As titles go, achieving quantum supremacy is among the best one can have. It doesn’t mean that you have control over the quantum world (sorry!), but it does signify you have a quantum computer that is faster at doing something than even the most powerful supercomputer on the planet.
In October 2019, Google announced quantum supremacy based on the results from their Sycamore quantum processor. It was able to perform one operation in 200 seconds that would take the best supercomputer 10,000 years. Chinese researchers now report in Science that their Jiuzhang processor takes 200 seconds to perform an operation that would take the best supercomputer 2.5 billion years.
Now, before we go any further we need to mention two caveats. Professor John Preskill who coined the term “quantum supremacy” explicitly stated that it can only be achieved if a quantum computer can do an operation a classical computer cannot do. But others argue that these huge processing-time differences are essentially showing the insurmountable separation between quantum and regular computers.
It is also important to stress that these are not quantum computers. We are still far away from a working quantum computer. These current machines are designed to do only one thing, but they do it exceptionally well. In the case of Jiuzhang, that one thing is called a Gaussian boson sampling algorithm. The problem has emerged over the last few years. It can’t be solved by classical computers in a short amount of time. Even an approximation of this problem is believed to be too hard for supercomputers. Hence why this particular approach is being considered a worthwhile test for quantum systems such as Jiuzhang.
There is a growing interest in quantum computers because they hold the promise of incredibly fast calculations that could allow us to solve extremely difficult scientific tasks and, for example, help us design more effective medical drugs.
Just like regular computer use bits, made of a series of zeros and ones, quantum computers use qubits (quantum bits), which can be zero, one, and a superposition of the two. The ability for a qubit to be in superposition opens this possibility of incredibly fast calculation.
But what is superposition? It is a quantum mechanical property in which, for example, two states can exist simultaneously, until the system is observed. This is often discussed using the concept of Schrodinger’s cat. In the famous thought experiment, a cat is locked in a box where a vial of poison can be activated by a quantum process. So if you don’t open the box, you have to conclude due to the nature of quantum mechanics that the cat is in superposition of being dead and alive.
The cat is not “dead or alive” but it is also not exactly “dead and alive”. There is nothing quite like it in the classical world we are used to. Employing this superposition is still limited to a few special problems but works such as this show we can do those well.