Microwave photonics with superconducting quantum circuits

In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.

Twisted photons can be used as alphabets to encode information beyond one bit per single photon. This ability offers great potential for quantum information tasks, as well as for the investigation of fundamental questions. In this review article, we give a brief overview of the theoretical differences between qubits and higher dimensional systems, qudits, in different quantum information scenarios. We then describe recent experimental developments in this field over the past three years. Finally, we summarize some important experimental and theoretical questions that might be beneficial to understand better in the near future. Photons possessing orbital angular momentum are promising for systems for realizing new quantum information applications. Quantum computing and communications are set to revolutionize information technology, but most systems studied to date are based on qubits —quantum analogs of classical bits that can take one of only two states. Manuel Erhard at the University of Vienna, Austria, and co-workers review progress in higher dimensional systems that use photons with orbital angular momentum, or twisted photons, as ‘qudits’, which can have any number of levels. They look at the advantages of such higher-dimensional systems, which include higher information capacity and greater protection from eavesdropping. The researchers then examine exciting developments in the field in the past two to three years, such as the creation of high-dimensional entanglement and optimal quantum cloning. Finally, they consider future challenges.

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Twisted photons: new quantum perspectives in high dimensions

Photonic quantum technologies represent a promising platform for several applications, ranging from long-distance communications to the simulation of complex phenomena. Indeed, the advantages offered by single photons do make them the candidate of choice for carrying quantum information in a broad variety of areas with a versatile approach. Furthermore, recent technological advances are now enabling first concrete applications of photonic quantum information processing. The goal of this manuscript is to provide the reader with a comprehensive review of the state of the art in this active field, with a due balance between theoretical, experimental and technological results. When more convenient, we will present significant achievements in tables or in schematic figures, in order to convey a global perspective of the several horizons that fall under the name of photonic quantum information.

https://iopscience.iop.org/article/10.1088/1361-6633/aad5b2/pdf

Photonic quantum information processing: a review

Quantum information science and quantum information technology have seen a virtual explosion world-wide. It is all based on the observation that fundamental quantum phenomena on the individual particle or system-level lead to completely novel ways of encoding, processing and transmitting information. Quantum mechanics, a child of the first third of the 20th century, has found numerous realizations and technical applications, much more than was thought at the beginning. Decades later, it became possible to do experiments with individual quantum particles and quantum systems. This was due to technological progress, and for light in particular, the development of the laser. Hitherto, nearly all experiments and also nearly all realizations in the fields have been performed with qubits, which are two-level quantum systems. We suggest that this limitation is again mainly a technological one, because it is very difficult to create, manipulate and measure more complex quantum systems. Here, we provide a specific overview of some recent developments with higher-dimensional quantum systems. We mainly focus on Orbital Angular Momentum (OAM) states of photons and possible applications in quantum information protocols. Such states form discrete higher-dimensional quantum systems, also called qudits. Specifically, we will first address the question what kind of new fundamental properties exist and the quantum information applications which are opened up by such novel systems. Then we give an overview of recent developments in the field by discussing several notable experiments over the past 2-3 years. Finally, we conclude with several important open questions which will be interesting for investigations in the future.

Twisted Photons: New Quantum Perspectives in High Dimensions

The ability to measure the orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there have been many studies on the measurement of OAM modes, it is difficult to quantitatively and instantaneously measure the power distribution among different OAM modes, let alone measure the phase distribution among them. In this work, we propose an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect. The original OAM mode distribution is mapped to an electrical spectrum of beat signals using a photodetector. The power and phase distributions of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement technique to other spatial modes, such as linear polarization modes. These results represent a new landmark in spatial mode analysis and show great potential for applications in OAM-based systems and optical communication systems with mode-division multiplexing. An instrument that can map the power and phase distributions of vortex light beams that have orbital angular momentum (OAM) has been built. The ability to measure the OAM distribution of vortex light is essential for OAM applications, but it is challenging to instantaneously measure the power and phase distribution of different OAM modes. The OAM spectrum analyzer developed by Jianji Dong of Huazhong University of Science and Technology and co-workers characterizes a beam with unknown OAM by sending it and a reference beam to a spinning object, which scatters the light, imparting a rotational Doppler frequency shift in the process. The scattered light is then passed through a mode filter and collected by a photodetector for analysis. The frequency shift of the scattered light reveals information about the OAM content of the unknown beam.

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Orbital angular momentum complex spectrum analyzer for vortex light based on the rotational Doppler effect.

The function to measure orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there are lots of works to measure OAM modes, it is difficult to measure the power distribution of different OAM modes quantitatively and instantaneously, let alone measure the phase distribution among them. In this work, we demonstrate an OAM complex spectrum analyzer, which enables to measure the power and phase distribution of OAM modes simultaneously by employing rotational Doppler Effect. The original OAM mode distribution is mapped to electrical spectrum of beating signals with a photodetector. The power distribution and phase distribution of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement to other spatial modes, such as linear polarization modes. These results represent a new landmark of spatial mode analysis and show great potentials in optical communication and OAM quantum state tomography.

Orbital angular momentum complex spectrum analyzer for vortex light based on rotational Doppler Effect

When a vortex beam with the spiral phase structure passes through a dynamic angular double slits (ADS), the interference pattern changes alternatively between destructive and constructive at the angular bisector direction of the ADS due to their phase difference. Based on this property, we experimentally demonstrate a simple method, which can precisely and efficiently determine the topological charge of vortex beams. What is more, this scheme allows determining both the modulus and sign of the topological charge of vortex beams simultaneously.

Probing the topological charge of a vortex beam with dynamic angular double slits

When a vortex beam with a spiral phase structure passes through dynamic angular double slits (ADS), the interference pattern changes alternatively between destructive and constructive at the angular bisector of the ADS. This change is due to their phase difference. Based on this property, we experimentally demonstrate a simple method to precisely and efficiently determine the topological charge of vortex beams. Furthermore, this scheme allows for the simultaneous determination of the modulus and the sign of the topological charge of the vortex beams.

Vortex beams with fractional topological charge (FTC) have many special characteristics and novel applications. However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit (ADS), the far-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.

Dongxu Chen

Papers

Orbital angular momentum complex spectrum analyzer for vortex light based on the rotational Doppler effect.

Orbital angular momentum complex spectrum analyzer for vortex light based on rotational Doppler Effect

Probing the topological charge of a vortex beam with dynamic angular double slits

Probing the topological charge of a vortex beam with dynamic angular double slits

Probing the fractional topological charge of a vortex light beam by using dynamic angular double slits