Low-noise computer-controlled current source for quantum coherence experiments

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We describe a dual current source designed to provide static flux biases for a superconducting qubit and for the Superconducting QUantum Interference Device (SQUID) which measures the qubit state. The source combines digitally programmable potentiometers with a stabilized voltage source. Each channel has a maximum output of ±1  mA, and can be adjusted with an accuracy of about ±1  nA. Both current supplies are fully computer controlled and designed not to inject digital noise into the quantum bit and SQUID during manipulation and measurement of the flux. For a 275  µA setting, the measured noise current is 2.6 parts per million (ppm) rms, in a bandwidth of 0.0017–10  Hz, from which we estimate dephasing times of hundreds of nanoseconds in the particular case of our own qubit design. By resetting the current every 10  min, we are able to reduce the drift to no more than 5  ppm at a current of 750  µA over a period of 3 days. The current source has been implemented without thermal regulation inside a radiofrequency-shielding room, and is used routinely in our quantum coherence experiments. ©2004 American Institute of Physics.