Download Silicon quantum dots for quantum information processing

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
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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
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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.