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Silicon quantum dots for quantum information processing Author: Ross Leon Supervisor: Prof. Andrew Dzurak, Dr. Alessandro Rossi Research Theme: The Digital Future Background and Motivation |0> Quantum dot presents if electrons are confined in a small region and energy is discrete. Electrons have a spin property. Two electrons can be coupled to form a Qubit, representing logic. Electrons occupy different quantum dots when representing different logics, hence one can measure electron charge instead of spin, this is called spin-charge conversion. Double Quantum Dot (DQD) double reservoir system allows us to measure current directly, current resonate when quantum dot energy changes and number of electrons changes. To isolated electron in DQD, Single reservoir system forbid direct current measurement. Dot 1 dot1 dot2 |1> Dot 2 Dot 1 Dot 2 Aim Method Remotely detect movement of electrons across regions in a double dot single reservoir system Distinguish electron movements within different parts of the system Optimise detection quality by changing detector position General methodology to simulate the result is shown as follows: reservoir SET reservoir SET reservoir Principle of Operation dot1 C1 dot2 Dot 1 Single Electron Transistor (SET) has its own reservoirs with continuous current flow. Imbalance capacitance between each dot and SET induce different effect in SET current Changing charge occupancy of each dot will affect SET current, either by: 1. Applying voltage to each quantum dot 2. Electrons tunnel from one region to another Dot 1 ISET Dot 2 Dot 2 FastCap: calculate mutual capacitance between all metals FCGUI2008 (Matlab program): 3D drawing Simon: simulate quantum effect. Note: an equivalent schematic circuit of the quantum device is built to perform simulations of the electrical characteristics Schematic Design C2 Physical dimension 3D Design 3D Model Capacitance Calculation Capacitance Matrix Simulator Result Review Results 1. Effect of Location of SET detector 2. Electron tunnelling detection The magnitude of the charge sensing signal depends on distance between SET and each dot whose occupancy changes. Detector position: Green and red lines in charge stability plot separates into regions with different electron numbers (Dot 1, Dot2). Number of electrons in both quantum dot in each Z-shape region are definite. Number of electrons in rhombus region in between depends on previous electron activities: a. Vertical displacement i. Distance (hence capacitance) between each dot and SET becomes comparable, hence unable to distinguish different type of electrons tunnelling behaviours. α β ii. Similar to a(i), tunnelling behaviours cannot be recognised. α a. Either of the quantum dot voltages increase, result in oblique SET current. b. Electron ‘jump’ into/out of either quantum dot, causes vertical jump in SET current a. Loading electrons (eg. sweeping left to right along blue dash line): only green lines in effect, ie. (M,N) b. Unloading electrons (eg. sweeping right to left along blue dash line) only red lines in effect, ie. (M,N+1) b. Horizontal displacement: i. SET current fluctuation magnitude is overwhelmed by background noise. In practice charge stability plot cannot be obtained. Measurement from SET detector current results in a sawtooth shape due to: a b β γ α Voltage sweep across black dash line results in all 3 types of electron tunnelling events. Different electron tunnelling events induce different magnitude of SET current jump. β reservoir Dot 1 γ β α Dot 2 Conclusion SET can be used to detect electron tunnelling in double dot single reservoir system. Different electron tunnelling events can be distinguished by configuring SET detector location. This work has a significant impact in readout information from a spin qubit. Future Work To investigate spin charge conversion and conduct experiments to detect and deduce spin state of a quantum dot To control spin state of current quantum dot system, and store information before readout.