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A Charge Parity Ammeter

N. J. Lambert, Megan Edwards, Chiara Ciccarelli, Andrew J. Ferguson

Published Jan 8, 2016• 3 citations
OpenAlex

Abstract

A metallic double-dot is measured with radio frequency reflectometry. Changes in the total electron number of the double-dot are determined via single electron tunnelling contributions to the complex electrical impedance. Electron counting experiments are performed by moni-toring the impedance, demonstrating operation of a single electron ammeter without the need for external charge detection. Single electron ammeters count each individual electron that passes. They enable the detec-tion of tiny (I < aA) currents;1 show the real-time dynamics of single electron tunnelling;2,3 and may be of use in metrology for the definition of the Ampere.4–8 These devices are made possible by high sensitivity, high bandwidth charge detection which can register the passage of an elec-tron through multiple island single electron devices.9 Typically the charge detector employed is the single electron transistor10 or quantum point contact,11–13 which may be operated in a radio-frequency impedance matching circuit to increase both bandwidth and sensitivity.14–16 In this Let-ter we show that use of external electrometry is unnecessary, simplifying the fabrication of a single electron ammeter. Rather, the internal tunnelling processes within a metallic double dot can be used to herald the passage of single electrons through the system, so the double dot acts as its own ∗To whom correspondence should be addressed 1 ar

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arXiv:0707.2946•Applied Physics Letters•Oct 15, 2007•295 citations
A metallic double-dot is measured with radio frequency reflectometry. Changes in the total electron number of the double-dot are determined via single electron tunnelling contributions to the complex electrical impedance. Electron counting experiments are performed by moni-toring the impedance, demonstrating operation of a single electron ammeter without the need for external charge detection. Single electron ammeters count each individual electron that passes. They enable the detec-tion of tiny (I < aA) currents;1 show the real-time dynamics of single electron tunnelling;2,3 and may be of use in metrology for the definition of the Ampere.4–8 These devices are made possible by high sensitivity, high bandwidth charge detection which can register the passage of an elec-tron through multiple island single electron devices.9 Typically the charge detector employed is the single electron transistor10 or quantum point contact,11–13 which may be operated in a radio-frequency impedance matching circuit to increase both bandwidth and sensitivity.14–16 In this Let-ter we show that use of external electrometry is unnecessary, simplifying the fabrication of a single electron ammeter. Rather, the internal tunnelling processes within a metallic double dot can be used to herald the passage of single electrons through the system, so the double dot acts as its own ∗To whom correspondence should be addressed 1 ar
We present time-domain electron counting studies on charge fluctuations in a mesoscopic system. In this measurement, a radio-frequency single-electron transistor (RF-SET) acting as a fast electrometer is capacitively coupled to a quantum dot (QD) electrostatically defined in a GaAs/AlGaAs heterostructure. Random telegraph signals (RTSs) were observed on the RF-SET and were interpreted as resulting from individual electrons tunneling into/out of the QD hence switching the QD charge state between N and N+1 electrons near the charge degeneracy point. Periodic behavior of the switching events is observed as the number of average electrons in the electron box is decreased one by one. The occupation time of the excess electron is directly measured and changes from a few microseconds to a few milliseconds as the tunneling resistance of the QD is increased. Histogram of the occupation time obtained from the time-domain electron counting measurements agrees well with the frequency-domain power spectrum analysis, both suggesting a Poissonian process at the charge degenerate point, in agreement with a model consisting of two charge states. Real time electron counting techniques can play a powerful role in studies on temporal electronic correlations, as well as quantum information processing schemes. Techniques that will improve the charge sensitivity of the RF-SET will also be discussed.
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.
Radio frequency (RF) reflectometry is a well understood measurement technique in common use by RF engineers today. This technique has been applied at low temperature to probe the impedance of nanoscale devices for over two decades. These measurements typically employ basic resonant impedance transformers to bring the device impedance close to the characteristic impedance of the measurement apparatus. In 2013 it was discovered by Colless et al. that reflectometry could be used to measure extremely small impedance changes in a gate coupled to a quantum dot to directly probe the charge state of the dot. This has simplified measurements of certain nanoscale devices including single electron boxes (SEBs) and semiconductor qubits. Despite the maturity of RF reflectometry measurements in general, researchers working at low temperature continue to make basic mistakes in their measurements. The root cause of most of these mistakes is the failure to properly calibrate the measurement apparatus. In some cases this leads researchers to unrealistic conclusions when trying to match measurements to theory. This dissertation demonstrates that a low temperature RF calibration can be very easy and straightforward using inexpensive, off the shelf SMA terminators as calibration standards. A proper calibration allows for direct quantitative comparisons between data measured on different reflectometers or between data and simulations. The consistency between measurements and simulations then allows for straightforward design and implementation of improved impedance transformers, increasing the SNR of extremely sensitive measurements, with simple data-based simulations. A new type of single electron device, the capacitively coupled RF single electron transistor (CSET) is proposed and demonstrated which can rival the traditional RF SET in sensitivity but integrates more effectively into arrays and may offer a level of resistance to electrostatic discharge damage.
This thesis will discuss the principles, techniques and applications of the Radio Frequency Single Electron Transistor (RF-SET). In the first part, the operating principles of Single Electron Transistors (SETS) in the normal and superconducting states will be introduced. The general techniques of fabricating and calibrating SETs will also be introduced. In the second part, two of our recent experiments are reviewed. One is related to the sensitivity and linearity of superconducting RF-SETs. We found that the RF-SET achieves the best balance of charge sensitivity and linearity in the subgap regime, as opposed to the usual preferred working point in the above-gap regime. The second experiment relates to the real-time counting of single electrons. We demonstrated that the RF-SET can be used as a fast and ultra-sensitive electrometer which can even detect tunneling of a single electron inside a tunable quantum dot (QD) formed in a two dimensional electron gas (2DEG).
By coupling a radio-frequency single-electron transistor (RF-SET) to a quantum dot (QD) in a GaAs/AlGaAs heterostructure, we have succeeded in detecting the tunneling of individual electrons on and off the QD on time scales as short as one microsecond. Using charge detection to probe the state of the QD allows us to nearly isolate the dot from its leads, thereby minimizing decoherence-inducing effects of the environment. We have extended these charge detection techniques to double quantum dots (DQDs) that can simultaneously be used to characterize the backaction of the RF-SET. The combined RF-SET/DQD system is well-suited to the development of charge- or spin-based quantum bits, and to investigation of the quantum measurement problem.