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ZERO RESISTANCE
Cool a wire, lose the resistance
Ordinary metals
Electrons scatter, so some resistance always remains.
Superconductors
Below a critical temperature Tc, the resistance is exactly zero.
1911
Heike Kamerlingh Onnes saw it first, in mercury at 4.2 K.
Zero resistance means a circuit can hold energy without losing it.
PERSISTENT CURRENT
A current that never stops
Copper ring
The current fades away as heat.
Superconducting ring
The current flows on with no measurable decay.
For qubits
Energy sloshing around a circuit isn't lost to resistance.
A lossless circuit is the starting point for a qubit.
MEISSNER EFFECT
Superconductors push out magnetic fields
Expulsion
Below Tc, surface currents cancel the field inside.
Levitation
A magnet can float above a superconductor.
Trapped flux
Vortices trapped in the film absorb energy, so chips need magnetic shields.
Superconductivity is more than zero resistance: it's a new state of matter.
MATERIALS
Which metals, and how cold?
Aluminium
1.2 K: the junction metal in nearly every superconducting qubit.
Nb, Ta, TiN
Higher Tc; used for pads, resonators and wiring.
Ceramics
Superconduct in liquid nitrogen, but are too lossy for qubits.
Qubit chips are made from a handful of well-understood metals.
WHY SO COLD?
Why 10 millikelvin?
Qubit energy
At 5 GHz, hf/kB is only about 0.24 K.
Heat flips qubits
The share kicked into 1 is about e−hf/kBT.
10 to 20 mK
Cold enough that almost every qubit starts in 0.
The fridge must be far colder than the metal needs, for the qubit's sake.
UNIT 15 STUDY COMPLETE
❄️
Ready for the Fab Challenge?
You've watched resistance vanish, kept a current flowing forever, pushed out a magnetic field and found out why qubits need millikelvin temperatures.