Decoherence Demo (with 2-slit interference demo) Scott Johnson Intel Press Play to begin

July 2010Scott Johnson, Schrödinger's Cat Paradox Why don’t we see superpositions of macroscopic objects like cats? We do see probabilities for macroscopic objects –Superpositions are different than probabilities Partial answer is decoherence Paradox: A seemingly contradictory statement that may nonetheless be true Box with cat Detector 1 releases poison Detector 2 prevents its release Beam splitter Source Mirror John Gribbon In Search of Schrödinger's Cat

PE x July 2010Scott Johnson, Harmonic Oscillator Gaussian wave packet in harmonic oscillator

July 2010Scott Johnson, One Particle Gaussian wave packet in time

July 2010Scott Johnson, Superposition One particle in two places

July 2010Scott Johnson, Superposition One particle in two places See the interference

July 2010Scott Johnson, Mixed State No interference

July 2010Scott Johnson, Compare Can a superposition become a mixed state? SuperpositionMixed State

July 2010Scott Johnson, Decoherence Yes! Decoherence turns a superposition into a mixed state

Other Particles Answer: our particle is interacting with other particles July 2010Scott Johnson,

July 2010Scott Johnson, Multi-Particle Wave Function One 1D particle requiresOne 1D wave function Two 1D particles requireTwo 1D wave functions? NO! One 2D particle requiresOne 2D wave function

July 2010Scott Johnson, Multi-Particle Wave Function 2 particles in 1D requires a 2D wave function! Particle 2 Particle 1 P2P2 P1P1

July 2010Scott Johnson, Classical Multi-Particle Two 1D particles can be tracked with a single point on a 2D plane

July 2010Scott Johnson, Classical Multi-Particle

July 2010Scott Johnson, Classical Multi-Particle

July 2010Scott Johnson, Classical Multi-Particle

July 2010Scott Johnson, Classical Multi-Particle

July 2010Scott Johnson, Quantum Multi-Particle These 2 particles are described by one 2D wave function Project (integrate) the 2D function onto an axis to get a 1D wave functions

July 2010Scott Johnson, Quantum Multi-Particle Sometimes the 2D function separates neatly into two 1D wave functions…

July 2010Scott Johnson, Quantum Multi-Particle But not in general These two particles are correlated or entangled –The 1D probability densities don’t have complete info

July 2010Scott Johnson, No Interaction Particles stay separable –Don’t need 2D function, but can plot one anyway

July 2010Scott Johnson, With Interaction Particles often don’t stay separable; become entangled

July 2010Scott Johnson, Decoherence Look under the hood…

July 2010Scott Johnson, Decoherence A superposition of P1…

July 2010Scott Johnson, Decoherence A superposition of P1… …drifts such that 2 “humps” no longer overlap

July 2010Scott Johnson, Decoherence Note P2 is gradually moved from ground state Real world  many other particles  many-D

July 2010Scott Johnson, Decoherence… …turns a superposition into a mixed state …is fast –A molecule interacting with heat photons in a lab vacuum will decohere in ~ seconds … Solves Schrödinger's Cat –Any macroscopic object will decohere long before we can see a macroscopic superposition

July 2010Scott Johnson, Measurement Still Has a Mystery Decoherence leaves us with two (or more) outcomes as proper (classical) probabilities –Probabilities are less mysterious than superpositions Does not say how nature chooses among these probabilities Does not say when the choice is made One step in understanding quantum mechanics

July 2010Scott Johnson, Q&A

July 2010Scott Johnson, Part 2 Two-Slit Interference

2-Slit Interference July 2010Scott Johnson, Source Barrier Screen

2-Slit Interference July 2010Scott Johnson,

Which Path? July 2010Scott Johnson,

Which Path? July 2010Scott Johnson,

One Slit July 2010Scott Johnson,

Einstein - Bohr July 2010Scott Johnson, Movable wall; measure recoil Source Crystal with inelastic collision Source No: Movement of slit washes out pattern No: Change in wavelength washes out pattern

Updated Experiment July 2010Scott Johnson, Source Excited electron Detector Ground state electron

Measurement Destroys Interference July 2010Scott Johnson, Path is measured at one or both slits:

What’s the Difference? July 2010Scott Johnson,

Model of Measurement July 2010Scott Johnson, P1 xP1 x P1 yP1 y P2P2 2D particle going through slits shown on this face in red Measurement particle shown along this axis Slices of 3D total function shown here in blue P2P2

Model of Measurement July 2010Scott Johnson, P1 xP1 x P1 yP1 y P2P2 P1 xP1 x P1 yP1 y P2P2 No measurement Measurement

Model of Measurement July 2010Scott Johnson, No measurement Measurement P1 xP1 x P1 yP1 y P2P2 P1 xP1 x P1 yP1 y P2P2

Model of Measurement July 2010Scott Johnson, No measurement Measurement P1 xP1 x P1 yP1 y P2P2 P1 xP1 x P1 yP1 y P2P2

Partial Measurement July 2010Scott Johnson, P1 xP1 x P1 yP1 y P2P2

Conclusion July 2010Scott Johnson, No measurement Measurement P1 xP1 x P1 yP1 y P2P2 P1 xP1 x P1 yP1 y P2P2

July 2010Scott Johnson, The End