Impact of noise reduction in measurement of a super conducting flux qubit at 4.2 K

A flux state of a super conducting flux qubit is evaluated at 4.2 K prior to measurement of quantum state at mK. At the 4.2 K measurement, a customized rod, where a sample is attached, is inserted in the Dewar vessel filled in the liquid helium. An elimination of artifact from an experimental system is crucial. In order to prevent signal quality deterioration by environmental noise, countermeasures have been taken by strengthening the electrostatic shield and introducing the twisted pair wiring. Low noise voltage source (LP6016) for bias current control of SQUID is also effective in suppressing signal fluctuation from SQUID. As a result, noise density at 50 Hz and 150 Hz in the measurement became 1/300 and 1/500 compared to the previous experiment, respectively. We evaluated the flux state in the super conducting flux qubit through an observation of signal in a readout-SQUID. This sample was fabricated by AIST Nb standard process 2, where J_c was 25 μA/μm². On a flux dependence of threshold current (I_th) in the readout-SQUID, standard deviations (σ) of I_th was 0.16. This indicates that we can control an operating point of the readout-SQUID in an accuracy of the measurement with an order less than 1 μA. Further, σ << 1 was evaluated when we observed flux conditions in the qubit.