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| 2. |
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| 3. |
- Bauch, Thilo, 1972, et al.
(författare)
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YBCOSQUIDs with unconventional current phase relation
- 2007
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Ingår i: Physica C: Superconductivity and its Applications. - : Elsevier BV. - 0921-4534. ; 463-465:1 October 2007, s. 979-984
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the dynamics of YBa2Cu3O7-delta,5 (YBCO) dc sperconducting quantum interference devices (SQUIDs) characterized by an unconventional Josephson current phase relation (CPR). We have focused on SQUID configurations with Josephson junctions where the lobe of the order parameter in one electrode is facing a node in the other electrode. This order parameter arrangement should enhance the appearance of a sin(2(p) term in the CPR. The response of the critical current of the dc SQUID, under the effect of an external magnetic field, has been measured in temperature, down to 20 mK. Our experimental data have been compared with numerical simulations of the SQUIDs dynamics by considering a CPR of a single junction of the form I(phi) = I-I sin(phi) - I-II sin(2 phi) where I-I, and I-II are, respectively, the first and second harmonic component. In our devices the values of the sin(2 phi) term are such that the fundamental state of the SQUID is naturally double degenerate. This is of great relevance for applications of d-wave SQUIDs in quantum information processing. (C) 2007 Elsevier B.V. All rights reserved.
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| 4. |
- Burnett, J., et al.
(författare)
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PC2: Identifying noise processes in superconducting resonators
- 2013
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Ingår i: 2013 IEEE 14th InternationalSuperconductive Electronics Conference, ISEC 2013. - 9781467363716 ; , s. Art. no. 6604284-
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Konferensbidrag (refereegranskat)abstract
- Extensive studies of dielectric loss due to two level fluctuators (TLFs) in superconducting resonators have provided routes for low loss resonators. The research is motivated not only by the use of resonators as detectors and in quantum information processing, but more generally due to TLFs being a source of noise and decoherence in all quantum devices. In this work a frequency locked loop was used to measure frequency fluctuations at timescales in excess of 104 seconds, thereby accurately probing the TLF induced low- frequency noise of the resonator. Our measurement method lead to very high statistical confidence even for very long timescales, and here we can therefore present results explicitly identifying power dependent flicker frequency noise (S = 1/fa where a=1) persisting down to the mHz level.
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| 6. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Charge Qubit Coupled to an Intense Microwave Electromagnetic Field in a Superconducting Nb Device: Evidence for Photon-Assisted Quasiparticle Tunneling
- 2013
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:13, s. Art. no. 137002-
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Tidskriftsartikel (refereegranskat)abstract
- We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.
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| 7. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Direct Identification of Dilute Surface Spins on Al2 O3: Origin of Flux Noise in Quantum Circuits
- 2017
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 118:5, s. 057703-
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Tidskriftsartikel (refereegranskat)abstract
- An on-chip electron spin resonance technique is applied to reveal the nature and origin of surface spins on Al2O3. We measure a spin density of 2.2×1017 spins/m2, attributed to physisorbed atomic hydrogen and S=1/2 electron spin states on the surface. This is direct evidence for the nature of spins responsible for flux noise in quantum circuits, which has been an issue of interest for several decades. Our findings open up a new approach to the identification and controlled reduction of paramagnetic sources of noise and decoherence in superconducting quantum devices.
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| 8. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Suppression of low-frequency charge noise in superconducting resonators by surface spin desorption
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Noise and decoherence due to spurious two-level systems located at material interfaces are long-standing issues for solid-state quantum devices. Efforts to mitigate the effects of two-level systems have been hampered by a lack of knowledge about their chemical and physical nature. Here, by combining dielectric loss, frequency noise and on-chip electron spin resonance measurements in superconducting resonators, we demonstrate that desorption of surface spins is accompanied by an almost tenfold reduction in the charge-induced frequency noise in the resonators. These measurements provide experimental evid ence that simultaneously reveals the chemical signatures of adsorbed magnetic moments and highlights their role in generating charge noise in solid-state quantum devices.
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| 9. |
- Geaney, S., et al.
(författare)
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Near-Field Scanning Microwave Microscopy in the Single Photon Regime
- 2019
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- The microwave properties of nano-scale structures are important in a wide variety of applications in quantum technology. Here we describe a low-power cryogenic near-field scanning microwave microscope (NSMM) which maintains nano-scale dielectric contrast down to the single microwave photon regime, up to 109 times lower power than in typical NSMMs. We discuss the remaining challenges towards developing nano-scale NSMM for quantum coherent interaction with two-level systems as an enabling tool for the development of quantum technologies in the microwave regime.
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| 10. |
- Keyser, Ailsa K.V., et al.
