24th October 2019

Google claims quantum supremacy

Google has announced that its 53-qubit 'Sycamore' processor has achieved quantum supremacy, performing a specific task in 200 seconds that would take the world's best supercomputers 10,000 years to complete.

Quantum supremacy is defined as the point when quantum computers become more capable than regular or "classical" computers. Researchers have been working for many years to achieve this milestone in computer science, using ever greater numbers of qubits – the quantum computing equivalent of bits that are found in your desktop PC, laptop, mobile and other devices.

Unlike the binary "bits" of classical computers (each a simple "0" or "1"), qubits can represent multiple values simultaneously, by exploiting the strange properties of subatomic phenomena. With sufficient numbers of qubits, quantum computers could tackle problems that are too complex and exponential for even the most powerful classical systems to handle, performing tasks in a matter of seconds that would take a binary computer thousands or even millions of years.

Just some of the many future applications may include:

• decoding of previously "uncrackable" codes;

• new ways to model financial data, with risk factors analysed in unprecedented detail, allowing better and safer investments;

• new ways to model the vast complexity of molecular and chemical interactions, leading to the discovery of new medicines and materials;

• vastly more powerful algorithms when data sets are very large, such as in searching images or video.



Last month, the Financial Times reported that "Google claims to have reached quantum supremacy with an array of 54 qbits out of which 53 were functional, which were used to perform a series of operations in 200 seconds that would take a supercomputer about 10,000 years to complete."

Yesterday, Google officially confirmed the claims. Researchers from the tech giant have just published a new study in Nature entitled Quantum supremacy using a programmable superconducting processor. In their paper, they describe a method called cross-entropy benchmarking to compare the quantum circuit's output ("bitstring") to its "corresponding ideal probability computed via simulation on a classical computer" to ascertain that the quantum computer is working correctly.

This milestone follows 20 years of research by project leader John Martinis and his group at the University of California, Santa Barbara, who partnered with Google to create the 'Sycamore' processor (pictured below). From the early development of a single superconducting qubit, they gradually progressed to more advanced systems including architectures of 72 and, with Sycamore, 53 qubits that exploit the bizarre properties of quantum mechanics.

Another company, D-Wave Systems, has previously announced quantum computers featuring hundreds of qubits. However, these rely on a technique called quantum annealing with high error rates and are generally not accepted by researchers as true "universal" quantum computers.



Google's Sycamore processor. Credit: Erik Lucero, Research Scientist and Lead Production Quantum Hardware

"We basically wanted to produce an entangled state involving all of our qubits as quickly as we can," explained Brooks Foxen, a graduate student researcher in the Martinis Group. "And so we settled on a sequence of operations that produced a complicated superposition state that, when measured, returns bitstring with a probability determined by the specific sequence of operations used to prepare that particular superposition. The exercise, which was to verify that the circuit's output corresponds to the sequence used to prepare the state, sampled the quantum circuit a million times in just a few minutes, exploring all possibilities – before the system could lose its quantum coherence."

"We performed a fixed set of operations that entangles 53 qubits into a complex superposition state," said Ben Chiaro, another graduate student researcher in the Martinis Group. "This superposition state encodes the probability distribution. For the quantum computer, preparing this superposition state is accomplished by applying a sequence of tens of control pulses to each qubit in a matter of microseconds. We can prepare and then sample from this distribution by measuring the qubits a million times in 200 seconds."

"For classical computers, it is much more difficult to compute the outcome of these operations, because it requires computing the probability of being in any one of the 2^53 (9,007,199,254,740,992) possible states, where the 53 comes from the number of qubits – the exponential scaling is why people are interested in quantum computing to begin with," said Foxen. "This is done by matrix multiplication, which is expensive for classical computers as the matrices become large."

"Quantum mechanical states do things that go beyond our day-to-day experience and so have the potential to provide capabilities and application that would otherwise be unattainable," commented Joe Incandela, UC Santa Barbara's vice chancellor for research. "The team has demonstrated the ability to reliably create and repeatedly sample complicated quantum states involving 53 entangled elements to carry out an exercise that would take millennia to do with a classical supercomputer. This is a major accomplishment. We are at the threshold of a new era of knowledge acquisition."

Quantum supremacy may have been achieved, but for Chiaro, Foxen, Martinis and other researchers, this is just the beginning. Correction of "noisy" qubits, reducing error rates, and improving coherence times could enable the simulation of fascinating phenomena in quantum mechanics, or unlock the vast potential in chemistry and materials science. Combined with ever-increasing numbers of qubits, even more impressive milestones await in the future.

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