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RESONATORS
A resonator on a chip
Coplanar waveguide
A strip of metal between two ground planes.
Quarter wave
Length sets frequency: about 4.5 mm for 6.6 GHz.
Meander
Folded up to fit beside the qubit.
A resonator is just a carefully cut length of line.
QUALITY FACTOR
How long does it ring?
Q = f/Δf
A narrow dip means a long ring.
Readout
Kept leaky (low Q) so the signal comes out fast.
Materials tests
A Q of a million reveals tiny losses.
Q is a fab's favourite measure of material loss.
DISPERSIVE READOUT
Listening to the qubit
Frequency shift
The qubit moves the resonator by ±χ.
Gentle
A weak tone reads the state without flipping it.
Best spot
Probe between the two dips.
We never measure the qubit directly, only its resonator.
SINGLE SHOT
One measurement, one answer
Two clouds
Shots of 0 and 1 form two blobs.
Trade-off
Longer beats the noise but lets the qubit decay.
State of the art
About 99% accuracy in a few hundred nanoseconds.
Readout has to be both fast and accurate.
AMPLIFIERS
Amplifying a whisper
Parametric amplifier
Close to the quantum limit, at 10 mK.
HEMT
A low-noise transistor amplifier at 4 K.
Isolators
Keep amplifier noise away from the qubits.
A chain of amplifiers turns a few photons into a 0 or a 1.
UNIT 19 STUDY COMPLETE
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Ready for the Fab Challenge?
You've tuned a resonator, read a qubit through it, optimised single-shot readout and followed the signal up the amplifier chain.