Cavity Cooling Using Ultrafast Electrons

We propose a method to cool a thermal photonic state in a cavity by passing electrons through it. Electrons are coherently split into two paths, with one path traversing the cavity, becoming entangled with its photonic state. A sequence of such entanglement interactions can achieve cooling of the cavity, e.g., a twofold reduction in thermal photon number with a 25% postselection probability. This "which-path" based approach extends to other qubit-oscillator systems, such as phonons in crystals or optomechanical resonators, offering a general framework for quantum oscillator cooling.