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arXiv:cond-mat/0204527 · Review of Scientific Instruments

Switching current measurements of large area Josephson tunnel junctions

Andreas J. Wallraff, Alexander V. Lukashenko, C. Coqui, A. Kemp, T. Duty, Alexey Vladimirovich Ustinov

Published Jul 23, 2003• 74 citations
arXivHTMLPDF

Abstract

We have developed a scheme for high resolution measurements of the switching current distribution of a current-biased Josephson tunnel junction using a timing technique. In the measurement setup digital control and read-out electronics are galvanically isolated from the analog sample bias electronics by an optical fiber link. We have successfully used this technique to investigate thermal activation and macroscopic quantum tunneling of the phase in a high-quality 5×5 μm2 Nb–Al/AlOx–Nb Josephson tunnel junction with a critical current of Ic≈325 μA. At temperatures above the cross over temperature of T★≈280 mK the escape is dominated by thermal activation. Due to the high quality factor of the junction (Q≈95), the escape temperature is noticeably affected by the thermal prefactor. At temperatures below T★, the value of which agrees well with the theoretical predictions, the escape of the phase by quantum mechanical tunneling is observed. The presented technique can be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.

DOI
10.1063/1.1588752

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We have developed a scheme for high resolution measurements of the switching current distribution of a current-biased Josephson tunnel junction using a timing technique. In the measurement setup digital control and read-out electronics are galvanically isolated from the analog sample bias electronics by an optical fiber link. We have successfully used this technique to investigate thermal activation and macroscopic quantum tunneling of the phase in a high-quality 5×5 μm2 Nb–Al/AlOx–Nb Josephson tunnel junction with a critical current of Ic≈325 μA. At temperatures above the cross over temperature of T★≈280 mK the escape is dominated by thermal activation. Due to the high quality factor of the junction (Q≈95), the escape temperature is noticeably affected by the thermal prefactor. At temperatures below T★, the value of which agrees well with the theoretical predictions, the escape of the phase by quantum mechanical tunneling is observed. The presented technique can be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.
Switching current distributions of an Nb/Al–AlO x /Nb Josephson junction are measured in a temperature range from 25 mK to 800 mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.
Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (∼0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.
This thesis examines Josephson tunnel junctions as candidate qubits for quantum computation. A large area current-biased junction, known as a phase qubit, uses the two lowest energy levels in a tilted washboard potential as the qubit states |0> and |1>. I performed experiments with 10 x 10 um^2 Nb/AlOx/Nb qubit junctions, with critical currents of roughly 30 uA. The state of a device was initialized by cooling below 50 mK in a dilution refrigerator. In order for quantum mechanical superpositions to be long-lived, it is necessary to isolate the junction from noisy bias leads that originate at room temperature. I studied two types of isolation: an LC filter, and a broadband scheme that used an auxiliary junction, resulting in a dc SQUID. One of the main goals of this work was to determine how well a simple Hamiltonian, derived assuming just a few lumped elements, describes the observed behavior of a macroscopic Josephson device, including coherent dynamics such as Rabi oscillations. I did this by comparing results to the expected behavior of ideal two-level systems and with more detailed master equation and density matrix simulations. I performed state manipulation by applying dc bias currents and resonant microwave currents, and through temperature control. The tunneling escape rate of the junction from the states |0> and |1> (zero voltage) to the running state (finite voltage) depends on the occupation probability of the energy levels and served as state readout. Experiments to measure the relaxation time T1 between |1> and |0> were performed by examining the dependence of the escape rate with temperature, yielding a maximum T1 = 15 ns. Measuring the decay to the ground state after applying a microwave pulse revealed at least two time constants, one of about 10 ns and another as long as 50 ns. The spectroscopic coherence time T2* was estimated to be roughly 5 ns by measuring resonance widths and the decay envelope of coherent Rabi oscillations was found to have a time constant T' = 10 ns over a wide range of conditions.
We report on the switching current distributions in nano-hybrid Josephson junctions made of InAs semiconductor nanowires. The temperature dependence of the switching current distribution can be understood through the motion of Josephson phase particles escaping from a tilted washboard potential, and the data could be fitted well by using the macroscopic quantum tunneling, thermal activation or phase diffusion models, depending on temperature. Application of the gate voltage to tune the Josephson coupling strength enable us to adjust the effective temperature for the escape process, and holds promising for developing gate-tunable superconducting phase qubits.
We report on the fabrication and millikelvin char acterization of Nb/Al–AlOx/Nb Josephson junctions directly de posited on X-cut LiNbO3 substrates. Motivated by the strong electro-optic and piezoelectric properties of LiNbO3, such devices are of interest for future hybrid superconducting–electro-optic architectures. The junctions were fabricated using standard processes involving DC magnetron sputtering followed by controlled thermal oxidation of the aluminum layer to form an AlOx tunnel barrier with reproducible junction areas. Insulation of the junctions was achieved by anodization and SiOx deposition. Electrical measurements performed at 30 mK reveal high-quality Josephson tunnel junction behavior with a critical current density ofJc= 70 A/cm2and a characteristic voltage reaching values up toVm= 286 mV. Differential conductance measurements exhibit pronounced quasiparticle peaks corresponding to a sum gap energy of 2Δ = (2.73 ± 0.05) meV, confirming the good quality of the Nb electrodes. Magnetic-field-dependent measure ments yield a Josephson penetration depth of λJ= 45 μm, confirming operation in the small-junction regime, with deviations from the ideal Josephson interference pattern attributed to non-uniform current distribution across the junction area. Compared to our previous characterization at 4.2 K, this work demonstrates reliable Josephson junction operation on LiNbO3 at dilution-refrigerator temperatures relevant for quantum circuits, supporting the suitability of lithium niobate as a promising substrate for future hybrid superconducting–electro-optic device implementations.
Josephson junctions based on Nb/AlOx/Nb trilayer technology have demonstrated excellent quality, exhibiting very low dissipation in the subgap region. This property is very important for all those experiments which have to deal with the quantum behavior of macroscopic variables in Josephson devices. In view of performing an experiment of macroscopic quantum coherence with a Josephson device, we measured the escape temperature and the return current of several junctions fabricated with different processes and having different characteristics, by cooling them from 4.2 to 0.3 K. We show that, for what concerns the process of escape from the zero-voltage state, no difference can be found among different junctions. On the other hand, the dissipation properties are strongly influenced by the current density, with lower current density resulting in lower intrinsic dissipation.