Figure 2

Three qubit spectroscopy. (a) Spectroscopy of the three transmon qubits. The x axis indicates the electrical current value driven through the larger magnetic coil mounted to the cavity. The y axis shows the frequency of the excitation tone sent to the input of the cavity together with the measurement tone. The color indicates transmission of the measurement tone through the cavity at its resonance frequency, recalibrated at each bias current with no excitation. The background transmission was subtracted for each bias current for clarity. The dashed black curves indicate a fit to the expected magnetic dependencies of the transmon frequencies allowing extraction of the mutual inductances between the coil and SQUID loops for each qubit. (b) Once the mutual inductance matrix between the coils and the qubits is extracted one can realize individual control of the qubit frequencies. In this example two of the qubits are kept at the chosen frequencies of 5.705 and 6.195 GHz, respectively, while the third qubit was made to change its frequency linearly with the bias parameters. The dashed black lines are shown as a guide for the eye. The inset shows a zoom in on the avoided level crossing observed around \((5.705 + 6.195)/2 = 5.950\mbox{ GHz}\) (the frequency is indicated by a dashed line in the inset).