Quantum Entanglement

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Quantum Entanglement EPR

• Experiments
• Entanglement
• Multi-entanglement
• Application
• Teleportation
• Cryptography

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Experiments

• Entanglement
• Interactions involving quantum mechanical
  particles, conserve various quantities.
• This leads to the pair being entangled

1982- Alain Aspect and colleagues in University
of Paris demonstrated for the first time quantum
entanglement. Proving that Einstein was wrong
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Experiments

• Multi-entanglement
• Entanglement has been confirmed in trillions
  of atoms at once. This is a so called collective
  process.
• 2001- A team led by Eugene Polzik in Denmark
  succeeded in entangling two clouds of caesium
  atoms. Previous record was four atoms!

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Quantum Teleportation

• This is achieved through entanglement.
• It was first achieved in 1997 when Prof. Anton
  Zeilinger at the university of Vienna used
  entanglement to teleport a photon 1m across the
  lab.

Experimental setup Bob and Alice share a pair of
entangled photons. But Alice has an additional
photon in an unknown state. (eg unknown
polarisation state, u)
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Quantum Teleportation
Bob
Alice
a
c
b
Photon a then takes on a real polarisation state,
u which is related to the initial state of
photon c.
Quantum Operation, entangles b c, thus
disentangling photon a.
Alice then communicates the results of her
operation to Bob
Result
Bob can then perform a local transformation to
recreate the state c.
a c
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Quantum Teleportation

• Photon a acquired a definite state as soon as
  the operation was done on Alice's photons, i.e.
  faster than light. But note that this
  teleportation process requires a classical
  channel of communication (i.e. a mobile phone.)
  to inform Bob of the operation and its result.
• Therefore no faster than light teleportation,
  regardless of what you may read on the internet!

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Quantum Cryptography
Encoding and deciphering messages based on a key.
Problems

• Bad guy can intercept the key!!
• Bad guy can determine the key using several
  messages.
• So need a unique key for each message.

Solution Quantum key distribution

• This field of endeavour exploits the fact that
  wave functions are fragile. A simple measurement
  will force a photon into a (random) definite
  state.
• This destroys the properties of entanglement.

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Quantum Cryptography
Bob
Alice
Entangled photons
Alice checks randomly for a particular
polarisation
Bob sends photons with 0, 90, 45, -45
polarisations
Bob sends photons with 0, 90, 45, -45
polarisations
Using a classical channel of communication
Alice sends the checks to Bob NOT the results
Chosen checks
Bob sends back which are correct
Alice now can deduce the key
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Quantum Cryptography

• What's the point?
• If the eavesdropper intercepts the message he/she
  wont know which test to perform, and they would
  incur errors!
• If any of the message was intercepted then the
  entanglement between Bob Alice's photons would
  be ruined. This would destroy the correlation in
  polarisation, and thus the eavesdropper would be
  detected!

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Conclusions

• Einstein may have been wrong to dismiss the
  strange process of entanglement. But he was still
  its founding father!
• Entanglement is one of the weirdest consequences
  of QM.
• We have known the rules of QM for some time now
  but entanglement is a step up the ladder, we are
  now learning to use these basic rules to exploit
  QM processes for use in real and useful
  applications

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Thank you for your time you can wake up
now!Sorry no questions cos I probably dont
know the answer.