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arXiv:1601.06250 · Nature Communications

On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom

Lan‐Tian Feng, Ming Zhang, Zhi‐Yuan Zhou, Ming Li, Xiao Xiong, Le Yu, Bao‐Sen Shi, Guo‐Ping Guo, Daoxin Dai A, Xi‐Feng Ren, Guang‐Can Guo

Published Jun 20, 2016• 138 citations
arXivHTMLPDF

Abstract

In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process.

DOI
10.1038/ncomms11985

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Abstract Integrated entangled photon pair source is an essential resource for both fundamental investigations and practical applications of quantum information science. Currently there have been several types of entanglement, among which the transverse-mode entanglement is becoming attractive because of its unique advantages. Here, we report an on-chip transverse-mode entangled photon pair source via the spontaneous four-wave mixing processes in a multimode silicon waveguide. Transverse-mode photon pairs are verified over multiple frequency channels within a bandwidth of ~2 THz, and a maximally entangled Bell state is also produced with a net fidelity of 0.96 ± 0.01. Our entangled photon pair source is the key element for quantum photonics based on transverse-mode, and also has the possibility to extend to higher-dimensional Hilbert space. Furthermore, the transverse-mode entanglement can be converted coherently to path and polarization entanglement, which paves the way to realizing highly complex quantum photonic circuits with multiple degrees of freedom.
In order to overcome the challenge of lacking polarization encoding in integrated quantum photonic circuits, we propose a scheme to realize arbitrary polarization manipulation of a single photon by integrating a single quantum emitter in a photonic waveguide. In our scheme, one transition path of the three-level emitter is designed to simultaneously couple with two orthogonal polarization degenerate modes in the waveguide with adjustable coupling strengths, and the other transition path of the three-level emitter is driven by an external coherent field. The proposed polarization converter has several advantages, including arbitrary polarization conversion for any input polarization, tunable working frequency, excellent antidissipation ability with high-conversion efficiency, and atomic-scale size. Our Letter provides an effective solution to enable the polarization encoding of photons that can be applied in the integrated quantum photonic circuits, and will boost quantum photonic chips.
Integrated quantum optics has the potential to markedly reduce the footprint and resource requirements of quantum information processing systems, but its practical implementation demands broader utilization of the available degrees of freedom within the optical field. To date, integrated photonic quantum systems have primarily relied on path encoding. However, in the classical regime, the transverse spatial modes of a multi-mode waveguide have been easily manipulated using the waveguide geometry to densely encode information. Here, we demonstrate quantum interference between the transverse spatial modes within a single multi-mode waveguide using quantum circuit-building blocks. This work shows that spatial modes can be controlled to an unprecedented level and have the potential to enable practical and robust quantum information processing.
Hybrid encodings, where multiple degrees of freedom are used to encode quantum information, can increase the size of the Hilbert space with minimal increase to hardware requirements. We show a reprogrammable integrated photonic device, with multimodal components designed to allow for control over the transverse electric modes. We use this device to generate qudit states entangled in the path and transverse electric mode degrees of freedom. We generate and verify a hyperentangled state with a fidelity of F HE =67.3±0.2% and a GHZ 4 -style state with a fidelity of F GHZ 4 =85.2±0.4%. We use our hyperentangled state in a single-copy entanglement distillation protocol on-chip, highlighting how this can be used to overcome inherent hardware noise with our hyperentangled state. We also classically simulate the application of probabilistic bit flip errors within this protocol, resulting in an average 9.1% increase in the fidelity of the distilled Bell state for up to a 50% probability of bit flip error. By utilizing degrees of freedom that are readily compatible with integrated photonics, our work highlights how this hybrid encoding demonstrates a first step in using the transverse electric mode to reduce the footprint of integrated quantum photonic experiments.