Quantum Information Science:
A Second Quantum Revolution
Christopher Monroe
18
56
Joint Quantum Institute
University of Maryland
Department of Physics
www.iontrap.umd.edu
Joint Quantum Institute
Quantum science
for tomorrow’s technology
Computer Science and Information Theory
Charles Babbage (1791-1871)
mechanical difference engine
Alan Turing (1912-1954)
universal computing machines
Claude Shannon (1916-2001)
quantify information: the bit
k
H   pi log2 pi
i 1
ENIAC
(1946)
The first solid-state transistor
(Bardeen, Brattain & Shockley, 1947)
Source: Intel
“There's Plenty of Room at the Bottom”
(1959)
Richard Feynman
“When we get to the very, very small world –
say circuits of seven atoms - we have a lot of
new things that would happen that represent
completely new opportunities for design.
Atoms on a small scale behave like nothing
on a large scale, for they satisfy the laws of
quantum mechanics…”
Quantum Mechanics: A 20th century revolution in physics
•
•
•
•
Why doesn’t the electron collapse onto the nucleus of an atom?
Why are there thermodynamic anomalies in materials at low temperature?
Why is light emitted at discrete colors?
....
Erwin Schrödinger (1887-1961)
Albert Einstein (1879-1955)
Werner Heisenberg (1901-1976)
The Golden Rules
of Quantum Mechanics
1. Quantum objects are waves and can be in
states of superposition.
“qubit”:
[0] & [1]
2. Rule #1 holds as long as you don’t look!
[0] & [1]
or
[0]
[1]
Most of 20th century quantum physics
concerned with rule #1:
• Wave mechanics
• Quantized energy
[ ]
ˆ
H [  ]  i 
t
• Low temperature phenomena
e.g., superfluidity, BEC
• Quantum Electrodynamics (QED)
e.g., magnetism of the electron:
ge = 2.00231930439 (agrees w/ theory to 12 digits)
• Nuclear physics
• Particle physics
A new science for the 21st Century?
Information
Quantum
Mechanics
20th Century
Theory
Quantum Information Science
21st Century
What if we store information in quantum
systems?
classical bit:
0 or 1
quantum bit: a[0] + b[1]
GOOD NEWS…
quantum parallel processing on 2N inputs
Example: N=3 qubits
 = a0 [000] + a1[001] + a2 [010] + a3 [011]
a4 [100] + a5[101] + a6 [110] + a7 [111]
f(x)
…BAD NEWS…
Measurement gives random result
e.g.,   [101]
f(x)
…GOOD NEWS!
quantum interference
quantum
logic
gates
depends
on all
inputs
Deutsch (1985)
Shor (1994) fast number factoring
Grover (1996) fast database search
N = pq
# articles mentioning
“Quantum Information”
or “Quantum Computing”
2000
1500
1000
Quantum
Computers
and Computing
Nature
Science
Phys. Rev. Lett.
Phys. Rev.
Institute of
Computer Science
Russian Academy
of Science
ISSN 1607-9817
500
0
…GOOD NEWS!
quantum interference
depends
on all
inputs
quantum
logic
gates
quantum [0]  [0] + [1]
NOT gate: [1]  [1]  [0]
quantum [0] [0]
XOR gate: [0] [1]
[1] [0]
[1] [1]




[0]
[0]
[1]
[1]
[0]
[1] e.g.,([0] + [1]) [0]  [0][0] + [1][1]
[1] superposition  entanglement
[0]
Ψ = [↑][↓]  [↓][↑]
John Bell (1964)
Any possible “completion” to
quantum mechanics will
violate local realism
just the same
Schrödinger’s Cat (1935)
[did decay][Alive] + [didn’t decay][Dead]
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
[H][H] & [T][T]
1
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
[H][H] & [T][T]
1
0
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
0
[H][H] & [T][T]
1
0
0
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
0
1
[H][H] & [T][T]
1
0
0
1
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
0
1
1
[H][H] & [T][T]
1
0
0
1
1
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
0
1
1
1
[H][H] & [T][T]
1
0
0
1
1
1
Entanglement: Quantum Coins
Two coins in a
quantum
superposition
1
0
0
1
1
1
0
.
.
.
[H][H] & [T][T]
1
0
0
1
1
1
0
.
