Qubits, the strange devices at the heart of a quantum computer that can be set to 0, 1, or both at once, could hardly be more different from the mechanical clockwork used in the earliest computers. Today, most quantum computers rely on qubits made out of tiny circuits of superconducting metal, individual ions, photons, or other things. But now, physicists have made a working qubit from a tiny, moving machine, an advance that echoes back to the early 20th century when the first computers employed mechanical switches.
“For many years, people were thinking it would be impossible to make a qubit from a mechanical system,” says Adrian Bachtold, a condensed matter physicist at the Institute of Photonic Sciences who was not involved in the work, published today in Science. Stephan Dürr, a quantum physicist at the Max Planck Institute for Quantum Optics, says the result “puts a new system on the map,” which could be used in other experiments—and perhaps to probe the interface of quantum mechanics and gravity.
A qubit can be any system that has two quantum states of different energies that can be isolated from all of its other states. For example, a superconducting qubit is a circuit that sloshes with unquenchable current and has a lower energy state representing 0 and a higher energy state representing 1. Applying microwaves of the right frequency, researchers can ease it into one state or the other, or any combination of two.
In theory, a tiny wriggling widget vibrating with mechanical motion could be a qubit, too. On the smallest scale, vibrations are quantized and consist of infinitesimal energy packets called phonons, just as light consists of photons of specific energies. However, at first blush, a mechanical oscillator is poorly suited for making a qubit.
The first hurdle is to get the device to sit as still as possible. Because of quantum uncertainty, a tiny object is never motionless, even at temperatures of absolute zero. Still, in 2010, physicists managed to chill a mechanical oscillator—a microscopic diving board that vibrated at 6 gigahertz—to its least energetic ground state. They even eased the widget into its next couple of states by feeding it energy one phonon at a time.
But a second challenge looms. A mechanical oscillator has “harmonic” energy states, spaced evenly like the rungs on a ladder. That makes it impossible to isolate and control two of them to form the qubit: A stimulus that drives one state to a higher state would also drive that higher state to the next higher one, and so on. The challenge “is whether you can make the energy levels unequally spaced enough that you can address two of them without touching the others,” says Yiwen Chu, a physicist at ETH Zürich (ETHZ).
For more than a decade, Dürr and other quantum physicists thought the issue was a showstopper. “We said, ‘It’s nice they can get to the ground state, but they have only this equally spaced ladder [of states]. It’s difficult to see how they’ll get out of that problem.’”
Now, Chu and her team have done just that by employing a two-part system. One part is a mechanical resonator that looks nothing like a diving board. On a waferlike sapphire crystal 400 micrometers thick, the researchers deposited a tiny dome of aluminum nitride, which would expand and contract in response to an oscillating voltage, sending vibrations into the material. Those vibrations would bounce between the crystal surfaces and ring for hundreds of millions of cycles before dying out.
The other part consisted of a superconducting qubit equipped with a tiny antenna, deposited on a similar sapphire crystal. The physicists stacked the crystals so the antenna sat above the aluminum nitride dome. That way the current sloshing in the superconducting qubit would excite vibrations in the mechanical oscillator.
Crucially, the researchers could tune the superconducting qubit’s oscillating current so its frequency was just slightly offset from that of the mechanical oscillator. As a result, the quantum states of the superconducting qubit melded slightly with those of the mechanical oscillator, forming a single system in which the energies of the hybridized states were no longer evenly spaced.
That induced “anharmonicity” allowed the researchers to isolate the two lowest energy states of the melded system as the 0 and 1 states of a qubit. Using the superconducting qubit as a controller, the ETHZ team showed it could achieve any combination of 0 and 1 in the mechanical qubit. “The main challenge was to find an optimal operating condition where we induce a strong enough anharmonicity while still keeping the mode mechanical,” says Igor Kladarić, a grad student at ETHZ.
The new mechanical qubit is unlikely to run more mature competition off the field any time soon. Its fidelity—a measure of how well experimenters can set the state they desire—is just 60%, compared with greater than 99% for the best qubits. For that reason, “it’s an advance in principle,” Bachtold says.
But Dürr notes that a mechanical qubit might serve as a supersensitive probe of forces, such as gravity, that don’t affect other qubits. And ETHZ researchers hope to take their demonstration a step further by using two mechanical qubits to perform simple logical operations. “That’s what Igor is working on now,” Chu says. If they succeed, the physical switches of the very first computers will have made a tiny comeback.
More: https://www.science.org/content/article/first-mechanical-qubit-quantum-computing-goes-steampunk
