Going gentle on mechanical quantum systems

When we think of quantum mechanical systems, we can 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 truly mechanical quantum systems; that is, massive objects in which mechanical motion such as vibration is quantized. In a series of major experiments, key features of quantum mechanics have been observed in mechanical systems, including energy quantization 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 manipulation of mechanical quantum objects, so that their quantum states can be controlled, measured and eventually exploited in device-like structures. Yiwen Chu’s group from the Department of Physics at ETH Zurich has now made significant progress in this direction. Writing in Nature Physics, they report extracting information from a mechanical quantum system without destroying the precious quantum state. This breakthrough paves the way for applications such as quantum error correction, and beyond.

Massive quantum mechanics

ETH physicists use a high-quality sapphire plate, just under half a millimeter thick, as the mechanical system. On its top is a thin piezoelectric transducer that can excite acoustic waves, which are reflected downwards and thus extend over a well-defined volume inside the slab. These excitations are the collective motion of a large number of atoms, but they are quantized (in units of energy called phonons) and can be subjected, in principle at least, to quantum operations in much the same way as the quantum states of atoms. , photons and electrons can be. Curiously, it is possible to interface the mechanical resonator with other quantum systems, and with superconducting qubits in particular. These are tiny electronic circuits in which electromagnetic energy states are quantized, and they are currently one of the main 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 thus to its mechanical quantum states.

In such hybrid qubit-resonator devices, the best of both worlds can be combined. Specifically, the highly developed computational capabilities of superconducting qubits can be used in synchrony with the robustness and long lifetime of acoustic modes, which can serve as quantum memories or transducers. For such applications, however, simply coupling the qubit and resonator states will not suffice. 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 more gentle and well-controlled way.

The non-destructive way

Demonstrating a protocol for such so-called non-​demolition quantum measurements is what Chu PhD students Uwe von Lüpke, Yu Yang and Marius Bild, in collaboration with Branco Weiss fellow Matteo Fadel and with the support of the project student of semester Laurent Michaud, have now realized. 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 depend on the number of phonons in the acoustic resonator, without the need to “touch” the mechanical quantum state directly – think of a theremin, the musical instrument in which 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 qubit spectrum is very difficult. There are strict requirements on how long quantum states can be maintained in both the qubit and the resonator, before they disappear due to imperfections and disturbances from outside. The team’s task was therefore to push back the lifetimes of the quantum states of the qubit and the resonator. And they succeeded, by making a series of improvements, including a judicious choice of the type of superconducting qubit used and the encapsulation of 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 smoothly extract the phonon number distribution in their acoustic resonator after exciting it with different amplitudes. Additionally, 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 phonon distribution. Obtaining this very specific information, but no other, is crucial in a number of applications of quantum technology. For example, a change in parity (a transition from an odd number to an even number or vice versa) may signal that an error has affected the quantum state and that correction is needed. Here it is essential, of course, that the state to be corrected is not destroyed.

Before an implementation of such error correction schemes is possible, however, further refinement of the hybrid system is required, in particular to improve the fidelity of operations. But quantum error correction is by far not the only use on the horizon. There is an abundance of exciting theoretical proposals in the scientific literature for quantum information protocols as well as fundamental studies that take advantage of the fact that acoustic quantum states reside in massive objects. These offer, for example, unique opportunities to explore the scope of quantum mechanics at the edge of large systems and to exploit quantum mechanical systems as a sensor.

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