Frequency Crowding
As fixed-frequency qubits multiply, their transition frequencies crowd together, causing addressing errors and constraining chip design.
Too many tones, too little room
Fixed-frequency superconducting qubits are addressed by microwave tones at their individual frequencies, which lie in a limited band (roughly 4 to 6 GHz) set by fabrication and coherence constraints. As a chip gains qubits, their frequencies must all fit in that band with enough separation that a pulse for one qubit does not drive another, or push a two-qubit gate off its needed detuning. This is frequency crowding.
Why it hurts
- Two qubits too close in frequency suffer strong crosstalk and swap errors
- Cross-resonance gates need specific control-target detunings; collisions ruin them
- Fabrication scatter in junction critical current makes exact frequencies hard to hit
- A single bad frequency can make several neighboring gates unusable
Frequency collisions
A frequency collision is when a fabricated frequency lands too near a neighbor's or near a harmful resonance condition. On a large chip the probability that at least one collision occurs grows with qubit count, so yield, the fraction of chips with no fatal collisions, falls unless frequencies are placed and made precisely. Laser annealing of junctions after fabrication can trim frequencies toward target to recover yield.
Design responses
- Sparse coupling graphs like heavy-hex that reduce the number of frequency constraints per qubit
- Tunable qubits or tunable couplers that sidestep fixed-frequency collisions
- Post-fabrication frequency trimming
- Careful frequency-allocation algorithms across the lattice
Frequency crowding is a concrete example of how scaling is not just adding qubits but managing the growing web of constraints among them, and it strongly shapes both architecture and fabrication targets.