Reducing error in quantum logic gates

Physicists at the University of Sydney, Dr Robin Harper and Professor Steven Flammia, have for the first time demonstrated improvement in quantum computers, thanks to the use of codes designed to detect and discard errors in the logic gates of such machines. Their work has been published in the journal Physical Review Letters.

Quantum technologies promise to revolutionise computing in the 21st century by performing calculations thought to be beyond the ability of the largest and fastest supercomputers. They will do this using the unusual properties of matter at the quantum scale that allow them to process information using quantum bits, or qubits. These are computing elements that utilise the fact that quantum objects can exist in an indeterminate state, known as superposition, and can become ‘entangled’ — a phenomenon describing behaviour not seen in conventional computers.

Quantum logic gates are formed by these entangled networks of qubits, and serve as the switches that allow quantum computers to run algorithms to process information and perform calculations. However, electronic ‘noise’ easily disrupts these indeterminate states, quickly producing errors in quantum computations. This makes the development of useful machines very difficult.

“Current devices tend to be too small, with limited interconnectivity between qubits, and are too ‘noisy’ to allow meaningful computations,” Dr Harper said. “However, they are sufficient to act as test beds for proof-of-principle concepts, such as detecting and potentially correcting errors using quantum codes.”

Furthermore, whereas the classical switches in your laptop or mobile phone can run for many years without error, at this stage quantum switches begin to fail after just fractions of a second.

“One way to look at this is through the concept of entropy,” Prof Flammia said. “All systems tend to disorder. In conventional computers, systems are refreshed easily and reset using DRAM and other methods, effectively dumping the entropy out of the system, allowing ordered computation.

“In quantum systems, effective reset methods to combat entropy are much harder to engineer. The codes we use are one way to dump this entropy from the system.”

Using codes to detect and discard errors on IBM’s quantum computer, Dr Harper and Prof Flammia showed error rates dropping from 5.8% to 0.60%. So rather than one in 20 quantum gates failing, just one in 200 would fail — an order of magnitude improvement.

“This is an important step forward to develop fault tolerance in quantum systems to allow them to scale up to meaningful devices,” Dr Harper said, “… [though] there is still a long way to go before the quantum community can demonstrate fault-tolerant computing.”

He added that other groups have shown improvements in other facets of quantum devices using codes. The next step is to synthesise and test these approaches on larger-scale devices of a few dozen qubits that enable the re-use and re-initialisation of qubits.

“These experiments are the first confirmation that the theoretical ability to detect errors in the operation of logical gates using quantum codes is advantageous in present-day devices — a significant step towards the goal of building large-scale quantum computers,” Dr Harper said.

Image credit: ©stock.adobe.com/au/josephsjacobs

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