We would like to realize a novel class of topologically protected quantum bits (qubits) with much longer coherence time and therefore smaller error rate that would allow for real application of JJ circuits for quantum information processing. This would overcome the major barrier for the quantum computer implementation in JJ circuits : the fact that the error rate of all existing superconducting qubits is far too high in order to implement scalable quantum computations. Read more
The physical system we study are circuits based on rhombi chains, a rhombus being a superconducting loop with four identical JJs (see Figure 1). These circuits allow to realise the general idea of a topologically protected qubit proposed by Kitaev[1,2,3,4]. Read more
We have measured the ground state energy of a rhombi chain in the classical and quantum regime as a function of bias phase difference over the chain . The ground state of the chain can be determined by a measurement of the chain’s current-phase relation I() : the supercurrent is given by the derivative of E0() with respect to . In order to measure the current-phase relation of a rhombi chain, we implemented an escape measurement using a large JJ which is connected in parallel to the chain (Fig. 1). Read more
Fig. 1 : a) SEM image of a rhombi chain (N = 8) in the closed superconducting circuit. b) Shunt junction. c) Single rhombus. d) Schematic of the circuit designed to measure the current-phase relation of a rhombi chain. e) At half flux frustration the classical eigenstates of a single rhombus are given by the clockwise and anticlockwise circulating currents.
Current research :
We are currently working on the experimental identification of the energy spectrum of circuits based on rhombi and SQUID chains.
Current PhD : Ioan Pop
Lev Ioffe, Michael Gershenson, Rutgers University, New Jersey
Bénoit Douçot, LPTHE-CNRS, Paris
 A. Kitaev, quant-ph/9707021, Annales Phys. 303 (2003) 2-30
 L. Ioffe et al., Nature, 415, 503, (2002)
 B. Douçot and J. Vidal, Phys. Rev. Lett. 88, 227005 (2002)
 S. Gladchenko et al, Nature Physics 5, No 1 (2008)
 I. Protopopov et al. Phys. Rev. B 70, 184519, (2004)
 K.A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)
MIDAS ( European Strep ”Macroscopic Interference Devices on Atoms and Solids”)