.
.
Comments on quantum coins:
1. Doesn’t violate relativity (superluminal communication):
no information transmitted in a random bit stream!
2. Application: Quantum Cryptography (a secure “one-time pad”)
+
plaintext
KEY
ciphertext
ciphertext
KEY +
plaintext
Quantum Superposition
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Superposition
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Superposition
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Entanglement
“Spooky action-at-a-distance”
(A. Einstein)
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Entanglement
“Spooky action-at-a-distance”
(A. Einstein)
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Entanglement
“Spooky action-at-a-distance”
(A. Einstein)
From Taking the Quantum Leap, by Fred Alan Wolf
Quantum Entanglement
“Spooky action-at-a-distance”
(A. Einstein)
From Taking the Quantum Leap, by Fred Alan Wolf
Trapped Atomic Ions
seven Yb+ ions
~2 mm
NIST-Boulder (D. Wineland)
U. Innsbruck (R. Blatt)
U. Maryland & JQI (C.M.)
171Yb+
qubit
1
Probability
Electronic
Excited State
(t ~ 8 nsec)
[]
0
0
5
10
15
20
25
# photons collected in 100 ms
[]
Hyperfine
Ground
States
~GHz
[]
“bright”
171Yb+
qubit
1
Probability
|
99.7%
detection
efficiency
Electronic
Excited State
(t ~ 8 nsec)
|
0
0
5
10
15
20
25
# photons collected in 100 ms
[]
Hyperfine
Ground
States
~GHz
[]
“dark”
Electronic
Excited State
[]
2
1
•
•
•
0
Hyperfine
Ground
States
[]
2
1
0
~GHz
•
•
•
~MHz
Mapping: (a[] + b[]) [0]m  [] (a[0]m + b[1]m)
Cirac and Zoller, Phys. Rev. Lett. 74, 4091 (1995)
Trapped Ion Quantum Computer
Internal states of these ions entangled
Cirac and Zoller, Phys. Rev. Lett. 74, 4091 (1995)
1 mm
Ion Trap Chips
NIST-Boulder
Au/Quartz
Maryland/LPS
GaAs/AlGaAs
Lucent/MIT
Al/Si/SiO2
Sandia
W/Si
Teleportation of
a single atom
from here…
to here…
we need
more
qubits..
Single electron quantum dots
Albert Chang (Duke Univ.)
Phosphorus atoms in Silicon
qubit stored in
31P nuclear spin
(31P: spin)
(28Si: no spin)
Si lattice
B. Kane, Nature 393, 133 (1998)
• LPS/U. Maryland
• Los Alamos
• entire country of Australia
Superconducting currents
quantized flux qubit states
H. Mooij (Delft, Netherlands)
Superconducting currents
R. Schoelkopf, Michel Devoret
Steve Girvin (Yale Univ.)
quantized charge qubit states
Doped impurities in glass
J. Wrachtrup (Stuttgart)
Fluorescence of
an array of single
impurities in diamond
Nitrogen + Vacancy
impurity in diamond
Quantum Computer Physical Implementations
works
1. Individual atoms and photons
ion traps
atoms in optical lattices
cavity-QED
2. Superconductors
Cooper-pair boxes (charge qubits)
rf-SQUIDS (flux qubits)
scales
3. Semiconductors
quantum dots
4. Other condensed-matter
electrons floating on liquid helium
single phosphorus atoms in silicon
N=1
N=1028
A new science for the 21st Century?
Information
Quantum
Mechanics
20th Century
Theory
Quantum Information Science
Physics
Electrical Engineering
Chemistry
Mathematics
Computer Science Information Theory
21st Century
Grad Students
Dave Hayes
Rajibul Islam
Simcha Korenblit
Andrew Manning
Jonathan Mizrahi
Steven Olmschenk
Jon Sterk
Postdocs
Ming-Shien Chang
Peter Maunz
Dmitry Matsukevich
Kihwan Kim
Wes Campbell
Le Luo
Qudsia Quraishi
Undergrads
Guillermo Silva
Andrew Chew
http://iontrap.umd.edu
Collaborators
Luming Duan (Michigan)
Jim Rabchuk (W. Illinois)
Keith Schwab (Cornell)
Vanderlei Bagnato (U. Sao Paulo)