Ground-state characterization of Nb charge-phase Josephson qubits

Abstract

We present investigations of Josephson charge-phase qubits inductively coupled to a radio-frequency driven tank-circuit enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits including junctions with linear dimensions of 60 nm and 80 nm are fabricated from Nb trilayer and allowing the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent at 4.2 K is explained in the framework of a quantum statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at 20 mK, demonstrating both the magnetic control of phase and the electrostatic control of charge on the qubit island.Comment: 8 pages, 8 figure

Current-phase relation and Josephson inductance in a superconducting Cooper-pair transistor

“…6 However, in superconducting Cooper pair transistors, where Coulomb energy plays an important role, the results on ͑derivatives of͒ CPRs have remained few and unpersuasive. [25][26][27] Closest to our work is the L J measurement of Ref. 27 but their result was distinctly affected by a mixing of the ground and excited states due to nonequilibrium quasiparticles.…”

“…Quasiparticles were also a problem in the determination of the ground state properties of Nb SCPTs in Ref. 25, where 1e periodicity in V g was observed contrary to our clean 2e-periodic modulation. Moreover, Born and co-workers have employed Josephson inductance to characterize qubits, 26 but their ratio of E J / E C ϳ 30...2 was too high to make observations similar to ours.…”

Quasiparticle transitions in charge-phase qubits probed by rf oscillations

“…The coupling coefficient and the tank circuit inductance L T ≈ 150 nH (see Ref. [20], where the behavior of an all-Nb circuit of similar design was investigated). Due to the coupling to the qubit, the effective inductance L eff of the resonance circuit is equal to L T − M 2 L −1 J (n, q, ϕ).…”

“…Our circuit comprises a Bloch transistor which is included in a superconducting ring inductively coupled to an rf-driven tank circuit [17,18,19,20]. In such an rf-SQUID configuration, the Bloch transistor is galvanically decoupled from the measuring circuit, which in general leads to a reduction in the density of non-equilibrium QPs that are able to enter the island.…”

“…The Bloch transistor, the loop and the tank circuit were fabricated by electron beam lithography on the same chip. The tank circuit inductor was fabricated on the basis of Nb technology [20,23], the qubit loop and the Bloch transistor by means of the two-angle Al shadow evaporation technique. No special precautions for the suppression of QP-poisoning of the island -such as, for example, the engineering of a barrier-like gap profile with the island gap value being greater than the electrode gap value [24,25], or the implementation of normal-metal QP traps [3] in the outer electrodes -were taken.…”