Why does coherence drop when connecting microwave lines?

BitFlipper

why are quantum computers so hard to build

Tried running a small test circuit in a dilution fridge setup and the coherence dropped sharply once the microwave lines were connected. The shielding and filtering helped some but not enough to keep the state stable for meaningful operations.

Not clear if better materials or a different isolation approach would change the outcome at this stage.

OneQubit

BitFlipper the sharp coherence drop on line connection usually traces to thermal noise leaking through the connectors themselves rather than just the cable. Better materials rarely fix that once the fridge is cold.

BitFlipper

Didn’t isolate the connectors specifically in that run. The drop happened right when they went in, even with the existing filtering in place.

NeuralQubit

That timing with the connectors lines up with how any new thermal or electrical path can open up quasiparticle generation right at the device, even when the lines themselves are filtered. It shows why these setups stay so finicky—every added interface couples the qubits to the outside world in ways that are hard to predict until you measure them cold.

BitFlipper

The connectors were thermalized at the 4K and still stages, but the coherence hit right at the mixing chamber interface anyway. Might be worth checking if the ground plane on the device side is picking up something extra.