F. Chiarello
Sapienza University of Rome
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Featured researches published by F. Chiarello.
Applied Physics Letters | 2005
M. Castellano; F. Chiarello; R. Leoni; D. Simeone; G. Torrioli; C. Cosmelli; P. Carelli
We discuss and demonstrate a prototype of superconducting transformer with a flux transfer function that can be varied in a wide range, by acting on a control parameter. The device is realized by inserting a small hysteretic superconducting quantum interference device (dc-SQUID) with unshunted junctions, working as a Josephson junction with flux-controlled critical current, parallel to a superconducting transformer; by varying the magnetic flux coupled to the dc-SQUID, the transfer function for the flux coupled to the transformer can be varied. This feature can prove particularly appealing in the field of quantum computing, where it could be exploited to achieve a controllable magnetic coupling among flux-based qubits. Measurements carried out on a prototype at 4.2K show a reduction factor of about 30 between the “on” and the “off” states. We discuss the system characteristics and the experimental results.
Superconductor Science and Technology | 2006
Maria Gabriella Castellano; Leif Grönberg; P. Carelli; F. Chiarello; C. Cosmelli; R. Leoni; S. Poletto; G. Torrioli; Juha Hassel; Panu Helistö
In order to integrate superconducting qubits with rapid-single-flux-quantum (RSFQ) control circuitry, it is necessary to develop a fabrication process that simultaneously fulfils the requirements of both elements: low critical current density, very low operating temperature (tens of millikelvin) and reduced dissipation on the qubit side; high operation frequency, large stability margins, low dissipated power on the RSFQ side. For this purpose, VTT has developed a fabrication process based on Nb trilayer technology, which allows the on-chip integration of superconducting qubits and RSFQ circuits even at very low temperature. Here we present the characterization (at 4.2 K) of the process from the point of view of the Josephson devices and show that they are suitable to build integrated superconducting qubits.
Physical Review Letters | 2004
Niels Grønbech-Jensen; M. Castellano; F. Chiarello; M. Cirillo; Cosmelli C; L. V. Filippenko; Russo R; Torrioli G
We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multivalued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.
IEEE Transactions on Applied Superconductivity | 2001
P. Carelli; Maria Gabriella Castellano; F. Chiarello; C. Cosmelli; R. Leoni; G. Torrioli
Among the various devices proposed as elements of a quantum computer, the rf-SQUID is a very promising candidate. In fact, systems based on this element can be adjusted in situ, can be coupled by means of superconducting transformers, can be prepared individually and measured with superconducting electronics. Moreover, many progresses were made in these years which showed quantum effects in this system. The present paper describes a complete device developed in order to get a direct measurement of the quantum coherent oscillation. The knowledge of this time, together with its limiting factors, is a prerequisite for fabricating a qubit based on rf-SQUIDs.
Journal of Applied Physics | 1996
M. Castellano; R. Leoni; G. Torrioli; F. Chiarello; C. Cosmelli; A. Costantini; G. Diambrini‐Palazzi; P. Carelli; R. Cristiano; L. Frunzio
Josephson junctions based on Nb/AlOx/Nb trilayer technology have demonstrated excellent quality, exhibiting very low dissipation in the subgap region. This property is very important for all those experiments which have to deal with the quantum behavior of macroscopic variables in Josephson devices. In view of performing an experiment of macroscopic quantum coherence with a Josephson device, we measured the escape temperature and the return current of several junctions fabricated with different processes and having different characteristics, by cooling them from 4.2 to 0.3 K. We show that, for what concerns the process of escape from the zero‐voltage state, no difference can be found among different junctions. On the other hand, the dissipation properties are strongly influenced by the current density, with lower current density resulting in lower intrinsic dissipation.
Physical Review Letters | 2007
M. Castellano; F. Chiarello; R. Leoni; F. Mattioli; G. Torrioli; P. Carelli; M. Cirillo; C. Cosmelli; A. de Waard; G. Frossati; Niels Grønbech-Jensen; S. Poletto
We report on experiments performed to probe quantum coherence in a system consisting of an rf-SQUID in which the Josephson junction is replaced by a small loop containing two junctions in parallel. At temperatures of the order of 10 mK the system may develop three potential energy wells, which modify the usual two well energy profile and thereby verify the qubit manipulation strategy. The appearance of the third potential well can be interpreted as evidence of a butterfly Catastrophe, namely, a catastrophe expected for a system described by four control parameters and one state variable.
Journal of Applied Physics | 1999
M. Castellano; G. Torrioli; F. Chiarello; C. Cosmelli; P. Carelli
We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly ...
IEEE Transactions on Applied Superconductivity | 2001
C. Cosmelli; P. Carelli; Maria Gabriella Castellano; F. Chiarello; R. Leoni; G. Torrioli
We have realized a hysteretic dc SQUID to measure the flux states of a rf SQUID inductively coupled to the hysteretic dc SQUID. This type of measurement is required whenever is necessary to make a non invasive measure of the state of the rf SQUID. This is the case of measurements of macroscopic quantum coherence or measurements on q-bits systems. With the proposed set up the hysteretic dc SQUID can switch from the superconducting to the normal state depending on the total applied magnetic flux. On this system we measured at 4.2 K the change on the switching probability of the dc SQUID by varying the flux state of the rf SQUID. The experimental results agree very well with the theoretical predictions showing that this device can be used to perform non invasive measurements of the rf SQUID flux state.
Applied Physics Letters | 2002
C. Cosmelli; F. Sciamanna; M. Castellano; R. Leoni; G. Torrioli; P. Carelli; F. Chiarello
The flux states of a rf superconducting quantum interference device (SQUID) can be used to investigate macroscopic quantum effects, the most challenging of which is the detection of quantum coherent oscillations, and are promising candidates for the implementation of quantum computing. The use of a proper readout system is of utmost importance for these purposes. In this letter we present experimental evidence of the possibility of using an underdamped, hysteretic dc SQUID to read out stroboscopically the flux states of a rf SQUID, allowing a single shot discrimination with an efficiency of 98%. The devices are all integrated on chip.
Physical Review B | 2009
A. B. Zorin; F. Chiarello
We propose a superconducting phase qubit on the basis of the radio-frequency SQUID with the screening parameter value