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Quantum Ratchets

Quantum Ratchets. Quantum Electronics For Engineers Instructor: Joel Therrien Presented By Ravi Bhatia. What’s 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.

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Quantum Ratchets

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  1. Quantum Ratchets Quantum Electronics For Engineers Instructor: Joel Therrien Presented By Ravi Bhatia

  2. What’s 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.

  4. 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!

  5. 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 -> Obey the Second Law of Thermodynamics. • The System must have Asymmetric Spatial Symmetry -> Asymmetric Ratchet teeth, Asymmetric potential.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 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.

  12. 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

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