Superconducting qubits have passed a key quantum test

For the first time, a superconducting circuit has passed a Bell test, the premier test in physics to confirm a system’s quantum behaviour. These circuits are used in quantum computers, and this test proves that their quantum bits are truly entangled.

When two particles are entangled, measuring the characteristics of one instantly affects the measured characteristics of the other in what is called a non-local correlation. When this happens, it means the effects of the entanglement must travel faster than light. The test for this strange quantum effect is called Bell’s inequality, which sets a limit on how often particles can end up in the same state by chance without the presence of actual entanglement. Violating Bell’s inequality is proof that a pair of particles are, in fact, entangled.

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Bell tests have been performed in many systems, but never on a superconducting circuit. For the test, the two entangled systems have to be far enough apart that a signal could not have travelled between them at the speed of light in the time it takes to measure both systems. This is difficult to test in a superconducting circuit, because the whole thing has to be kept at temperatures close to absolute zero. For the first time, Simon Storz at the Swiss Federal Institute of Technology in Zurich and his colleagues have managed to perform a Bell test on such a circuit.

They connected the two entangled parts of the circuit, called quantum bits or qubits, using microwaves sent through a chilled 30-metre-long aluminium tube, while keeping each qubit in its own individual refrigerator. They then used a random number generator to decide what sort of measurement to make on the qubits to avoid any human bias.

The researchers made more than 4 million measurements at a rate of 12,500 measurements per second – a speed necessary to make sure each pair of measurements occurred faster than light could travel down the tube between the two qubits. Analysing all of those data points together, they found with high certainty that Bell’s inequality was violated and the qubits were truly undergoing what Albert Einstein termed “spooky action at a distance”, as was expected.

“The test confirms the platform’s ability to exploit these unique quantum features for technological applications,” says Storz. The success of connecting the qubits across 30 metres is particularly promising for quantum computing and encryption, he says. “This is a potential path towards scaling up superconducting circuit-based quantum computers, for instance in future quantum supercomputer-like centres.”