Final Report Summary - HYBMQC (Macroscopic quantum dynamics and coherence in hybrid superconducting circuits for quantum computation)
Dr. Longobardi has carried out his research activity from Oct 1st 2009 to June 30th 2011 within the framework of the Project "Macroscopic quantum dynamics and coherence in hybrid superconducting circuits for quantum computation" (Marie Curie Reintegration Grant, FP7-PEOPLE-2009-RG number 248933).
The main objective of the 3-years project was to demonstrate the feasibility of a qubit of sufficient quality to form the building blocks of a quantum information hybrid technology, partly based on the Josephson effect in high Tc superconductors (HTS). The project has several intermediate milestones distributed in the three years, ranging from time flight measurements on different types of junctions to the study of dissipation in various systems and specifically in HTS junctions. The main responsibilities of Dr. Longobardi have been to design various experiments aimed to the final target, to implement the experimental set-up to perform quantum measurements and to carry out measurements. He has benefited of the experience and the active collaboration of all members of the hosting group to design and realize the samples and for the background on HTS, of two students (a Ph.D. and an undergraduate) who have been working with him the whole time and of a series of international collaborations of the hosting group on the themes of the project. For important personal reasons Dr. Longobardi has preferred to move back to USA, and the project has been interrupted at half time of the whole duration. The project can be however considered very successful in the sense that 1) significant results and most of intermediate targets have been achieved (this can be understood by the scientific report, by the list of manuscripts, which have been already published or are currently in preparation); 2) Dr. Longobardi has made during the Marie Curie experience important steps forward for setting his scientific career (also documented by his new academic position in USA); 3) Dr. Longobardi has acquired experience on novel topics of solid state physics and transferred his know-how in quantum measurements on superconducting Josephson junctions to young students.
First step has been the implementation of the experimental set-up for measurements down to 20 mK: a third set of electronic filters has been installed at the 50 mK stage to reduce noise during measurements and new electronics to perform switching current distribution measurements has been installed and optimized. This is now a solid platform to perform any transport measurements.
At the same time we have been carrying out systematic measurements both on low (LTS) and high (HTS) critical temperature superconductors Josephson junctions. LTS first served to calibrate the new set-up, and we have then focused on special LTS Josephson junctions (JJs), NbN/MgO/NbN with MgO barriers of about 1nm, characterized by very low values of the critical current density down to 3 A/cm2. NbN may present some advantages with respect to Nb junctions for the realization of superconducting qubits, because of possible reluctance to form intrinsic two-level systems, and is a reference system also for HTS JJs. They have both relatively short fast non-equilibrium electron-phonon relaxation times and higher gap values, when compared with traditional junction technologies based on Nb, Al, and Pb. More importantly in our project, they are characterized by moderately damped regime (MDR) in analogy to HTS JJs. The moderately damped nature of these junctions generates a characteristic diffusive phase dynamics, analogous to what happens in HTS JJs. This regime is quite distinct from the well-known case of underdamped systems (Q > 10), and apparently quite common in junctions characterized by low critical currents (Ic). Thermal fluctuations assist in premature switching into the resistive state and, on the other hand, help in retrapping back to the superconducting state. In view of a more and more relevant use of nanotechnologies in quantum superconducting electronics and therefore of low values of Ic, studies on MDR can offer novel insights on dissipative effects on Josephson junctions, and inspire appropriate designs to respond to specific circuit requirements. For these NbN junctions, a physical picture emerges of moderately damped junctions, with a damping substantially independent of the frequency and able to sustain macroscopic quantum tunnelling at lower temperatures.
Switching current probabilities have been measured down to 20 mK for different HTS JJs. Junctions are in particular YBCO grain boundaries (GBs), and different configurations characterized by a variable orientation of the interface with respect to the electrodes have been explored. The GB forming the barrier changes as a function of the interface orientation and determines different barrier transparency and levels of dissipation. Apart from canonical quantum and thermal regimes, we have found evidence of a tunable moderately damped regime. HTS JJs seem to offer complementary functionalities when compared to LTS systems and some more flexibility in tuning the crossover temperature of the phase diffusion regime.
The comparative analysis of LTS and HTS systems has allowed to draw some relevant conclusions on the dynamical junction parameters, which determine MDR, and to better define the fingerprints of MDR. A change in the sign of the derivative of the second moment of the distribution at a turn-over temperature T* and a modification of the shape of the distributions at temperature around T* (which can be parameterized by the skewness, proportional to the third central moment of the distribution) are robust signatures of the phase diffusion regime.
