Luigi Longobardi - Academia.edu (original) (raw)
Papers by Luigi Longobardi
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interferenc... more At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis
IEEE Transactions on Applied Superconductivity, 2014
ABSTRACT
Physica C-Superconductivity and Its Applications, 2002
We present the design of an integrated planar SQUID device in view of macroscopic quantum coheren... more We present the design of an integrated planar SQUID device in view of macroscopic quantum coherence (MQC) experiments. This device is made up by an rf SQUID, as a probe to test the MQC, and a readout system based on a dc SQUID sensor coupled to the probe. To minimize the electromagnetic noise from environment, gradiometric configuration of rf SQUID, dc SQUID and a flux transformer is included. Preliminary results on the decay from the metastable flux state for different rf SQUID-based devices in the classical limit are shown.
International Journal of Modern Physics B, 2003
We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Cohe... more We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit).
International Journal of Modern Physics B, 2003
Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been desi... more Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been designed, fabricated and characterized. The basic design of the sensor is based on the Ketchen-type magnetometer. A moat surrounding the washer have been inserted to reduce 1/f noise due to hopping of flux vortices, trapped in the SQUID body during the cool-down process. To decrease flux motions in the pick-up coil structure a turn width as small as possible has been used. Further Josephson junctions with high areas have been employed to reduce the 1/f arising from critical currant fluctuations. An additional positive feedback (APF) circuit and a feedback coil for Flux-Locked-Loop (FLL) have been integrated on the same chip to avoid any additional noise due to an external APF circuit. Details about design criteria and experimental results concerning voltage-flux characteristics and noise performances of the devices are presented. Due to a low magnetic field noise, such devices well meet the requirements of ultra-low frequency noise applications.
Superconductor Science & Technology, 2007
... Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA ... more ... Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA E-mail: Douglas.Bennett@sunysb.edu Received 12 June 2007, in final form 13 July 2007 Published 18 October 2007 Online at stacks.iop.org/SUST/20/S445 ...
Superconductivity Centennial Conference 2011, 2012
An experimental investigation of the critical current noise in underdamped niobium based Josephso... more An experimental investigation of the critical current noise in underdamped niobium based Josephson junctions by a technique based on the switching current measurements is reported. By swiping the junction with a current ramp we measure the critical current switching using the standard time of flight technique. In such a way it is possible to obtain a digital sampling of the critical current as a function of the time I c (t) and the corresponding fluctuation ΔI c =I c (t) -<I c (t)> with a sampling frequency given by the ramp frequency. Computing the Fast Fourier Transform module it is possible to evaluate the power spectral density of the critical current fluctuation S ΔIc (f). Measurements on Josephson junctions having an area ranging from (4x4) μm 2 to (40x40) μm 2 in the temperature range from 4.2 K to 1.2 K are reported. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. A very good agreement between the experimental data and the Nyquist noise of the junction normal resistance is shown. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits. Pacs: 74.50+r, 85.25.Dq, 85.25.Cp, 74.78.Na
ABSTRACT Self-assembled nanoscale channels may naturally arise in the growth process of grain bou... more ABSTRACT Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.
ABSTRACT We report on measurements of the switching current probability both in the thermal and q... more ABSTRACT We report on measurements of the switching current probability both in the thermal and quantum regime on moderately damped NbN/MgO/NbN Josephson junctions with very low critical current density (similar or equal to 2A/cm(2)). We observed three distinct regimes: below 90 mK the width of the switching distributions is constant within the experimental error, while from 90 mK to 1.6 K the width increases with temperature, this change of behavior signaling the standard transition from macroscopic quantum tunneling (MQT) to thermal activation (TA) regime respectively. Above 1.6 K we find effects of anti-correlation between the bath temperature and thermal fluctuations since the width of the histograms starts to collapse, and indications of a transition to a third regime called Phase Diffusion (PD) are evident, such as the change of the switching distributions symmetry. Experimental data have been analyzed through numerical fitting of the switching probability and Monte Carlo simulations, and we found a good agreement with theoretical expectations based on multiple re-trapping processes, which strongly depend on the junction damping. The parameters which completely characterize the dynamics of such moderately damped Josephson junctions have been also extracted. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
Physical Review B, 2003
We present an experimental study of the effective dissipation of a fully integrated Nb/AlOx/Nb Jo... more We present an experimental study of the effective dissipation of a fully integrated Nb/AlOx/Nb Josephson device consisting of a rf superconducting quantum interference device (SQUID) coupled to a readout system based on a dc SQUID sensor. In the classical regime data on the decay rate from the metastable flux states of rf SQUID are reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of building a macroscopic quantum device.
