Resonator-free readout of a unimon*

Unimon [1] is a recently invented high-anharmonicity superconducting qubit, which is immune to low-frequency charge noise owing to its island-free design. It presents a grounded λ/2 coplanar waveguide resonator with a Josephson junction in the middle of the central conductor. Anharmonicity of this qubit is enhanced by a partial cancellation of the inductive energy of the central conductor by the Josephson energy of the flux-biased junction. Due to the distributed nature of the unimon, it is essentially multi-mode. For a symmetric unimon with the junction exactly in the middle of the central conductor only half of these modes are non-linear and can be used for the quantum computation while the rest are linear. However, these intrinsic linear modes can be used for dispersive readout of the unimon without introducing an additional resonator. The required coupling between the linear and nonlinear modes can be achieved by making the unimon slightly asymmetric. Coupling of the readout mode to the external transmission line can be implemented via two-point coupler which protects the qubit state from dissipation. This work presents an important step for development of quantum logic for novel superconducting qubits.