Measurement of quantum phaseslips in a Josephson junction chain
Josephson junction chains attract currently a lot of interest due to their possible applications in metrology or quantum information. For example, under microwave irradiation of frequency f, such chains could exhibit current quantization I=2nef where 2e is the charge of a Cooper pair and n is an integer number. They could be used for the definition of a new quantum current standard. In view of the potential applications, we have measured the ground state of a Josephson junction chain. Here we have analysed our results in terms of “quantum phaseslips”, the central phenomenon governing these superconducting networks.
A Josephson junction is constituted of two superconducting electrodes separated by an insulating layer. A phase difference between the two electrodes φ yields a supercurrent through the junction I(f)=I_{c} sin(φ). I_{c} maximum value of this supercurrent is fixed by the junction Josephson energy E_{J} which depends on the superconducting gap of the electrodes and on the junction normal state resistance. The junction charging energy E_{C}=q^{2}/2C (C is its capacitance) induces quantum fluctuation on the phase difference φ across the junction.These quantum phase fluctuations induce socalled “quantum phaseslips” where the phase across the junction changes by 2π. If a phaseslip occur ocross a junction embedded in a chain with many other junctions, new phenomena appear. Here, a quantum phaseslip on one junction leads to a collective response of all the others. 
This collective behaviour implies the formation of new energy levels ε_{n}(δ).Here δ is the overall phase difference over the chain given by the sum of all junctions’ phase differences φ_{i}. For a large phaseslip rate υ, i.e. the number of quantum phaseslips per second, the currentphase relation of a Josephson junction chain, I_{chain}(δ)=I_{c}^{chain} sin(δ) has the same form as for a single junction but with an effective critical current Icchain. This critical current is determined by the number of junctions in the chain and the quantum phaseslip rate.
In the measurements, the ground state of a Josephson junction chain containing 6 SQUIDs is determined by measuring the critical current of the chain shunted by a readout junction (Fig.1). As each junction is a SQUID, the E_{J}/E_{C} ratio can be tuned in situ by changing the magnetic flux Φ_{S} through the SQUIDs. Fig. 2 shows the chain’s critical current as a function of Φ_{S}. With increasing flux, the critical current is suppressed due to the increase of quantum fluctuations. From our measurements, the quantum phaseslip rate υ is deduced.
Quantum phaseslips in Josephson junction networks offer interesting prospects in the realisation of topologically protected qubits and for current standards.

Further reading :
 Measurement of the effect of quantum phaseslips in a Josephson junction chain, I. Pop, . I. Protopopov, F. Lecocq, Z. Peng, B. Pannetier, O. Buisson, W. Guichard, arXiv:0912.2417 accepted for publication in Nature Physics
 Measurement of the currentphase relation in Josepshon junction rhombi chains, I. Pop, K. Hasselbach, 0. Buisson, W. Guichard, I. Protopov, B. Pannetier, Phys. Rev. B, Phys. Rev. B 78, 104504 (2008)
Cet article est repris du site C’NANO RhôneAlpes