---
abstract: 'We analyze in detail the proposal for a two-qubit gate for travelling single-photon qubits recently presented by C. Ottaviani *et al*. \[Phys. Rev. A **73**, 010301(R) (2006)\]. The scheme is based on an ensemble of five-level atoms coupled to two quantum and two classical light fields. The two quantum fields undergo cross-phase modulation induced by electromagnetically induced transparency. The performance of this two-qubit quantum phase gate for travelling single-photon qubits is thoroughly examined in the steady-state and transient regimes, by means of a full quantum treatment of the system dynamics. In the steady-state regime, we find a general trade-off between the size of the conditional phase shift and the fidelity of the gate operation. However, this trade-off can be bypassed in the transient regime, where a satisfactory gate operation is found to be possible, significantly reducing the gate operation time.'
author:
- 'S. Rebić,'
- 'C. Ottaviani, G. Di Giuseppe, D. Vitali and P. Tombesi'
title: Assessment of a quantum phase gate operation based on nonlinear optics
---

Introduction {#sec:intro}
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A promising system for quantum computation is to use single photons to encode quantum information [@chuang95]. This is due to the photon’s robustness against decoherence and the 