Deterministic and cascadable conditional phase gate for photonic qubits

TL;DR

This paper proposes a high-fidelity, cascadable conditional phase gate for photonic qubits using an atomic V-system and quantum Zeno effect, overcoming previous limitations related to noise and phase shift magnitude.

Contribution

It introduces a novel conditional phase gate model that maintains high fidelity for small phase shifts by employing principal mode projection and the quantum Zeno effect.

Findings

High-fidelity conditional phase gate demonstrated for small phase shifts

Cascadable gate achieved using principal mode projection

Overcomes noise issues associated with large nonlinear phase shifts

Abstract

Previous analyses of conditional \phi-phase gates for photonic qubits that treat cross-phase modulation (XPM) in a causal, multimode, quantum field setting suggest that a large (~\pi rad) nonlinear phase shift is always accompanied by fidelity-degrading noise [J. H. Shapiro, Phys. Rev. A 73, 062305 (2006); J. Gea-Banacloche, Phys. Rev. A 81, 043823 (2010)]. Using an atomic V-system to model an XPM medium, we present a conditional phase gate that, for sufficiently small nonzero \phi, has high fidelity. The gate is made cascadable by using using a special measurement, principal mode projection, to exploit the quantum Zeno effect and preclude the accumulation of fidelity-degrading departures from the principal-mode Hilbert space when both control and target photons illuminate the gate.