Systems in which mechanical motion is controlled at the level of individual quanta become a promising quantum technological platform. New experimental work now establishes how the quantum properties of such systems can be measured without destroying the quantum state – a key ingredient for harnessing the full potential of mechanical quantum systems.
When thinking about quantum mechanical systems, individual photons and well-isolated ions and atoms or electrons propagating through a crystal may come to mind. More exotic in the context of quantum mechanics are truly mechanical quantum systems; that is, massive objects in which mechanical motion is quantized, such as vibration. In a series of initial experiments in mechanical systems, quintessential quantum-mechanical features, including energy quantization and entanglement, were observed. However, in order to use such systems in basic research and technological applications, the observation of quantum properties is only the first step. Next is to master the handling of mechanical quantum objects so that their quantum states can be monitored, measured, and eventually used in device-like structures. Yiwen Chu’s group from the Solid State Physics Laboratory at ETH Zurich has made significant progress in this direction. Write to Physics of nature, they report extracting information from a mechanical quantum system without destroying a valuable quantum state. This progress paves the way for applications such as quantum error correction and more.
Massive quantum mechanics
ETH physicists use as their mechanical system a plate of high-quality sapphire just under half a millimeter thick. At its top is a thin piezoelectric transducer that can excite acoustic waves, which are reflected at the bottom and thus propagate to a well-defined volume inside 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 at least quantum operations in much the same way as the quantum states of atoms. , photons and electrons can be. Interestingly, the mechanical resonator can be combined with other quantum systems, and in particular with superconducting qubits. The latter are tiny electronic circuits in which states of electromagnetic energy are quantized, and they are now one of the leading platforms for building scalable quantum computers. The electromagnetic fields associated with the superconducting circuit allow the qubit to be connected to a piezoelectric transducer of an acoustic resonator and thus to its mechanical quantum states.
In such hybrid qubit-resonator devices it is possible to unite the best of two worlds. In particular, the highly developed computing capabilities of superconducting qubits can be used synchronously with the reliability and long life of acoustic modes that can serve as quantum memory or transducers. For such applications, however, simply combining qubit and resonator states will not suffice. For example, a simple measurement of the quantum state in a resonator destroys it, making repeated measurements impossible. Instead, you need the ability to extract information about the mechanical quantum state in a softer, well-controlled way.
The non-destructive way
Demonstrating the protocol for so-called quantum measurements without wear is what PhD students Chu Uwe von Lupke, Yu Yang and Marius Bild achieved, working with Branko Weiss’s colleague Mateo Fadel and with the support of semester project student Laurent Misho. 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 directly “touch” the mechanical quantum state – think of the term, a musical instrument whose height 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 very difficult. There are strict requirements for how long quantum states can persist in both the qubit and the resonator before they disappear due to imperfections and perturbations from the outside. Thus, the task of the team was to push the lifetime of both the qubit and the quantum states of the resonator. And they have 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 provide dense electromagnetic shielding.
Quantum information based on the need to know
By successfully converting their system to the desired operating mode (known as the “strong dispersion mode”), the team was able to accurately highlight the distribution of phonon numbers in its acoustic resonator after excitation with different amplitudes. Moreover, they demonstrated a way to determine in one dimension whether the number of phonons in a resonator is even or odd – the so-called parity measurement – without studying anything else about phonon distribution. Obtaining such very specific information, but no other, is crucial in a number of quantum applications. For example, a change in evenness (transition from an odd number to an even number or on the contrary) may signal that the error has affected the quantum state and that correction is needed. Here, of course, it is essential not to destroy the condition that needs to be corrected.
However, before the implementation of such error correction schemes will be possible, further improvement of the hybrid system is needed, in particular to improve the accuracy of operations. But quantum error correction is far from the only use on the horizon. In the scientific literature, there are many interesting theoretical proposals for quantum information protocols, as well as for basic research, which is beneficial from the fact that acoustic quantum states are in massive objects. They provide, for example, unique opportunities to study the field of quantum mechanics within large systems and to use mechanical quantum systems as a sensor.