Download - 7 Quantum Universe. Quantum Communication
Big Questions in Science
Big Questions in Science, fall 2012. SdH, AUC 2
Big Questions in Science, fall 2012. SdH, AUC 3
4
http://www.upscale.utoronto.ca/PVB/Harrison/SternGerlach/SternGerlach.html
Stern-Gerlach experiment
Big Questions in Science, fall 2012. SdH, AUC
Superpositions of electrons:
Big Questions in Science, fall 2012. SdH, AUC 5
Radiatioactive decay is random. Same true for photon emission by atom:
unpredictable.
Big Questions in Science, fall 2012. SdH, AUC 6
Big Questions in Science, fall 2012. SdH, AUC
Traveling wave Frequency (and velocity) well-defined
Position ill-defined
Wave pulse Position well-defined
Frequency (and velocity) ill-defined
Planck’s constant:
7
Planck’s constant:
The larger the object, the smaller the uncertainty.
For large objects, probabilities are sharply peaked.
8
Big Questions in Science, fall 2012. SdH, AUC
Quantum physics seem to have bearing on music as well. Anyone who's ever played in a blues band will confirm that, while you know where the drummer is (in the pub), there's no way of knowing what time he will show up for rehearsal (or sound check, or whatever). This is known in quantum mechanics as Heisenberg's Uncertainty Principle : "The more precisely the POSITION is determined, the less precisely the MOMENTUM is known".
Big Questions in Science, fall 2012. SdH, AUC 9
• First bank transfer encoded via quantum crypto for entangled photons
• Information via glass fiber cable from Vienna City Hall to Bank Austria Creditanstalt branch office “Schottengasse”
10 Big Questions in Science, fall 2012. SdH, AUC
Big Questions in Science, fall 2012. SdH, AUC
“God does not play dice”
11
Big Questions in Science, fall 2012. SdH, AUC
Determinism: “I would always be able to keep my
deterministic faith for fundamental phenomena” (Lorentz)
Causality Can compute, but what does it mean? “If I am not
satisfied with current state of problem it is because I do not understand yet physical meaning of its solution. What Heisenberg has said is mathematically unexceptionable, but the point in question is that of physical interpretation” (Schrödinger).
“Anschaulichkeit”: “The way Dirac has formulated
Schrödinger’s theory [leads to] a more advanced renunciation of Anschaulichkeit, a fact very characteristic of symbolic methods in quantum theory” (Bohr).
12
Big Questions in Science, fall 2012. SdH, AUC 13
Big Questions in Science, fall 2012. SdH, AUC
14
Big Questions in Science, fall 2012. SdH, AUC
• No influences faster than light. • Particle must have velocity
before measurement. • Violation of • Quantum mechanics is
incomplete.
15
Einstein:
There is no faster-than-light interaction in this experiment.
Experimental set-up defines the meaning of ‘velocity’.
Change in position: from ‘to measure is to interact’ to ‘measurement context’.
16 Big Questions in Science, fall 2012. SdH, AUC
Faster-than-light interaction is not needed. We only have correlations.
Entanglement: particles correlated and system behaves as a whole rather than sum of parts.
Bell’s theorem (‘64): If velocities defined before measured, this implies certain inequality. QM violates this: experiment confirms quantum mechanics (Aspect ‘82).
Big Questions in Science, fall 2012. SdH, AUC 17
18 Big Questions in Science, fall 2012. SdH, AUC
Big Questions in Science, fall 2012. SdH, AUC 20
• First bank transfer encoded via quantum crypto for entangled photons
• Information via glass fiber cable from Vienna City Hall to Bank Austria Creditanstalt branch office “Schottengasse”
21
Record: 144 km in La Palma, Tenerife
Big Questions in Science, fall 2012. SdH, AUC 22
Big Questions in Science, fall 2012. SdH, AUC 24
Share a randomly generated key
110100010 110100010
Big Questions in Science, fall 2012. SdH, AUC 25
Can be used for secure communication. Eavesdropper destroys correlations.
26 Big Questions in Science, fall 2012. SdH, AUC
Photons:
Big Questions in Science, fall 2012. SdH, AUC 27
28 Big Questions in Science, fall 2012. SdH, AUC
Interference means change No possible ‘unvisible’ eavesdropper Applications:
Banking
Confident information
Collaboration fundamental research with industry (and government).
Key feature: entanglement (“spooky action at a distance”)
Big Questions in Science, fall 2012. SdH, AUC 29
Counter-intuitive aspects: “If you are not confused by quantum physics then you
haven’t really understood it.” (Niels Bohr) “I think I can safely say that nobody understands
quantum mechanics.” (Richard Feynman). After WWII: “Shut up and calculate”. Imposed by increasing numbers of students: “Such
classroom numbers, Berkeley’s department chair exclaimed to his dean, were ‘a disgrace and should not be tolerated at any respectable university’… The larger the class, the less time spent talking through the big issues.” (David Kaiser)
Big Questions in Science, fall 2012. SdH, AUC 30
Big Questions in Science, fall 2012. SdH, AUC 32
Complementarity: waves and particles Interference, superpositions Heisenberg’s uncertainty principle
Probability No determinism Applications: cryptography, quantum computing
Big Questions in Science, fall 2012. SdH, AUC 33
Big Questions in Science, spring 2012. SdH, AUC 34
Modern science
Time and relativity
Radiation and QM
Atoms, forces
Our cosmic origins
Classical physics
Big Questions in Science, fall 2012. SdH, AUC 34
Outline (or summarize in a diagram) EPR’s line of argument against quantum mechanics.
What where they contending?
Describe the experiment in a few lines.
What were the key concepts and what do they mean?
What assumptions did they make?
How did they arrive at their conclusion?
Can you reformulate the paradox in terms of spin?
Big Questions in Science, fall 2012. SdH, AUC 35
Download from BB: “Transgressing the Boundaries”. Skim through the paper. Have a quick look at the
introduction and first section. Questions: 1. What is, according to the introduction, the relation
between physics and power? 2. What is the announced aim of the paper? 3. Do the sections have any particular structure
(argumentative, logical, etc.)? 4. Are any of the suggested arguments convincing? 5. What, in your opinion, is the author trying to argue?
Big Questions in Science, fall 2012. SdH, AUC 36