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Quantum Mechanics may not make sense but it is true!

Quantum Mechanics may not make sense but it is true!. Prof. Miles Padgett FRSE Dept. Physics and Astronomy University of Glasgow m.padgett@physics.gla.ac.uk. Which slit did the photon go through?. The double slit experiment gives interference fringes

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Quantum Mechanics may not make sense but it is true!

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  1. Quantum Mechanicsmay not make sensebut it is true! Prof. Miles Padgett FRSE Dept. Physics and Astronomy University of Glasgow m.padgett@physics.gla.ac.uk

  2. Which slit did the photon go through? • The double slit experiment gives interference fringes • Reducing the light intensity still gives fringes • Fire one photon at a time, STILL get interference fringes! (Taylor 1910)

  3. How does Quantum Mechanicsexplain this? • To get interference fringes the photon must pass through both slits at once! • (“Standard”) Quantum Mechanics says that until it is observed a photon (or anything) can follow multiple paths • The probability is treated as a wave which, like all waves, can interfere

  4. Quantum Mechanics is about probability • The heart of Quantum Mechanics tells us “do the same thing twice” but still get different answers • e.g. double slits, • “fire” a single photon through the slits • it can land anywhere • probability of finding it at any position is proportional to the “many photon” intensity • Einstein thought this was tosh • “God does not play dice with nature”

  5. But Einstein was wrong! • 1980s experiments by Aspect • One Photon is split into two (within a non-linear optical crystal) • The two photons head off in opposite directions • What happens?

  6. The photon pairs When measured they’re one polarisation or the other Mix the photons at part-silvered mirror Split photons have opposite polarisations Both new photons are a “mix” of polarisation

  7. Measuring polarisation • Shine light at a polariser, does it get through? • Malus’s law • transmission a cos2q • For bright beam it gives the fraction of light transmitted • For single photons it gives the probability of transmission

  8. Measuring polarisation - the answer • Measuring absorbs the photon • Trying to pass the photon through a polariser answers the question • is the polarisation of the photon aligned to the polariser Y/N?

  9. Where is the polarisation decided? Option 1: polarisation mix defined at beam splitter Option 2: polarisation mix defined only when measured

  10. Option 1

  11. Option 2 If transmitted then If then Therefore never transmitted

  12. And the answer is • Option 2 is true! • Measuring A INSTANTANEOUSLY defines B • Quantum entanglement • “Spooky” action at a distance • Mixed states “superpositions” do exist and can travel (many km’s at least) • It seems Einstein was wrong • Does it work with anything other than light? (See Physics World Aug. 2002)

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