Researchers in Switzerland have developed a mechanical qubit using an acoustic wave resonator, representing a pivotal advance in quantum acoustodynamics. While the qubit is not yet suitable for quantum logic operations, scientists anticipate that further progress could enable its use in quantum sensing and quantum memory applications.

Current quantum computing platforms, such as trapped ions and superconducting qubits, rely on quantum electrodynamics, where quantum information is stored in electromagnetic states and transmitted via photons. In contrast, quantum acoustodynamics encodes quantum information in the quantum states of mechanical resonators. These resonators interact with their environment through quantized vibrations, or phonons, which cannot travel through a vacuum. This isolation gives mechanical resonators significantly longer lifetimes compared to their electromagnetic counterparts, making them promising candidates for quantum memory technologies.

Figure 1. Quantum Sensor

John Teufel from the US National Institute for Standards and Technology (NIST) and his team, recipients of Physics World's 2021 Breakthrough of the Year award, demonstrated quantum entanglement of two mechanical resonators using light. “If you entangle two drums, their motion is correlated beyond vacuum fluctuations,” explains Teufel. “You can achieve highly quantum phenomena, but for applications like quantum computing, you'd need these systems to exhibit nonlinearity at the single-photon level, enabling them to hold a single excitation like a qubit. That’s not a regime we typically work in,” he adds. Figure 1 shows Quantum Sensor.

Previously Unattainable

Several research groups, including Yiwen Chu’s team at ETH Zurich, have successfully interfaced electromagnetic qubits with mechanical resonators and used qubits to induce quantized mechanical excitations. However, the creation of a true mechanical qubit had remained unattainable until now. A functional qubit requires two distinct energy levels, analogous to the 1 and 0 states of a classical bit, which can be initialized and maintained in a coherent superposition without interference from additional energy levels.

This condition is satisfied when a system possesses unevenly spaced energy levels—a characteristic inherent in atoms or ions and one that can be engineered in superconducting qubits. When photons of the precise transition energy are used to drive a qubit, they induce Rabi oscillations, causing the population of the upper energy level to oscillate periodically. However, acoustic resonators, as harmonic oscillators, have evenly spaced energy levels, making this behavior challenging. “Whenever we prepared a phonon mode in a harmonic oscillator, it would jump by one energy level,” explains Igor Kladarić, a PhD student in Chu’s group.

In their recent breakthrough, Kladarić and colleagues utilized a superconducting transmon qubit coupled to an acoustic resonator fabricated on a sapphire chip. By slightly detuning the frequency of the superconducting qubit from that of the mechanical resonator, they achieved a coupling that altered the frequencies of the resonator's ground state and first excited state. This modification effectively established the desired two-level system within the resonator.

Exchanging Excitations

The researchers injected microwave signals tuned to the frequency of the mechanical resonator, converting them into acoustic signals via piezoelectric aluminum nitride. “We measured it the same way as before,” explains Kladarić. “We aligned the superconducting qubit with the mechanical qubit to exchange an excitation back into the superconducting qubit and then simply read out the superconducting qubit.”

Their measurements confirmed that the mechanical resonator exhibited Rabi oscillations between the ground and first excited states, with a leakage probability of less than 10% into the second excited state. This demonstrated that the system functioned as a genuine mechanical qubit.

The team is now focused on enhancing the mechanical qubit to make it viable for quantum information processing and exploring its potential applications in quantum sensing. “These mechanical systems are very massive and can couple to degrees of freedom that single atoms or superconducting qubits cannot, such as gravitational forces,” explains Kladarić.

John Teufel is impressed by the Swiss team’s breakthrough, stating, “There are very few strong nonlinearities in nature that are also clean and not lossy. The challenge for any technology is to create something that is both highly nonlinear and long-lived. Achieving that means you’ve developed a very good qubit.” He adds, “This is the first mechanical resonator to exhibit nonlinearity at the single quantum level. While it’s not yet a spectacular qubit, the key achievement of this work is showing that this technology can behave like a qubit.”

Warwick Bowen from Australia’s University of Queensland shares his enthusiasm, telling Physics World: “The creation of a mechanical qubit has been a dream of the quantum community for decades—taking the most classical of systems, like a macroscopic pendulum, and transforming it into the most quantum of systems, effectively an atom.”

Source: Science

Cite this article:

Janani R (2024), A Mechanical Qubit Has Potential Applications in Quantum Sensors and Quantum Memory Systems, AnaTechmaz, pp.161