When quantum computers eventually reach larger scales,
they’ll probably remain pretty precious resources, locked away in
research institutions just like our classical supercomputers. So anyone
who wants to perform quantum calculations will likely have to do it in
the cloud, remotely accessing a quantum server somewhere else. A new double-blind cryptography method
would ensure that these calculations remain secret. It uses the
uncertain, unusual nature of quantum mechanics as a double advantage.
Entangled Qubits
Clusters of entangled qubits allow remote quantum computing to be
performed on a remote server, while keeping the contents and results
hidden.
Imagine you’re a developer and you have some code you’d like to run
on a quantum computer. And imagine there’s a quantum computer maker who
says you can run your code. But you can’t trust each other — you, the
developer, don’t want the computer maker to rip off your great code, and
the computer builder doesn’t want you to peep its breakthrough machine.
This new system can satisfy both of you.
Stefanie Barz and colleagues at the University of Vienna’s Center for
Quantum Science and Technology prepared an experimental demonstration
of a blind computing technique, and tested it with two well-known
quantum computing algorithms. Here’s how it would work: You, the developer, prepare some quantum bits,
in this case photons that have a polarity (vertical or horizontal)
known only to you. Then you would send these to the remote quantum
server. The computer would entangle the qubits with even more qubits,
using a quantum entangling gate — but the computer wouldn’t know the
nature of the entangled states, just that they are in fact entangled.
The server is “blind” to the entanglement state, and anyone tapping into
the server would be blind, too.
Imagine the computer tries to snoop on the qubits and see their
entanglement, which could then be used to extract the information they
carry. You’d be able to tell, because of the laws of quantum mechanics.
The cat is both dead and alive until you check whether it’s dead or
alive, and then it’s one or the other. If your photon has a specific
state, you’d be able to tell that it was spied upon.
Back to the entangled bits. The actual information processing takes
place via a sequence of measurements on your qubits. These measurements
would be directed by you, based on the particular states of each qubit
(which, again, only you know). The quantum server would run the
measurements and report the results to you. This is called
measurement-based quantum computation. Then you’d be able to interpret
the results, based on your knowledge of the qubits’ initial states. To
the computer — or any interceptor — the whole thing would look utterly
random.
Since you know the entangled state on which the measurements were
made, you can be certain whether the server really was a quantum
computer. And you wouldn’t have to disclose your algorithm, the input or
even the output — it’s perfectly secure, the researchers write in their
paper, published online today in Science.
Blind quantum computation is more secure than classical blind
computation, which relies on tactics like the backward factoring of
prime numbers, said Vlatko Vedral, a researcher at the University of
Oxford who wrote a Perspective piece explaining this finding.
“The double blindness is guaranteed by the laws of quantum physics,
instead of the assumed difficulty of of computational tasks as in
classical physics,” Vedral writes.
The Vienna team argues their simulation is a potentially useful technique for future cloud-based quantum computing networks.
“Our experiment is a step toward unconditionally secure quantum
computing in a client-server environment where the client’s entire
computation remains hidden, a functionality not known to be achievable
in the classical world,” they write.
From popsci
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