Universal and Ultrafast Quantum Computation Based on Free-Electron-Polariton Blockade

Cavity QED, wherein a quantum emitter is coupled to electromagnetic cavity modes, is a powerful platform for implementing quantum sensors, memories, and networks. However, due to the fundamental trade-off between gate fidelity and execution time, as well as limited scalability, the use of cavity QED for quantum computation was overtaken by other architectures. Here, we introduce a new element into cavity QED-a free charged particle, acting as a flying qubit. Using free electrons as a specific example, we demonstrate that our approach enables ultrafast, deterministic, and universal discrete -variable quantum computation in a cavity -QED -based architecture, with potentially improved scalability. Our proposal hinges on a novel excitation blockade mechanism in a resonant interaction between a free -electron and a cavity polariton. This nonlinear interaction is faster by several orders of magnitude with respect to current photon -based cavity -QED gates, enjoys wide tunability and can demonstrate fidelities close to unity. Furthermore, our scheme is ubiquitous to any cavity nonlinearity, either due to light -matter coupling as in the Jaynes -Cummings model or due to photon -photon interactions as in a Kerr -type many -body system. In addition to promising advancements in cavity -QED quantum computation, our approach paves the way towards ultrafast and deterministic generation of highly entangled photonic graph states and is applicable to other quantum technologies involving cavity QED.