Josephson Vortex Dynamics at Millikelvin Temperatures

A. Wallraff, A. Kemp, and A. V. Ustinov

Physikalisches Institut III, Friedrich-Alexander-UniversitŠt 
Erlangen-NŸrnberg, D-91058 Erlangen

We experimentally study the dynamics of a single Josephson vortex in a
tilted periodic potential.  Applying a bias current uniformly to a
shaped long Josephson junction subject to an in-plane external
magnetic field, metastable vortex-states are created.  The escape of
the vortex from these states is experimentally investigated by
measuring the statistical distribution of the junction critical
current, i.e.~the vortex depinning current.  At high temperatures, the
thermal activation of the vortex is observed.  As temperature and
damping is reduced, the macroscopic quantum properties of Josephson
vortices, such as energy level quantization and quantum tunneling, are
predicted to appear [2,3].  Here we report on our current experimental
work to observe these effects at millikelvin temperatures.  Our
interest in this macroscopic quantum system is related to the
possibility of using quantum states of Josephson vortices for
performing quantum computation.  We have suggested that a vortex
trapped in a double-well potential in a narrow long junction can be
used as a scalable and well-controllable qubit [3].

[1] T. Kato and M. Imada, J. Phys. Soc. Japan 65, 2963 (1996).
[2] A. Shnirman, E. Ben-Jacob, and B. Malomed, Phys.  Rev.  B 56,
14677 (1997).
[3] A. Wallraff et al. , J. Low Temp.  Phys. 188, 543 (2000).