“
Anyone who can contemplate
quantum mechanics without getting
dizzy hasn
’
t understood it.
”

Niels Bohr
The Quantum Information Revolution
Paul Kwiat
Kwiat’s
Quantum
Clan (2012)
Graduate Students:
Rebecca Holmes
Aditya
Sharma
Trent Graham
Brad Christensen
Kevin
Zielnicki
Mike
Wayne
Courtney
Byard
Undergraduates
:
Daniel
Kumor
David
Schmid
Jia
Jun (“JJ”) Wong
Cory
Alford
Joseph Nash
David Rhodes
Visit Prof:
Hee
Su Park
Post

Doc:
Jian
Wang
A New Science!
Quantum
Mechanics
Information
Science
Quantum Information Science
20th Century
21st Century
Quantum
Information
Fundamental physics
Decoherence
Quantum
捬c獳s捡c
Entanglement
Ultimate control over
“
污牧l
”
獹獴s浳
兵慮瑵Q 浥瑲潬潧y
䵥慳畲M浥湴猠扥b潮o
瑨攠捬c獳s捡c 汩l楴
Non

invasive measurements
Measurements on quantum
systems
Quantum cryptography
Secure key distribution
(even between
non

speaking parties)
Quantum computation
Factoring
Simulating other quantum
systems (>30bits)
Error correction
Quantum communciation
Teleportation
Linking separated
quantum systems
(
“
焮q湥瑷潲o
”
)
Quantum
Metrology
Quantum
Computing
Quantum
Cryptography
“
Things should
be made as
simple as
possible, but not
any simpler.
”
What
I
do…
Unravel the mysteries
of the universe…
Quantum Optics
Light
???
Quantum…
a.
very small
b.
very big (e.g., “quantum leap”)
c.
an
unsplittable
parcel/bundle of
energy
d.
a cool buzzword to get more
funding, more papers, more
people at your Sat. am physics
lecture, etc.
e.
all of the above
1905: Einstein made a ‘quantum
leap’ and proposed that light
was really made of particles
with tiny energy given by
E = h f = h c/
l
frequency
6.6 x 10

34
J

s
wavelength
Power input
Partially transmitting
mirror
How many photons per second are emitted from a
1

mW
laser (
l
=635nm
)?
How do we reconcile this notion that light comes in
‘
packets
’
with
our view of an electromagnetic wave, e.g., from a laser??
photon
hc
E
l
1240eVnm
2eV
635 nm
Power output: P = (# photons/sec) x E
photon
3
15 1
19
10 J 1eV 1photon
(# photons/sec) 3.1 10 s
s 1.6 10 J 2eV
photon
P
E
Visible light
Physics 214: Lect. 7
Example
:
“Counting photons”
1
mW
red laser
3 x 10
15
photons/sec =
This is an incredibly huge number
–
your eye basically
cannot resolve this many individual photons (though
the rods can detect single photons!
).
And
you
MAY be able to see just one photon!!
3,000,000,000,000,000/sec
Formation of Optical Images
l
For large light intensities
, image
formation by an optical system can be
described by classical optics.
Exposure time
l
However,
for very low light intensities
, one can see the
statistical and random nature of image formation.
l
Use an extremely sensitive CCD camera that can detect single photons.
A. Rose, J. Opt. Sci. Am. 43, 715 (1953)
"God does not play
dice with
the universe."
“
It seems to me that the idea of a personal God is an
anthropological concept which I cannot take seriously.
”
Photon only detected in one output.
Equally likely to be transmitted or
reflected
–
cannot tell which
.
A
b
eamsplitter
…
But how do we *know* there’s
only ONE photon…
Quantum random

number generator
!
•
completely unpredictable
•
patented
•
commercially available
“0”
“1”
“
The important thing is
not to stop questioning.
”

A. Einstein
Quantum
Interrogation
“
Yes, yes, already, Warren! …
There
is
film in the camera!
”
The problem…
Measure

film

absorber

atom
without

exposing

heating

exciting it
WHY was Einstein’s 1905
proposal that light was made of
particles such a profound leap
that
almost no one believed him
?
Because everyone KNEW that light
was really waves.
One of the strangest features of QM:
all particles can behave like waves…
Interference of
waves (e.g.,
water, sound, …
)
Superposition (adding together)
of
waves
Waves add up:
“Constructive interference”
Waves cancel:
“Destructive interference”
Light:
Particle
or
Wave
?
1675: Newton
“
proved
”
the light was made
of
“
corpuscles
”
1818: French Academy science contest
l
Fresnel proposed
interference
of
light.
l
Judge Poisson
knew
light was made of
particles:
“
Fresnel
’
s ideas ridiculous
”
If
Fresnel ideas were correct, one would
see a bright spot in the middle of the
shadow of a disk.
Judge Arago decided to
actually do the experiment…
Conclusion (at the time):
Light
must
be a wave, since
particles don
’
t
interfere!
Only, now we know
that
they
must!
Single

