How Quantum Computer Hardware Actually Works
Quantum algorithms get the spotlight, but the machine underneath is a masterclass in engineering. A superconducting quantum processor lives at the bottom of a dilution refrigerator — cooled to roughly 15 millikelvin, a hundred times colder than outer space, with signal cables stitched across seven temperature stages and physics-tight component placement at every step. This guide walks through the whole stack, with direct links to the interactive 3D cryostat designer, the component catalog, validation rules, and preset architectures.
Why does it have to be so cold?
A superconducting qubit is a tiny microwave resonator whose ground-to-excited transition frequency sits around 5 GHz. For quantum operations to work, the qubit has to start life in its ground state with high probability — which means the surrounding thermal photon occupation must be far below 1. At 300 K, there are roughly 1,250 thermal photons at 5 GHz; at 15 mK, the occupation drops to about 10-7. That is the basic reason every superconducting-qubit company in the world is shipping its chips on a dilution-refrigerator stack.
Getting cold is one problem. Staying cold while pumping microwave signals in and out is the harder one. Every coaxial line is also a heat pipe from room temperature to the qubit. That is the engineering story this page tells.
The seven temperature stages
A typical Bluefors or Oxford dilution refrigerator has stages at roughly 300 K, 50 K, 4 K, 1 K (still), CP (~100 mK), and MXC (~15 mK). Each plate has a finite cooling budget — the MXC typically offers only ~20 µW of continuous cooling power. That budget controls every downstream design decision: what can be placed on the stage, how much attenuation can dissipate there, and how long the signal cables can run between stages.
Open the 3D cryostat designer to see the stages rendered at scale, or read the cooling chain lesson for the thermodynamics of how the 3He/4He mixture gets you below 100 mK.
Signal chains: XY, flux, readout, DC
A single transmon qubit needs four distinct signal paths:
- XY drive — microwave tones at the qubit frequency that rotate the state on the Bloch sphere. Stainless-steel semi-rigid coax with staged attenuators (10 dB at 50 K, 10 dB at 4 K, 20–30 dB at MXC). The attenuators double as thermalization: the Johnson-Nyquist noise temperature at the qubit equals the temperature of the coldest attenuator.
- Flux bias — low-frequency DC + shaped pulses that tune the qubit frequency. Low-pass filtered at cold stages to suppress high-frequency noise that would modulate the qubit and reduce T2.
- Readout — microwave pulses at the resonator frequency. The return path is the bottleneck: a single-photon signal reflected off the readout cavity must survive until a HEMT amplifier at 4 K can lift it above the noise floor. Circulators at MXC provide 20 dB of reverse isolation so amplifier noise doesn't back-act on the qubit.
- DC / auxiliary — twisted-pair bias lines for heaters, thermometers, and slow control. Twisted pairs are chosen because solid coax shields conduct far more heat per unit cross-section than superconducting NbTi.
The full signal-type reference, with typical cables, stages, and anchors, is in the component catalog.
The components that make it work
Every component in the chain has a specific thermal and RF role. Clicking through to the detail page for any of these will show allowed stages, typical specifications, and the sources they were modeled from:
- AttenuatorReduce drive-line thermal noise and signal power. Thermalize coaxial line at each stage.
- HEMT AmplifierFirst cryogenic amplification of weak readout signals returning from the qubit/resonator.
- CirculatorProtect the qubit and resonator from amplifier back-action noise on the readout return line.
- Low-pass FilterSuppress out-of-band noise and high-frequency leakage on filtered cryogenic lines, especially flux and auxiliary low-frequency paths.
- Feedthrough PanelProvide a physically realistic hermetic panel entry or thermalization flange interface for cryogenic RF and DC signal paths.
What can go wrong, and how we check
Good cryogenic wiring isn't just “connect A to B.” There are tight physics-backed constraints at every step: minimum cable bend radius (to protect the dielectric under thermal contraction), minimum service reserve (stainless steel contracts ~0.3% from 300 K to 4 K, so cables need slack), panel feedthrough capacity, stage-plate diameter, and attenuator placement budgets (a 20 dB attenuator at MXC wastes thermal capacity that could have been spent at 4 K).
The system-checks reference documents ~20 validation rules — each with the threshold, the formula that derives it, the code that implements it in the studio, and citations to peer-reviewed papers. It reads like a cheat-sheet for anyone designing a real fridge.
Preset architectures to start from
We ship a few realistic baseline cryostats you can open in the studio, inspect, and fork:
- Compact Transmon BaselineMinimal single-channel transmon cryostat baseline with one XY, one flux, one readout-in, and one readout-out chain.
- Dense 5-Qubit TransmonRealistic 5-qubit transmon system with 25+ signal chains and 130+ cable routes. Demonstrates wiring density, panel occupancy, bundle congestion, and thermal budget pressure in a production-class cryostat.
- HDW 10-Qubit Thermal StressHigh-density 10-qubit transmon system with 210 cable routes and 10 HEMT amplifiers. Designed to stress-test thermal budgets on a ProteoxLX-class dilution refrigerator with realistic wiring density.
Design your own
The 3D Cryostat Wiring Studio runs entirely in the browser. Drop in stages, mount components, route signal cables, watch every validation rule update live, and run thermal simulations to see how heat flows through the structure. It's the only interactive tool of its kind, and it's free to use.