Perfect qubit transfer no longer needs a laser pulse

A fundamental limit of 4/e^2 ≈ 0.54 previously capped the excited-state occupation of the receiving qubit in quantum networks, regardless of distance or the strength of connections between qubits. Now, researchers have demonstrated a way to bypass this constraint without relying on an external laser, a technique previously proposed by Cirac et al. The team achieved this by engineering the dispersion relation of the waveguide itself, providing the necessary phase shift at each frequency to time-reverse the propagating pulse. This passive method, utilizing only linear and standard photonic engineering, "suggests new opportunities of photonic design in quantum optics," and highlights engineered pulse propagation as a powerful tool in waveguide quantum…