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?