Quantum Computing Secret

You may not need a quantum computer of your own to securely use quantum computing in the future. For the first time, researchers have shown how even ordinary classical computer users could remotely access quantum computing resources online while keeping their quantum computations securely hidden from the quantum computer itself.

Tech giants such as Google and IBM are racing to build universal quantum computers that could someday analyze millions of possible solutions much faster than today’s most powerful classical supercomputers. Such companies have also begun offering online access to their early quantum processors as a glimpse of how anyone could tap the power of cloud-based quantum computing. Until recently, most researchers believed that there was no way for remote users to securely hide their quantum computations from prying eyes unless they too possessed quantum computers. That assumption is now being challenged by researchers in Singapore and Australia through a new paper published in the 11 July issue of the journal Physical Review X.

“Frankly, I think we are all quite surprised that this is possible,” says Joseph Fitzsimons, a theoretical physicist for the Centre for Quantum Technologies at the National University of Singapore and principal investigator on the study. “There had been a number of results showing that it was unlikely for a classical user to be able to hide [delegated quantum computations] perfectly, and I think many of us in the field had interpreted this as evidence that nothing useful could be hidden.”

The technique for helping classical computer users hide their quantum computations relies upon a particular approach known as measurement-based quantum computing. Quantum computing’s main promise relies upon leveraging quantum bits (qubits) of information that can exist as both 1s and 0s simultaneously—unlike classical computing bits that exist as either 1 or 0. That means qubits can simultaneously represent and process many more states of information than classical computing bits.

In measurement-based quantum computing, a quantum computer puts all its qubits into a particular state of quantum entanglement so that any changes to a single qubit affect all the qubits. Next, qubits are individually measured one by one in a certain order that specifies the program being run on the quantum computer. A remote user can provide step-by-step instructions for each qubit’s measurement that encode both the input data and the program being run. Crucially, each measurement depends on the outcome of previous measurements.

Fitzsimons and his colleagues figured out how to exploit this step-wise approach to quantum computing and achieve a new form of “blind quantum computation” security. They showed how remote users relying on classical computers can hide the meaning behind each step of the measurement sequence from the quantum computer performing the computation. That means the owner of the quantum computer cannot tell the role of each measurement step and which qubits were used for inputs, operations, or outputs.

The finding runs counter to previous assumptions that it was impossible to guarantee data privacy for users relying on ordinary classical computers to remotely access quantum computers. But Fitzsimons says that early feedback to the group’s work has been “very positive” because the proposed security mechanism—described as the “flow ambiguity effect”—is fairly straightforward.