Quantum Information Science:

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SidneyPacific Seminar
Quantum Information Science Putting quantum
to work
weirdness
Andrew LandahlHP/MIT Postdoctoral Fellow
October 8, 2003
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Information
Information is Relative
0 1
011110001001111
H e l l o
100001110110000
W x z z p
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Information
Information is Physical
Must obey physical law!
SPEEDLIMIT
670,616,629
MPH
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Energy
Energy is Physical
Governed by Thermodynamical Law
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Moores Law
Graph www.intel.com
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Moores Law
Uncharged 0
Charged 1
Half-charged Error!
One charge Quantum!
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Quantum Mechanics
Niels Bohr (1885-1962)
Anyone who can contemplate quantum mechanics
without getting dizzy hasnt properly understood
it.
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Quantum Weirdness

• Wave-particle duality

• Heisenberg Uncertainty

• Nonlocal States

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Quantum Weirdness

• Superconductivity

• Bose-Einstein Condensates

• Photoelectric Effect

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Quantum Information
Quantum bits Many more possibilities!
Images Bryan Christie Design, Scientific American
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Qubits
Examples

• Photon polarization
• Electron spin
• Nuclear spin
• Two-level atom

0
1
01
0-1
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Qubits
Qubits vs. bits

• Computational basis
• Superpositions
• Interference
• Entanglement
• Nonclonability

Could ponder forever, or
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Quantum Information Science
Schumacher Criteria

• Physical Resource.

B) Information Processing Task.
C) Criterion for successful completion.
How much A needed to achieve B subject to C?
Put the weirdness to work!!
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Bit storage
Q How many bits (C) can be stored (B)in a qubit
(A)?
A One. Holevo, 1973
After a measurement, all one has is a bit!
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Quantum Data Compression
Q How many qubits (C) can be stored (B)in a
qubit (A)?
A The quantum entropy, Schumacher, 1993
½
Compressible to 0.601qubits/signal
½
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No-cloning Theorem
Q Can one (C) make a copy (B) of a qubit (A)?
A Not if the state is unknown. Wooters Zurek,
1982.
COPY(x,0) (x, x)
QCOPY( , ) ( , ) ( , )
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Teleportation
Q Can one (C) send qubits (B) using only bits
and entanglement (A)?
A Yes. Bennett et al., 1993.
1 qubit
2 bits
B
time
1 qubit
A
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Superdense coding
Q How many (C) bits can one send (B) using only
1 qubit and entanglement (A)?
A Two. Bennett Wiesner, 1992.
2 bits
1 qubit
B
time
2 bits
A
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Quantum cryptography
Q Can one (C) send uncrackable secrets (B) using
qubits (A)?
A Yes. Ekert, 1991.

• Alice sends random qubits in random bases.

• Bob measures in random bases.

• Alice announces bases.

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Quantum computing
Q Can one (C) compute faster (B) by using qubits
instead of bits (A)?
A Yes.

• Computing the parity. Deutsch, 1985
• Factoring a number. Shor, 1994
• Searching an unordered list. Grover, 1995
• Simulating quantum systems. Lloyd, 1996

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Quantum parallelism
f(0) f(1)
Musical chord
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Experimental Progress
Cavity QED
Liquid NMR
Quantum Dot
Lattice BEC
Ion Trap
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Quantum error correction
Q Can one (C) protect qubits from noise (B) by
using entanglement (A)?
A Yes. Shor, 1996 Steane, 1996.
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The Road Ahead

• About 50 years since the ENIAC. Ubiquitous
  quantum computers in 50 more?
• Useful quantum crypto here today.
• Small quantum computers here today.
• Language of quantum information has inspired and
  is inspiring novel experiments

Weird, man.