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Physica Scripta

Reading Out Charge Qubits with a Radio-Frequency Single-Electron-Transistor

Kevin Bladh, David Gunnarsson, Göran Johansson, A. K?ck, Goran P. Wendin, A. Aassime, Marian A. Taslakov, Marian A. Taslakov, Per Delsing

Published Jan 1, 2002• 16 citations
Publisher

Abstract

We describe fabrication and measurements of Single Electron Transistors operated in the Radio Frequency mode (RF-SET). We demonstrate very high sensitivity for RF-SETs and we evaluate the back-action from an RF-SET, when used as a read-out device for charge qubits. We conclude that single-shot read-out is possible with a signal to noise ratio greater than one, and that niobium qubits would substantially improve the signal to noise ratio. Furthermore we describe how the RF-SET could be used to read out a differential qubit in a gradiometer coupling.

DOI
10.1238/physica.topical.102a00167

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Science•May 22, 1998•776 citations
We describe fabrication and measurements of Single Electron Transistors operated in the Radio Frequency mode (RF-SET). We demonstrate very high sensitivity for RF-SETs and we evaluate the back-action from an RF-SET, when used as a read-out device for charge qubits. We conclude that single-shot read-out is possible with a signal to noise ratio greater than one, and that niobium qubits would substantially improve the signal to noise ratio. Furthermore we describe how the RF-SET could be used to read out a differential qubit in a gradiometer coupling.
We study the radio-frequency single-electron transistor (rf-SET) as a readout device for charge qubits. We measure the charge sensitivity of an rf-SET to be 6.3microe/sqrt[Hz] and evaluate the backaction of the rf-SET on a single Cooper-pair box. This allows us to compare the needed measurement time with the mixing time of the qubit imposed by the measurement. We find that the mixing time can be substantially longer than the measurement time, which would allow readout of the state of the qubit in a single shot measurement.
The radio-frequency single-electron transistor (rf-SET) possesses key requirements necessary for reading out a solid state quantum computer. This work explores the use of the rf-SET as a single-shot readout device in the presence of 1∕f and telegraph charge noise. For a typical spectrum of 1∕f noise we find that high fidelity, single-shot measurements are possible for signals Δq>0.01e. For the case of telegraph noise, we present a cross-correlation measurement technique that uses two rf-SETs to suppress the effect of random switching events on readout. We demonstrate this technique by monitoring the charge state of a metal double dot system on microsecond time scales. Such a scheme will be advantageous in achieving high readout fidelity in a solid-state quantum computer.
The radio-frequency superconducting single electron transistor (RF-SET) is a highly sensitive and fast electrometer operating near the quantum limit. The back-action current noise of the SET induces voltage fluctuations on the system it is coupled to, dephasing its charge states. Here we propose to study the back action of the RF-SET by means of a double quantum dot (DQD) and, an RF-SET electrostatically coupled to one of the two dots. The inelastic current through the DQD is very sensitive to the spectral density of the voltage fluctuations in the nearby environment. By properly choosing the dot size and inter-dot tunnel barrier thickness, one can cause inelastic process to dominate the transport processes. A measurement of the inelastic current through the double dot system can then be used to calculate the spectral density of noise associated with the RF-SET and hence its back action. The design, fabrication and characterization of an RF-SET/DQD system is discussed in this work. Real-time studies of electron dynamics in coupled dots are also addressed.
We present results on a double-island single-electron transistor (DISET) operated at radio frequency (rf) for the fast and highly sensitive detection of charge motion in the solid state. Using an intuitive definition for the charge sensitivity, we compare a DISET to a conventional single-electron transistor (SET). We find that a DISET can be more sensitive than a SET for identical, minimum device resistances in the Coulomb blockade regime. This is of particular importance for rf operation, where ideal impedance matching to 50-Ω transmission lines is only possible for a limited range of device resistances. We report a charge sensitivity of 5.6×10−6e∕Hz for a rf-DISET, together with a demonstration of the single-shot detection of small (⩽0.1e) charge signals on microsecond time scales.
A new type of electrometer is described that uses a single-electron transistor (SET) and that allows large operating speeds and extremely high charge sensitivity. The SET readout was accomplished by measuring the damping of a 1.7-gigahertz resonant circuit in which the device is embedded, and in some ways is the electrostatic "dual" of the well-known radio-frequency superconducting quantum interference device. The device is more than two orders of magnitude faster than previous single-electron devices, with a constant gain from dc to greater than 100 megahertz. For a still-unoptimized device, a charge sensitivity of 1.2 x 10(-5) e/hertz was obtained at a frequency of 1.1 megahertz, which is about an order of magnitude better than a typical, 1/f-noise-limited SET, and corresponds to an energy sensitivity (in joules per hertz) of about 41 Planck's over 2pi.