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.
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.
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.
A hysteretic dc-superconducting quantum interference device (SQUID) is used to trace the flux characteristic of a tunable rf-SQUID, the basic element for the realization of superconducting flux qubits. This allows important simplifications of circuitry and electronics in developing devices for quantum computing, by eliminating the necessity of more complex magnetometers. A hysteretic dc-SQUID is usually operated as a comparator, distinguishing only which one of two adjacent flux states is occupied by the rf-SQUID. The necessary sensitivity, moreover, is usually reached only at temperatures in the mK range. However, by exploiting the statistical properties of the current–voltage curve in the region where the switching from the zero-voltage state occurs, it is possible to obtain an accurate tracing of the input flux, even at a relatively high temperature (a few Kelvin). In our case, the input signal is given by the internal flux of a tunable rf-SQUID, the building block of a flux qubit.
We studied the escape process from the zero-voltage state to the running state in hysteretic dc-SQUIDs made with trilayer Nb-AlOx-Nb junctions. We analyze the experimental results, according to the thermal activation model, for devices having similar parameters except for the dimensionless inductance β L , which however is always very small. The interest in hysteretic dc-SQUIDs is in the possibility of using them to perform non-invasive measurements of the flux states in rf-SQUIDs: this achievement would be very important both in tests of macroscopic quantum properties of SQUIDs and in measurements on a class of quantum bits that has been recently proposed, based on magnetic flux states in SQUIDs.
The flux state quantum bit (qubit) is promising for a solid state implementation of scalable quantum computing. The simplest flux state qubit consists of an rf SQUID with two fluxoid states, which can be readout with a dc SQUID - the most sensitive magnetic flux detector. Efficient readout with less back-action is desirable for quantum computing. In this work, we report measurements of the switching flux and switching current distributions of under damped dc SQUIDs. The data show that single shot readout of flux qubit with very high efficiency (>99%) can be realized using underdamped hysteretic dc SQUIDs.
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.
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.