Using phononic bandgap materials to suppress decoherence in quantum computers

Quantum computers have the potential of outperforming classical computers on some optimization and computational tasks. Compared to classical systems, however, quantum systems are more prone to errors, as they are more sensitive to noise and prone to so-called decoherence.

Decoherence is a process via which a quantum system loses its quantum properties due to interactions with its surrounding environment, causing a loss of quantum information and errors. This loss of coherence can be caused by a range of external disturbances, including material defects, temperature fluctuations and electromagnetic fields.

In recent years, physicists and engineers worldwide have been trying to devise effective methods to reduce decoherence and thus improve the reliability of quantum computers. The sources of decoherence in solid-state systems include so-called two-level systems (TLSs), which are a class of material defects that arise from the random switching between two energy states, which can disrupt the stability of qubits.

Researchers at University of California Berkeley and the Lawrence Berkeley National Laboratory have introduced a new possible method to mitigate the decoherence induced by TLSs, via what is known as phononic engineering. Their proposed approach, outlined in a Nature Physics paper, entails the use of a carefully designed phononic bandgap metamaterial that could suppress the interactions of qubits with TLSs.

“We are interested in understanding and engineering microscopic processes that lead to decoherence in superconducting qubits,” Alp Sipahigil, senior author of the paper, told Phys.org. “Solid-state quantum hardware is fabricated from imperfect materials and needs to consider how qubits interact with material defects and phonons. This study was inspired by the following question: ‘What would happen if a superconducting qubit is forbidden from losing energy via phonon radiation?'”

Sipahigil and his colleagues used phononic engineering, an approach that entails controlling phonons (i.e., quantized vibrations) in a solid material, to create a new phononic bandgap metamaterial. They then successfully used this material to suppress the interactions of superconducting qubits with TLSs.

“When a superconducting qubit decays, the energy is often lost to the environment as phonons in an irreversible Markovian process,” explained Sipahigil. “We suppressed this phonon radiation process by embedding a superconducting qubit in a phononic metamaterial where phonon emission is forbidden. We used measurements on the qubit to identify the lifetime extension of material defects called TLSs and a transition to non-Markovian qubit relaxation.”

In initial tests, the researchers found that their newly engineered phononic bandgap material extended the duration for which a qubit can maintain an excitation, also known as relaxation time. Moreover, they observed that qubits in the material exhibited non-Markovian behavior, which could open new possibilities for the control of quantum systems.

“We found that engineering the phonon bath of a superconductor can modify its dynamics,” said Sipahigil. “Engineering the electromagnetic environment of a qubit is well-established and is routinely done. Our work shows that co-designing the phononic environment might be necessary to improve superconducting qubit performance further.”

This recent study by Sipahigil and their colleagues introduces a new promising design strategy that could help to reduce decoherence in solid-state quantum computing systems. In the future, it could inspire other research groups to also explore the potential of phononic engineering strategies for improving the performance and stability of quantum systems.

“We identified a regime where qubit miniaturization and phonon engineering could dramatically improve qubit performance,” added Sipahigil. “We are now developing compact qubits that co-design the electromagnetic and phonon environments to achieve longer lifetimes.”

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