The possibility to master YBCO GB junctions on the scale of hundreds of nm using both a 'classical' top-down approach or intrinsic self-assembling of nano-channels, has been important to achieve the necessary level of accuracy in the control of the structures to carry out a systematic analysis. Switching current spectra viceversa turn to be unique fingerprints of characteristic transport quantum modes in these systems (also from a morphological point of view).
HTS JJs seem to offer complementary functionalities when compared to LTS systems and some more flexibility in tuning the crossover temperature of the phase diffusion regime, despite the presence of additional sources of dissipation still to be completely defined. The Q dissipation factor is at the moment not much higher than 10. We believe that higher Q values (and therefore lower levels of dissipation) can be achieved for biepitaxial junctions, when the GB width will be reduced to the size of a single facet (of the order of 100-200 nm), being the current limit for the width about 500 nm.
Numerical codes have been developed to support the conclusions of the experimental measurements. Most of results on NbN/MgO/NbN junctions have been already published, while the manuscripts on switching current measurements on YBCO junctions are currently in progress.
Progress in the control of the properties of HTS GB JJs and a better understanding of their dynamical parameters has allowed the design of a HTS rf-SQUID (Superconducting QUantum Interference Device), aimed to be on a longer time scale the cell of the HTS qubit. The use of the superconducting loop where the junction is embedded, guarantees to reach a further level of isolation from direct bias lines. This way the bias can be executed by simply inductively coupling an on-chip superconductive coil to the qubit. The first design fully with high Tc superconductive technology is also the basis to then move on to the hybrid structure.
Significant steps have been also made on the realization of hybrid structures, where at the moment we have fully integrated and hybridized YBCO cells with InAs nanowires, which work as barriers. Up to now nanowires are of InAs, and the final structure is technologically quite demanding due to the structural complexity of both components of the junctions. The fabrication process can be applied to all sorts of barriers and hybrid structures, and paves the way to the LTS/HTS hybrid devices able to match a wide range of demand of quantum superconducting electronics.
In conclusions the work realized by Dr. Luigi Longobardi within the framework of the Project "Macroscopic quantum dynamics and coherence in hybrid superconducting circuits for quantum computation" (Marie Curie Reintegration Grant, FP7-PEOPLE-2009-RG number 248933), and partly supported by projects already active in the hosting group on related topics, has some impact on the possibility to use novel superconducting quit cells in nano-circuits. Efficiency clearly demonstrated for superconducting qubits based on classical junctions platforms can be partly extended to novel materials with possible novel functionalities
The main objective of the 3-years project was to demonstrate the feasibility of a qubit of sufficient quality to form the building blocks of a quantum information hybrid technology, partly based on the Josephson effect in high Tc superconductors (HTS). The project has several intermediate milestones distributed in the three years, ranging from time flight measurements on different types of junctions to the study of dissipation in various systems and specifically in HTS junctions. The main responsibilities of Dr. Longobardi have been to design various experiments aimed to the final target, to implement the experimental set-up to perform quantum measurements and to carry out measurements. He has benefited of the experience and the active collaboration of all members of the hosting group to design and realize the samples and for the background on HTS, of two students (a Ph.D. and an undergraduate) who have been working with him the whole time and of a series of international collaborations of the hosting group on the themes of the project. For important personal reasons Dr. Longobardi has preferred to move back to USA, and the project has been interrupted at half time of the whole duration. The project can be however considered very successful in the sense that 1) significant results and most of intermediate targets have been achieved (this can be understood by the scientific report, by the list of manuscripts, which have been already published or are currently in preparation); 2) Dr. Longobardi has made during the Marie Curie experience important steps forward for setting his scientific career (also documented by his new academic position in USA); 3) Dr. Longobardi has acquired experience on novel topics of solid state physics and transferred his know-how in quantum measurements on superconducting Josephson junctions to young students.
First step has been the implementation of the experimental set-up for measurements down to 20 mK: a third set of electronic filters has been installed at the 50 mK stage to reduce noise during measurements and new electronics to perform switching current distribution measurements has been installed and optimized. This is now a solid platform to perform any transport measurements.
