Physicists at UC Santa Barbara have made an important advance in quantum mechanics using a superconducting electrical circuit. The finding is reported in this week’s issue of the journal Nature.

The researchers showed that they could detect the quantum correlations in the results of measurements of entangled quantum bits, using a superconducting electrical circuit. The correlations are stronger than can be obtained using classical (non-quantum mechanical) physics, and according to the physicists, this illustrates that the oddities of quantum mechanics clearly extend to macroscopic systems. The work is part of an ongoing collaboration between the UCSB laboratories of John Martinis and Andrew Cleland.

The results of measurements in quantum mechanics are intrinsically unpredictable, according to the theory of quantum mechanics, and yet still contain very strong correlations, in contradiction with classical physics. In particular, measurements of “entangled states,” such as a pair of particles with opposite spins, allow stringent tests of the predicted discrepancy between quantum and classical physics, as described by the “Bell inequalities.” Measuring such a discrepancy is known as a “Bell violation.”

According to quantum theory, Bell violations should be detectable using “qubits,” superconducting quantum bits, but measuring these violations is technologically challenging. Martinis, Cleland, and their colleagues have overcome these challenges, and report a clear violation of Bell’s inequality with two entangled superconducting qubits. Thus, they have demonstrated that this macroscopic electrical circuit is a quantum system.