The Review of scientific instruments, 2013
Careful filtering is necessary for observations of quantum phenomena in superconducting circuits ... more Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.
International Workshop on Superconducting Nano-Electronics Devices, 2002
Superconductor Science and Technology, 2012
YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO ... more YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA/cm 2 at T = 4.2 K and of 10 MA/cm 2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices. arXiv:1201.1886v1 [cond-mat.supr-con]
Superconductor Science and Technology, 2011
We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on l... more We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on low loss substrates. Excellent transport properties have been reproducibly found, with remarkable values of the quality factor I c R n (with I c the critical current and R n the normal state resistance) above 10 mV, far higher than the values commonly reported in the literature for high temperature superconductor (HTS) based Josephson junctions. The outcomes are consistent with a picture of a more uniform grain boundary region along the current path. This work supports a possible implementation of grain boundary junctions for various applications including terahertz sensors and HTS quantum circuits in the presence of microwaves.
Quantum Information Processing, 2009
We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid ... more We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID, described by an double-well potential, has independent, in situ, controls for the tilt and barrier height of the potential. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise and is consistent with independent measurement of these quantities obtained by microwave spectroscopy, resonant tunneling between fluxoid wells and decay of the excited state. The oscillation's waveform is compared to analytical results obtained for finite decay rates and detuning and averaged over low frequency flux noise.
Physical Review Letters, 2012
Dissipation encodes the interaction of a quantum system with the environment and regulates the ac... more Dissipation encodes the interaction of a quantum system with the environment and regulates the activation regimes of a Brownian particle. We have engineered grain boundary biepitaxial YBaCuO junctions to drive a direct transition from a quantum activated running state to a phase diffusion regime. The crossover to the quantum regime is tuned by the magnetic field and dissipation is described by a fully consistent set of junction parameters. To unravel phase dynamics in moderately damped systems is of general interest for advances in the comprehension of retrapping phenomena and in view of quantum hybrid technology.
Physical Review Letters, 2012
Physical Review B, 2004
We present an experimental characterization of Nb/AlOx/Nb Josephson devices in the quantum regime... more We present an experimental characterization of Nb/AlOx/Nb Josephson devices in the quantum regime. Josephson junctions and fully integrated Josephson devices consisting of an rf SQUID coupled to a read-out system based on a dc SQUID have been measured. Data on the decay rates from the metastable states show evidence of macroscopic quantum tunneling with a very low dissipation level and
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interferenc... more At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis
IEEE Transactions on Applied Superconductivity, 2014
ABSTRACT
Physica C-Superconductivity and Its Applications, 2002
We present the design of an integrated planar SQUID device in view of macroscopic quantum coheren... more We present the design of an integrated planar SQUID device in view of macroscopic quantum coherence (MQC) experiments. This device is made up by an rf SQUID, as a probe to test the MQC, and a readout system based on a dc SQUID sensor coupled to the probe. To minimize the electromagnetic noise from environment, gradiometric configuration of rf SQUID, dc SQUID and a flux transformer is included. Preliminary results on the decay from the metastable flux state for different rf SQUID-based devices in the classical limit are shown.
International Journal of Modern Physics B, 2003
We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Cohe... more We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit).
International Journal of Modern Physics B, 2003
Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been desi... more Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been designed, fabricated and characterized. The basic design of the sensor is based on the Ketchen-type magnetometer. A moat surrounding the washer have been inserted to reduce 1/f noise due to hopping of flux vortices, trapped in the SQUID body during the cool-down process. To decrease flux motions in the pick-up coil structure a turn width as small as possible has been used. Further Josephson junctions with high areas have been employed to reduce the 1/f arising from critical currant fluctuations. An additional positive feedback (APF) circuit and a feedback coil for Flux-Locked-Loop (FLL) have been integrated on the same chip to avoid any additional noise due to an external APF circuit. Details about design criteria and experimental results concerning voltage-flux characteristics and noise performances of the devices are presented. Due to a low magnetic field noise, such devices well meet the requirements of ultra-low frequency noise applications.
Superconductor Science & Technology, 2007
... Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA ... more ... Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA E-mail: Douglas.Bennett@sunysb.edu Received 12 June 2007, in final form 13 July 2007 Published 18 October 2007 Online at stacks.iop.org/SUST/20/S445 ...
