Cryostat platform: Bluefors Ultra-Compact LD 294 mm
A single-qubit transmon cryostat with four signal chains traversing six temperature stages from 300 K to ~15 mK. This is the simplest realistic configuration for a superconducting qubit experiment: one XY microwave drive, one flux bias, one readout stimulus, and one readout return, each attenuated and filtered at progressively colder stages to suppress thermal noise below the quantum noise floor.
A dilution refrigerator cools in discrete stages. Each plate intercepts heat from the cables above and adds attenuation or filtering so thermal noise decreases as you approach the qubit at ~15 mK.
Hermetic SMA feedthrough panels on the vacuum flange provide the boundary between room-temperature electronics and the cryostat interior.
First cold attenuation on XY and readout-in lines intercepts the bulk of 300 K Johnson-Nyquist noise. Other chains pass through with thermal anchoring.
HEMT amplifier provides ~30 dB first-stage gain on readout-out. Additional attenuators on drive lines and a low-pass filter on flux.
Continued attenuation and filtering at ~800 mK. Moderate cooling power (~20 uW) supports lightweight passive components.
Penultimate attenuation at ~100 mK. Very limited cooling power (~200 nW) restricts this to low-dissipation components.
Qubit chip, final attenuators, flux filter, and readout circulators at ~15 mK. Every microwatt of heat leak is critical.
Carries 4-8 GHz microwave pulses that rotate the qubit state on the Bloch sphere. Attenuated at every stage so thermal noise from 300 K does not reach the qubit.
Delivers a weak microwave probe tone to the readout resonator. Probe power at the device must stay below one photon to avoid disturbing the qubit.
Returns qubit state information to room temperature. Circulators at MXC block amplifier back-action; a HEMT at 4 K adds ~30 dB gain at 2-5 K noise temperature.
DC/low-frequency line that tunes qubit frequency via SQUID loop flux. Heavily low-pass filtered at cold stages to reject high-frequency noise.
Staged attenuation ensures the effective noise temperature at the qubit is set by the coldest attenuator (~15 mK), not the 300 K electronics.
The HEMT amplifier lives at 4 K because it dissipates ~10 mW — far exceeding the microwatt cooling budget of millikelvin stages.
Circulators provide non-reciprocal isolation: readout signals pass from qubit to amplifier, but amplifier noise is blocked from reaching the qubit.
Cable materials transition from stainless steel at warm stages to NbTi superconductor below 4 K, trading RF loss for dramatically lower thermal conductivity.