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THE KEY COMPONENT
A sandwich of aluminium and oxide
Two electrodes
Aluminium strips that cross.
The barrier
1 to 2 nm of aluminium oxide between them.
Tiny
The overlap is about 0.2 × 0.2 µm.
Every superconducting qubit is built around this one structure.
TUNNELLING
Crossing an insulator
Quantum leakage
The pair wave leaks through the thin barrier.
Exponential
0.1 nm more oxide cuts the current by about e (2.7 times).
Sensitive
Atom-scale control decides the qubit's frequency.
The barrier must be uniform almost atom by atom.
JOSEPHSON EFFECTS
Current with no voltage
DC effect
Below Ic, current flows with zero voltage.
AC effect
Above it, a voltage V makes it oscillate at 2eV/h.
1962
Brian Josephson predicted both (Nobel Prize, 1973).
Qubits always run below the critical current.
NONLINEARITY
An inductor with a twist
Phase
The current sets the phase: I = Ic sin φ.
Inductance
LJ grows as the current grows.
The key
Nonlinear and lossless: exactly what a qubit needs.
The junction is the only circuit element that is both nonlinear and lossless.
THE SQUID
Two junctions make a tunable one
SQUID
A small loop containing two junctions.
Flux tunes it
EJ falls as |cos(πΦ/Φ0)|.
Sweet spot
At the top of the curve, flux noise matters least.
Tunable qubits and couplers are built from SQUIDs.
UNIT 17 STUDY COMPLETE
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Ready for the Fab Challenge?
You've explored a junction in 3D, tunnelled through its barrier, pushed it past its critical current, seen its nonlinear inductance and tuned a SQUID.