dc SQUID based Johnson Noise Measurements of a 3k$\Omega $ Resistor at mK temperatures

Johnson noise measurement is a useful technique for primary thermometry of charge carriers. However, Johnson noise of k$\Omega $ resistors at mK temperatures is lower than typical noise of room temperature electronics. Therefore, lower noise amplifiers are needed to make such measurements. Low temperature amplifiers based on cooled High Electron Mobility Transistors (HEMTs) are often employed, however these offer low noise operation at MHz frequencies only. We present an alternative circuit which operates at low frequencies which is based on a dc SQUID. We demonstrate that our circuit does not contribute appreciable noise to the Johnson noise of a 3.25 k$\Omega $ resistor down to 16mK, enabling Johnson noise thermometry.