Quantum computers will never surpass conventional computers of today, unless they can correct errors that disrupt fragile quantum states of their qubits. A Google team took the next big step towards making quantum computing practice by demonstrating the first system capable of correcting these errors.
The breakthrough of Google origin with hiring a quantum computing research group at the University of California, Santa Barbara in the fall of 2014. UCSB researchers had already constructed a quantum circuits system made with sufficient accuracy to to the error correction a superconducting ability. This realization led the way early for researchers now many employees to Google to build a system to correct errors that naturally arise during quantum computing operations. Their work is detailed in the March 4, 2015 issue of the journal Nature.
"This is the first time the natural qubit errors arising from the environment have been corrected," said Rami Barends, quantum electronics engineer at Google. "This is the first device that can correct its own mistakes."
Quantum computers have the potential to perform multiple simultaneous calculations based on quantum bits, or qubits, which can represent information both 1 and 0 at the same time. This gives quantum computing a big advantage over conventional computers today that rely on the bits that can represent 1 or 0.But a huge challenge in building practical quantum computers is to preserve the fragile quantum states of qubits long enough to perform calculations. The solution that Google and UCSB demonstrated is a quantum error correction code that uses a simple conventional process of correcting errors that arise in quantum computing operations.These codes can not directly detect errors in qubits without disturbing the fragile quantum states. But they get around this problem by relying on entanglement, a physical phenomenon that allows a single qubit to share information with many other qubits via a quantum connection. The codes exploit entanglement with an architecture that includes qubits "Measuring" entangled qubits with "data" neighbors.The Google team and UCSB developed a quantum correction code specific error called "area code." They ultimately hope to build an area code 2-D architecture based on a checkerboard arrangement of qubits, so that "white squares" represent the qubits that perform data operations and "black boxes" of measurement represent qubits that detect errors in neighboring qubits.For now, the researchers tested the area code in a "repetition code" simplified architecture that involves linear 1-D array of qubits. Their unprecedented demonstration of error correction using a repetition code architecture comprising nine qubits. They tested the repetition code by the equivalent of 90,000 test runs to collect the necessary statistics on its performance."This validates years and years of theory to show that the error correction can be a practical possibility," said Julian Kelly, quantum electronics engineer at Google.Equally important, the demonstration of the team showed that the correction error rate actually improved when they increased the number of qubits in the system. This is great news for quantum computing because it shows that large-qubit systems will not necessarily collapse under a pile more and more mistakes. This means that large quantum computing systems could be practical.For example, the team compared the rate of a single physical qubit with the rate of several qubits that work together to perform logical operations error error. When they used the code matrix repeated five qubits, the rate of logical error was 2.7 times lower than the error rate of single physical qubit. A wider range of nine qubits showed more improvement with a logical error rate 8.5 times less than the error rate of single physical qubit. As Kelly says:
"One, we wanted to show a system where qubits cooperate and outperforming any single qubit. This is an important step. Even more exciting than that, when we go from five to nine qubits, it gets better. If you want to get a rate of error Additionally, you add more qubits. "This first error correction demonstration shows a clear way forward to expanding the size of quantum computing systems. But on its own terms, but it is still well below the error correction rates needed to make practical quantum computing, said Austin Fowler, quantum electronics engineer at Google. The team would need to improve the error correction rate of 5 to 10 times in order to make truly practical quantum computing.However, the current system of quantum computing has managed to maintain consistency in the quantum states of its qubits despite all the uncorrected errors-a fact that has allowed researchers to feel optimistic about the future.The last error correction demonstration would work mainly with universal quantum computer logic gate; systems that represent superfast versions of classic "model holder" computers today. But Google has also invested in other "quantum annealing" approach to Canadian company D-Wave.D-Wave quantum annealing machines have sacrificed some consistency across qubit rapidly in size to the 512-qubit D-Wave Two Machine system that overshadows the most experimental quantum computing systems containing only a few qubits. Google turned to John Martinis, a physics professor at the University of California, Santa Barbara, with its former researchers now on the payroll of Google UCSB, trying to build a more stable version of quantum annealing D machines -wave.