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When a quantum computer can produce results that would take thousands of years to produce out of a classical computer, an obvious question arises: if you've given the wrong answer, how would you know? That's a question to which University of Vienna boffins have turned their attention to.

A computation involving a handful of qubits can be checked by a classical computer, because it can iterate through the possible states one-by-one. Some other quantum computations are also checkable in the classical world: for example, if we produce a quantum computer with enough power to factor very long cryptographic keys, the result would be testable against the original message.

However, quantum computing boffins assure us that just 300 qubits should represent more possible states than there are atoms in the visible universe, making the job of delivering “provably correct” results a challenge. Some proposals to overcome this go so far as to create entanglements between entire quantum computers, something which reaches far beyond any current technology.

The University of Vienna's Philip Walther, Stefanie Barz and their collaborators, proposed a scheme called “blind quantum computing” in a paper in Nature, and now, the same group says it has demonstrated the technique at a small scale.

The basic idea is simple: the calculation includes traps, intermediate steps in a calculation for which the “classical” answer can be known in advance.

Meanwhile, the quantum computer actually carrying out the calculation has no idea what it's doing. As explained at Science Magazine: “A quantum computer receives qubits and completes a task with them, but it remains blind to what the input and output were, and even what computation it performed … The test is designed in such a way that the quantum computer cannot distinguish the trap from its normal tasks”.

The trap is designed show an error while the quantum computer is working.

The trick is that the researchers didn't embed their traps into a classical calculation. Rather, they used a four-qubit quantum computer as the verifier, to perform a “blind Bell test” against a second quantum computer. In their Nature paper, they claim the experiment “is independent of the experimental quantum-computation platform used”.

It's just an experiment a this stage. As Scott Aaronson of MIT told Science, “this currently has the status of a fun demonstration proof of concept, rather than anything that's directly useful yet”, but such demonstrations are “necessary steps” towards useful quantum computers.

The University of Vienna is quite capable at producing “fun demonstrations” of quantum computing. Earlier this year, it produced a real-time visualisation of the emergence of entanglement. ®

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