quantum crystals quantum choreography and quantum...
TRANSCRIPT
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Quantum CrystalsQuantum Choreographyand Quantum Computing
Chalk Talk - KITP 4/12/2006
MPA Fisher
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CrystalsQuartz - SiO2
Amethyst (purple)Quartz (clear)Citrine (yellow)
• In almost all rocks• Principle constituent of glass• Key component of computer chips• Gem stones
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Crystals come in many shades
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And in many shapes
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And with many “uses”:
Age old vanity, to new age voodoo
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Crystals “enter” academia …
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“Look” inside crystals
Periodic array of atoms
Many arrangements Many different atoms
Many, many crystals
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“Quantum Crystals”
Outer shell electrons can often move from one atom to another
Electrons are so light that theirmotion thru crystal is always Quantum Mechanical!
Atoms:
Tiny Nucleus
Why “Quantum”??
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Electrons are Particles AND Waves
a=3 x 10-10 meters
Atoms
Electron “particles” (with spin)
Inside Crystals:Quantum “wave” mechanics of 1023 electron “particles”
Electron wavelength much greaterthan spacing between atoms- even at room temperature!
Electron “waves”
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Landau says - “Electrons do their own thing”
• Explains why copper conducts and whyquartz does not, and why silicon is asemiconductor and much, much more
• Enabled the computer revolution!
Landau’s Theory of Quantum Crystals
Lev Landau
up-spinelectron
But sometimes they don’t…
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Each electron gets stuck
Many “Complex” Crystals are “bad actors”
Landau - “complex crystals should conduct - like copper.”But because the electrons are too crowded they do NOT.
“Mott Insulators”
Electrons spins canflop around ….Quantum mechanically
But how???Sir Neville Mott
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“males and females”
Electrons are - • “homophobes”• basically shy Antiferromagent
(boring)
“Quantum Docey Doe”
“Quantum Choreography”
Electrons like to dance
chemical (or “valence”) bond Valence Bond Crystal
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“ORDER”
Antiferromagnetic ORDER
Valence Bond Crystal ORDER
ORDER is Boring!
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Electrons like to “swing”
“resonate”
foursome
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Electrons like to “swing”
“resonate”
foursome
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Electrons like to “swing”
“resonate”
foursome
“Quantum Grand Right and Left”
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“Quantum Grand Right and Left”
Resonating Valence Bond state (RVB) PW Anderson
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“Quantum Grand Right and Left”
Resonating Valence Bond state (RVB) PW Anderson
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“Quantum Grand Right and Left”
Resonating Valence Bond state (RVB) PW Anderson
“Spin Liquid”
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Breakups - break valence bonds
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Separation
Loners - “Spinons” Spin of electron but not charge
Electron “Fractionalization”: “Spin-Charge separation”
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Dejected and ejected: Dope in holes
“Holons” Charge of electron but not spin
Electron “Fractionalization”: “Spin-Charge separation”
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As the componentsof computers get
smaller,we are approachingthe limit in whichquantum effects
become important.
Motivation:
Can spin liquids be “useful” (make money)?
“Quantum Computers”
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Is this a problem? … or an opportunity?
Richard Feynamn (1981)
A computer which operates“coherently” on a quantum systemcan be much, much, much morepowerful than ANY imaginableconventional computer
Massive quantum parallelism: Computing simultaneously in many “parallel” universes,interfering and measuring in one
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Upside of Quantum Computing - Power
Peter Shor (1994)
A quantum computer could readily(prime) factorize a 300 digit number!
World record: 200 digit number factorized after 170 CPU years ona Pentium (running at a 109 cycles/sec - 1019 cycles in total)
Prime Factorization: Most secure encryption method
In 1994 Peter Shor developed analgorithm for prime factorization on aQuantum computer
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Downside of Quantum Comp. - “Decoherence”
Even tiny error (noise) can destroy the delicate quantumsuperposition (ie. “decohere”) ruining the calculation
Quantum wave function is very, very “fragile”
Electron “particle”Electron “wave”
Noise
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Topology: a branch of mathematics concerned with thoseproperties of geometric configurations which are unaltered bylocal deformations.
The first two loops can be deformedinto each other, but the third cannot.It is a non-trivial knot.
A trick: Exploit Topology
Knots and Braiding
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Resonating Valence Bond: Simplest quantum state with “topological” particles!
Idea: Use “Quantum” Knots!
“A quantum system having particles with “topological” character would be automatically protected against errors caused by local disturbances”
Alexei Kitaev (1995)
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“Topological” Particles in RVB State
vison
holon
Braid holonaround vison
• Spinons and holons (with “electric charge”)• “Visons” (with “magnetic charge”)
Quantum Braid
Minus sign
Created a Quantum Knot
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More exotic topological states
Quantum states supporting particles with
“Non Abelian particles”
Braid 1 and 21
2 3
Order of braids matters!
Multiple braids:
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“Topological Quantum Computing”
Bill Gates enters the “hardware” game!
Station Q at UCSB (Michael Freedman)
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“Braid” non Abelian particles -decoherence free quantum computing
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• Quantum Crystals 1023 electrons “particles” with quantum “wave” motion
• Quantum Choreography
Attempt to ascertain the fundamental rules underlying the
quantum behavior of electrons in crystals
• Quantum Computing (Topological) Attempt to use quantum states with “topological”
choreography to perform decoherence free quantum computing
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Global property unaffected by local dynamics
Red line intersects an even number of bonds
Recall the “cartoon” of the RVB state
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Global property unaffected by local dynamics
Red line intersects an even number of bonds
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Global property unaffected by local dynamics
Red line intersects an even number of bonds
Two quantum states:
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How is RVB Topological? Two states on a cylinder, a surface with non-trivial topology
“Vison” thru hole
Take holon around cylinder -wave function changes sign
Holon