Measurement of geometric dephasing using a superconducting qubit

Talking to friends in a crowded venue is sometimes difficult: the information one tries to get across gets washed out in the noisy environment. In this respect, quantum systems are no different. When they interact with the environment, they are subject to dephasing which ultimately destroys the information they hold. This time around, we have used superconducting circuits to demonstrate that dephasing in quantum systems comes in two distinct flavours: not only the well-studied dynamic dephasing (originating in the stochastic evolution of the dynamic phase of the system's wave function), but also geometric dephasing. As a quantum system evolves in time, it traces out a path in its projective Hilbert space, thereby acquiring a geometric phase proportional to the encircled area. In an open quantum system the system’s path will fluctuate, resulting in geometric dephasing. Depending on the direction along which this path is traversed, the system is either subject to even more dephasing or it regains lost coherence, which makes the geometric nature of this type of dephasing manifest. Shaping the paths of the system’s state could therefore be used to improve the coherence properties of the quantum system.

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S. Berger, M. Pechal, P. Kurpiers, A. A. Abdumalikov, C. Eichler, J. A. Mlynek, A. Shnirman, Y. Gefen, A. Wallraff, and S. Filipp, Nat Commun (2015)