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In search of that quantum advantage

December is a hectic time of year for everyone, but quantum physicists seem to be especially busy. In particular, two new studies suggest researchers may be getting closer to achieving “quantum advantage”, the point at which quantum computing can solve a problem that no classical computing can in a feasible amount of time – that is, solve a problem that is otherwise computationally impossible.

In Europe, Danish and German researchers have built a chip that is a promising advancement in the race to build the world’s first photonic quantum computer, while a team from China used an unusual experimental set-up to demonstrate quantum advantage for the second time ever.

The first study, published in Science Advances, describes the development of a nanochip less than a tenth the width of the human hair yet capable of producing hundreds of stable photons, the fundamental particle of light, that can be used to store huge amount of data – and therefore form the hardware of quantum computers, like transistors in today’s computers.

“We now possess the tool that makes it possible to build a quantum simulator that can outperform a classical computer,” explains co-author Peter Lodahl, quantum physicist at the University of Copenhagen’s Niels Bohr Institute.

Practically speaking, reaching this milestone involves simultaneously controlling at least 50 quantum bits, known as qubits – analogous to the binary bits of zeroes and ones in classical computers. Google achieved this for the first time last year using superconducting qubits, but this new research shows it’s possible using photons.

“It is certainly an interesting and exciting result,” says Till Weinhold, a quantum physicist from Australia’s University of Queensland, who was not involved with the study. “It is closing the gap to the boundary where we can start to exploit a quantum computational advantage, but we are not there yet.”

Photonic quantum computing is just one type of quantum computing, which differ depending on the “building blocks” that make up qubits. Using photons was all the rage back in the 2000s, but the field became limited by inefficient photon sources and detectors, as well as the tricky problem of getting single photons to interact with each other.

Most big players in the quantum race today – like Google, IBM and Microsoft – have instead been focusing on using superconducting architectures, such as Google’s 53-qubit Sycamore chip.

“A superconducting quantum computer uses electric charge or the flow of currents to perform its computation, while a photonic quantum computer uses photons or light,” says Weinhold.

“One key difference between the two architectures thus becomes how information is detected, transmitted and manipulated, and the second is the speed. Optical frequencies are significantly higher and have the potential for an overall faster operation – similar to the difference between the classical copper telecommunication network and the fibre network for the NBN.”

The Danish team and its equipment. Credit: Niels Bohr Institute

Photonic computing is again near the top of the candidate list for quantum computation, thanks to several advances over the past few years – including more efficient photon detectors, improvements in single photon sources, and breakthroughs in novel ways to prevent errors when encoding qubits.

In fact, a team led by Jian-Wei Pan at the University of Science and Technology of China in Shanghai has just made the first definitive demonstration of quantum advantage using photons.

They used a distinctly untraditional computer – a novel tabletop set-up of lasers, mirrors, prisms and photon detectors – to perform a technique called Gaussian boson sampling. The calculation took just 200 seconds – but would have been impossible on classical computers. The team estimates that the Sunway TaihuLight, the third most powerful supercomputer in the world, would have taken a staggering 2.5 billion years.

Their results are published in Science and far surpass Google’s demonstration; Pan’s team observed 76 photons interacting.

However, unlike Google’s Sycamore, the Chinese team’s photonic quantum computer is not programmable and so can’t be used for solving more practical problems.

In contrast, back at the University of Copenhagen, the team has built a programmable nanochip that can act as a photon source – but hasn’t been able to test it.

“It would cost us 10 million Euro to perform an actual experiment that simultaneously controls 50 photons, as Google did with superconducting qubits,” says Ravitej Uppu, from the paper’s lead author.

“In the meantime, we will use our photon sources to develop new and advanced quantum simulators to solve complex biochemical problems that might, for example, be used to develop new medicines.”

Weinhold notes that while this is an exciting enabling tool, there is still a long way to go.

“The losses in the system are still too high for it to be actually useful to operate a photonic quantum computation that surpasses the capability of a classical supercomputer,” he explains.

“While their ability to generate and extract the photons is impressive, it is a combination of the remaining losses, the additional circuit losses and the distinguishability of the single photons that will need to collectively surpass the derived thresholds before a quantum advantage can be manifested.”

But photonic quantum computing is back in the race and may even be capable of overtaking the competition.