NIST scales up quantum computing

Researchers at the National Institute of Standards and Technology (NIST, Boulder Colo.) said they demonstrated continuous quantum operations using a trapped-ion processor.

The quantum computer maintained a 94 percent accuracy rate in multistep operations for quantum bits. These qubits passed intermediate results to the next processing stage for up to 15 seconds.

NIST's quantum artithmetic-logic unit demonstrated sustained operations that proved the feasibility of large-scale quantum computers.

"We had already shown all the individual operations before, but this demonstration shows that we can perform repeated, sustained quantum computing using a trapped-ion processor that transports results about 960 microns--almost a millimeter," said Jonathan Home, a NIST post-doctoral researcher.

The sequence of quantum operations used five arithmetic-logic operations--four single-qubit operations and a two-qubit operation--involving 10 transport operations. The sequence of operations took about 20 milliseconds and was repeated 3,150 times for each of 16 different starting states. The sustained operations could be performed for as long as 15 seconds before errors occurred. The current prototype used no error-correction procedures, but was constructed to test the limits of trapped-ion quantum computers.

Trapped-ion quantum computers store qubits on electrically charged atoms--NIST used beryllium ions--which were isolated inside a dark slit between the gold-covered alumina wafers measuring just 3.5 millimeters long and 200 microns wide. The trapped beryllium ions were moved among six zones in the trap. Operations were preformed using lasers to initialize the ions in a known quantum state, store data as operands, perform several one- and two-qubit operations during which intermediate results were transported between traps and then read out the final result.

"Next, we want to improve our accuracy, which involves building better, more powerful lasers," said Homes, "as well as demonstrate operations using more qubits in more complicated computing tasks."

Previous attempts at sustained quantum operations were foiled by stray magnetic fields that disturbed the delicate quantum states where qubits maintain a superposition of both 0 and 1 simultaneously.

NIST solved the problem using "sympathetic cooling," whereby partner magnesium ions were employed as refrigerants to cool the beryllium ions after they were transported and by storing qubit data in special quantum states that are naturally resistant to unwanted alterations by stray magnetic fields.

NIST's 94 percent accuracy rate will have to be raised to 99.9 percent to make quantum computing commercially feasible, the researchers said.