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Quantum Mechanics and Quantum Computation


What is quantum computing? Exploiting the strange phenomena in quantum mechanics to process information. Quantum computers can be in a superposition of several states ... – PowerPoint PPT presentation

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Title: Quantum Mechanics and Quantum Computation

Quantum MechanicsandQuantum Computation
  • Sarah Brandsen

  • If quantum mechanics hasnt profoundly shocked
    you, you havent understood it yet.
  • -Niels Bohr

  • What is Quantum Mechanics?
  • Phenomena
  • Quantization
  • Wavefunctions and Schrodingers Cat
  • Quantum Computing
  • What is Computing?
  • Why might Quantum Computers be useful?
  • National Security and Quantum Computers
  • Quantum Key Distribution (QKD)
  • Polarized Photons, Heisenberg Uncertainty,
    No-Cloning Theorem
  • Is QKD a perfect form of encryption?
  • When will we have quantum computers?

What is Quantum Mechanics, really?
  • Our attempt to understand the universe on
    extremely small (nanoscopic) scales
  • This leads to strange and unintuitive results-
  • Momentum and energy can be quantized
  • Light and matter can both exhibit wave and
    particle behaviour

Most Famous Observation in Physics- Schrodingers
  • In fact, the mere act of opening the box will
    determine the state of the cat, although in this
    case there were three determinate states the cat
    could be in these being Alive, Dead, and Bloody
  • - Terry Pratchett

Set-up for Schrodingers Cat
  • Place cat in box with a purely random radioactive
    source that has 50 of going off (killing the
    cat) within an hour
  • At the end of the hour, right before you open the
    box, the cats state is completely unknown
  • Therefore, the cat is considered to be both alive
    and dead at the same time until it is observed.

  • The wavefunction represents all possible outcomes
    and their probabilities

Square this to find probability of cat being
alive 50
State of cat being alive
Measurement and Zombie Cats
  • If cats can be in superpositions of dead and
    alive, why dont we see zombie cats?
  • According to a standard interpretation,
    measurement collapses the wavefunction into a
    single state!

Quantum Computing- Active Research based on
Quantum Mechanics
What is computing?
  • Goal-oriented activity
  • Store information in bits (0 or 1) and processes
    the information similarly to computation by hand
  • Very effective for many tasks, but has time
    limitations for other tasks

Example- Computer Chess Game
  • Input- Your move of the chess piece
  • Process- Based on your move, the computer
    calculates a response
  • Output- Computer makes a move, which appears on
    your screen

What is quantum computing?
  • Exploiting the strange phenomena in quantum
    mechanics to process information
  • Quantum computers can be in a superposition of
    several states, so you can process information in

Bit vs. Qubit
  • Superposition of classical states, so you can
    contain as many inputs as you want and process
    in parallel!

Measurement problems?
  • Quantum computing is probabilistic- we could run
    exactly the same program twice and see different
  • But you can repeatedly initialize, run, and

Thought Experiment
  • Impossible task-- find a small X that has been
    marked on one book in the Library of Congress in
    less than five minutes
  • But what if there were several million parallel
    Library of Congress and each one had a parallel
    self? If you could coordinate with all of the
    parallel selves, you might be successful.

Why Quantum Computing is a Big Deal
  • Can solve enormous tasks in a reasonable amount
    of time finding a needle in a haystack
  • True random number generation
  • Implications for artificial intelligence?
  • National security

More about National Security
  • Our daily lives depend on encryption
  • Encodes message so that they are useless to
    anyone who doesnt know the cipher
  • Prevents eavesdroppers on mobile phone
    conversations, protects information being
    transferred over the network, etc.
  • Symmetric key scheme

Another kind of Encryption
  • Banking, medical records, business records, and
    government records often use RSA encryption tools
  • This relies on the difficulty of breaking a very
    large number into two prime factors

Factors of a very large number!
Safe For now
  • In 2009, computer scientists were able to crack
    the primes for a 768-bit number over the span of
    two years and using hundreds of computers
  • Many encryption keys are 1024 bits, so it would
    even take 1000 times longer to crack!
  • However, quantum computers could use a process
    called Shors algorithm to crack very large
    encryption keys

Quantum Key Distribution (QKD)
  • QKD depends on three properties
  • Photon Polarization
  • Heisenberg uncertainty principle
  • No-cloning theorem

Photon Polarization
  • Unpolarized (normal) photons can spin in all of
    their directions at once
  • Can be polarized by passing through a filter
  • Cant be re-polarized without losing information

Uncertainty Principle
  • Some kinds of knowledge are fundamentally
  • For example, the more precisely you measure a
    particles position the less precisely you can
    know its momentum.
  • It is also impossible to know a photons spin in
    two directions at once!

