Researchers at the Swiss Federal Institute of Technology (ETH) in Zurich have developed the first-ever fully functional mechanical qubit. This incredible quantum innovation is a two-in-one system combining the abilities of a mechanical oscillator and a superconducting qubit.

Compared to the traditional virtual qubits that are created using multiple physical qubits and error-correcting codes to protect quantum information, mechanical qubits are real, physical systems that don’t need this extra layer of protection. 

This makes mechanical qubits simpler to use because they don’t rely on complex encoding or multiple qubits working together to function reliably. Moreover, mechanical qubits also have much longer lifespans than virtual qubits which pop up and disappear in a blink.

“The longer lifetime of mechanical quantum states should prove useful in establishing quantum acoustics as a platform for enhanced quantum technologies,” the researchers note.

The ETH team suggests that their mechanical qubit could help scientists overcome some of the major hurdles associated with realizing feasible quantum computing and sensing applications.

The challenge with mechanical qubit

Any quantum system that has two distinct energy states that are separable or can be isolated from other energy levels, is referred to as a qubit. For example, virtual qubits can exist as a superposition of 0 and 1. 

Similarly, superconducting qubit, which is a physical electronic device, also has lower and higher energy states represented by 0 and 1 respectively. However, while developing a virtual and a superconducting qubit is relatively straightforward, creating a functional mechanical qubit has been a challenge for scientists for years. 

“For many years, people were thinking it would be impossible to make a qubit from a mechanical system,” Adrian Bachtold, a physicist at the Institute of Photonic Sciences, told AAAS.

Virtual qubits are anharmonic, meaning that their energy levels are unevenly spaced, which allows for the superposition of different quantum states. However, mechanical resonators, devices that are generally considered for making mechanical qubits, come with evenly-spaced energy levels —- making it difficult to isolate two energy states.

Scientists have been puzzled by this question — “How you make the energy levels unequally spaced enough that you can address two of them without touching the others,” Yiwen Chu, one of the researchers and a physicist at ETH Zürich, said.

A two-part quantum system solved the problem

The study authors developed a two-part system to solve the energy gap problem. The first part, a mechanical resonator made of aluminum nitride was fitted on a sapphire crystal. 

When an oscillating voltage is applied through this arrangement, the aluminum nitride dome expands and contracts, producing vibrations that travel through the material, ringing between the surfaces of the crystal for millions of cycles before fading. 

Right above the mechanical resonator, the researchers placed another sapphire crystal containing a superconducting qubit with a tiny antenna positioned above the aluminum nitride dome. 

When electrical current flows through the superconducting qubit, it generates vibrations in the mechanical resonator. This interaction allows the researchers to control and adjust the energy levels of the resonator.

Therefore, by coupling the resonator with the qubit in this way, the researchers successfully modify the previously evenly spaced energy gaps (harmonic) into uneven ones (anharmonic), creating the first-ever functional mechanical qubit. 

The ETH team now plans to use two such mechanical qubits to execute some logical operations. Hopefully, they will achieve desirable results and their efforts will contribute to the development of practical quantum computing applications.

The study is published in the journal Science.

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