(författare)
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Pulsed electron spin resonance of an organic microcrystal by dispersive readout
- 2020
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Ingår i: Journal of Magnetic Resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 321
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Tidskriftsartikel (refereegranskat)abstract
- We establish a testbed system for the development of high-sensitivity Electron Spin Resonance (ESR) techniques for small samples at cryogenic temperatures. Our system consists of a NbN thin-film planar superconducting microresonator designed to have a concentrated mode volume to couple to a small amount of paramagnetic material, and to be resilient to magnetic fields of up to 400mT. At 65mK we measure high-cooperativity coupling (C≈19) to an organic radical microcrystal containing 1012 spins in a pico-litre volume. We detect the spin–lattice decoherence rate via the dispersive frequency shift of the resonator. Techniques such as these could be suitable for applications in quantum information as well as for pulsed ESR interrogation of very few spins to provide insights into the surface chemistry of, for example, the material defects in superconducting quantum processors.
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| 11. |
- Leppäkangas, Juha, 1979, et al.
(författare)
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Effects of quasiparticle tunnelling in a circuit-QED realization of a strongly driven two-level system
- 2013
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 1361-6455 .- 0953-4075. ; 46:22, s. Art. no. 224019-
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally and theoretically study the frequency shift of a driven cavity coupled to a superconducting charge qubit. In addition to previous studies, here we also consider drive strengths large enough to energetically allow for quasiparticle creation. Quasiparticle tunnelling leads to the inclusion of more than two charge states in the dynamics. To explain the observed effects, we develop a master equation for the microwave dressed charge states, including quasiparticle tunnelling. A bimodal behaviour of the frequency shift as a function of gate voltage can be used for sensitive charge detection. However, at weak drives, the charge sensitivity is significantly reduced by nonequilibrium quasiparticles, which induce transitions to a non-sensitive state. Unexpectedly, at high-enough drives, the quasiparticle tunnelling enables a very fast relaxation channel to the sensitive state. In this regime, the charge sensitivity is thus robust against externally injected quasiparticles and the desired dynamics prevail over a broad range of temperatures. We find very good agreement between the theory and experiment over a wide range of drive strengths and temperatures.
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| 12. |
- Lindström, Tobias, 1976
(författare)
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d-wave Josephson Junctions
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The properties of d-wave YBCO Josephson junctions and dc-SQUIDs with high misorientation angles have beenstudied experimentally .The predominant d-wave symmetry of the pairing wavefunction in high-Tc superconductors has, to a large extent, beena hinderance in applications, but it also makes it possible to fabricate devices with novel properties that may be suitablefor applications in the quantum regime; one particulary interesting property is a strong second harmonic component in the current-phase relation ofjunctions with the electrodes oriented in a node-lobe arrangement; so-called 0-45 junctions.The work has been focused on the properties of grain-boundary 0-45 dc-SQUIDs: Their dc-properties as wellas the effects of a strong second harmonic component on the dynamics have been studied experimentally andmodelled numerically. The experimental results can be explained by taking into account the meandering ofthe grain boundary which in combination with a strong orientational dependence of the transport properties due to thed-wave symmetry results in interfaces with properties that change dramatically over a length scale of 50-100 nm.The properties of grain-boundary YBCO 0-45 junctions have been studied in the quantum regime. Macroscopicquantum tunnelling has been demonstrated in these junctions and microwave spectroscopy is here used to verifythe presence of discrete energy levels. The quality factor of the junction has been experimentally determined to 40+-10, a much higher value than previously believed to be possible in high-Tc structures.Finally, the possibility of using high-T$_c$ structures to build qubits is discussed. There is now a renewedinterest in this possibility, especially in light of our recent results whichdemonstrate that the inherent dissipation mechanisms are much less detrimental to coherent phenomena than previously feared.
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| 13. |
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| 14. |
- Lindström, Tobias, 1976
(författare)
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Dynamics of high-angle grain boundary YBCO Josephson junctions
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes experimental investigations of properties of high-angle YBa2Cu3O7−δ(YBCO)bicrystal Josephson junctions and SQUIDs fabricated on SrTiO3-substrates. The main focus of theinvestigation has been on the effects of the predominant d-wave symmetry of the superconductingwavefunction in YBCO on transport properties and dynamics.At a high-angle grain-boundary interface between two high-temperature superconductors Andreevstates can form, the current carried by these states hasπ-periodic component which flows in a direc-tion opposite of the usual Josephson current. Asymmetric high-angle grain boundaries also exhibit acritical current which is four orders of magnitude lower than symmetric lower-angle junctions, thiscan be attributed to the node-lobe arrangement of the the superconducting order parameter.High-angle grain-boundary dc-SQUIDs that have been studied which exhibit unusual dynamicssuch as a relative ”shift” of the positions of the positive and negative modulation and a highly non-sinusoidal dependence on the external field. This behavior vanished when moving to very narrowjunctions. These result are explained using a semi-classical model which assumes the presence ofa2ndharmonic in the current-phase relation. Numerical simulations confirm that this model is inqualitative agreement with experimental results.The properties of sub-micron sized junctions have also been studied. These exhibit some un-usual behavior. These junctions have been used to study the tunnelling spectra since the high normalresistance means that it is possible to study energies close to the gap.Finally, some general properties of high-angle Josephson Junctions are discussed. It is arguedthat some seemingly inconsistent experimental results can be explained using a multi-channel modelwhich accounts for the wiggling and faceting of the interface.
