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Applied Physics Letters

Stroboscopic single-shot detection of the flux state in a radio frequency superconducting quantum interference device

C. Cosmelli, F. Sciamanna, Maria Gabriella Castellano, Roberto Leoni, G. Torrioli, P. Carelli, F. Chiarello

Published Apr 29, 2002• 14 citations
Publisher

Abstract

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.

DOI
10.1063/1.1474605

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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.
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.
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 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 present a readout method for superconducting flux qubits. The qubit quantum flux state can be measured by determining the Josephson inductance of an inductively coupled dc superconducting quantum interference device (dc-SQUID). We determine the response function of the dc-SQUID and its back-action on the qubit during measurement. Due to driving, the qubit energy relaxation rate depends on the spectral density of the measurement circuit noise at sum and difference frequencies of the qubit Larmor frequency and SQUID driving frequency. The qubit dephasing rate is proportional to the spectral density of circuit noise at the SQUID driving frequency. These features of the back-action are qualitatively different from the case when the SQUID is used in the usual switching mode. For a particular type of readout circuit with feasible parameters we find that single shot readout of a superconducting flux qubit is possible.
We propose a single shot quantum measurement to determine the state of a Josephson charge quantum bit (qubit). The qubit is a Cooper pair box and the measuring device is a two junction superconducting quantum interference device (dc SQUID). This coupled system exhibits a close analogy with a Rydberg atom in a high Q cavity, except that in the present device we benefit from the additional feature of escape from the supercurrent state by macroscopic quantum tunneling, which provides the final readout. We test the feasibility of our idea against realistic experimental circuit parameters and by analyzing the phase fluctuations of the qubit.