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Optics Express

Coherent frequency down-conversions and entanglement generation in a Sagnac interferometer

Yunning Lu, Shaoyan Gao, Aiping Fang, Fuli Li, Fuli Li, Muhammad Suhail Zubairy

Published Jun 29, 2017• 10 citations
Publisher

Abstract

A waveguide loop coupled to two external line waveguides by a 50/50 beam splitter forms a Sagnac interferometer. We consider the situation where two Λ-type three-level emitters are symmetrically coupled to the loop of a Sagnac interferometer and a single photon is input through one end of the line waveguides. Since the incoming photon is always in a superposition of the clockwise and counterclockwise modes of the loop and the two emitters are positioned symmetrically with respect to the input port of photon, the processes of photon scattering at the two emitters are symmetric and coherent. When the separation of the emitters and the coupling strengths of the emitters with the waveguide loop take some special values, due to quantum interference, a frequency down-conversion can certainly happen at one of the two emitters during the photon scattering but one cannot know at which emitter the frequency down-conversion takes place. This indistinguishability of the coherent frequency down-conversion processes can result in the generation of the symmetric or antisymmetric two-qubit maximally entangled states of the emitters. In the present scheme, a single photon comes in and goes out of the waveguide loop, and no photon localization modes exists. The entangled states result from the coherent frequency down-conversion processes of the emitters. Thus, the resulting entangled states are stable if the two lower-lying states of the emitters have no decay. We also investigate the influence of the dissipation of the emitters and the finite bandwidth of an input photon wavepacket on the success probability of entanglement generation, and find that the present scheme is robust to these effects and feasible with current available technologies.

DOI
10.1364/oe.25.016151

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A waveguide loop coupled to two external line waveguides by a 50/50 beam splitter forms a Sagnac interferometer. We consider the situation where two Λ-type three-level emitters are symmetrically coupled to the loop of a Sagnac interferometer and a single photon is input through one end of the line waveguides. Since the incoming photon is always in a superposition of the clockwise and counterclockwise modes of the loop and the two emitters are positioned symmetrically with respect to the input port of photon, the processes of photon scattering at the two emitters are symmetric and coherent. When the separation of the emitters and the coupling strengths of the emitters with the waveguide loop take some special values, due to quantum interference, a frequency down-conversion can certainly happen at one of the two emitters during the photon scattering but one cannot know at which emitter the frequency down-conversion takes place. This indistinguishability of the coherent frequency down-conversion processes can result in the generation of the symmetric or antisymmetric two-qubit maximally entangled states of the emitters. In the present scheme, a single photon comes in and goes out of the waveguide loop, and no photon localization modes exists. The entangled states result from the coherent frequency down-conversion processes of the emitters. Thus, the resulting entangled states are stable if the two lower-lying states of the emitters have no decay. We also investigate the influence of the dissipation of the emitters and the finite bandwidth of an input photon wavepacket on the success probability of entanglement generation, and find that the present scheme is robust to these effects and feasible with current available technologies.
A single-photon frequency conversion process with a Sagnac interferometer (Bradford et al 2012 Phys. Rev. Lett. 108 103902 and Bradford and Shen 2012 Phys. Rev. A 85 043814) is described by the input–output formalism. The result shows that a perfect frequency conversion can be obtained in an appropriate parameter regime via constructive interference. Based on the frequency-conversion process, an alternative scheme is provided for the generation of entanglement between distant particles including two-body entanglement and multi-body Greenberger–Horne–Zeilinger entanglement. We study the effect of the parameters' deviation and spontaneous decay of the excited state and waveguide mode on the transmission amplitudes, and also on the frequency-conversion efficiency. In addition, the influence of the experimental parameters on the fidelity of the entangled state is discussed.
Generation of multi-photon entangled states with high efficiency in integrated photonic quantum systems is still a big challenge. The usual three-photon generation efficiency based on the third-order nonlinear effect is extremely low. Here, we propose a scheme to generate three-photon correlated states, which are entangled states in frequency space and bound states in real space, with high efficiency. This method relies on two crucial processes. On one hand, by employing a Sagnac interferometer, an incident photon can be transformed into a symmetric superposition of the clockwise and counterclockwise modes of the Sagnac loop, which can then be perfectly absorbed by the emitter. On the other hand, the coupling strengths of the two transition paths of the emitter to the Sagnac loop are set to be equal, under which the absorbed photon can be emitted completely from the cascaded transition path due to quantum interference. By adjusting the coupling strengths among the three transition paths of the emitter and the waveguide modes, we can control the spectral entanglement and spatial separation among the three photons. Our proposal can be used to generate three-photon entangled states on demand, and the efficiency can be higher than 90% with some practical parameters, which can find important applications in integrated quantum information processing.
We present a quantum nanophotonic scheme to achieve efficient single-photon frequency conversion. This mechanism is essential for integrated nanophotonics, as it can provide access to frequency regimes in which no single-photon sources are currently available. Moreover, such a device could be used as the basis of a photonic frequency-shift-keyed quantum information scheme. The proposed scheme uses a Sagnac interferometer to exploit quantum interference between two transition pathways in a three-level quantum dot. In the proposed scheme, an input photon induces a complete population state transfer on the Lambda-type quantum dot, causing a frequency shift in the outgoing photon. The Sagnac interferometer is used to put the input photon into a superposition of counterpropagating states which interfere at the quantum dot, providing the necessary quantum interference to make the process efficient. We have developed a real-space theoretical approach and a computationally efficient pseudospectral numerical method to invegtigate the full spatiotemporal dynamics of the scattering process. It is shown that the efficiency of the frequency-conversion process approaches unity in the ideal case, and is greater than 80% even in the presence of realistic dissipation.
Abstract A single-photon frequency conversion scheme is proposed to generate three-dimensional entanglement between a quantum emitter and a photon with different frequencies. In the scheme, after scattered by a four-level quantum emitter on Sagnac loop, input photon is divided into three parts and each has its own frequency via constructive interference. In addition, through tuning system parameters, we can achieve equal probability amplitudes of output photons which mean the realization of three-dimensional entanglement. Our calculations show that the present scheme is robust against the deviation of experimental and decoherence.
Based on constructive interference in Sagnac waveguide loop, an efficient scheme is proposed for selective frequency conversion and multifrequency modes W entanglement via input–output formalism. We can adjust the probability amplitudes of output photons by choosing parameter values properly. The tunable probability amplitude will lead to the generation of output photon with a selectable frequency and W photonic entanglement of different frequencies modes in a wide range of parameter values. Our calculations show the present scheme is robust to the deviation of parameters and spontaneous decay.