silicon-based quantum computation cheuk chi lo kinyip phoa dept. of eecs, uc berkeley c191 final...
Post on 20-Dec-2015
225 views
TRANSCRIPT
Silicon-based Quantum Computation
Cheuk Chi Lo Kinyip Phoa
Dept. of EECS, UC Berkeley
C191 Final Project PresentationNov 30, 2005
Silicon-based Quantum Computation: Presentation Outline
I. Introduction
II. Proposals for Silicon Quantum Computers
III. Physical Realization: Technology and Challenges
IV. Summary and Conclusions
Introduction: Why Silicon?
We know silicon from years of building classical computers
Donor nuclear spins are well-isolated from environment low error rates and long decoherence time
Integration of quantum computer with conventional electronics
Scalability advantages?
Introduction: DiVincenzo’s Criteria
1. Well-defined qubits
2. Ability to initialize the qubits
3. Long decoherence time
4. Manipulation of qubit states
5. Read-out of qubit states
6. Scalability (~105 qubits)
II. Overview of Silicon Quantum Computation Architectures
Silicon Quantum Computer Proposals
Shallow Donor Qubits Deep Donor Qubits Silicon-29 Qubits
Exchange Coupling
Magnetic Dipolar Coupling
Electron Shuttling
Silicon Shallow Donor Qubits: Qubit Definition and State Manipulation
barrier
Silicon-28
Control gate
A-Gate(Hyperfine Interaction)
J-Gate(Exchange Coupling)
magnetic dipolar coupling
S-Gates(Electron shuttling)
BDC
BAC
BE Kane, Nature, 393 14 (1998)AJ Skinner et al, PRL, 90 8 (2003)R de Sousa et al, Phys Rev A, 70 052304 (2004)
Spin Resonance
Qubit
Summary of Silicon Shallow Donor Qubits
Qubit: donor nuclear spin or hydrogenic qubit (nucleus + electron spins)
Initialization: Recycling of nuclear state read-out + nuclear spin-state flip via interaction with donor electron
Decoherence time: e.g. at 1.5K nucleus spin T1 > 10 hours electron spin T1 > 0.3hours
Qubit Manipulation Single Qubit Manipulation: hyperfine interaction + spin
resonance Multi-qubit Interaction: Exchange coupling, Magnetic
dipolar coupling or Electron shuttling Read-out: Transfer of nucleus spin state to donor electron
via hyperfine interaction, then read-out of electron spin state
Physical Realization of a Si QC
Some common features that must be realized in a shallow donor Si QC are:Array of single, activated 31P atoms:Single-spin state read-out: Integrated control gatesProcess Variations
Formation of Ordered Donor Arrays
JL O’Brien et al, Smart Mater. Struct., 11 741 (2002)
“Top-down” single ion implantation
“Bottom up” STM based Hydrogen Lithography
T Schenkel et al, APR, 94(11) 7017 (2003)
Spin-State Read-out with SET’s & Fabrication of Control Gates
Read-out Challenges: i. SET’s are susceptible to 1/f and telegraphic
noises (from the random charging and discharging of defect/trap states in the silicon host)
ii. alignment and thermal budget of SET’s with the donor atom sites also present as a fabrication challenge.
Read-out: Spin state Charge state (e.g. measurement by SET)
Control Gate Challenges: Qubit-qubit spacing requirements for different coupling
mechanisms: Exchange Coupling: 10-20nm Magnetic Dipolar Coupling: 30nm Electron Shuttling: >1m
State-of the art electron beam lithography: can do ~10nm, but not dense patterns Qubit interaction control gates extremely challenging!
(L Chang, PhD Thesis, EECS)
(UNSW)
Process Variations
(IBM)
Process Variations may arise from:
i. substrate temperature gradient,
ii. uneven reagent use during fabrication,
iii. differences in material thermal expansion
iv. strain induced by the patterning of the substrate (leads to uncertainty in ground state donor electron wavefunction, due to incomplete mixing of states)
Consequences:
i. Need careful tuning and initialization of qubits
ii. Limit of scalability?
iii. Introduce strain in silicon intentionally?
• lifts degeneracy of electronic state less vulnerable to process variations
Silicon Deep Donors Proposal
Excited State
Ground State
Optical Excitation
Bi Er Bi
Bi Er Bi
Bi Er Bi
AM Stoneham et al, J. Phys.: Condens. Matter, 15 (2003), L447
Initialization, Manipulationand Readout?
Initialization by polarized light or injection of polarized electron both are not very possible under room temperature
Manipulation with microwave pulses like the work by Charnock et. al. on N-V centers in diamond
Readout optically detection of photons emitted potentially require detection of single photon
Disorderness of donor ion Irreproducibility and difficult to address qubits
Decoherence Time andThermal Ionization
Summary of Silicon Deep Donor Qubits
Qubit: deep donor (e.g. Bismuth) nuclear spin, proposed to work at room temperature.
Initialization: Optical pumping or injection of polarized electron, questionable in feasibility.
Decoherence time: fraction of nanosecond at room temperature
Qubit Manipulation: via applying intense microwave pulse, like N-V centers in diamond
Read-out: optical readout of photon emitted from transition between two states
Silicon-29 Quantum Computer Overview
NMR-type quantum computer
Initialize with circularly polarized light
Manipulating qubits by Dysprosium (Dy) magnet
Readout using MRFM CAI
TD Ladd et. al. , PRL, 89(1) 017901, 2002
Decoherence Times
Long decoherence time (T1 and T2)Reported T1 as large as 200 hours,
measured in darkExperimentally find T2 as long as 25
secondsT2 is reduced by the presence of 1/f
noise due to the traps at lattice defects and impurities
Summary of Silicon NMR quantum computer
Qubit: Chains of silicon-29 isotope for ensemble measurement
Initialization: Optical pumping with circularly polarized light Decoherence time: measured as long as 200 hours in dark
at 77K for T1 but only 25 seconds for T2
Qubit Manipulation: combination of static magnetic field and RF magnetic field
Read-out: with cantilever, performing MRFM CAI
Problem:RF Coil, Dy Magnet & MRFM
The deposition method of Dy magnet is not outlined! It won’t be trivial to incorporate
The cantilever tip for MRFM is not included in the schematic. How to insert it?
TD Ladd et. al. , PRL, 89(1) 017901, 2002
Summary and Conclusions
Several proposals for implementing quantum computer in silicon Shallow donor (phosphorus) qubit Deep donor (bismuth) qubit Silicon-29 NMR quantum computer
Difficulties faced in each proposals Arguments on the feasibility Most experimental efforts are on shallow donor qubits Convergence with conventional electronics processing
requirements: Currently: 90nm technology node (~45nm features) 22nm technology node in 2016! Strained-silicon: hot topic of research in semiconductor industry Narrower transistor performance window with ordered dopants Single-electron transistors and other nanoelectronics
(http://www.ITRS.net)
Thank You
Thank You!