MIT makes a significant step toward the full realization of quantum computation

MIT researchers show a way to reduce errors in two-qubit gates dramatically. This is a significant step towards fully realizing quantum computation.

Researchers at MIT have made a significant step forward in realizing quantum computation. They demonstrated a technique that eliminates common mistakes in the most fundamental operation of quantum algorithms, called the “gate” or two-qubit operation.

Youngkyu Sung is an MIT graduate student studying electrical engineering and computer science. He is the lead author of a paper published in Physical Review X on June 16, 2021. We have shown a way to reduce these errors dramatically.

The processing of information in quantum computers is a delicate process that the fragile qubits perform. They are susceptible to decoherence and the loss of their quantum mechanic behavior. Sung’s previous research with MIT Engineering Quantum Systems revealed that tunable couplers had been developed. They allow researchers to switch two-qubit interactions on/off to control their operations while preserving fragile qubits. Google cited the tunable coupler concept as a significant breakthrough in their recent demonstration of quantum computing’s superiority over classical computing.

However, fixing error mechanisms is like peeling an onion. Each layer exposes the next. Even with tunable couplers, two-qubit gates still had errors due to unwanted interactions between qubits. These unwanted interactions were not noticed before the invention of tunable couplers. These residual errors, which increase in number with qubits or gates, can hinder the development of larger-scale quantum processors. This Physical Review X paper offers a new way to reduce these errors.

“We have taken the tunable-coupler concept further and demonstrated near 99.9 percent fidelity for two major types of two-qubit gates, Controlled-Z and iSWAP,” says William D. Oliver. He is an associate professor of electrical and computer science, MIT Lincoln Laboratory fellow, and Center for Quantum Engineering director. Oliver also serves as associate director of the Research Laboratory of Electronics. This laboratory houses the Engineering Quantum Systems group. “Higher-fidelity gates allow for more operations, which allows one to implement more complex algorithms on larger scales.”

Researchers harnessed the higher energy levels of the coupler to eliminate qubit-qubit interactions that could cause errors. These higher energy levels were not used in previous research, but they did cause non-negligible 2-qubit interactions.

“Better control of the coupler is key to tailoring qubit-qubit interaction as desired.” Engineering the multilevel dynamics can help achieve this,” Sung states.

The next generation of quantum computers will be error-corrected. This means that more qubits will now be available to increase the reliability of quantum computation.

Oliver says that Qubit errors can be addressed actively by adding redundancy. However, this process is only possible if the gates are sufficiently good, above a certain threshold, dependent on the error correction protocol. “The current most permissive entries are 99 percent. In practice, however, one should seek gate fidelities higher than 99 percent to live with acceptable levels of hardware redundancy.

Oliver says that the MIT Lincoln Laboratory research devices were crucial to achieving the demonstrated gains in fidelity for the two-qubit operations.

He says, “Fabricating high coherence devices is step 1 to implementing high fidelity control.”

Sung states that the high error rates in two-qubit gate gates limit quantum hardware. This makes it challenging to run quantum applications. These are often complex problems for classical computers, such as solving optimization and quantum chemistry.

Until now, only small molecules were simulated on quantum computers, simulations that are much easier on classical computers.

He says, “In this way, our new approach for reducing the two-qubit gate error is timely in quantum computation and helps to address one of today’s most critical quantum hardware problems.”

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