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“ Anyone who can contemplate quantum mechanics without getting dizzy hasn ’ t understood it. ” --Niels Bohr. The Quantum Information Revolution Paul Kwiat. DARPA. Kwiat’s Quantum Clan (2012) Graduate Students: Rebecca Holmes Aditya Sharma Trent Graham Brad Christensen

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slide1

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

--Niels Bohr

slide3

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

slide4

A New Science!

Quantum

Mechanics

Information

Science

20th Century

Quantum Information Science

21st Century

slide5

Quantum computation

Factoring

Simulating other quantum

systems (>30bits)

Ultimate control over

Error correction

“large” systems

Quantum

classical

Quantum

Quantum communciation

Information

Teleportation

Linking separated

quantum systems

(“q. network”)

Quantum cryptography

Secure key distribution

(even between

non-speaking parties)

Fundamental physics

Entanglement

Decoherence

Quantum metrology

Measurements beyond

the classical limit

Non-invasive measurements

Measurements on quantum

systems

slide6

Quantum Metrology

Quantum Cryptography

Quantum Computing

what i do
What I do…

Unravel the mysteries of the universe…

Quantum Optics

Light

???

quantum
Quantum…

very small

very big (e.g., “quantum leap”)

an unsplittable parcel/bundle of energy

a cool buzzword to get more funding, more papers, more people at your Sat. am physics lecture, etc.

all of the above

slide10
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

wavelength

6.6 x 10-34J-s

frequency

physics 214 lect 7 example counting photons
Physics 214: Lect. 7Example: “Counting photons”

Visible light

Power output: P = (# photons/sec) x Ephoton

How do we reconcile this notion that light comes in ‘packets’ with our view of an electromagnetic wave, e.g., from a laser??

Partially transmitting mirror

Power input

How many photons per second are emitted from a 1-mW laser (l=635nm)?

1 mw red laser
1 mW red laser

3 x 1015 photons/sec =

3,000,000,000,000,000/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!!

slide13

Formation of Optical Images

  • However, for very low light intensities, one can see the statistical and random nature of image formation.
    • Use an extremely sensitive CCD camera that can detect single photons.

A. Rose, J. Opt. Sci. Am. 43, 715 (1953)

Exposure time

  • For large light intensities, image formation by an optical system can be described by classical optics.
slide14

But how do we *know* there’s only ONE photon…

A beamsplitter…

  • “1”

Photon only detected in one output.

Equally likely to be transmitted or reflected –cannot tell which.

  • “0”

"God does not play dice with the universe."

  • Quantum random-number generator!
  • completely unpredictable
  • patented
  • commercially available

“It seems to me that the idea of a personal God is an anthropological concept which I cannot take seriously.”

quantum interrogation

“Yes, yes, already, Warren! …

There is film in the camera!”

Quantum Interrogation

The problem…

Measure -film -absorber -atom without -exposing -heating -exciting it

slide17
WHY was Einstein’s 1905 proposal that light was made of particles such a profound leapthat 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…
superposition adding together of waves
Superposition (adding together) of waves

Waves add up:

“Constructive interference”

Waves cancel:

“Destructive interference”

light particle or wave
Light: Particle or Wave?

1675: Newton “proved” the light was made of “corpuscles”

1818: French Academy science contest

  • Fresnel proposed interference of light.
  • 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.
slide21

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
Single-Photon Interference:

Exposure time

Photons

  • Question: what if we reduce the source intensity so that at most one particle (photon) is in the apparatus at a time?

?

  • 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”!
slide23

mirror

beam-

splitter

Optical Interferometers

  • Interference arises when there are two (or more) ways for something to happen, e.g., sound from two speakers reaching your ear.
  • 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.
  • Two common types:

Mach-Zehnder: Michelson :

beam-

splitter

mirror

quantum interrogation1

“Yes, yes, already, Warren! …

There is film in the camera!”

Quantum Interrogation

The problem…

Measure -film -absorber -atom without -exposing -heating -exciting it

slide25

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)

slide26

Now place an absorbing object in one arm…

50% chance that photon is absorbed by object50% chance it isn’t  25% chance D2 fires “interaction-free measurement” of object

slide27

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
slide28

Quantum Metrology

WpdrvalL&wz;xcuymnzx

Quantum Cryptography

Quantum Computing

cryptography make messages so that only the intended recipient can understand them
Cryptography: Make messages so that only the intended recipient can understand them…
  • public key encryption: Standard, but not provably secure; relies on difficulty of factoring (e.g., 15 = 3x5)
  • secret key encryption: PROVABLY secure as long as
    • no one else has the key
    • the key is never reused

Quantum Cryptography = Quantum Key Distribution

slide30

Quantum Cryptography

Cryptography

KEY:

…010001010011101001…

XOR(Cipher,Key)

Message

XOR(Message,Key)

Cipher

EVE

ALICE

BOB

Cipher:

…0110010110100010…

quantum cryptography use a different property of light polarization
Quantum Cryptography:Use a different property of light– polarization!

