[QIP-Sem] Special Seminar Wednesday at 4pm

[Terry Orlando]

Special Seminar
Wednesday August 25th at 4pm
13-3028 (conference room on the third floor of Building 13)


Circuit-QED with Superconducting Flux Qubits.
Matteo Mariantoni
Walther-Meissner-Institut,


in collaboration with Markus J. Storcz3, Frank K. Wilhelm, Achim Marx, and
Rudolf Gross



The symbiosis of cavity-quantum-electrodynamics (cQED) [1] and superconducting
quantum bit (qubit) circuits has gained great interest lately. This scheme 
can drastically
improve the decoherence properties of superconducting qubits and allows for
the manipulation and reading-out of qubits more easily than existing 
architectures.
Wallraff et al. recently proposed [2, 3, 4] coupling a superconducting charge
qubit to a one-dimensional transmission line resonator. They have shown 
that the
transmission line resonator acts as a cavity whose photons can be strongly 
coupled
to the quantized excitations of an adjacent qubit. As a consequence, a 
dispersive
read-out of the qubit can be performed. The transmission line resonator can 
also
be used to prepare the state of the qubit, control it, and entangle 
additional qubits.
Moreover, the qubit lifetime is expected to be effectively enhanced.
A realizable design proposal of a microcavity for superconducting flux 
qubits does
not yet exist, although flux qubits are promising candidates for quantum 
computing
because of their long decoherence times and their simple design. We propose to
embed a three-Josephson-junctions flux qubit [5] into a high quality factor 
microstrip
resonator [6, 7, 8] which will serve as our cavity. The microstrip 
resonator can
be fabricated using a Nb thin-film coil separated from a Nb ground-plane by an
insulating layer. The microstrip resonator is inductively coupled to a 
qubit placed
inside the innermost turn of the coil. Furthermore, the microstrip can also 
serve
as a dc-flux-bias line. A similar structure has been used in Refs. [7, 8] 
to probe
and control flux qubits in a different way. In these experiments, the 
resonator used
was made of lumped elements and was extremely detuned from the characteristic
frequency of the qubit, thus yielding a very weak cavity-qubit coupling. We 
calculate
the vacuum Rabi frequency for the coupling [1, 3, 4, 9, 10] between the cavity
mode and the qubit pseudo-spin and show that, with an accurate design of the
microstrip resonator, the strong coupling limit of cQED can be achieved. 
Moreover,
we investigate the control and read-out of flux qubits in the cavity, 
propose a scheme
for coupling several qubits by way of cavity photons, and analyze the 
characteristic
decay rates of the system.
References
[1] J. M. Raimond, M. Brune, and S. Haroche, Rev.Mod. Phys. 73, 565 (2001).
[2] A.Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J.Majer, 
S. Kumar,
S. M. Girvin, and R. J. Schoelkopf, accepted for publication in Nature 
(2004), condmat/
0407325.
[3] S. M. Girvin, R.-S. Huang, A. Blais, A.Wallraff, and R. J. Schoelkopf, 
Proceedings of
Les Houches Summer School, Session LXXIX, Quantum Entanglement and Information
Processing (2003), cond-mat/0310670.
[4] A. Blais, R.-S. Huang, A.Wallraff, S. M. Girvin, and R. J. Schoelkopf, 
Phys. Rev.A69
(2004), cond-mat/0402216.
[5] J. E.Mooij, T.P. Orlando, L. Levitov, L. Tian, C. H. van der Wal, and 
S. Lloyd, Science
285, 1036 (1999).
[6] M.M¨uck and J. Clarke, J. Appl. Phys. 88, 6910 (2000).
[7] E. Il’ichev, N. Oukhanski, A. Izmalkov, Th.Wagner, M. Grajcar, H.-G. Meyer,
A.Yu. Smirnov, A. Massen van den Brink, M. H. S. Amin, and A. M. Zagoskin, 
Phys.
Rev. Lett. 91, 097906 (2003).
[8] A. Izmalkov, M. Grajcar, E. Il’ichev, Th.Wagner, H.-G. Meyer, A.Yu. 
Smirnov,
M. H. S. Amin, A. Massen van den Brink, and A. M. Zagoskin, cond-mat/0312332.
[9] C.-P.Yang, S.-I Chu, and S. Han, Phys. Rev.A67, 042311 (2003).
[10] C.-P.Yang, S.-I Chu, and S. Han, Phys. Rev. Lett. 92, 117902 (2004).
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