Retrospective test for quantum computers can build trust

January 24, 2018, National University of Singapore
This cartoon illustrates how a quantum computation performed over the cloud could be verified after completion with the help of a network of quantum computers. Researchers in Singapore and Japan have published in Physical Review Letters a proposal for such a scheme. Credit: Liu Jia & Aki Honda / Centre for Quantum Technologies, National University of Singapore

Tech companies are racing to make commercial quantum computers. A new scheme from researchers in Singapore and Japan could help customers establish trust in buying time on such machines—and protect companies from dishonest customers.

Quantum computers have the potential to solve problems that are beyond the reach of even today's biggest supercomputers, in areas such as drug modelling and optimisation.

"Our approach gives a way to generate a proof that a computation was correct, after it has been completed," explains Joseph Fitzsimons, a principal investigator at Singapore's Centre for Quantum Technologies and Assistant Professor at the Singapore University of Technology and Design. Fitzsimons carried out the work with colleague Michal Hajdusek and collaborator Tomoyuki Morimae, who is at Kyoto University in Japan. Their proposals are published in Physical Review Letters.

Quantum computers today are bulky, specialised machines that require careful maintenance, meaning that people are more likely to access machines owned and operated by a third party than to have their own—like a quantum version of a cloud service. Customers sending off data and programmes to a quantum computer will want to check that their instructions have been carried out as they intended. This problem of verification has been tackled before, but previous solutions required the customer to interact with the quantum computer while it was running the computation.

That kind of back-and-forth communication isn't necessary in the new scheme. "If you receive a result that look fishy, you can choose to verify the result, essentially retrospectively," says Fitzsimons. Verification guards against a quantum computer that does not perform correctly because of an accidental fault or even malicious tampering.

The improvement comes from how the calculation is checked. "The approach is completely different. We try to produce a state which can be used as a witness to the correctness of the computation. The previous approaches had some kind of trap built into the computation that gets checked as you go along," explains Fitzsimons.

The witness state registers each step of the computation. This means it must have as many bits as the computation has steps. For example, if a computation has 1000 steps, on 100 qubits, the witness would need to be 1100 qubits long.

The research team present two post-hoc verification schemes, based on different ways of testing the witness state. The first requires the customer to be able to send and measure quantum bits. In practice, this means they would need some specialized hardware and a line for sending these qubits to the owner of the quantum computer. The customer then measures the witness directly.

In the second scheme, the customer can be without any quantum tools—communication over the regular internet would do—but the quantum doing the calculation must be networked with five other quantum computers that help to check the witness state, playing a role as provers.

"It will be difficult to do an experiment to demonstrate post-hoc verification, but maybe not impossible", says Fitzsimons. A challenge is the size of the computers available today—the biggest are around 50 qubits. Another is that the networked setups required for the prover schemes don't exist—at least not yet.

The researchers wrap up their paper by pointing out an interesting advantage of the post-hoc verification scheme: It's not only the customer who could check that a computation was carried out correctly. The scheme allows public verifiability. The could be checked by a trusted third party, such as a court. This could protect the company if, say, a customer claimed the computation was not done correctly to avoid paying for the service.

Explore further: Why you might trust a quantum computer with secrets—even over the internet

More information: Joseph F. Fitzsimons et al, Post hoc Verification of Quantum Computation, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.040501

Related Stories

A new kind of quantum computer

November 6, 2017

Quantum mechanics incorporates some very non-intuitive properties of matter. Quantum superposition, for example, allows an atom to be simultaneously in two different states with its spin axis pointed both up and down, or ...

Quantum computing on the move

November 7, 2017

A future quantum computer, using quantum bits, or qubits, might be able to solve problems that are not tractable for classical computers. Scientists are currently struggling to build devices with more than a few qubits, as ...

Ion qubits offer early glimpse of quantum error detection

November 9, 2017

Computers based on quantum physics promise to solve certain problems much faster than their conventional counterparts. By utilizing qubits—which can have more than just the two values of ordinary bits—quantum computers ...

Refrigerator for quantum computers discovered

May 8, 2017

The global race towards a functioning quantum computer is on. With future quantum computers, we will be able to solve previously impossible problems and develop, for example, complex medicines, fertilizers, or artificial ...

Recommended for you

Tunable diamond string may hold key to quantum memory

May 22, 2018

A quantum internet promises completely secure communication. But using quantum bits or qubits to carry information requires a radically new piece of hardware—a quantum memory. This atomic-scale device needs to store quantum ...

Research reveals how order first appears in liquid crystals

May 22, 2018

Liquid crystals undergo a peculiar type of phase change. At a certain temperature, their cigar-shaped molecules go from a disordered jumble to a more orderly arrangement in which they all point more or less in the same direction. ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

rrwillsj
1 / 5 (1) Jan 24, 2018
I don't care how many quarky angels you get to dance on the quantum pin.

I don't believe you. I don't know any reason I should believe you . And the more you wheedle me too trust you? The faster my trust in your good intentions, declines. And, if I do not trust fallible humans building and coding these machines?

I sure as hell ain't going to trust any machine that tries to convince me that it is trustworthy!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.