Progress towards error correction in quantum computers

The Google and UCSB researchers showed they could program groups of qubits—devices that represent information using fragile quantum physics—to detect certain kinds of error, and to prevent those errors from ruining a calculation. The new advance comes from researchers led by John Martinis, a professor at the University of California, Santa Barbara, who last year joined Google to set up a quantum computing research lab.

Much quantum computing research focuses on trying to get systems of qubits to detect and fix errors. Martinis’s group has demonstrated a piece of one of the most promising schemes for doing this, an approach known as surface codes. The researchers programmed a chip with nine qubits so that they monitored one another for errors called “bit flips,” where environmental noise causes a 1 to flip to a 0 or vice versa. The qubits could not correct bit flips, but they could take action to ensure that they did not contaminate later steps of an operation

Researchers from Google and the University of California, Santa Barbara, used this chip to demonstrate a crucial method needed to make quantum computers reliable.

Journal Nature – State preservation by repetitive error detection in a superconducting quantum circuit

The elements still required are not trivial, though. The bit flips that Martinis and colleagues took on can be addressed using classical algorithms that work on a conventional computer. A trickier kind of error, where a quantum property of a qubit known as “phase” is altered by environmental noise, can only be tackled using more complex algorithms that exploit quantum effects. Austin Fowler, a quantum electronics engineer with Google, says the group is now working on that, and on demonstrating error checking on more than nine qubits

Quantum computing becomes viable when a quantum state can be protected from environment-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and quantum error correction (QEC) is capable of identifying and correcting them. Adding more qubits improves the preservation of states by guaranteeing that increasingly larger clusters of errors will not cause logical failure—a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural step towards the two-dimensional surface code QEC scheme, and track errors as they occur by repeatedly performing projective quantum non-demolition parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 when using five of our nine qubits and by a factor of 8.5 when using all nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger–Horne–Zeilinger state. The successful suppression of environment-induced errors will motivate further research into the many challenges associated with building a large-scale superconducting quantum computer.

SOURCES – Technology Review, Journal Nature

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