Protection of mechanical quantum systems

Protection of mechanical quantum systems

Schonung mechanischer Quantensysteme

Light micrograph of the acoustic resonator seen from above (two larger disks, the inside of which is the piezoelectric transducer) and the antenna connected to the superconducting qubit (white structure). Source: Adapted from von Lüpke et al, natural physics (2022). DOI: 10.1038/s41567-022-01591-2.

When one thinks of quantum mechanical systems, one might think of single photons and well isolated ions and atoms, or electrons propagating through a crystal. More exotic in the context of quantum mechanics are genuinely mechanical quantum systems; that is, massive objects in which mechanical motion such as vibration is quantized. A series of pioneering experiments observed fundamental quantum mechanical features in mechanical systems, including energy quantization and entanglement.

However, to use such systems in fundamental studies and technological applications, the observation of quantum properties is only a first step. The next task is to master the manipulation of mechanical quantum objects so that their quantum states can be controlled, measured and ultimately used in device-like structures. Yiwen Chu’s group at the Department of Physics at ETH Zurich has now made great progress in this direction. registered mail natural physicsthey report the extraction of information from a mechanical quantum system without destroying the precious quantum state. This advance paves the way to applications such as quantum error correction and beyond.

Massive quantum mechanics

The ETH physicists use a high-quality sapphire plate almost half a millimeter thick as the mechanical system. A thin piezoelectric transducer sits on its upper side, which can excite acoustic waves that are reflected on the underside and thus propagate over a precisely defined volume within 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 be subjected to quantum operations, at least in principle, in very similar ways to how the quantum states of atoms, photons and electrons can be.

Interestingly, it is possible to connect the mechanical resonator to other quantum systems, in particular to superconducting qubits. The latter are tiny 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 enable the coupling of the qubit with the piezoelectric transducer of the acoustic resonator and thus with its mechanical quantum states.

In such hybrid qubit resonator devices, the best of both worlds can be combined. In particular, the sophisticated computational capabilities of superconducting qubits can be exploited in sync with the robustness and long lifetime of acoustic modes that can serve as quantum memories or transducers. However, for such applications it is not enough to just couple qubit and resonator states. 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 smoother, well-controlled way.

Schonung mechanischer Quantensysteme

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

The non-destructive way

The Chu doctoral students Uwe von Lüpke, Yu Yang and Marius Bild, together with Branco Weiss scholarship holder Matteo Fadel and with the support of semester project student Laurent Michaud, have now succeeded in demonstrating a protocol for such so-called quantum non-demolition measurements. 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 where the pitch depends on the position of the musician’s hand without 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 for how long the quantum states can be maintained in both the qubit and the resonator before they fade away due to imperfections and outside interference. So the team’s task was to extend the lifetime of both the qubit and the resonator quantum state. And they succeeded by making a number of improvements, including 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 on a need-to-know basis

After successfully bringing their system into the desired regime of operation (known as the “strong dispersive regime”), the team was able to smoothly extract the phonon number distribution in its acoustic resonator after being excited at different amplitudes. They also demonstrated a way to determine with a single measurement whether the number of phonons in the resonator is even or odd – a so-called parity measurement – without learning anything about the distribution of the phonons. Obtaining such very specific information, but no other, is crucial for a number of quantum technological applications. For example, a change in parity (a transition from an odd number to an even number or vice versa) can signal that an error has affected the quantum state and correction is needed. Of course, it is essential here that the state to be corrected is not destroyed.

However, before such error correction schemes can be implemented, further refinement of the hybrid system is required, in particular to improve the accuracy of the operations. But quantum error correction is far from the only application on the horizon. The scientific literature abounds with exciting theoretical proposals for quantum information protocols, as well as fundamental studies that benefit from the fact that the quantum acoustic states reside in massive objects. These offer, for example, unique opportunities to explore the scope of quantum mechanics at the limit of large systems and to make mechanical quantum systems usable 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 Acoustodynamics, natural physics (2022). DOI: 10.1038/s41567-022-01591-2

Citation: Going Gentle on Mechanical Quantum Systems (2022, May 13) Retrieved May 14, 2022 from

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