.

However, in view of the use of such systems in fundamental studies and technological applications, observing quantum properties is only a first step. The next is to master the handling of mechanical quantum objects so that their quantum states can be monitored, measured and ultimately exploited in device-like structures. Yiwen Chu’s group in the Department of Physics at ETH Zurich has now made great strides in this direction. Signing up Nature physics, they report the extraction of information from a mechanical quantum system without destroying the precious quantum state. These advancements are paving the way for applications such as quantum error correction and more.

Massive Quantum Mechanics

The ETH physicists use as their mechanical system a high-quality slab of sapphire, slightly less than half a millimeter thick. At the top is a thin piezoelectric transducer that can generate acoustic waves, which are reflected at the bottom and extend over a well-defined volume in the plate. These excitations are the collective motion of a large number of atoms, but they are quantized (in units of energy known as phonons) and can in principle be subjected to quantum operations in much the same way as the quantum states of atoms, photons and electrons. can be.

Intriguingly, it is possible to couple the mechanical resonator with other quantum systems, and in particular with superconducting qubits. The latter are small electronic circuits in which electromagnetic energy states are quantized, and they are currently one of the leading platforms for building scalable quantum computers. The electromagnetic fields associated with the superconducting circuit allow the coupling of the qubit to the piezoelectric transducer of the acoustic resonator, and thereby to its mechanical quantum states.

Such hybrid qubit resonator devices can combine the best of both worlds. In particular, the highly developed computational capabilities of superconducting qubits can be used in synchronization with the robustness and longevity of acoustic modes, which can serve as quantum memories or transducers. However, for such applications, coupling qubit and resonator states is not enough. For example, a simple measurement of the quantum state in the resonator destroys it, making repeated measurements impossible. What is needed instead is the ability to extract information about the mechanical quantum state in a gentler, well-controlled way.

Zacht voor mechanische kwantumsystemenNature Physics (2022). DOI: 10.1038/s41567-022-01591-2.”/>

The flip chip bonded hybrid device, with the acoustic resonator chip on top of the superconducting qubit chip. The bottom chip is 7mm long. Credit: Adapted from von Lüpke et al, Nature physics (2022). DOI: 10.1038/s41567-022-01591-2.

The Non-Destructive Path

Demonstrating a protocol for such so-called quantum no-demolition measurements is what Chu’s PhD students Uwe von Lüpke, Yu Yang and Marius Bild, in collaboration with Branco Weiss colleague Matteo Fadel and with support from semester project student Laurent Michaud, have now achieved. In their experiments, there is no direct energy exchange between the superconducting qubit and the acoustic resonator during the measurement. Instead, the properties of the qubit are made dependent on the number of phonons in the acoustic resonator, without having to directly “touch” the mechanical quantum state – think of a theremin, the musical instrument in which the pitch depends on the position of the hand. of the musician without making physical contact with the instrument.

Creating a hybrid system in which the state of the resonator is reflected in the spectrum of the qubit is a major challenge. There are strict requirements on how long the quantum states can be sustained, both in the qubit and in the resonator, before fading due to imperfections and outside disturbances. So the task for the team was to increase the lifetimes of both the qubit and resonator quantum states. And they succeeded by making a series of improvements, including careful choice of the type of superconducting qubit used and encapsulating the hybrid device in a superconducting aluminum cavity to ensure tight electromagnetic shielding.

Quantum information on a need-to-know basis

After successfully pushing their system into the desired operational regime (known as the “strong dispersive regime”), the team was able to gently extract the phonon number distribution in their acoustic resonator after activating it at different amplitudes. In addition, they demonstrated a way to determine in a single measurement whether the number of phonons in the resonator is even or odd – a so-called parity measurement – without learning anything else about the distribution of phonons. Obtaining such very specific information, but no other, is crucial in a number of quantum technology applications. For example, a change in parity (a transition from an odd to an even number or vice versa) may indicate that an error has affected the quantum state and that correction is needed. It is of course essential that the condition to be corrected is not destroyed.

However, before an implementation of such error correction schemes is possible, further refinement of the hybrid system is necessary, in particular to improve the reliability of the operations. But quantum error correction is far from the only use on the horizon. There is a plethora of exciting theoretical proposals in the scientific literature for both quantum information protocols and fundamental studies that take advantage of the acoustic quantum states being contained in massive objects. For example, these offer unique possibilities to explore the scope of quantum mechanics in the limit of large systems and to exploit the mechanical quantum systems as sensors.

How to test the limits of quantum mechanics

More information:
Uwe von Lüpke et al, Parity measurement in the highly dispersive regime of circuit quantum acoustics, Nature physics (2022). DOI: 10.1038/s41567-022-01591-2

Quote: Soft to mechanical quantum systems (2022, May 13) retrieved May 14, 2022 from https://phys.org/news/2022-05-gentle-mechanical-quantum.html

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