When is quantum supremacy achieved?

Monday, 11 november 2019 | Redacción CEU

In recent years, science and technology have gone from taking decisive steps to making enormous strides. It has advanced so fast that it is increasingly difficult for us to anticipate what effects the different discoveries that impacts on our society may have. Artificial Intelligence, Big Data, biometrics, the Internet of Things, self-driving cars, 5G networks, Blockchain, Mixed Reality, nanotechnology, 3D printing, etc. Nowadays, there are many examples of disruptive advances. All of them are currently being developed, and some are even beginning to have practical applications. However, this seems to be the tip of the iceberg in relation to everything that might happen in the near future. Nature Magazine’s study is a proof of this. This journal has just published in an article that Google has achieved quantum supremacy. What exactly has this company done?

The paper published in the journal Nature aims to be a first empirical test on quantum supremacy. To understand exactly what Google has achieved, it is first necessary to have some basic notions about what quantum computing is. It is a completely different model from the conventional one. In classical computing, the minimum units, which are bits, have a value of 0 or 1, while in quantum computing, the minimum units, which are qubits, present these values simultaneously. It is pretty much like Schörindinger's cat, which is alive and dead at the same time until the moment that someone opens its box. This may seem something very far from reality, but it is precisely the kind of state that can be seen at microscopic scales, specifically, in particles.

Quantum computing takes advantage of two principles: that of superposition, thanks to which qubits can adopt several values at the same time, and that of entanglement, which explains that if a qubit undergoes a change, this will also be reflected in the rest. These are very interesting qualities, because they make it possible to multiply the capacity of representation and calculation exponentially.

Has Google really accomplished quantum supremacy?

This grandiose and to some extent extravagant term of quantum supremacy gives name to a key event: the moment when a quantum computer will be capable of performing an operation which is unmanageable for a classic computer. This definition might have different nuances, but the most widespread idea is that by "unmanageable" we understand a feasible timeframe for a quantum computer, but unfeasible for a conventional computer due to its extension in time. In this context, what Nature's article asserts is that Google has been able to develop a processor which is capable of calculating in 3 minutes and 20 seconds an operation that would take thousands of years to calculate for the most powerful conventional computer known so far, the Summit created by IBM.

To carry out this task, the Google team developed a Sycamore processor consisting of 54 qubits. Its goal was to perform a pattern identification operation on a series of random numbers. Far from seeking a practical use, the experiment was aimed at checking if it was possible to achieve quantum supremacy with Google means. Finally, due to a failure in one of the qubits, the test was carried out with 53 qubits. In a first test, Sycamore needed 200 seconds, while Summit performed the operation in 130 seconds (although it took five extra hours to verify the result). It was when the complexity of the test increased that Sycamore reached the significant result.

Apart from Google, other major tech companies compete in the quantum computing race. One of them is IBM. Precisely, this firm does not consider that its competitor has accomplished the long-awaited milestone of quantum supremacy. It ensures that to obtain the result of this last test, Summit computer only needs two and a half days. In such a case, the Google experiment would continue being relevant, since they are still seconds versus days, but it would become an manageable period of time, so the goal of quantum supremacy would not have been achieved.