© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

IEEE Transactions on Applied Superconductivity

Superconducting tunnel junction structures designed for qubit realizations

Detlef Born, Th. Wagner, Wolfram Krech, M. Grajcar, Uwe Hübner

Published Jun 1, 2003• 1 citation

Abstract

In view of future macroscopic quantum experiments, we have fabricated prototypes of charge qubits (with tunable Josephson coupling and capacitive gate) provided with capacitively coupled readout single-electron transistor electrometers. The devices are prepared by electron beam direct-writing in conjunction with material deposition by sputtering. All leads are made of Al, and the AlO/sub x/ tunnel barriers are generated by local thermal oxidizing of the surface of the base striplines. The island electrodes are made of Al or Nb in a subsequent deposition step. In the case of Nb it is suitable to protect the barriers by a thin sputtered Al film first. The small-area (100 nm/spl times/100 nm) tunnel junctions have capacitances in the order of a few 10/sup -16/ F; their resistances can be adjusted in a wide range from k/spl Omega/ to M/spl Omega/ with an on-chip spread of less than 10%. The (double) superconducting gap energy was determined to be about 0.4 meV for Al and 1.4 meV for Nb, respectively. Measuring the so-called Coulomb staircase, we have demonstrated the operation of both qubit boxes and sub-e electrometers.

DOI: 10.1109/tasc.2003.814127

Related papers

Understanding the behavior of superconducting Coulomb-blockade electrometers

D. Song, A. Amar, Christopher J. Lobb +1 more

We report measurements and quantum mechanical simulations of the behavior of superconducting two-junction Coulomb-blockade electrometers biased near the superconducting gap. Our quantum model allows coherent superpositions of states with different number of charges on the island. It predicts maximum current modulation /spl delta/I for devices with tunneling resistance R/sub /spl Sigma///spl ap/64/spl pi//spl Delta//spl planck//sup 2/C/sub /spl Sigma///sup 2//(e/sup 6//spl tau/), where /spl tau/ is the time for which the system maintains quantum coherence, C/sub /spl Sigma// is the total island capacitance and /spl Delta/ is the superconducting energy gap. For comparison, we built Al-AlO/sub x/-Al electrometers with R/sub /spl Sigma// from 61 k/spl Omega/ to 1.7 M/spl Omega/ and C/sub /spl Sigma// ranging from 0.9 fF to 8 fF. We find good agreement for /spl tau//spl ap/0.35 nsec.

IEEE Transactions on Applied Superconductivity•Jun 1, 1997•0 citations

Characterization of superconducting devices spanning the charging to Josephson regimes

Maria Gabriella Castellano, Roberto Leoni, G. Torrioli +6 more

Charge states in mesoscopic Josephson devices are considered promising candidates for a solid-state implementation of quantum computing. We studied three superconducting single electron transistors (SSETs) fabricated with small Al/AlO x /Al tunnel junctions, which, with a different choice of the construction parameters, can realize both the quantum bit and the readout system for the qubit charge state. We varied the junction capacitance and barrier transparency, making the Josephson energy E J of the device smaller, comparable and larger than the charging energy E C : the first case is suitable to build the qubit, while the latter can be suitable for the qubit readout.

Superconductor Science and Technology•Nov 21, 2001•1 citation

Fabrication and dc characteristics of small-area tantalum and niobium superconducting tunnel junctions

Dean W. Face, D. E. Prober

We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.

Journal of Applied Physics•Oct 15, 1987•12 citations

Fabrication of high-quality submicron Nb/Al-AlO x /Nb tunnel junctions

Haifeng Yu, Cao Wen-Hui, Zhu Xiao-Bo +6 more

Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.

Chinese Physics B•Aug 1, 2008•7 citations

Single-charge devices with ultrasmall Nb∕AlOx∕Nb trilayer Josephson junctions

R. Dolata, Hansjorg Scherer, Alexander B Zorin +1 more

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.

arXiv:cond-mat/0410288•Journal of Applied Physics•Feb 4, 2005•37 citations

Fabrication of ultrasmall Nb-AlOx-Nb Josephson tunnel junctions

John M. Martinis, Ronald H. Ono

We describe a fabrication process to make Nb-AlOx-Nb edge junctions with areas as small as 0.0022 μm2 and with current densities from 10 to 24 000 A/cm2. The junction conductance was low for voltages below the superconducting energy gap which indicates good quality tunnel barriers. Coulomb gap measurements obtained when the junctions were in the normal state were used to find the junction capacitance. Junction capacitance as small as 0.18 fF has been measured.

Applied Physics Letters•Aug 6, 1990•34 citations

In view of future macroscopic quantum experiments, we have fabricated prototypes of charge qubits (with tunable Josephson coupling and capacitive gate) provided with capacitively coupled readout single-electron transistor electrometers. The devices are prepared by electron beam direct-writing in conjunction with material deposition by sputtering. All leads are made of Al, and the AlO/sub x/ tunnel barriers are generated by local thermal oxidizing of the surface of the base striplines. The island electrodes are made of Al or Nb in a subsequent deposition step. In the case of Nb it is suitable to protect the barriers by a thin sputtered Al film first. The small-area (100 nm/spl times/100 nm) tunnel junctions have capacitances in the order of a few 10/sup -16/ F; their resistances can be adjusted in a wide range from k/spl Omega/ to M/spl Omega/ with an on-chip spread of less than 10%. The (double) superconducting gap energy was determined to be about 0.4 meV for Al and 1.4 meV for Nb, respectively. Measuring the so-called Coulomb staircase, we have demonstrated the operation of both qubit boxes and sub-e electrometers.

We report measurements and quantum mechanical simulations of the behavior of superconducting two-junction Coulomb-blockade electrometers biased near the superconducting gap. Our quantum model allows coherent superpositions of states with different number of charges on the island. It predicts maximum current modulation /spl delta/I for devices with tunneling resistance R/sub /spl Sigma///spl ap/64/spl pi//spl Delta//spl planck//sup 2/C/sub /spl Sigma///sup 2//(e/sup 6//spl tau/), where /spl tau/ is the time for which the system maintains quantum coherence, C/sub /spl Sigma// is the total island capacitance and /spl Delta/ is the superconducting energy gap. For comparison, we built Al-AlO/sub x/-Al electrometers with R/sub /spl Sigma// from 61 k/spl Omega/ to 1.7 M/spl Omega/ and C/sub /spl Sigma// ranging from 0.9 fF to 8 fF. We find good agreement for /spl tau//spl ap/0.35 nsec.

Charge states in mesoscopic Josephson devices are considered promising candidates for a solid-state implementation of quantum computing. We studied three superconducting single electron transistors (SSETs) fabricated with small Al/AlO x /Al tunnel junctions, which, with a different choice of the construction parameters, can realize both the quantum bit and the readout system for the qubit charge state. We varied the junction capacitance and barrier transparency, making the Josephson energy E J of the device smaller, comparable and larger than the charging energy E C : the first case is suitable to build the qubit, while the latter can be suitable for the qubit readout.

We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.

Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.