| 1. |
- Adamyan, Astghik, 1984, et al.
(författare)
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Kinetic inductance as a microwave circuit design variable by multilayer fabrication
- 2015
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Ingår i: Superconductor Science and Technology. - : IOP Publishing. - 0953-2048 .- 1361-6668. ; 28:8
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Tidskriftsartikel (refereegranskat)abstract
- We report on the development of a reliable NbN/Al/Nb/NbN multilayer fabrication technique for combining design elements with and without kinetic inductance in superconducting microwave circuits. As a proof-of-concept we demonstrate the application of the proposed technique to build a slow microwave propagation line matched to 50 Ω terminals. Fabrication details along with the design and measurements are discussed. At 8 GHz the presented device operates as a dc controllable full-turn phase shifter. We suggest that by exploiting the kinetic inductance as a design variable one can greatly improve operation parameters for a variety of standard microwave designs such as step-impedance filters and resonators.
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| 2. |
- Adamyan, Astghik, 1984, et al.
(författare)
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Superconducting microwave parametric amplifier based on a quasi-fractal slow propagation line
- 2016
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 119:8
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Tidskriftsartikel (refereegranskat)abstract
- Quantum limited amplifiers are sought after for a wide range of applications within quantum technologies and sensing. One promising candidate is the travelling wave parametric amplifier which exploits the non-linear kinetic inductance of a superconducting transmission line. This type of microwave amplifier promises to deliver a high gain, a quantum limited noise performance over several GHz bandwidth, and a high dynamic range. However, practical realizations of this type of device have so far been limited by fabrication defects, since the length of the superconducting transmission line required for achieving substantial parametric gain is on the order of similar to 1m. Here, we report on a design for a microwave traveling wave amplifier based on a slow propagation line comprising a central strip with high kinetic inductance and quasi-fractal line-to-ground capacitors. Due to an enhanced per unit length inductance (73 nH cm(-1)) and capacitance (15 pF cm(-1)), the line has a microwave propagation velocity as low as 9.8 x 10(8) cm s(-1). This translates into parametric gain up to 0.5 dB cm(-1) and a total gain of 6 dB for just a similar to 10 cm long transmission line. Moreover, the flexibility of the presented design allows balancing the line inductance and capacitance in order to keep the characteristic impedance close to 50 Omega and to suppress standing waves, both factors being essential in order to implement a practical parametric amplifier in the microwave domain.
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| 3. |
- 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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| 4. |
- Christensen, M. A., et al.
(författare)
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Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes
- 2014
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 2:48, s. 10428-10438
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Tidskriftsartikel (refereegranskat)abstract
- Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on pi-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that pi-extended TTFs based on the indenofluorene core, prepared by Horner-Wadsworth-Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral.cation]) and pi-dimer ([cation.cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin-Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6- , BF4-, and TaF6-) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed - as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.
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| 5. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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A near-field scanning microwave microscope based on a superconducting resonator for low power measurements
- 2013
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 84:2, s. 023706-
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Konferensbidrag (refereegranskat)abstract
- We report on the design and performance of a cryogenic (300 mK) near-field scanning microwave microscope. It uses a microwave resonator as the near-field sensor, operating at a frequency of 6 GHz and microwave probing amplitudes down to 100 μV, approaching low enough photon population (N ∼ 1000) of the resonator such that coherent quantum manipulation becomes feasible. The resonator is made out of a miniaturized distributed fractal superconducting circuit that is integrated with the probing tip, micromachined to be compact enough such that it can be mounted directly on a quartz tuning-fork, and used for parallel operation as an atomic force microscope (AFM). The resonator is magnetically coupled to a transmission line for readout, and to achieve enhanced sensitivity we employ a Pound-Drever-Hall measurement scheme to lock to the resonance frequency. We achieve a well localized near-field around the tip such that the microwave resolution is comparable to the AFM resolution, and a capacitive sensitivity down to 6.4 × 10−20 F/rtHz, limited by mechanical noise. We believe that the results presented here are a significant step towards probing quantum systems at the nanoscale using near-field scanning microwave microscopy.
