Quantum modulation of a coherent state with a single electron spin

The interaction of quantum objects lies at the heart of fundamental quantum physics and is key to a wide range of quantum information technologies. Photon-quantum-emitter interactions are among the most widely studied. Two-qubit interactions are generally simplified into two quantum objects in static well-defined states. In this work we explore a fundamentally new dynamic type of spin-photon interaction. We demonstrate the modulation of a coherent narrowband laser by a coherently evolving spin in the ground state of a quantum dot. What results is a quantum modulation of the output phase (either 0 or <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>π</a:mi></a:math> but no values in between), and a new quantum state of light that cannot be described classically.