Photon-Number Insensitive Qubits with Magic Transitions

The need to operate at dilution-refrigerator temperatures is one of the biggest scaling limitations of superconducting qubits. Josephson junctions that can provide high coherence above 1K are currently under development by many groups, but even with the use of these junctions, the conventional circuit QED architecture still requires the readout resonator to be cold, in order to eliminate qubit-dephasing due to photon number fluctuations. Here we present an alternative to challenging high-frequency mm-wave operation using a C-shunted flux qubit and transmon pair. The opposing anharmonicities of the qubits allows the dispersive shift from the readout resonator to vanish outside of the straddling regime. At such an operating point, the readout resonator coincides with a magic frequency [1], and the qubit transition frequency is independent of photon number, enabling long T2 times even in thermally occupied resonators.