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| 6. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Accurate Real-Time Monitoring of Quality Factor and Center Frequency of Superconducting Resonators
- 2014
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Ingår i: IEEE Transactions on Applied Superconductivity. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2515 .- 1051-8223. ; 24:6
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we track both center frequency and quality factor of a superconducting microwave resonator with very high precision using a modified Pound-Drever-Hall frequency locking scheme. The quality factor measurements can be done with high enough bandwidth to study different processes in a superconducting resonator in real time. With this added functionality, the frequency readout remains unaffected, with typical accuracy around 10-30 Hz/root Hz, whereas the cavity loss rate can be determined to within 0.1% on a millisecond time scale (34 dB/root Hz). As we demonstrate, this can be particularly useful for electron spin resonance measurements, where we achieve a minimum number of detectable spins coupled to the resonator N = 5 . 10(5) spins/root Hz, based on quality factor measurements. It also enables the study of fast dynamic processes, for example, vortex dynamics, in these devices on a short time scale.
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| 7. |
- 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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| 8. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Coherent interaction with two-level fluctuators using near field scanning microwave microscopy
- 2015
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- Near field Scanning Microwave Microscopy (NSMM) is a scanning probe technique that non-invasively can obtain material properties on the nano-scale at microwave frequencies. While focus has been on developing room-temperature systems it was recently shown that this technique can potentially reach the quantum regime, opening up for applications in materials science and device characterization in solid state quantum information processing. In this paper we theoretically investigate this new regime of NSMM. Specifically we show that interaction between a resonant NSMM probe and certain types of two-level systems become possible when the NSMM probe operates in the (sub-) single photon regime, and we expect a high signal-to-noise ratio if operated under the right conditions. This would allow to detect single atomic material defects with energy splittings in the GHz range with nano-scale resolution, provided that individual defects in the material under study are well enough separated. We estimate that this condition is fulfilled for materials with loss tangents below tan delta similar to 10(-3) which holds for materials used in today's quantum circuits and devices where typically tan delta
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| 9. |
- 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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| 10. |
- de Graaf, Sebastian Erik, 1986
(författare)
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Fractal superconducting resonators for the interrogation of two-level systems
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we use high-Q superconducting thin-film microwave resonators to interact with several types of quantum mechanical two-level systems. Such a resonator is used as the central building block in a cryogenic near-field scanning microwave microscope (NSMM) to reach a completely new regime of NSMM operation. In this regime where the superconducting resonator is only populated with a small number of photons, we demonstrate a capacitance sensitivity down to 64x10^(-21) Farad/rtHz and nanoscale resolution, which is sufficient to apply this scanning probe technique to quantum coherent objects. Such a 'coherent'-NSMM enables several new applications: for example to study the interaction of the NSMM probe with two-level defects in samples and to characterize artificial two-level systems (qubits), which eventually could lead to better understanding of decoherence mechanisms in superconducting quantum circuits. We demonstrate the ability to reach this regime in a sample consisting of a Cooper-pair box (CPB) weakly coupled to a superconducting resonator. In the strong driving regime we observe Landau-Zener-Stückelberg interference and we discover a new type of relaxation mechanism in the strongly driven CPBthat involves pair breaking and quasiparticle tunneling. It results in a recovered parity of the CPB and a population inversion of the dressed states. Not only does this demonstrate the applicability of NSMM for qubit characterization, but the quasiparticle mediated population inversion also becomes suitable for robust charge sensing in a scanning probe setup. To integrate the superconducting resonator onto our NSMM probe we develop a new type of resonator design - the fractal design - that have a very small external dipole moment allowing for a compact resonator. Another advantage of the fractal resonator is its resilience to magnetic fields. We show that the fractal resonator, after further optimization, can maintain quality factors above 10^5 in applied fields of more than 400 mT, something that becomes particularly useful for the interrogation of spin ensembles coupled to the resonator. We demonstrate that it is possible to detect down to 5x10^5 spins/rtHz in a very small volume coupled to a fractal resonator. Furthermore, the low dipole moment of the fractal resonator allows us to also introduce DC bias into the resonator without degrading its Q-factor. This is an important technological step that allows us to interact with new materials where spins can be quickly and locally manipulated using electric fields and we demonstrate the first steps in this direction with ensembles of manganese doped ZnO nanowires and frustrated molecular Cu spin triangles.The measurements achieve a very high sensitivity thanks to the Pound-Drever-Hall locking technique used. We develop this technique such that both resonance frequency and quality factor can be monitored with very high accuracy in real time. The demonstrated stability is ~30 Hz/rtHz for frequency readout and we can determine the Q-factor with a precision of 34 dB/rtHz.
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