Delei Chen

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Quantum Computer

• Delei Chen
• Nov 7th 2008, Stockholm

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(No Transcript)
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Problem

• At current rate transistors will be as small as
  an atom
• As scale becomes too small, matters obey the
  rules of quantum mechanics

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Quantum Computer(QC)

• 1970s and 1980s Theorists proposed idea of
  quantum computers
• What is a QC?A QC is a device for computation
  that makes use of quantum mechanical phenomena to
  perform operations on data, implemented with
  quantum binary digits (qubits) in particle spin
  states.

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Classical Computers VS. Quantum Computers

• Qubit (quantum bit) 0 or 1 or
• Register Hold all number3 qubits
  ???? represent 0-7
• Operation( Simultaneous)

• Bit 0 or 1
• Register Hold one number3 bits000,001
  represent 0-7
• Operation

F(00) F(01) F(10) F(11)
00 01 10 11
F(x) processor
F(x) Processor
001
F(001)
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Implements of Qubits
Spin-based quantum computer
A Silicon-based nuclear spin quantum computer,
B.E. Kane, Nature, May 14, 1998
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Spin-based QC (Kane Proposal)

• The qubits are the nuclear spins of the 31P
  donors
• Gates (A) control the strength of the hyperfine
  interactions
• Gates (J) turn on and off electron-mediated
  coupling between the nuclear spins
• A globally applied a.c magnetic field Bac flips
  nuclear spins at resonance

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Single Spin Operations
Voltage-controlled oscillator
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Two Spin Operations
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Spin measurement

• Measurement
• Both electrons bound to same donor
• Differential voltage in A-gates results in charge
  motion
• Current measured via capacitive techniques
• Signal lasts entire decoherence time
• Measurement of single qubit via magnetic field

Cross-section of Kane Quantum Computer Nature May
1998, Kane
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Criteria(DiVincenzo checklist)

• Be a scalable physical system with well-defined
  qubits, ?a0gtb1gt
• Be initializable to a simple fiducial state such
  as ?00000gt
• Have much longer decoherence time than operation
  time
• Have a universal set of quantum gates
• Qubit specific measurements

D. P. DiVincenzo, in Mesoscopic Electron
Transport, eds. Sohn, Kowenhoven, Schoen (Kluwer
1997), p. 657, cond-mat/9612126 The Physical
Implementation of Quantum Computation, Fort. der
Physik 48, 771 (2000), quant-ph/0002077.
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Thank you