First secure quantum computer is blind to its own bits

The first secure quantum computer has been made by combining entanglement, a bizarre property of tiny particles, with the power of apparent randomness.

The technique is similar to quantum cryptography, which guarantees the secrecy of a message sent from one place to another, but in this instance guarantees the privacy of data-processing. It could enable code-breakers, governments or private individuals to harness the power of a quantum server remotely without having to worry that the owner can snoop on their data or calculations.

Quantum computers exploit the ability of quantum particles to be in more than one state at the same time. This allows the computer to check many possible solutions to a problem simultaneously.

If this capability can be scaled up, it could allow quantum computers to solve problems that are beyond the power of classical computers. Nobody has yet succeeded in building a useful quantum computer, but if they do, such computers will be expensive and rare. So it is unlikely that people, or even government departments, will have their own.

Fragile qubits

Renting time on them remotely, though, presents a new problem: how to ensure that whatever the remote user is doing is hidden from the person or company who owns the computer. Enter blind quantum computation, first outlined theoretically in 2009. It combines two tricks to ensure that a computer owner can detect nothing about the data it receives, the algorithm it executes or the result it finds.

The first is entanglement, the ability to link two quantum particles no matter how far apart they are. Entanglement is hugely fragile: sneeze and it vanishes. As a result, if an eavesdropper measures any properties of an entangled qubit, his or her presence will be obvious.

However, all quantum computers already have entangled qubits, and this alone can't provide complete security. An eavesdropper could still glean some information in the process of being detected.

So blind quantum computing has an added twist. The remote user must encode the programs to be run on the computer in such a way that it looks random but in fact is not. The quantum computer still runs the program but if the computer owner intercepts the result, he or she would not be able to make sense of it. The user, of course, can decode the result that is returned by reversing the encryption process.

Doubly blind

For the first time Stefanie Barz at the University of Vienna in Austria and colleagues have demonstrated such blind quantum computing using a photon-based quantum computer.

They created strings of photons that looked random but were actually encoded versions of two programs: Deutsch's algorithm, which looks for regularities in certain mathematical functions, and Grover's algorithm, which searches an unsorted database.

They beamed these strings at the quantum computer. It ran the algorithms but because of encryption there was no way to detect it was doing this just by examining the quantum computer. Only when the results were returned could they be decoded and checked. "It's a new level of security," says Barz.

The secrecy is two-way. The technique also ensures that the user cannot know anything about the quantum computer. "You don't learn anything about their technology or how it works. It's a kind of double-blindess," says Vlatko Vedral, a quantum physicist at the University of Oxford, who was not involved in the work.

This may seem like an extreme form of secrecy, but Vedral says that various government and military organisations need to guarantee the secrecy of their data and calculations on timescales of 30 to 50 years. "The only way of doing that is to use blind computing," he says.

Journal reference: Science, DOI: 10.1126/science.1214707

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