Dephasing and heat currents in superconducting qubits

by Janine Splettstoesser (Chalmers University)

Monday 2 Feb 2015, 11:00 → 12:00 Europe/Stockholm

Nordita West (122:026)

The heat current flowing through a temperature-biased Josephson junction is a periodic function of the phase difference between the electrodes. This effect has recently been demonstrated experimentally [1], in a dc-SQUID exposed to a temperature gradient. An altogether different application of the phase sensitivity of the supercurrent in SQUIDs is the realisation of a flux qubit [2], where the phase sensitivity of the device is used to implement qubit operations. We combine these two intriguing studies on the phase-sensitivity in superconducting rings [3,4]: in particular, we demonstrate that the heat current through a qubit depends on its quantum state. We show that in turn, the state-sensitive heat current has an impact on the qubit state, yielding a source of decoherence, which has until now been disregarded. I will here present studies on two different types of flux qubits, the Delft qubit and the recently developed fluxonium [5]. They are both made of a superconducting loop interrupted by Josephson junctions, which we here assume to be subject to a temperature gradient. We have found that even small temperature gradients can lead to dephasing times of the order of microseconds for the Delft-qubit design. We furthermore show that the dephasing due to thermal currents is less damaging for the fluxonium qubit. This can be attributed to its small inductive energy, stemming from the superinductance realized by a chain of Josephson junctions. [1] F. Giazotto and M. J. Martinez-Perez, Nature 492, 401 (2012). [2] J. E. Mooij, et al., Science 285,1036 (1999). [3] S. Spilla, F. Hassler, and J. Splettstoesser, New J. Phys. 16, 045020 (2014). [4] S. Spilla, F. Hassler, A. Napoli, and J. Splettstoesser, in preparation. [5] V. Manucharyan, et al., Science 326, 113 (2009).