Prospects for Quantum Computing

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Prospects forQuantum Computing
IEDM, 12/11/00
David DiVincenzo, IBM
Thanks to IBM colleagues, ARO/NSA/ARDA
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CMOS Device Performance
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Back to basics
Fundamental carrier of information the bit
Possible bit states
0
1
or
Fundamental carrier of quantum information the
qubit
Possible qubit states any superposition
described by the wavefunction
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Quantum superpositions in electrical engineering

To understand a bit can be in both states at the
same time, think about an electron can be in
two (and more) places at the same time.
(1953)
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Brillouin, Wave Propagation in Periodic
Structures (electric filters and crystal lattices)

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Quantum mechanics of electrons in crystals
Electron motion Superposition of (many) paths

B
A
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Superpositions and quantum gates
x f(x)
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Exploiting superposition the Deutsch algorithm
General form of two-bit gate
x x y y?f (x)
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Exploiting superposition the Deutsch algorithm

x x y y?f?(x)
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Exploiting superposition the Deutsch algorithm
x x y y?f?(x)
? f1 or f3
Can we narrow down to f1 or f2 in one run of
the gate?
No, classically.
Yes, with quantum superpositions!
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Exploiting superposition the Deutsch algorithm
x x y y?f1(x)
0? 1?
0? - 1?
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Exploiting superposition the Deutsch algorithm
x x y y?f2(x)
0? 1?
0? - 1?
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Exploiting superposition the Deutsch algorithm
x x y y?f3(x)
0? 1?
0? - 1?
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Exploiting superposition the Deutsch algorithm
x x y y?f4(x)
0? 1?
0? - 1?
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Exploiting superposition the Deutsch algorithm
http//www.aip.org/pt/vol-53/iss-7/p11.html N.
David Mermin, The contemplation of quantum
computation, Physics Today, July, 2000.
x x y y?f?(x)
0? 1?
0? - 1?
Seeing 0? 1? in x register means f1 or f2.
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Fast Quantum Computation
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Quantum computationwith chloroform NMR
Cl
Cl
Cl
Deutsch algorithm demonstrated.
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5 qubit 215 Hz Q. Processor
( Vandersypen, Steffen, Breyta, Yannoni, Cleve,
Chuang, July 2000 Physical Rev. Lett. )

• 5-spin molecule synthesized

• First demonstration of a fast
• 5-qubit algorithm

• Pathway to 7-9 qubits

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Measured spectra
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Physical systems actively consideredfor quantum
computer implementation

• Electrons on liquid He
• Small Josephson junctions
• charge qubits
• flux qubits
• Spin spectroscopies, impurities in semiconductors
• Coupled quantum dots
• Qubits spin,charge,excitons
• Exchange coupled, cavity coupled

• Liquid-state NMR
• NMR spin lattices
• Linear ion-trap spectroscopy
• Neutral-atom optical lattices
• Cavity QED atoms
• Linear optics with single photons
• Nitrogen vacancies in diamond

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Five Criteria for physical implementation of a
quantum computer

• Well defined extendible qubit array -stable
  memory
• Preparable in the 000 state
• Long decoherence time (gt104 operation time)
• Universal set of gate operations
• Single-quantum 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,
quant-ph/0002077.
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Concept device spin-resonance transistorR.
Vrijen et al, Phys. Rev. A 62, 012306 (2000)
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Quantum-dot array proposal
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Concept device spin-resonance transistorR.
Vrijen et al, Phys. Rev. A 62, 012306 (2000)
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Five Criteria for physical implementation of a
quantum computer

• Well defined extendible qubit array -stable
  memory
• Preparable in the 000 state
• Long decoherence time (gt104 operation time)
• Universal set of gate operations
• Single-quantum 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,
quant-ph/0002077.
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Conclusions Prospects for Quantum Computing

• alternative theoretical paradigm for computing
• enables the solution of new classes of problems
• becomes possible at the near-atomic scale
• new electronic devices may be the way to go

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IBM Yorktown group
Nabil Amer (manager) Charlie Bennett David
DiVincenzo John Smolin Barbara Terhal
(postdoc) Debbie Leung (postdoc) Ashish Thapliyal
(student, UCSB) Elitza Maneva (student, Cal
Tech) Collaborators visitors Peter Shor
(ATT) Patrick Hayden (student, Oxford) P., M.
and R. Horodecki (Gdansk) J. Kempe, D. Bacon, and
B. Whaley (Berkeley)