Go gentle with mechanical quantum systems

Go gentle with mechanical quantum systems

Πηγαίνετε ήπια με τα μηχανικά κβαντικά συστήματα

Optical microscope image of the acoustic resonator seen from above (two larger disks, the inside of which is the piezoelectric converter) and the antenna connected to the superconducting qubit (white structure). Credit: Adapted by von Lüpke et al, Physics of Nature (2022). DOI: 10.1038 / s41567- 022-01591-2.

When we think of quantum mechanical systems, individual photons and well-isolated ions and atoms or electrons that propagate through a crystal may come to mind. More exotic in the context of quantum mechanics are the real mechanical quantum systems. that is, massive objects in which mechanical motion, such as vibration, is quantized. In a series of basic experiments, quintessential quantum mechanical properties have been observed in mechanical systems, including quantization of energy and entanglement.

However, in order to use such systems in fundamental studies and technological applications, the observation of quantum properties is only a first step. The next step is to master the handling of mechanical quantum objects so that their quantum states can be controlled, measured, and ultimately utilized in device-like structures. Yiwen Chu’s team at the Physics Department at ETH Zurich has now made significant progress in this direction. Writing inside Physics of Nature, report the extraction of information from a mechanical quantum system without destroying the valuable quantum state. This advancement paves the way for applications such as quantum error correction and more.

Huge quantum mechanics

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

Surprisingly, it is possible to interconnect the mechanical tuner with other quantum systems, especially superconducting qubits. The latter are tiny electronic circuits in which electromagnetic energy states are quantized and are currently one of the leading platforms for the construction of scalable quantum computers. The electromagnetic fields associated with the superconducting circuit allow the qubit to be coupled to the piezoelectric transducer of the acoustic tuner and therefore to its mechanical quantum states.

In such qubit-resonator hybrid devices, the best of two worlds can be combined. In particular, the highly developed computing capabilities of superconducting qubits can be used to synchronize with the robustness and longevity of audio modes, which can serve as quantum memories or converters. For such applications, however, simply pairing qubit and coordinator states will not suffice. For example, a simple measurement of the quantum state in the coordinator destroys it, making repetitive measurements impossible. Instead, what is needed is the ability to extract information about the mechanical quantum state in a more gentle, well-controlled way.

Πηγαίνετε ήπια με τα μηχανικά κβαντικά συστήματα

The hybrid device connected to a flip-chip, with the audio tuning chip over the superconducting qubit chip. The lower chip is 7 mm long. Credit: Adapted by von Lüpke et al, Physics of Nature (2022). DOI: 10.1038 / s41567-022-01591-2.

The non-destructive route

Demonstrating a protocol for such so-called quantum non-demolition measurements is what Chu Uwe von Lüpke, Yu Yang and Marius Bild, PhD students, have now achieved, working with Branco Weiss’s collaborator Matteo Fadel Laurent Michaud. In their experiments there is no direct energy exchange between the superconducting qubit and the acoustic tuner during the measurement. In contrast, the properties of the qubit depend on the number of phonons in the acoustic resonator, without having to “touch” the mechanical quantum state directly – think of a thermin, the musical instrument on which the pitch depends on the position of the musician’s hand without has physical contact with the organ.

Creating a hybrid system in which the coordinator state is reflected in the qubit spectrum is extremely difficult. There are strict requirements for how long quantum states can be maintained in both the qubit and the coordinator before they disappear due to imperfections and disturbances from the outside. Thus, the task for the team was to boost the lifespan of both the qubit quantum states and the coordinator. And they succeeded by making a number of improvements, including a careful selection of the type of superconducting qubit used and encapsulating the hybrid device in a superconducting aluminum cavity to ensure tight electromagnetic shielding.

Quantum information based on the need for knowledge

Having successfully pushed their system into the desired operating mode (known as the “strong dispersion mode”), the team was able to gently extract the number of phonons in its resonator after stimulating it with different widths. In addition, they showed a way to determine in a single measure whether the number of phonons in the coordinator is even or odd – a so-called parity measure – without learning anything else about phonon distribution. Obtaining such very specific information, but not others, is vital 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 signal that an error has affected the quantum state and needs to be corrected. Here it is necessary, of course, not to damage the situation to be corrected.

However, before enabling such debugging schemes can be implemented, further improvement of the hybrid system is required, in particular to improve the fidelity of the functions. But quantum error correction is by far not the only use on the horizon. There is a wealth of fascinating theoretical propositions in the scientific literature for quantum information protocols as well as for fundamental studies that benefit from the fact that acoustic quantum states are found in vast objects. These provide, for example, unique opportunities to explore the field of quantum mechanics at the edge of large systems and to utilize mechanical quantum systems as sensors.

How to test the limits of quantum mechanics

More information:
Uwe von Lüpke et al, Equivalence measurement in the strong scattering regime of the quantum acoustics of the circuit, Physics of Nature (2022). DOI: 10.1038 / s41567-022-01591-2

Reference: Going gentle on mechanical quantum systems (2022, 13 May) Retrieved 14 May 2022 from https://phys.org/news/2022-05-gentle-mechanical-quantum.html

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