Flux bias lines

Control your Qubits!

Flux lines for superconducting qubits provide signals to tune qubits or tunable couplers. The key requirements to these lines are, short risetimes as well as low (contact) resistance to reduce both active and passive heating.

General Specifications

Storage temperature0 K to 320 K
Operating temperature0 K to 400 K (gradient)
ApplicationsQubit flux for superconducting qubits or similar
Number of channels8
Transmission typeStripline
Connector typeSMA female / SMP
Primary flex materialsPolyimide, NbTi
Minimum bending radius 5 mm
Required length for 180° longitudinal rotation 10 mm
Vacuum feedthrough connection ​KF25/40, ISO 100
side and top loader ports
Vacuum feedthrough leak rate < 10-9 mbar L/s

Cable overview

Used to

Optimised for

Key Features

The product is provided with coaxial SMA/SMP connector blocks to interface to the room-temperature electronics and the chipholder at the mK stage. Vacuum feedthroughs are designed to carry multiple flexes and are compatible with most fridge types. The chain of integrated attenuators and low pass filters is configured for flux bias currents, shielding the bias loop from undesired high frequency noise while allowing short risetimes. Below the 100 mK plate, integrated infrared filtering is positioned and thermalized with a slope of -1.5 dB/GHz to further filter out high frequency radiation.

The passive heatload is 600 pW per channel at 10 mK due to the superconducting material used. Microwave performance of the flux lines is shown in the figure below.

IR filter thermalisation clamp at the 10 mK temperature stage

Microwave Response

A flux line I/O chain allows base-band (0-2 GHz) access to your circuit, typically terminated by an inductive component on your chip like a flux loop. With a DC bias current one tunes the frequency of the qubit or the magnitude of the coupler. In time domain one can tune this in nanosecond timescale.

The Cri/oFlex® flux line is configured completely for this task. With a low pass filter at 1 GHz, a 20 dB attenuator, and a strong IR-filter, this setup allows you to control your qubit without opening unwanted noise channels.


Cross talk

Cross talk is what we experience daily. Whether it is picking up unwanted conversations in the train, or a phone connection suffering from interference. 

Cri/oFlex® has been designed such that each qubit is addressed individually. Learn more about cross talk in our core technology section.

How to


Cri/oFlex® into your system?

Cri/oFlex® can fit in your system in many ways and Delft Circuits has multiple approaches to find a perfect fit for your wishes, options range from a one-stop-shop in cabling solutions or a full service I/O provider designing a specific solution for your I/O challanges.

Need some help picking the right option?

First questions to ask yourself is how your system is currently wired. Do you have a port available or full loader you would like to install, upgrade or add wiring to?

For ports Delft Circuits recommends choosing from our wiring ports solution, if you want to fill a single port with more cables or get a lower heatload etc. etc. and already know exactly what cables you want pick the Wiring ports option. if you want a hassle free solution with installation, service, calibration. We recommend going for the turn-key solution and see what else we can do for you.

Delft Circuits recommends

Wiring ports

Delft Circuits recommends


For ports Delft Circuits recommends choosing from our wiring ports solution, if you want to fill a loader with more cables or get a lower heatload etc.. If you want a hassle free solution with installation etc. etc. we recommend going for the turn-key solution and see what else we can do for you

Delft Circuits recommends

Delft Circuits recommends

If you are building a new system we recommend looking at everything we have to offer for you, next to just I/O we can help you design, help you optimize, do cryogenic calculations, calibrations and much more.

Delft Circuits recommends

Dive deeper into

Cri/oFlex® technology

Learn all about what makes our technology specially fitted for the quantum industry.