We report on development of a set-up for measuring intrawell dynamics in a Nb-based rf-SQUID qubit described by a double well potential, by rapidly tilting the potential, allowing escape to the adjacent well with high probability for an excited state but low probability for the ground state. The rapid tilt of the double well potential is accomplished via a readout flux pulse inductively coupled to the qubit from a microstrip transmission line on a separate chip suspended above the qubit chip. The readout pulse is analogous to the current bias pulse used to readout phase qubits and hysteretic dc-SQUID magnetometers. The coupling between the transmission line and the qubit is carefully controlled via a window in the ground plane between the signal conductor of the microstrip and the qubit loop. Since the high frequency transmission lines are on a separate chip, they can be independently characterized and reused for different qubit samples. Clean flux pulses as short as 5 ns with rise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than 10 8 s −1 , the lifetime of the excited state, and coherent oscillations between the ground and excited states within the same well.
We report on development of a set-up for measuring intrawell dynamics in a Nb-based rf-SQUID qubit described by a double well potential, by rapidly tilting the potential, allowing escape to the adjacent well with high probability for an excited state but low probability for the ground state. The rapid tilt of the double well potential is accomplished via a readout flux pulse inductively coupled to the qubit from a microstrip transmission line on a separate chip suspended above the qubit chip. The readout pulse is analogous to the current bias pulse used to readout phase qubits and hysteretic dc-SQUID magnetometers. The coupling between the transmission line and the qubit is carefully controlled via a window in the ground plane between the signal conductor of the microstrip and the qubit loop. Since the high frequency transmission lines are on a separate chip, they can be independently characterized and reused for different qubit samples. Clean flux pulses as short as 5 ns with rise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than 10 8 s −1 , the lifetime of the excited state, and coherent oscillations between the ground and excited states within the same well.
We report on the development and test of an integrated system composed of a flux qubit and a rapid single-flux quantum (RSFQ) circuit that allows qubit manipulation. The goal is to demonstrate the feasibility of control electronics integrated on the same chip as the qubit, in view of the application in quantum computation with superconducting devices. RSFQ logic relies on the storage and transmission of magnetic flux quanta and can be profitably used with superconducting qubits because of the speed, scalability, compatibility with the qubit fabrication process and low temperature environment. While standard RSFQ circuitry is well assessed, the application to quantum computing requires a complete rescaling of parameter values, in order to preserve the qubit coherence and reduce the power dissipation. In the system presented in this paper, the qubit role is played by a superconducting loop interrupted by a small dc SQUID, usually called a double SQUID, which behaves as a tunable rf-SQUID. Its energy potential has the shape of a double well, with the barrier between the wells controlled by magnetic flux applied to the inner dc SQUID. Here for the first time we report measurements at a base temperature of 370 mK in which flux control pulses with desired characteristics were supplied by a RSFQ circuit fabricated using non-standard parameters in the same chip as the qubit.
The observation and study of macroscopic quantum coherence (MQC), i.e. the coherent superposition and evolution of macroscopically distinct quantum states, is of great importance for both our understanding of nature and for quantum engineering, for example in quantum computing. Recently a first indication of MQC in a system of SQUIDs has been indirectly observed by means of spectroscopic techniques. Applications like quantum computing, however, require the manipulation and the observation of the time evolution of the MQC states. We have realized a system of SQUIDs for the study of MQC. Our system, based on trilayer Nb/AlOx/Nb technology, consists of a double rf-SQUID, source of the MQC state, inductively coupled to a non-hysteretic dc-SQUID magnetometer and to a hysteretic dc-SQUID threshold detector, used for the read-out of the magnetic flux state. The magnetometer is used to test and monitor the rf-SQUID behaviour, while the hysteretic dc-SQUID is used to perform fast measurements with a reduced back action on the rf-SQUID. We present measurements performed at 4.2 K and at 20 mK to characterize the single devices and their overall behaviour, and a scheme for the use of a set of SQUIDs as a qubit system.
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 has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.
We present a technique to control the quantum state of a rf-SQUID qubit. We propose to employ a stream of single flux quantum (SFQ) pulses magnetically coupled to the qubit junction to momentarily suppress its critical current. This effectively lowers the barrier in the double-well rf SQUID potential thereby increasing the tunneling oscillation frequency between the wells. By carefully choosing the time interval between SFQ pulses one may accelerate the interwell tunneling rate. Thus it is possible to place the qubit into a chosen superposition of flux states and then effectively to freeze the qubit state. We present both numerical simulations and analytical time-dependent perturbation theory calculations that demonstrate the technique. Using this strategy one may control the quantum state of the rf SQUID in a way analogous to the /spl pi/ pulses in other qubit schemes.
In recent years, the quantum behaviour of Josephson devices has been the object of thorough experimental investigation, mainly with the objective of studying quantum mechanics in macroscopic objects. At present, the same quantum properties are being exploited with the aim of the physical implementation of quantum bits and quantum registers. Here we present measurements on a system containing potential qubits, namely a rf SQUID and a hysteretic dc SQUID, magnetically coupled, under microwave excitation; the devices are realized on a single chip with trilayer Nb/AlO x /Nb technology. On this system we have performed a set of measurements to test the dc SQUID response to short pulses of microwaves ranging from 2 to 32 GHz. A first analysis of our results indicates the presence of population oscillations in the hysteretic dc SQUID. This result is very promising in view of using SQUIDs for more complex qubits systems.
Superconducting qubits demonstrate good perspectives as scalable blocks for quantum information processing. These systems can be individually readout, addressed, and controlled, making them some of the most attractive thin-film qubits. Quantum measurements should be performed in order to readout the final state of flux qubits. To observe the dynamics of such systems, one requires a fast magnetometer with exceptional sensitivity and low back-action. We address the problem of reducing the back-action of an RF SQUID readout circuit, which may significantly destroy a Josephson flux qubit in the case of the circuits integration. The main sources of such back-action in conventional RF SQUID and DC SQUID circuits are low-ohmic shunting resistors ensuring high damping. The advantage of flux qubits is the presence of an optimal bias point at which, in the first-order approximation, the device is immune to fluctuations in readout and control lines. Nonetheless, even such systems have characteristics limited by a second-order dissipation. The dissipation can be eliminated by using an “ideal parametric readout” based on an RF SQUID acting in non-hysteretic adiabatic mode. In this paper, we propose an RF SQUID in a non-hysteretic regime as a fast, sensitive, and scalable readout system with low back-action for measurements of a single MW photon counter based on a flux qubit.