Photon
Interference:
l
Question:
what if we reduce the
source intensity so that at most
one particle (photon)
is in the
apparatus
at a time?
Exposure time
l
Answer:
Just like in the
“
optical
image formation
”
, given enough time,
the
classical interference pattern
will gradually build up from a huge #
of seemingly random
“
events
”
!
Photons
?
Optical Interferometers
l
Interference arises when there are two (or more) ways
for something to happen, e.g.,
sound from two speakers
reaching your ear.
l
An interferometer is a device using mirrors and
“
beam
splitters
”
(half light is transmitted, half is reflected) to
give two separate paths
for light to get from the source
to
the detector
.
l
Two common types:
Mach

Zehnder
:
Michelson :
mirror
beam

splitter
beam

splitter
mirror
Quantum
Interrogation
“
Yes, yes, already, Warren! …
There
is
film in the camera!
”
The problem…
Measure

film

absorber

atom
without

exposing

heating

exciting it
The solution…
(
Elitzur
&
Vaidman
, 1993)
Use
dual
wave

particle nature of quantum
objects (
“
wavicles
”
)
Single photon always shows up at D1
(complete destructive interference to D2)
Now place an absorbing object in one arm…
50% chance that photon is absorbed by object
50% chance it isn
’
t
㈵┠%桡湣攠䐲晩牥猠
“
interaction

free measurement
”
of object
Quantum Interrogation
•
Optimizing
reflectivities
50% efficiency
•
By combining these techniques with the
“
quantum Zeno effect
”
(making repeated very
weak interactions), the efficiency can in
principle be pushed to 100%: no photons
absorbed by the absorbing object!
[85% demonstrated to date]
•
Imaging semi

transparent objects does
not
readily yield a gray

scale
Quantum
Metrology
Quantum
Computing
Quantum
Cryptography
Wpdrval
L&wz;xcuymnzx
Cryptography:
Make messages so that only the
intended recipient can understand
them…
1.
public key encryption: Standard, but
not provably secure; relies on
difficulty of factoring (e.g., 15 = 3x5)
2.
secret key encryption: PROVABLY
secure as long as
a.
no one else has the key
b.
the key is never reused
Quantum Cryptography = Quantum Key Distribution
ALICE
BOB
Cipher:
…0110010110100010…
XOR(Cipher,Key)
Message
EVE
KEY:
…010001010011101001…
Quantum Cryptography
Cryptography
XOR(Message,Key)
Cipher
Quantum Cryptography:
Use a different property of light
–
polarization!
Prob
(horizontally polarized photon pass horizontal polarizer): 1
Polarization:

the
oscillation direction of the
light

property of each photon

can measure with polarizers, calcite, etc.
Prob
(horizontally polarized photon pass vertical polarizer): 0
Prob
(diagonally pol. photon pass horizontal polarizer): 1/2
Prob
(diagonally pol. photon pass vertical polarizer): 1/2
How to Make “Entangled” Coins
We don’t know WHICH crystal created the pair of
photons, but we know they both came from the
same
crystal
they
MUST have the same polarization.
“Spontaneous
DownConversion
”:
high energy
parent photon can
split into two
daughter photons
(with same
polarization)
•
Eve cannot
“
瑡t
”
瑨攠汩湥l
灨潴潮猠瑨慴潮
’
琠
浡m攠e琠t漠䉯戠慲攠湯琠灡牴映 桥h步y
•
Eve cannot
“
捬潮c
”
瑨攠灨潴潮
景牢楤f敮e批
basic quantum mechanics
•
Measurements by Eve necessarily have a
chance (
25
%
)
to disturb the quantum state
†
䅬楣A湤 䉯B 捡渠摥瑥捴 敲牯牳 楮⁴桥i步y!
If the bit error rate is too high, they simply discard
the key. No message is ever compromised.
What about Eavesdropping?
Current Free

Space QKD Distance Record:
144
km between
LaPalma
and
Tenerife
Last week news item: they used the
entangled photons to teleport the
state of a photon between the
islands
–
world distance record for
quantum teleportation!
QKD Goes Commercial…
Since we can seemingly
“
see
”
without
“
looking
”
using quantum interrogation, does this mean an
eavesdropper could use it to defeat quantum
cryptography?
No! It turns out that even making the gentlest
measurement possible, if the eavesdropper gains
any information, she disturbs the state.
Or if she is so gentle so as not to disturb the state,
then she gets no information.
Quantum key distribution is secure against
any
attack allowed by the laws of physics!
Quantum Interrogation vs.
Quantum Cryptography
Imagination is more
important than knowledge
Source: Intel
Moore
’
猠䱡L
The first solid

state transistor
(Bardeen, Brattain & Shockley, 1947)
INTEL
Pentium 4
transistor
The Ant and
the Pentium
~100 million transistors
Size of an atom
~ 0.1nm
Superposition
Interference
Wave

particle
duality
Intrinsic
randomness in
measurement
Entanglement
2

level atom
:
g
e
spin

1/2
:
polarization:
H
V
Binary digit
Quantum bit
“
bit
”
“
qubit
”
0, 1
0
1
0
1
Physical realization of qubits
any 2 level system
All 2

level systems are created equal, but some are
more equal than others!
Quantum communication
photons
Quantum storage
atoms, spins
Scaleable
circuits
superconducting
systems
“
Quantum
”
phenomena
“
Entanglement
”
Ⱐ,湤n瑨攠獣慬楮朠瑨慴t牥獵汴猬l楳 瑨攠
key to the power of quantum computing.
•
Classically
, information is stored in a bit register: a 3

bit
register can store
one
number, from 0
–
7.
•
a

000
+ b

001
+ c

010
+ d

011
+ e

100
+ f

101
+ g

110
+ h

111
•
Result:

