Volume: 3 Issue 6
7 Article(s)

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Engineering photonic angular momentum with structured light: a review | Article Video
Jian Chen, Chenhao Wan, and Qiwen Zhan
Structured light with inhomogeneous phase, amplitude, and polarization spatial distributions that represent an infinite-dimensional space of eigenstates for light as the ideal carrier can provide a structured combination of photonic spin and orbital angular momentum (OAM). Photonic spin angular momentum (SAM) interactions with matter have long been studied, whereas the photonic OAM has only recently been discovered, receiving attention in the past three decades. Although controlling polarization (i.e., SAM) alone can provide useful information about the media with which the light interacts, light fields carrying both OAM and SAM may provide additional information, permitting new sensing mechanisms and light–matter interactions. We summarize recent developments in controlling photonic angular momentum (AM) using complex structured optical fields. Arbitrarily oriented photonic SAM and OAM states may be generated through careful engineering of the spatial and temporal structures of optical fields. Moreover, we discuss potential applications of specifically engineered photonic AM states in optical tweezers, directional coupling, and optical information transmission and processing.
Advanced Photonics
  • Publication Date: Nov. 17, 2021
  • Vol. 3, Issue 6, 064001 (2021)
Quantum entanglement on photonic chips: a review
Xiaojiong Chen, Zhaorong Fu, Qihuang Gong, and Jianwei Wang
Advanced Photonics
  • Publication Date: Dec. 07, 2021
  • Vol. 3, Issue 6, 064002 (2021)
Entanglement-based quantum key distribution with a blinking-free quantum dot operated at a temperature up to 20 K | On the Cover
Christian Schimpf, Santanu Manna, Saimon F. Covre da Silva, Maximilian Aigner, and Armando Rastelli
Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.
Advanced Photonics
  • Publication Date: Dec. 17, 2021
  • Vol. 3, Issue 6, 065001 (2021)
Research Articles
Direct characterization of coherence of quantum detectors by sequential measurements
Liang Xu, Huichao Xu, Jie Xie, Hui Li, Lin Zhou, Feixiang Xu, and Lijian Zhang
The quantum properties of quantum measurements are indispensable resources in quantum information processing and have drawn extensive research interest. The conventional approach to revealing quantum properties relies on the reconstruction of entire measurement operators by quantum detector tomography. However, many specific properties can be determined by a part of the matrix components of the measurement operators, which makes it possible to simplify the characterization process. We propose a general framework to directly obtain individual matrix elements of the measurement operators by sequentially measuring two noncompatible observables. This method allows us to circumvent the complete tomography of the quantum measurement and extract the required information. We experimentally implement this scheme to monitor the coherent evolution of a general quantum measurement by determining the off-diagonal matrix elements. The investigation of the measurement precision indicates the good feasibility of our protocol for arbitrary quantum measurements. Our results pave the way for revealing the quantum properties of quantum measurements by selectively determining the matrix components of the measurement operators.
Advanced Photonics
  • Publication Date: Nov. 29, 2021
  • Vol. 3, Issue 6, 066001 (2021)
Dynamical learning of a photonics quantum-state engineering process | Article Video
Alessia Suprano, Danilo Zia, Emanuele Polino, Taira Giordani, Luca Innocenti, Alessandro Ferraro, Mauro Paternostro, Nicolò Spagnolo, and Fabio Sciarrino
Experimental engineering of high-dimensional quantum states is a crucial task for several quantum information protocols. However, a high degree of precision in the characterization of the noisy experimental apparatus is required to apply existing quantum-state engineering protocols. This is often lacking in practical scenarios, affecting the quality of the engineered states. We implement, experimentally, an automated adaptive optimization protocol to engineer photonic orbital angular momentum (OAM) states. The protocol, given a target output state, performs an online estimation of the quality of the currently produced states, relying on output measurement statistics, and determines how to tune the experimental parameters to optimize the state generation. To achieve this, the algorithm does not need to be imbued with a description of the generation apparatus itself. Rather, it operates in a fully black-box scenario, making the scheme applicable in a wide variety of circumstances. The handles controlled by the algorithm are the rotation angles of a series of waveplates and can be used to probabilistically generate arbitrary four-dimensional OAM states. We showcase our scheme on different target states both in classical and quantum regimes and prove its robustness to external perturbations on the control parameters. This approach represents a powerful tool for automated optimizations of noisy experimental tasks for quantum information protocols and technologies.
Advanced Photonics
  • Publication Date: Dec. 13, 2021
  • Vol. 3, Issue 6, 066002 (2021)

About the Cover

The image on the cover depicts the generation of polarization-entangled photon pairs by an optically excited semiconductor quantum dot. The dot is placed in a p-i-n diode structure, supplied with a forward-bias voltage, to control its internal charge configuration and to allow the generation of an uninterrupted stream of photon pairs. These photon pairs are used to perform quantum key distribution between two parties, located in different buildings, connected via a 350-metre-long optical fibre. The generated keys were used to encrypt and decrypt a message in an information theoretically secure manner.