Google has built a quantum processor, in which 72 superconductor qubits are combined into a two-dimensional array.
This processor uses the same technology as the previous 9-qubit quantum computer, built by the company and having a low percentage of errors in the calculations. A new development company presented at the annual meeting of the American Physical Society in Los Angeles, reported in the company's blog.
The main obstacle preventing the construction of a quantum computer with a large number of qubits is the errors that inevitably arise when computing, reading, and writing information into qubits because of the destruction of their quantum state. The more qubits, the higher the probability that the qubit will interact with its "neighbor", and the more often errors will occur. Strictly speaking, the time of de-coherence (decay of the superposition) of the system rapidly decreases with increasing number of constituent components.
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Nevertheless, scientists have already came up with several ways with which you can deal with this problem, and built quantum computers, which include several dozen qubits. So, during the last year several groups reported the creation of quantum computers consisting of 49 (IBM), 51 (Mikhail Lukin's group) and 53 (Christopher Monroe's group) qubits. Moreover, with the help of built computers, scientists discovered new effects that could not be calculated on classical computers. Thus, scientists have almost reached quantum superiority.
At the same time, in June 2016 a group of researchers from Google under the leadership of John Martinis (John Martinis) built a quantum computer consisting of nine superconductor qubits (qubits based on the Josephson junction) connected in a chain. The main feature of this computer was high reliability: when reading error conditions, there were about one out of a hundred, with the operation of a single-qubit gate, the error probability was about 0.1 percent, and for a two-qubit valve, less than 0.6 percent . This allowed scientists to assume that in the future the system can be easily scaled.
Indeed, Google reported the creation of a 72-qubit quantum processor, built in the same way as its 9-qubit predecessor. In a new computer, the qubits are not connected in a chain, but form two square 6 × 6 arrays located one above the other. This allows you to track and correct errors that occur during the calculation. While the company does not disclose the detailed characteristics of the device, but claims that it allows you to achieve quantum superiority.
To confirm their statement, scientists theoretically estimated under what conditions the constructed system can not be modeled on a classic computer. According to their calculations, for this purpose, at least 49 qubits should be included in it, the "depth" (circuit depth) should exceed 40 qubits, and the probability of an error in the two-qubit logic element should not be more than 0.5 percent. For a built-in computer, these requirements are met.
In November last year (2017), two groups of scientists implemented the algorithm in different ways to track and correct errors that inevitably arise in quantum computing, even earlier, IBM reported about the development of a processor with automatic error correction. In December, physicists from Australia and the Netherlands proposed an integrated circuit for a quantum computer that can easily accommodate several thousand qubits based on quantum dots, and in January, experimentally created such qubits.
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