[institut] Seminar Fizickog fakulteta

[Mirzeta Savic]

Gost  prof. dr Zorana Radovica,  Dr Taro Yamashita sa Department of Physics, 
Tohoku University, Japan, odrzace seminar pod nazivom:

"Josephson qubit with ferromagnetic Josephson junction"

Abstract:  In recent years, several quantum computing devices (qubits) based 
on the Josephson effect have been demonstrated.  We propose a new qubit with 
a ferromagnetic Josephson junction in a pi-state [1,2].  The qubit consists 
of a superconducting ring with a ferromagnet and an insulator.  In the ring, 
superconductor insulator/superconductor (SC/I/SC) junction  is an ordinary 
Josephson junction (0-junction) whereas the metallic 
superconductor/ferromagnet/superconductor (SC/FM/SC) junction is a 
pi-junction.  The phase dependence of the Josephson coupling energy in  the 
pi-junction is nonsinusoidal because of the metallic contact.  Due  to the 
competition between the insulating 0-junction and the metallic pi-junction, 
the free energy of the ring has double minima as a function of the 
superconducting phase. By quantum tunneling between the two degenerate 
states,the bonding and the antibonding states are formed in the ring.  The 
coherent states are utilized as a quantum bit.  For the quantum computing, 
the so-called "universal gate", which consists of a single qubit rotation 
gate and a controlled-NOT (CNOT) gate, is required. In the qubit, the single 
qubit rotation is caused by icrowave resonance  between the coherent states 
(Rabi oscillation), and CNOT gate is also realized by using the inductive 
coupling between the qubits.  The qubit has the following advantages: (i) 
the simple geometry, i.e., only two Josephson junctions are used, (ii) the 
qubit is constructed without an external magnetic field, and only a small 
external magnetic field is needed in the measurement of the state. 
Furthermore, because of these advantages, (iii) the size of the qubit can be 
reduced.  This makes the large-scale integration possible, and leads to a 
reduction of the decoherence due to the coupling to the environment.  This 
proposal also offers a new route for spin-electronics to quantum computing.
[1] T. Yamashita et al., Phys. Rev. Lett. 95, 097001 (2005).
[2] T. Yamashita et al., cond-mat/0507199.

Seminar ce se odrzati u ponedeljak 20. februara 2006. godine u 13:30 casova, 
na Fizickom fakultetu, Studentski trg 12, sala 661, III sprat.