Researchers in Switzerland have created a mechanical qubit using an acoustic wave resonator, marking a significant step forward in quantum acoustodynamics. The qubit is not good enough for quantum logic operations, but researchers hope that further efforts could lead to applications in quantum sensing and quantum memories.
Contemporary quantum computing platforms such as trapped ions and superconducting qubits operate according to the principles of quantum electrodynamics. In such systems, quantum information is held in electromagnetic states and transmitted using photons. In quantum acoustodynamics, however, the quantum information is stored in the quantum states of mechanical resonators. These devices interact with their surroundings via quantized vibrations (phonons), which cannot propagate through a vacuum. As a result, isolated mechanical resonators can have much longer lifetimes that their electromagnetic counterparts. This could be particularly useful for creating quantum memories.
John Teufel of the US’s National Institute for Standards and Technology (NIST) and his team shared Physics World’s 2021 Breakthrough of the Year award for using light to achieve the quantum entanglement of two mechanical resonators. “If you entangle two drums, you know that their motion is correlated beyond vacuum fluctuations,” explains Teufel. “You can do very quantum things, but what you’d really want is for these things to be nonlinear at the single-photon level – that’s more like a bit, holding one and only one excitation – if you want to do things like quantum computing. In my work that’s not a regime we’re usually ever in.”
Hitherto impossible
Several groups such as Yiwen Chu’s at ETH Zurich have interfaced electromagnetic qubits with mechanical resonators and used qubits to induce quantized mechanical excitations. Actually producing a mechanical qubit had proved hitherto impossible, however. A good qubit must have two energy levels, akin to the 1 and 0 states of a classical bit. It can then be placed (or initialized) in one of those levels and remain in a coherent superposition of the two without other levels interfering.
This is possible if the system has unevenly spaced energy levels – which is true in an atom or ion, and can be engineered in a superconducting qubit. Driving a qubit using photons with the exact transition energy then excites Rabi oscillations, in which the population of the upper level rises and falls periodically. However, acoustic resonators are harmonic oscillators, and the energy levels of a harmonic oscillator are evenly spaced. “Every time we would prepare a phonon mode into a harmonic oscillator we would jump by one energy level,” says Igor Kladarić, who is a PhD student in Chu’s group.
In the new work, Kladarić and colleagues used a superconducting transmon qubit coupled to an acoustic resonator on a sapphire chip. The frequency of the superconducting qubit was slightly off-resonance with that of the mechanical resonator. Within being driven in any way, the superconducting qubit coupled to the mechanical resonator and created a shift in the frequencies of the ground state and first excited state of the resonator. This created the desired two-level system in the resonator.
Swapping excitations
The researchers then injected microwave signals at the frequency of the mechanical resonator, converting them into acoustic signals using piezoelectric aluminium nitride. “The way we did the measurement is the way we did it beforehand,” says Kladarić. “We would simply put our superconducting qubit on resonance with our mechanical qubit to swap an excitation back into the superconducting qubit and then simply read out the superconducting qubit itself.”
The researchers confirmed that the mechanical resonator undergoes Rabi oscillations between the first and second excited states, with less than 10% probability of leakage into the second excited state, and was therefore a true mechanical qubit.
The team is now working to improve the qubit to the point where it could be useful in quantum information processing. They are also interested in the possibility of using the qubit in quantum sensing. “These mechanical systems are very massive and so…they can couple to degrees of freedom that single atoms or superconducting qubits cannot, such as gravitational forces,” explains Kladarić.
Teufel is impressed by the Swiss team’s accomplishment, “There are a very short list of strong nonlinearities in nature that are also clean and not lossy…The hard thing for any technology is to make something that’s simultaneously super-nonlinear and super-long lived, and if you do that, you’ve made a very good qubit”. He adds, “This is really the first mechanical resonator that is nonlinear at the single quantum level…It’s not a spectacular qubit yet, but the heart of this work is demonstrating that this is yet another of a very small number of technologies that can behave like a qubit.”
Warwick Bowen of Australia’s University of Queensland told Physics World, “the creation of a mechanical qubit has been a dream in the quantum community for many decades – taking the most classical of systems – a macroscopic pendulum – and converting it to the most quantum of systems, effectively an atom.”
The mechanical qubit is described in Science.
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