Measurement of the current-phase relation in Josephson junction rhombi chains

We present low-temperature transport measurements in one-dimensional Josephson junctions rhombi chains. We have measured the current-phase relation of a chain of eight rhombi. The junctions are either in the classical phase regime with the Josephson energy much larger than the charging energy, E_J≫E_C , or in the quantum phase regime where E_J/E_C≈2 . In the strong Josephson coupling regime ( E_J≫E_C≫k_BT ) we observe a sawtoothlike supercurrent as a function of the phase difference over the chain. The period of the supercurrent oscillations changes abruptly from one flux quantum Φ₀ to half the flux quantum Φ₀/2 as the rhombi are tuned in the vicinity of full frustration. The main observed features can be understood from the complex energy ground state of the chain. For E_J/E_C≈2 , we do observe a dramatic suppression and rounding of the switching current dependence which we found to be consistent with the model developed by Matveev [Phys. Rev. Lett. 89, 096802 (2002)] for long Josephson junctions chains.