Systems in which mechanical motion is controlled at the
level of individual quanta are emerging as a promising quantum-technology
platform. New experimental work now establishes how quantum properties of such
systems can be measured without destroying the quantum state — a key ingredient
for tapping the full potential of mechanical quantum systems.
When thinking about quantum mechanical systems, single
photons and well-isolated ions and atoms may spring to mind, or electrons
spreading 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.
In a series of seminal experiments, quintessential
quantum-mechanical features have been observed in mechanical systems, including
energy quantization and entanglement. However, with a view to putting such
systems to use in fundamental studies and technological applications, observing
quantum properties is but a first step. The next one is to master the handling
of mechanical quantum objects, so that their quantum states can be controlled,
measured, and eventually exploited in device-like structures.
The group of Yiwen Chu in the Laboratory of Solid State
Physics at ETH Zurich has now made major progress in that direction. Writing in
the journal Nature Physics, they report the extraction of information from a
mechanical quantum system without destroying the precious quantum state. This
advance paves the path to applications such as quantum error correction, and
beyond.
The ETH physicists employ a slab of high-quality sapphire, a little under half a millimeter thick, as their mechanical system. On its top sits a thin piezoelectrical transducer that can excite acoustic waves, which are reflected at the bottom and thus extend across a well-defined volume inside the slab. 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 operations in very much the same ways as the quantum states of atoms, photons and electrons can be.
Intriguingly, it is possible to interface the mechanical
resonator with other quantum systems, and with superconducting qubits in
particular. 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 to
the piezoelectrical transducer of the acoustic resonator, and thereby to its
mechanical quantum states.
In such hybrid qubit–resonator devices, the best of two
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 acoustical modes, which can serve as quantum
memories or transducers. For such applications, however, merely coupling qubit
and resonator states will be not enough. For example, a straightforward
measurement of the quantum state in the resonator destroys it, making repeated
measurements impossible. What is needed instead is the capability to extract
information about the mechanical quantum state in a more gentle,
well-controlled manner.
Demonstrating a protocol for such so-called quantum non-demolition measurements is what Chu’s doctoral students Uwe von Lüpke, Yu Yang, and Marius Bild, working with Branco Weiss fellow Matteo Fadel and with support from semester project student Laurent Michaud, 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 to depend on the number of phonons in the acoustic resonator, with no need to directly ‘touch’ the mechanical quantum state — think about a theremin, the musical instrument in which the pitch depends on the position of the musician’s hand 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 highly challenging. There are
stringent demands on how long the quantum states can be sustained both in the
qubit and in the resonator, before they fade away due to imperfections and
perturbations from the outside. So the task for the team was to push the
lifetimes of both the qubit and the resonator quantum states. And they
succeeded, by making a series of improvements, including a careful choice of the
type of superconducting qubit used and encapsulating the hybrid device in a
superconducting aluminum cavity to ensure tight electromagnetic shielding.
Having successfully pushed 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 exciting it with different amplitudes. Moreover, they demonstrated a way to determine in one 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-technological applications. For instance, a change in parity (a transition from an odd to an even number or vice versa) can signal that an error has affected the quantum state and that correcting is needed. Here it is essential, of course, that the to-be-corrected state is not destroyed.
Before an implementation of such error-correction schemes is
possible, however, further refinement of the hybrid system is necessary, in
particular, to improve the fidelity of the 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 for fundamental studies that benefit
from the fact that the acoustic quantum states reside in massive objects. These
provide, for example, unique opportunities for exploring the scope of quantum
mechanics in the limit of large systems and for harnessing the mechanical
quantum systems as a sensor.
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