For the first time, a superconducting circuit has passed a Bell test, the main test in physics to confirm the quantum behavior of a system. These circuits are used in quantum computers, and this test shows that their quantum bits are indeed entangled.
When two particles are entangled, the measured features of one instantly affect the measured features of the other in what is called non-local correlation. When this happens, it means that the entanglement effects must travel faster than light. The proof of this strange quantum effect is called Bell’s inequality, which places 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 is, in fact, entangled.
Bell tests have been done on many systems, but never on a superconducting circuit. For the test, the two entangled systems must be far enough apart that a signal cannot travel between them at the speed of light in the time it takes to measure both systems. This is hard to prove in a superconducting circuit, because everything 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 managed to perform a Bell test on such a circuit.
They connected the two interlocking parts of the circuit, called quantum bits or qubits, using microwaves sent through a 30-meter-long chilled aluminum tube, while keeping each qubit in its own individual refrigerator. They then used a random number generator to decide what kind 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 needed to ensure that each pair of measurements occurred faster than light could travel down the tube between the two qubits. By analyzing all those data points together, they found with high certainty that Bell’s inequality was violated and that the qubits were actually experiencing what Albert Einstein called “spooky action at a distance,” as 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 over 30 meters is particularly promising for quantum computing and encryption, she says. “This is a potential path towards scaling up quantum computers based on superconducting circuits, for example, in future centers similar to quantum supercomputers.”