Superconducting quantum bits

Clarke, John; Wilhelm, Frank K.

doi:10.1038/nature07128

Cited by 1,880 publications

(1,806 citation statements)

References 77 publications

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“…The search for longer coherence times of superconducting qubits brought the study of finite-frequency electromagnetic properties of mesoscopic superconductors to the forefront of experimental research [1][2][3][4][5][6] . The majority of experiments until recently was performed on structures using Josephson junctions as "weak" superconductors, and substantial progress in recognizing the coherencelimiting mechanisms was achieved.…”

Section: Pacs Numbersmentioning

confidence: 99%

“…These processes are represented by one arrow of type 1 and one of type 2 in the Figure 1. The real part of Y (0) 2 (ω) is given by: 6) and it vanishes for ω < ∆ + E A , as for these frequencies the processes "1+2" are energetically not allowed. Finally, there are processes in which two quasiparticles on Andreev level are created.…”

Section: Admittance Of a Single-channel Junction At T = 0 (Equilmentioning

confidence: 99%

“…These transitions are represented by the arrow 3 in Figure 1. We can write it in the form resembling that of Y 2 (ω): 6) where the real part of Y (0) 5 (ω) is given by:…”

Section: Admittance Of a Single-channel Junction In The Presence mentioning

confidence: 99%

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Frequency-dependent admittance of a short superconducting weak link

2013

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We consider the linear and non-linear electromagnetic responses of a nanowire connecting two bulk superconductors. Andreev states appearing at a finite phase bias substantially affect the finite-frequency admittance of such a wire junction. Electron transitions involving Andreev levels are easily saturated, leading to the nonlinear effects in photon absorption for the sub-gap photon energies. We evaluate the complex admittance analytically at arbitrary frequency and arbitrary, possibly non-equilibrium, occupation of Andreev levels. Special care is given to the limits of a single-channel contact and a disordered metallic weak link. We also evaluate the quasi-static fluctuations of admittance induced by fluctuations of the occupation factors of Andreev levels. In view of possible qubit applications, we compare properties of a weak link with those of a tunnel Josephson junction. Compared to the latter, a weak link has smaller low-frequency dissipation. However, because of the deeper Andreev levels, the low-temperature quasi-static fluctuations of the inductance of a weak link are exponentially larger than of a tunnel junction. These fluctuations limit the applicability of nanowire junctions in superconducting qubits

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Section: Pacs Numbersmentioning

confidence: 99%

Section: Admittance Of a Single-channel Junction At T = 0 (Equilmentioning

confidence: 99%

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Frequency-dependent admittance of a short superconducting weak link

2013

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“…1 In the case of qubits, this magnetic flux noise places fundamental limits on the performance and scalability of such architectures. Low frequency flux noise is widely thought to be due to fluctuations of magnetic impurities local to the superconductor wiring [2][3][4] but the identity of these impurities and the physical mechanism producing the observed fluctuations is not known.…”

Section: Introductionmentioning

confidence: 99%

Evidence for temperature-dependent spin diffusion as a mechanism of intrinsic flux noise in SQUIDs

et al. 2014

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The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron spin impurities, but the frequency and temperature dependence observed in experiments do not agree with the usual 1/f models. We present theoretical calculations and experimental measurements of flux noise in rf-SQUID flux qubits that show how these observations, and previous reported measurements, can be interpreted in terms of a spindiffusion constant that increases with temperature. We fit measurements of flux noise in sixteen devices, taken in the 20 − 80 mK temperature range, to the spin-diffusion model. This allowed us to extract the spin-diffusion constant and its temperature dependence, suggesting that the spin system is close to a spin-glass phase transition.

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“…Superconducting quantum computation is a viable and scalable approach to achieving a quantum computation scheme. Because of their solid state nature, their macroscopic phase coherence and their similarity to conventional semiconductor circuits, Josephson junction circuits are a promising technology within superconducting quantum computation schemes [13][14][15][16]. A typical flux qubit [16][17][18][19] consists of an isolated superconductive junction and a SQUID.…”

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confidence: 99%

Character and fabrication of Al/Al2O3/Al tunnel junctions for qubit application

Shen

Zhu

et al. 2012

Chin. Sci. Bull.

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The superconductive Josephson junction is the key device for superconducting quantum computation. We have fabricated Al/ Al 2 O 3 /Al tunnel junctions using a double angle evaporation method based on a suspended shadow mask. The Al 2 O 3 junction barrier has been formed by introducing pure oxygen into the chamber during the fabrication process. We have adjusted exposure conditions by changing either the oxygen pressure or the oxidizing time during the formation of tunnel barriers to control the critical current density J c and the junction specific resistance R c . [8][9][10][11][12]. Quantum computation entails a new computation system that exceeds its classical counterpart. It is anticipated there will be an exponential speed up in solving problems which are difficult for classical computers, such as prime factorization. This prospect has fuelled much research into various experimental realizations of a quantum computer. Superconducting quantum computation is a viable and scalable approach to achieving a quantum computation scheme. Because of their solid state nature, their macroscopic phase coherence and their similarity to conventional semiconductor circuits, Josephson junction circuits are a promising technology within superconducting quantum computation schemes [13][14][15][16]. A typical flux qubit [16][17][18][19] consists of an isolated superconductive junction and a SQUID. In a SQUID the tunnel junctions are arranged in parallel to form a superconducting loop. The superconductive Josephson junction is the key device for superconducting quantum computation. The most commonly used materials for Josephson junction qubits are Aluminum (Al) and Niobium (Nb), because of the purity of their superconducting state as well as their relatively low transition temperatures (T c = 1.2 K and 9.3 K, respectively) [20]. We have chosen to work with aluminum for several reasons. Firstly, high quality aluminum oxide has been shown to be less detrimental to Josephson junction qubits than other materials [21], with longer coherence times seen in aluminum-based devices [22]. Also, the fabrication of Al is flexible because of the fact that its melting point (660°C) is significantly lower than that of other popular superconducting materials, such as Nb (2477°C). For qubit applications, we have explored the electronic properties of the Al/Al 2 O 3 /Al tunnel junction, including the critical current density J c and the junction specific resistance R c , which depend on the oxygen pressure and the oxidation

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Superconducting quantum bits

Cited by 1,880 publications

References 77 publications

Frequency-dependent admittance of a short superconducting weak link

Frequency-dependent admittance of a short superconducting weak link

Evidence for temperature-dependent spin diffusion as a mechanism of intrinsic flux noise in SQUIDs

Character and fabrication of Al/Al2O3/Al tunnel junctions for qubit application

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