Silicon-based Quantum Computation

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Silicon-based Quantum Computation
C191 Final Project Presentation Nov 30, 2005

• Cheuk Chi Lo
• Kinyip Phoa
• Dept. of EECS, UC Berkeley

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Silicon-based Quantum Computation Presentation
Outline

1. Introduction
2. Proposals for Silicon Quantum Computers
3. Physical Realization Technology and Challenges
4. Summary and Conclusions

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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?

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Introduction DiVincenzos Criteria

• Well-defined qubits
• Ability to initialize the qubits
• Long decoherence time
• Manipulation of qubit states
• Read-out of qubit states
• Scalability (105 qubits)

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II. Overview of Silicon Quantum Computation
Architectures
Silicon Quantum Computer Proposals
Shallow Donor Qubits
Deep Donor Qubits
Silicon-29 Qubits
Electron Shuttling
Exchange Coupling
Magnetic Dipolar Coupling
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Silicon Shallow Donor Qubits Qubit Definition
and State Manipulation
Spin Resonance
J-Gate (Exchange Coupling)
A-Gate (Hyperfine Interaction)
Control gate
barrier
Silicon-28
Qubit
S-Gates(Electron shuttling)
magnetic dipolar coupling
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)
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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 gt 10 hours
• electron spin T1 gt 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

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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 gates
• Process Variations

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Formation of Ordered Donor Arrays
Top-down ? single ion implantation
T Schenkel et al, APR, 94(11) 7017 (2003)
Bottom up ? STM based Hydrogen Lithography
JL OBrien et al, Smart Mater. Struct., 11 741
(2002)
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Spin-State Read-out with SETs Fabrication of
Control Gates
Read-out Spin state ? Charge state (e.g.
measurement by SET)

• Read-out Challenges
• SETs are susceptible to 1/f and telegraphic
  noises (from the random charging and discharging
  of defect/trap states in the silicon host)
• alignment and thermal budget of SETs with the
  donor atom sites also present as a fabrication
  challenge.

(UNSW)

• Control Gate Challenges
• Qubit-qubit spacing requirements for different
  coupling mechanisms
• Exchange Coupling 10-20nm
• Magnetic Dipolar Coupling 30nm
• Electron Shuttling gt1?m
• 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)
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Process Variations

• Process Variations may arise from
• substrate temperature gradient,
• uneven reagent use during fabrication,
• differences in material thermal expansion
• strain induced by the patterning of the substrate
  (leads to uncertainty in ground state donor
  electron wavefunction, due to incomplete mixing
  of states)

• Consequences
• Need careful tuning and initialization of qubits
• Limit of scalability?
• Introduce strain in silicon intentionally?
• lifts degeneracy of electronic state ? less
  vulnerable to process variations

(IBM)
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Silicon Deep Donors Proposal
Excited State
Bi
Er
Bi
Optical Excitation
Ground State
Bi
Er
Bi
Bi
Er
Bi
AM Stoneham et al, J. Phys. Condens. Matter, 15
(2003), L447
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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

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Decoherence Time andThermal Ionization
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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

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Silicon-29 Quantum Computer Overview
Manipulating qubits by Dysprosium (Dy) magnet
Initialize with circularly polarized light
NMR-type quantum computer
Readout using MRFM CAI
TD Ladd et. al. , PRL, 89(1) 017901, 2002
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Decoherence Times

• Long decoherence time (T1 and T2)
• Reported T1 as large as 200 hours, measured in
  dark
• Experimentally find T2 as long as 25 seconds
• T2 is reduced by the presence of 1/f noise due to
  the traps at lattice defects and impurities

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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

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ProblemRF Coil, Dy Magnet MRFM
The deposition method of Dy magnet is not
outlined! It wont 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
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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)
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Thank You

• Thank You!