Superconductivity Centennial Conference 2011, 2012
An experimental investigation of the critical current noise in underdamped niobium based Josephso... more An experimental investigation of the critical current noise in underdamped niobium based Josephson junctions by a technique based on the switching current measurements is reported. By swiping the junction with a current ramp we measure the critical current switching using the standard time of flight technique. In such a way it is possible to obtain a digital sampling of the critical current as a function of the time I c (t) and the corresponding fluctuation ΔI c =I c (t) -<I c (t)> with a sampling frequency given by the ramp frequency. Computing the Fast Fourier Transform module it is possible to evaluate the power spectral density of the critical current fluctuation S ΔIc (f). Measurements on Josephson junctions having an area ranging from (4x4) μm 2 to (40x40) μm 2 in the temperature range from 4.2 K to 1.2 K are reported. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. A very good agreement between the experimental data and the Nyquist noise of the junction normal resistance is shown. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits. Pacs: 74.50+r, 85.25.Dq, 85.25.Cp, 74.78.Na
ABSTRACT Self-assembled nanoscale channels may naturally arise in the growth process of grain bou... more ABSTRACT Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.
ABSTRACT We report on measurements of the switching current probability both in the thermal and q... more ABSTRACT We report on measurements of the switching current probability both in the thermal and quantum regime on moderately damped NbN/MgO/NbN Josephson junctions with very low critical current density (similar or equal to 2A/cm(2)). We observed three distinct regimes: below 90 mK the width of the switching distributions is constant within the experimental error, while from 90 mK to 1.6 K the width increases with temperature, this change of behavior signaling the standard transition from macroscopic quantum tunneling (MQT) to thermal activation (TA) regime respectively. Above 1.6 K we find effects of anti-correlation between the bath temperature and thermal fluctuations since the width of the histograms starts to collapse, and indications of a transition to a third regime called Phase Diffusion (PD) are evident, such as the change of the switching distributions symmetry. Experimental data have been analyzed through numerical fitting of the switching probability and Monte Carlo simulations, and we found a good agreement with theoretical expectations based on multiple re-trapping processes, which strongly depend on the junction damping. The parameters which completely characterize the dynamics of such moderately damped Josephson junctions have been also extracted. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
Physical Review B, 2003
We present an experimental study of the effective dissipation of a fully integrated Nb/AlOx/Nb Jo... more We present an experimental study of the effective dissipation of a fully integrated Nb/AlOx/Nb Josephson device consisting of a rf superconducting quantum interference device (SQUID) coupled to a readout system based on a dc SQUID sensor. In the classical regime data on the decay rate from the metastable flux states of rf SQUID are reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of building a macroscopic quantum device.
The Review of scientific instruments, 2013
Careful filtering is necessary for observations of quantum phenomena in superconducting circuits ... more Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.
International Workshop on Superconducting Nano-Electronics Devices, 2002
Superconductor Science and Technology, 2012
YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO ... more YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA/cm 2 at T = 4.2 K and of 10 MA/cm 2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices. arXiv:1201.1886v1 [cond-mat.supr-con]
Superconductor Science and Technology, 2011
We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on l... more We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on low loss substrates. Excellent transport properties have been reproducibly found, with remarkable values of the quality factor I c R n (with I c the critical current and R n the normal state resistance) above 10 mV, far higher than the values commonly reported in the literature for high temperature superconductor (HTS) based Josephson junctions. The outcomes are consistent with a picture of a more uniform grain boundary region along the current path. This work supports a possible implementation of grain boundary junctions for various applications including terahertz sensors and HTS quantum circuits in the presence of microwaves.
Quantum Information Processing, 2009
We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid ... more We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID, described by an double-well potential, has independent, in situ, controls for the tilt and barrier height of the potential. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise and is consistent with independent measurement of these quantities obtained by microwave spectroscopy, resonant tunneling between fluxoid wells and decay of the excited state. The oscillation's waveform is compared to analytical results obtained for finite decay rates and detuning and averaged over low frequency flux noise.
Physical Review Letters, 2012
Dissipation encodes the interaction of a quantum system with the environment and regulates the ac... more Dissipation encodes the interaction of a quantum system with the environment and regulates the activation regimes of a Brownian particle. We have engineered grain boundary biepitaxial YBaCuO junctions to drive a direct transition from a quantum activated running state to a phase diffusion regime. The crossover to the quantum regime is tuned by the magnetic field and dissipation is described by a fully consistent set of junction parameters. To unravel phase dynamics in moderately damped systems is of general interest for advances in the comprehension of retrapping phenomena and in view of quantum hybrid technology.
Physical Review Letters, 2012
Physical Review B, 2004
We present an experimental characterization of Nb/AlOx/Nb Josephson devices in the quantum regime... more We present an experimental characterization of Nb/AlOx/Nb Josephson devices in the quantum regime. Josephson junctions and fully integrated Josephson devices consisting of an rf SQUID coupled to a read-out system based on a dc SQUID have been measured. Data on the decay rates from the metastable states show evidence of macroscopic quantum tunneling with a very low dissipation level and