Frequency Multiplexing of Qubit Readout
Assigning each qubit a distinct resonator frequency lets many qubits share one readout line, cutting the wiring and heat load that limit scaling.
Sharing a wire
If every qubit needed its own dedicated readout cable running the full height of the cryostat, wiring count and its heat load would grow prohibitively with qubit number. Frequency multiplexing avoids this by letting many qubits share a single transmission line, each distinguished by the frequency of its own readout resonator.
How it works
Each qubit is coupled to a resonator tuned to a slightly different frequency. All the resonators connect to one shared feedline. A single broadband probe signal containing many frequency tones interrogates all of them at once; each qubit imprints its state on the amplitude and phase of its own tone. On the way out, a broadband amplifier such as a traveling-wave parametric amplifier handles all the tones together, and room-temperature electronics digitize the combined signal and separate the tones by frequency.
What sets the limit
The number of qubits that can share one line is set by how finely resonator frequencies can be spaced without crosstalk and by the bandwidth of the amplifier and digitizer. Fabrication spread in resonator frequencies, limited amplifier bandwidth, and the need to avoid collisions all cap the multiplexing factor, typically at tens of qubits per line with current technology, and improving this is an active engineering goal.
Why it matters for cooling
Every qubit removed from its own dedicated line is one fewer thermal bridge and one fewer amplifier chain contributing to the cold budget. Multiplexing is therefore one of the principal tools for bending the cooling-demand curve, working alongside cryo-CMOS to make large processors thermally feasible.
- Each qubit gets a resonator at a distinct frequency
- Many resonators share one feedline probed by multiple tones
- Limited by frequency spacing and amplifier bandwidth
- Cuts wiring and heat load, aiding scaling