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Lombardi, Floriana, 1967, et al.
(författare)
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Quantum properties of d-wave YBa2Cu3O7-δ Josephson junction
- 2006
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Ingår i: Physica C: Superconductivity and its Applications. - : Elsevier BV. - 0921-4534. ; 435:1-2, s. 8-11
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Tidskriftsartikel (refereegranskat)abstract
- We have measured the transport properties of YBa2Cu3O7-δ (YBCO) dc SQUIDs with Josephson junctions characterized by a combination of 45° [0 0 1] tilt and α [1 0 0] tilt grain boundary with α = 0°, ±10° and 45°. By increasing the component of transport in the YBCO c-axis direction (increasing values of α), the SQUIDs showed an enhanced quality factor. At the same time, for all the devices the response under an external magnetic field presented the main features of an unconventional Josephson current phase relation with a pronounced sin 2 component. Switching current measurements at low temperature (10th of mK) also revealed that the dynamics of grain boundary junction resulting from a combination of 45° [0 0 1] and 45° [1 0 0] tilt is dominated by macroscopic quantum tunnelling of the superconductive phase. The features are quite encouraging for applications of d-wave junctions in quantum circuitry.
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| 19. |
- Malisa, Anayesu, 1965, et al.
(författare)
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Magnesium diboride nanobridges fabricated by electron-beam lithography
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:12, s. 1-5
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Tidskriftsartikel (refereegranskat)abstract
- MgB2 nanobridges were fabricated by e-beam lithography and Ar-ion beam milling. Nanobridges of widths ranging from 60 nm to 1 μm and 3 μm in length were realized by Ar-ion beam milling using amorphous carbon as etching mask. The processing did not harm the superconducting properties appreciably. High values of the critical current density, more than 10 MA cm2, were measured for bridges with widths down to 60 nm. Current-voltage (I-V) characteristics showed a behavior typical of a bridge going normal, after the critical current is exceeded, and remaining normal as the current is decreased to a lower switch back value due to Joule heating. We could also observe switching behavior in some bridges indicating formation of normal hotspots in the bridges before they returned to their superconducting state. Alternative explanations may include natural grain boundaries in the film or the movement of Abrikosov vortices. The current-voltage (I-V) characteristics showing critical current densities up to 5× 107 A/cm2 indicates excellent film properties in the nanobridges.
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| 20. |
- Ranjan, V., et al.
(författare)
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Spin-Echo Silencing Using a Current-Biased Frequency-Tunable Resonator
- 2022
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 129:18
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Tidskriftsartikel (refereegranskat)abstract
- The ability to control microwave emission from a spin ensemble is a requirement of several quantum memory protocols. Here, we demonstrate such ability by using a resonator whose frequency can be rapidly tuned with a bias current. We store excitations in an ensemble of rare-earth ions and suppress on demand the echo emission ("echo silencing") by two methods: (1) detuning the resonator during the spin rephasing, and (2) subjecting spins to magnetic field gradients generated by the bias current itself. We also show that spin coherence is preserved during silencing.
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| 21. |
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| 22. |
- Saxena, Ruchi, et al.
(författare)
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Electronic confinement of surface states in a topological insulator nanowire
- 2022
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 106:3
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the confinement of electronic surface states in a model of a topological insulator nanowire. Spin-momentum locking in the surface states reduces unwanted backscattering in the presence of nonmagnetic disorder and is known to counteract localization for certain values of magnetic flux threading the wire. We show that intentional backscattering can be induced for a range of conditions in the presence of a nanowire constriction. We propose a geometry for a nanowire that involves two constrictions and show that these regions form effective barriers that allow for the formation of a quantum dot. We analyze the zero-temperature noninteracting electronic transport through the device using the Landauer-Büttiker approach and show how externally applied magnetic flux parallel to the nanowire and electrostatic gates can be used to control the spectrum of the quantum dot and the electronic transport through the surface states of the model device.
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