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PAIRING
Two electrons become a pair
The lattice
A moving electron pulls the positive ions together.
Attraction
A second electron is drawn to that positive trail.
Cooper pair
Two electrons acting as one particle of charge 2e.
Through the lattice, electrons that normally repel can attract.
ONE WAVE
One giant quantum wave
Above Tc
Unpaired electrons with random phases.
Below Tc
Pairs share one phase and move in step.
Macroscopic
One quantum state spread across a whole circuit.
That's how a circuit you can see can behave like an atom.
THE GAP
It costs energy to break a pair
Gap 2Δ
About 3.5 kBTc: roughly 90 GHz for aluminium.
Qubit photons
4 to 8 GHz: far too weak to break pairs.
Low loss
Microwaves slosh around without being absorbed.
The gap protects the qubit's energy from the metal itself.
QUASIPARTICLES
When pairs break anyway
Causes
Stray infrared light, heat and cosmic rays.
Effect
Quasiparticles tunnel through the junction and cause errors.
Defences
Shields, filters and quasiparticle traps.
Even at 10 mK, a few broken pairs can limit a qubit.
FLUX QUANTUM
Flux comes in packets
Φ0 = h/2e
About 2.07 × 10−15 webers.
Proof of pairs
Its value showed in 1961 that pairs carry the current.
For qubits
Flux through loops is measured in units of Φ0.
Quantum rules show up in a ring you could hold in your hand.
UNIT 16 STUDY COMPLETE
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Ready for the Fab Challenge?
You've paired up electrons, formed a condensate, met the energy gap and the quasiparticles that cross it, and trapped flux quanta.