Engineering giant transmon molecules as mediators of conditional two-photon gates

Abstract: Artificial atoms non-locally coupled to waveguides -- the so-called giant atoms -- offer new opportunities for the control of light and matter. In this work, we show how to use an array of non-locally coupled transmon "molecules" to engineer a passive photonic controlled gate for waveguide photons. In particular, we show that a conditional elastic phase shift between counter-propagating photons arises from the interplay between direction-dependent couplings, engineered through an interplay of non local interactions and molecular binding strength; and the nonlinearity of the transmon array. We analyze the conditions under which a maximal $\pi$-phase shift -- and hence a CZ gate -- is obtained, and characterize the gate fidelity as a function of key experimental parameters, including finite transmon nonlinearities, emitter spectral inhomogeneities, and limited cooperativity. Our work opens the use of giant atoms as key elements of microwave photonic quantum computing devices.