Quantum Ratchets

1
Quantum Ratchets

• Quantum Electronics For Engineers
• Instructor Joel Therrien
• Presented By Ravi Bhatia

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Whats a Ratchet?

• A device which allows motion in one direction.
• Consists of a gear with Asymmetric Teeth and a
  Locking Mechanism that allows the gear to move in
  one direction only.
• Everyday Examples Screwdrivers Winches,
  Bicycle Transmissions.

3
Feynman Ratchet

• A thought Experiment by Richard Feynman,
  Lectures on Physics, 1963.
• Can we obtain directed motion from random thermal
  fluctuation of particles?
• Dream of the Free Energy!
• System consists of a microscopic ratchet and vane
  placed in equilibrium, such that it is affected
  by the random motion of gas particles hitting the
  vane.

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Microscopic Ratchet and Vane Mechanism

• Asymmetric teeth make the ratchet move in one
  direction under random collisions of gas
  molecules.

• Direct Violation of the Second Law of
  Thermodynamics. Not Possible!

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Key Properties of Ratchet Effect

• Ratchet Effect The ability to get directed
  motion or transport from random fluctuations of
  particles.
• The System must not be in Thermal Equilibrium -gt
  Obey the Second Law of Thermodynamics.
• The System must have Asymmetric Spatial Symmetry
  -gt Asymmetric Ratchet teeth, Asymmetric
  potential.

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Ratchet Effect in Electronics

• Using Noise to achieve rectified current.
• Electrons in an asymmetric but spatially periodic
  potential wells.
• Random Diffusion of electrons is converted into a
  net motion to the left.
• Thermal equilibrium broken by switching ratchet
  potential on and off.

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Quantum Tunneling Ratchets

• Asymmetric Potential implemented using
  Semiconductor Heterostructures.
• MBE growth of Alternating GaAs/AlGaAs Layers.
• Band bending due to conduction band offset
  produces a triangular shaped potential well.

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AlGaAs/GaAs Heterostructure

• Fig a) Offset in conduction band causes electrons
  to flow down to the GaAs layer.
• Fig b) Only the first Level is populated at
  cryogenic temperatures forming a 2DEG.

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Behavior of Tunneling Ratchets

• Rectification of a randomly varying signal.
• Electron Flow direction is a function of
  Temperature.
• Positive Voltage to the ratchet makes the wells
  to the right shallower.

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• High Temperature Positive voltage causes high
  energy electrons to predominantly move to the
  right
• Low Temperature Electrons tunnel through the
  barrier and move to the Left

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What Have We Achieved using Quantum Tunneling
Ratchets?

• Fundamentally Opposite Quantum and Classical
  Behavior.
• Tunneling contributes to the particle flow at low
  temperatures.
• Rectification of Random Signals or Noise at
  Cryogenic or High Temperatures.

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References

• Mesoscopic Quantum Ratchets and the
  thermodynamics of Energy Selective Electron Heat
  Engines, Doctoral Thesis, Tammy E.
  Humphrey,2003, Univ of New South Wales.
• Quantum Clockwork, New Scientist 2000, Michael
  Brooks.
• Quantum Ratchets, Doctoral Thesis,2005, Joel
  Peguiron.
• Theory of Chaotic Hamiltonian Ratchets,
  Doctoral Thesis, Mathhew R. Sherwood,2004,
  University of London.
• How about free Perpetual Energy?
  http//home.wanadoo.nl/perpetual/index.html