Metastable States of Josepshon Vortices:
Thermal Processes and Quantum Effects

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

Physikalisches Institut III,
Universitaet Erlangen-Nuernberg, D-91058 Erlangen

M. V. Fistul

MPI für Physik komplexer Systeme, Dresden

We experimentally study the dynamics of a single Josephson vortex in a
tilted periodic potential.  In such a potential metastable vortex-states
are generated due to the interaction of the vortex with a pinning site and
a uniformly distributed bias current.  The pinning is induced by the
interaction of the vortex with an inhomogeneity in the junction or by
shaping a junction subject to a small external magnetic field [1].  At high
temperatures, we investigate the thermal escape of the vortex out of the
metastable state.  Reducing temperature and damping the macroscopic quantum
properties of Josephson vortices, such as energy level quantization and
quantum tunneling, are expected to manifest themselves [2,3].  We report
our current experimental efforts to observe these effects.  Our interest in
such a macroscopic quantum system is related to the possibility of using
quantum states of Josephson vortices for performing quantum computation. 
We suggested that a fluxon trapped in a double-well potential in a narrow
long junction can be used as a qubit [1].

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