Classical:
one N

bit number

Quantum:
2
N
(all possible) N

bit numbers
•
N.B.
:
A 300

qubit
register can simultaneously store
2
300
~ 10
90
numbers
1
0
1
Quantum Mechanically
, a register of 3 entangled
qubits
can store all of these numbers in superposition:
That’s a BIG number
10
9
0
=
This is more than the total number of particles
in the Universe!
1,000,000,000,000,000,000,
000,000,000,000,000,000
,
000,000,000,000,000,000
,
000,000,000,000,000,000
,
000,000,000,000,000,000
Some important problems benefit from this
exponential scaling, enabling solutions of
otherwise insoluble problems.
A hard problem: factoring large integers
:
For example, it is hard to factor
167,659
But an elementary school student can
easily multiply
389 x 431 = 167,659
This asymmetry in the difficulty of factoring
vs. multiplying is the basis of public key
encryption, on which everything from on

line
transaction security to ensuring diplomatic
secrecy depends.
Quantum Computing
’
s
“
killer app.
”
RSA digits
PC
(1
GHz)
Blue
Waters
(10PF)
Quantum
Computer
(1
GHz)
129
4 months
1 sec
10 sec
225
300,000
yr
12 days
100 sec
300
10
16
yr
(>> universe)
20 million
yr
200 sec
The difficulty (impossibility) of factoring large numbers
(and the ease of creating a large number from its
factors) is the basis of public key encryption (which
nearly everyone uses for secure transmission today).
Quantum algorithms enable one to factor numbers
into their prime constituents MUCH faster:
state labels
“
”
,
“
”
Atom in different energy states:
atom
energy states:
atom in
state
atom in
state
atom in
superposition state
shorthand
“
wave function
”
representation
=
+
Probability of measuring , P
= 

2
Probability of measuring , P
= 

2
0
0
+ 1
0
0
1
rest
state of motion
Collective motion: the
“
quantum data bus
”
laser
0
0
0
0
1
rest
+ moving
Science News
Quantum Computing Explored
Sep 12 2001 @ 08:10
American computer scientists are studying the possibility to
build a super fast computer based on quantum physics.
Technology requirements
•
Set of qubits isolated from environment.
•
“
Quantum information bus
”
to connect qubits.
•
Reliable read

out method.
Essential Dichotomy
Need WEAK coupling to
environment to avoid
decoherence
, but you also
need STRONG coupling to at
least some external modes in
order to ensure high speed and
reliability.
Why it might not work…
Quantum Information Timeline
0
5
10
~15
20?
25??
Time (years)
Difficulty/Complexity
Quantum
Measurement
Quantum
Communication
The known
Quantum
Computation
The expected
The unlikely
–
impossible?
Quantum
Sensors?
The as yet unimagined!!!
Quantum
Engineered
Photocells?
Quantum
Widgets
Quantum
Games & Toys
“
Why is it that nobody
understands me
, and
everybody likes me?
”
–
A.E.
•
Interconnected multi

trap
structure
•
Route ions by controlling
electrode potentials
•
Processor sympathetically
cooled
•
No individual optical
addressing during two

qubit
gates
(can do gates in strong
trap
晡f琩
•
One

qubit gates in subtrap
•
Readout in subtrap
Multiplexed Ion Trap
Architecture
control electrodes
0
200
400
600
800
1000
1
10
100
1000
1
10
4
1
10
5
1
10
6
1
10
7
1
10
8
1
10
9
1
10
10
1
10
11
1
10
12
1
10
13
1
10
14
1
10
15
1
10
16
1
10
17
1
10
18
1
10
19
1
10
20
1
10
21
1
10
22
1
10
23
1
10
24
Quantum factoring and cryptography
Classical
~ e
AL
# of instructions
# of bits, L, factored
Quantum
~
L
3
RSA129
~ 10
9
operations:
seconds
~ 10
17
instructions:
8 months
the RSA cryptosystem:
•
polynomial
work to
encrypt/decrypt
•
exponential
work to
break = factoring
•
BUT quantum factoring is
only
polynomial work
•
“
latency
”
㨠:楬氠楮景牭f瑩t渠
敮e特灴敤p瑯t慹 扥b捵牥
慧a楮獴畴s牥 煵慮瑵洠
捯浰畴敲c?
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