Google has published the details of its demonstration for ‘quantum supremacy’. The company’s quantum chip has performed a calculation in two hundred seconds that would take a classical computer ten thousand years. IBM has reservations about the claim.
Google CEO Sundar Pichai speaks of a ‘hello world’ moment and the biggest milestone yet to make quantum computing a reality. The company is publishing the findings of its demonstration in a paper in Nature titled Quantum supremacy using a programmable superconducting processor. It states that the research team used its 53-qubit Sycamore quantum processor to sample an instance of a quantum circuit a million times. The quantum processor took three minutes and twenty seconds to do this, while Google’s benchmarks estimate that time at about ten thousand years for a modern supercomputer based on classical computing.
According to Google, this constitutes experimental quantum supremacy. Quantum scientists use this term to indicate the point at which quantum systems can complete computational tasks that can no longer be completed within a reasonable time by classical computers. Until now, quantum calculations were limited in nature and supercomputers were able to simulate them. Scientists expect that with quantum processors of sufficient size, from about fifty qubits, the tipping point could be reached.
Google attributes the achievement of this quantum supremacy point to the quality of its Sycamore processor and in particular the properties of the gates. The Sycamore processor is made up of 54 qubits arranged diagonally in a grid and connected to four neighbors via special links. Google can adjust the degree of that linking. Because one of the 54 qubits was not functioning, Google performed the demonstration with 53 qubits and 86 links. The qubits are so-called transmons that work on the basis of superconductivity.
The scientists designed a pseudo-random quantum circuit for their test. Sampling the output of that circuit produced strings of bits with a certain probability of randomness. As the number of qubits and gates used increases, it becomes exponentially more difficult for classical computers to calculate that probability.
Attempts to simulate the calculation at the German Jülich and American Summit supercomputers, a lack of working memory threw a spanner in the works. A more limited calculation where Summit, the world’s most powerful supercomputer, samples three million bit strings, would take a year. Using Google Cloud servers, the calculation would take 50 trillion core hours and a petawatt hour of energy. “To put this in perspective, it took the quantum processor 600 seconds to sample the circuit three million times, the speed being limited by the communication of control hardware,” Google said. The net computation time without that limitation would be only thirty seconds.
According to Google, the findings of the demonstration can be used to generate proven random numbers. In the computing world, random number generators are used for many applications, such as security. In addition, Google expects that applications for machine learning and chemistry will be possible.
IBM, which also has a 53-qubit quantum processor, already published an objection on Monday in the run-up to Google’s announcement against the claim that the moment of quantum supremacy would have been reached. IBM claims that a simulation is possible in which a classical computer only takes two and a half days, using not only working memory but also storage memory to store and manipulate the calculation. IBM also declares that it is not a proponent of the term quantum supremacy, because quantum computers will never dominate classical computers, but rather cooperate with each their own strengths.