Quantum No-Cloning Theorem
  • Suppose you can create multiple identical
    properties of a quantum state
  • Then you can measure position in one state and
    momentum in another
  • This violates the Heisenberg uncertainty

BB84 Protocol
  • Alice chooses a random polarization and records
    her result
  • Bob also chooses a random polarization and
    records his result

BB84 Protocol
  • At the end, Alice can read off what polarizing
    filters she used and they can keep sets that
  • This will give them a private code!

What if Eve was eavesdropping?
  • Eve would have to randomly measure the photon to
    obtain information.
  • Half of the time she will choose the wrong
    filter. Then if Bob chooses the right filter, his
    result will be random and might be different from
  • Bob and Alice can compare a small portion of
    their code over the classical channel and see
    whether Eve was eavesdropping

Problems with Quantum Encryption
  • Noisy and imperfect equipment might make Bob and
    Alices measurements disagree
  • Imperfect photon source helps Eve eavesdrop

Problems with Quantum Computation
  • Quantum systems are very delicate
  • Decoherence (interactions with the environment)
    can quickly collapse the wavefunction

When will quantum computers be made?
  • 1998- First three qubit computer
  • 2012- Decoherence suppressed for 2 seconds at
    room temperature
  • 2013- Decoherence suppressed for 39 minutes at
    room temperature in an ensemble of 3 billion

Thank you for Listening
  • Consider a ball rolling off into a valley, bound
    on the other side by a higher hill. Classic
    physics states that the ball cannot have
    sufficient energy to roll over from the other
    side of the higher hill unless an external
    velocity is imparted on this ball, but according
    to quantum tunneling, there is the minute
    possibility that this ball will turn out to be
    that one coincidence out of a million and DIG
    THROUGH the hill instead of rolling over.

Black Hole Information Paradox
  • The reason that this lost information is such a
    major issue for quantum mechanics once again ties
    into thermodynamics. In quantum mechanics,
    information is related to the thermodynamic
    concept of order. If information is lost, then
    order is lost meaning that entropy (disorder)
    is increased. This means that the black holes
    would begin generating heat, rising up to
    billions of billions of degrees in mere moments.
    Though Leonard Susskind and others realized this
    in the mid-1980s, they couldnt find the flaws in
    Hawkings reasoning that would prove him wrong.
  • he redid some of his earlier calculations and
    found that it was possible that, as an object
    fell into a black hole, it would disturb the
    black holes radiation field. The information
    about the object could seep out, though probably
    in mangled form, through the fluctuations in this
  • it may be possible that all the information
    within the black hole is also encoded in some
    form on the surface area of the black hole.

Uncertainty Principle in Pictures
More Precise Position, Less Precise Momentum
Precisely determined momentum, unknown position
Quantum Entanglement- Spooky Action At A
  • ?Particles can be connected, even across great
  • ?Example- laser beam fired through specific
    crystal can cause individual photons to split
    into pairs of entangled photons
  • When you measure the spin of particle A, you
    determine the spin of particle B virtually

Alternative View- Many Worlds Approach
  • Wavefunction doesnt magically collapse upon
  • Instead, we start with a superposition
  • After measuring, the superposition is still
    there but the different branches can no longer
    interact with each other
  • This is due to decoherence

Many-Worlds Interpretation
  • Schrodingers cat is alive in one world and dead
    in another
  • When you open the box you can say which world
    you are in. You are either in world-1 or world-2,
    but both the worlds exist.

Phenomena 1- Quantization
  • In 1900 Max Planck started a revolution in
    physics without realizing it
  • Blackbody- an object that perfectly absorbs and
    emits radiation
  • Experimental data could be fit by assuming that
    energy was quantized!

More Quantization
Electrons restricted to specific energy levels
More Quantization
  • Energy
  • Momentum/Velocity

What is light?
  • Debate goes all the way back to Aristotle and
  • Lots of evidence for light to be a wave

Light as a Particle
  • More intense light doesnt lead to more intense
  • 1905- Einstein hypothesized that light may be
    composed particle-like pieces (photons)
  • Existence of photons later confirmed (1923)

What about matter?
  • Familiar with definition of matter as a
    collection of particles (for example, a
  • We have never observed baseballs diffracting
    around corners

Double Slit Experiment
  • Light produces interference patterns when passed
    through a slit

Electrons can also act like a wave
  • Interference builds up over time

  • Momentum, energy can be quantized
  • Light can act like a wave and a particle
  • Matter can act like a group of particles and a

Even Stranger- Double Slit Experiment Revisited
  • If particle detectors are positioned at the slit,
    the interference pattern will disappear
  • We cannot observe something acting as a particle
    and wave at once

Quantum Mechanics in Pop Culture
New Age Religion
Obscure Math
Image Source Church of the Cosmos
We are immortal timeless beings of light in an
infinitely expanding omniverse filled with
intelligent life.
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