Polarization: --the oscillation direction of the light

--property of each photon

--can measure with polarizers, calcite, etc.

Prob(horizontally polarized photon pass horizontal polarizer): 1

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

slide32

How to Make “Entangled” Coins

“Spontaneous

DownConversion”:

high energy parent photon can split into two daughter photons

(with same polarization)

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.

slide33

What about Eavesdropping?

  • Eve cannot “tap” the line  photons that don’t make it to Bob are not part of the key
  • Eve cannot “clone” the photon  forbidden by basic quantum mechanics
  • Measurements by Eve necessarily have a chance (25%) to disturb the quantum state
  •  Alice and Bob can detect errors in the key!
  • If the bit error rate is too high, they simply discard the key. No message is ever compromised.
current free space qkd distance record 144 km between lapalma and tenerife
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!

slide36

Quantum Interrogation vs. Quantum Cryptography

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!

slide39

Moore’s Law

Source: Intel

slide40

The first solid-state transistor

(Bardeen, Brattain & Shockley, 1947)

slide41

The Ant and

the Pentium

~100 million transistors

INTEL

Pentium 4

transistor

Size of an atom

~ 0.1nm

slide42

Binary digit Quantum bit

“bit”“qubit”

0, 10101

Physical realization of qubits  any 2 level system

2-level atom: ge

spin-1/2: 

polarization: HV

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

Superposition

Interference

Wave-particle duality

Intrinsic randomness in measurement

Entanglement

slide43

“Entanglement”, and the scaling that results, is the key to the power of quantum computing.

1

0

1

Classically

, information is stored in a bit register: a 3-bit

register can store

one

number, from 0 – 7.

Quantum Mechanically, a register of 3 entangled qubits can store all of these numbers in superposition:

a

|

000

+ b

|

001

+ c

|

010

+ d

|

011

+ e

|

100

+ f

|

101

+ g

|

110

+ h

|

111

ñ

ñ

ñ

ñ

ñ

ñ

ñ

ñ

Result:

Classical:

one N-bit number

--

--

Quantum:

2

(all possible) N-bit numbers

N

register can simultaneously store

N.B.

:

A 300-

qubit

2300 ~ 1090 numbers

that s a big number
That’s a BIG number

1090 =

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

This is more than the total number of particles in the Universe!

Some important problems benefit from this exponential scaling, enabling solutions of otherwise insoluble problems.

slide45

A hard problem: factoring large integers:

For example, it is hard to factor 167,659

But an elementary school student can easily multiply389 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.

slide46

Quantum Computing’s “killer app.”

Quantum algorithms enable one to factor numbers into their prime constituents MUCH faster:

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).

slide47

Atom in different energy states:

state labels

“”, “”

energy states:

atom in

state

atom in

state

atom

atom in

superposition state

shorthand “wave function”

representation

 =   + 

Probability of measuring , P = ||2

Probability of measuring , P= ||2

slide48

laser

|rest + |moving

|0

|0

|0

|0

|1

Collective motion: the “quantum data bus”

state of motion

|rest

|0

|0 + |1

|0

|0

|1

slide49

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.

slide50

Why it might not work…

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.

quantum information timeline
Quantum Information Timeline

Quantum

Computation

The expected

The unlikely – impossible?

Difficulty/Complexity

The known

Quantum

Measurement

Quantum

Communication

0 5 10 ~15 20? 25??

Time (years)

Quantum

Widgets

Quantum

Engineered Photocells?

Quantum

Sensors?

The as yet unimagined!!!

Quantum

Games & Toys

multiplexed ion trap architecture
Multiplexed Ion Trap Architecture

control electrodes

  • 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  fast)
  • One-qubit gates in subtrap
  • Readout in subtrap
slide54

Quantum factoring and cryptography

# of instructions

Classical ~ eAL

  • the RSA cryptosystem:
  • polynomial work to encrypt/decrypt
  • exponential work to break = factoring
  • BUT quantum factoring is only polynomial work

~ 1017 instructions: 8 months

Quantum~ L3

~ 109 operations: seconds

RSA129

# of bits, L, factored

  • “latency”: will information encrypted today be secure against future quantum computers?