At the same time we have been carrying out systematic measurements both on low (LTS) and high (HTS) critical temperature superconductors Josephson junctions. LTS first served to calibrate the new set-up, and we have then focused on special LTS Josephson junctions (JJs), NbN/MgO/NbN with MgO barriers of about 1nm, characterized by very low values of the critical current density down to 3 A/cm2. NbN may present some advantages with respect to Nb junctions for the realization of superconducting qubits, because of possible reluctance to form intrinsic two-level systems, and is a reference system also for HTS JJs. They have both relatively short fast non-equilibrium electron-phonon relaxation times and higher gap values, when compared with traditional junction technologies based on Nb, Al, and Pb. More importantly in our project, they are characterized by moderately damped regime (MDR) in analogy to HTS JJs. The moderately damped nature of these junctions generates a characteristic diffusive phase dynamics, analogous to what happens in HTS JJs. This regime is quite distinct from the well-known case of underdamped systems (Q > 10), and apparently quite common in junctions characterized by low critical currents (Ic). Thermal fluctuations assist in premature switching into the resistive state and, on the other hand, help in retrapping back to the superconducting state. In view of a more and more relevant use of nanotechnologies in quantum superconducting electronics and therefore of low values of Ic, studies on MDR can offer novel insights on dissipative effects on Josephson junctions, and inspire appropriate designs to respond to specific circuit requirements. For these NbN junctions, a physical picture emerges of moderately damped junctions, with a damping substantially independent of the frequency and able to sustain macroscopic quantum tunnelling at lower temperatures.
Switching current probabilities have been measured down to 20 mK for different HTS JJs. Junctions are in particular YBCO grain boundaries (GBs), and different configurations characterized by a variable orientation of the interface with respect to the electrodes have been explored. The GB forming the barrier changes as a function of the interface orientation and determines different barrier transparency and levels of dissipation. Apart from canonical quantum and thermal regimes, we have found evidence of a tunable moderately damped regime. HTS JJs seem to offer complementary functionalities when compared to LTS systems and some more flexibility in tuning the crossover temperature of the phase diffusion regime.
The comparative analysis of LTS and HTS systems has allowed to draw some relevant conclusions on the dynamical junction parameters, which determine MDR, and to better define the fingerprints of MDR. A change in the sign of the derivative of the second moment of the distribution at a turn-over temperature T* and a modification of the shape of the distributions at temperature around T* (which can be parameterized by the skewness, proportional to the third central moment of the distribution) are robust signatures of the phase diffusion regime.
The possibility to master YBCO GB junctions on the scale of hundreds of nm using both a 'classical' top-down approach or intrinsic self-assembling of nano-channels, has been important to achieve the necessary level of accuracy in the control of the structures to carry out a systematic analysis. Switching current spectra viceversa turn to be unique fingerprints of characteristic transport quantum modes in these systems (also from a morphological point of view).
HTS JJs seem to offer complementary functionalities when compared to LTS systems and some more flexibility in tuning the crossover temperature of the phase diffusion regime, despite the presence of additional sources of dissipation still to be completely defined. The Q dissipation factor is at the moment not much higher than 10. We believe that higher Q values (and therefore lower levels of dissipation) can be achieved for biepitaxial junctions, when the GB width will be reduced to the size of a single facet (of the order of 100-200 nm), being the current limit for the width about 500 nm.
Numerical codes have been developed to support the conclusions of the experimental measurements. Most of results on NbN/MgO/NbN junctions have been already published, while the manuscripts on switching current measurements on YBCO junctions are currently in progress.
Progress in the control of the properties of HTS GB JJs and a better understanding of their dynamical parameters has allowed the design of a HTS rf-SQUID (Superconducting QUantum Interference Device), aimed to be on a longer time scale the cell of the HTS qubit. The use of the superconducting loop where the junction is embedded, guarantees to reach a further level of isolation from direct bias lines. This way the bias can be executed by simply inductively coupling an on-chip superconductive coil to the qubit. The first design fully with high Tc superconductive technology is also the basis to then move on to the hybrid structure.
Significant steps have been also made on the realization of hybrid structures, where at the moment we have fully integrated and hybridized YBCO cells with InAs nanowires, which work as barriers. Up to now nanowires are of InAs, and the final structure is technologically quite demanding due to the structural complexity of both components of the junctions. The fabrication process can be applied to all sorts of barriers and hybrid structures, and paves the way to the LTS/HTS hybrid devices able to match a wide range of demand of quantum superconducting electronics.
In conclusions the work realized by Dr. Luigi Longobardi within the framework of the Project "Macroscopic quantum dynamics and coherence in hybrid superconducting circuits for quantum computation" (Marie Curie Reintegration Grant, FP7-PEOPLE-2009-RG number 248933), and partly supported by projects already active in the hosting group on related topics, has some impact on the possibility to use novel superconducting quit cells in nano-circuits. Efficiency clearly demonstrated for superconducting qubits based on classical junctions platforms can be partly extended to novel materials with possible novel functionalities