A Data Bus for Quantum Computers

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Quantum physicists are now laying the groundwork for a "quantum bus," which can teleport quantum information between the memory and processor components of future quantum computers.

Classical computers switch transistors between one state or another to symbolize data as ones and zeros. Quantum computers use quantum bits or qubits that, because of the surreal nature of quantum mechanics, can be in a state of superposition where they can essentially behave as both one and zero.

The superpositions that qubits adopt let them hold two states at once. If two qubits are quantum-mechanically linked, or entangled, they can hold four states simultaneously; three qubits, eight states; and so on. In theory, a quantum computer with 300 qubits could hold more states than there are atoms in the visible universe. Algorithms can use such entangled qubits to run an extraordinary amount of calculations in an instant.

Environmental disturbances and other noise can easily disrupt superpositions. As such, quantum computers need to carry out techniques known as error correction codes to protect them from interference. For instance, the information from a single "logical" qubit can spread to several highly entangled "physical" qubits to reduce the chances that any environmental disturbance will tamper with the information in question.

"Generally, the big challenge in quantum computation is that quantum objects are extremely fragile," says Hendrik Poulsen Nautrup, a quantum physicist at the University of Innsbruck in Austria. "Theoretically, we can use quantum error correction to protect those objects."

Just as classical computers have components that can serve as memory and processors, so too will future quantum computers need qubits that can either store data or perform operations. These quantum memory and quantum processor components will need to withstand different thresholds of noise—quantum memory must prove robust against change, while quantum processors must prove more flexible to change. As such, quantum memory and quantum processors require different error correction codes.

Previously, researchers have built small-scale quantum processors and quantum memory. However, given the different protocols used to encode information within these two different kinds of components, scientists now face the challenge of how to pass that information between them.

Now, Poulsen Nautrup and a team of researchers have designed a way to make such hybrid architectures viable. The scientists recently detailed their findings in the journal Nature Communications. "We can indeed envision a quantum computer with separate components that are used for different purposes like processing and memory," he says.

The new technique essentially involves modifying some of the physical qubits encoding each logical qubit. This strategy, which the researchers call subsystem lattice surgery, can temporarily "sew" different logical qubits together—for instance, those used in quantum memory or quantum processors. Once coupled, information can then be teleported from one system to the other.

"In principle, labs across the globe already have all the tools available to perform a proof-of-principle experiment," Poulsen Nautrup says. He and his colleagues are now collaborating with experimental physicist Rainer Blatt's group at the University of Innsbruck to perform their own. "I can't tell you too much about it because it is underway and has not been completed yet, but the intermediate results are sure exciting," he says.