gressed rapidly in recent years in the hope of addressing these needs. This field of research covers many topics, and is developing at an unprecedented scale with the involvement of researchers from many different leading-edge science and technology fields. Although it is very hard to predict how things will turn out in the future, it seems that as far as a hardware is concerned, important roles will be played by photonic qubits for quantum communication, nuclear spin qubits for quantum memory, and electron spin qubits for quantum information processing to connect them together. This paper introduces the latest research trends relating to the production of photonic qubits used for quantum communication and computation, with particular reference to the results obtained by the author’s group. In quantum cryptography, 1)
gressed rapidly in recent years in the hope of addressing these needs. This field of research covers many topics, and is developing at an unprecedented scale with the involvement of researchers from many different leading-edge science and technology fields. Although it is very hard to predict how things will turn out in the future, it seems that as far as a hardware is concerned, important roles will be played by photonic qubits for quantum communication, nuclear spin qubits for quantum memory, and electron spin qubits for quantum information processing to connect them together. This paper introduces the latest research trends relating to the production of photonic qubits used for quantum communication and computation, with particular reference to the results obtained by the author’s group. In quantum cryptography, 1)
Photons are very promising resources for the whole field of quantum information processing. Bulk optics are currently not suitable for the realization of complex quantum optical schemes due to several drawbacks. The ongoing miniaturization of photonic structures due to the availability of sophisticated nanofabrication has provided huge opportunities for physical research of novel phenomena in nanophotonic systems and quantum technological applications. These include quantum information processing such as quantum computing, quantum communication, quantum metrology, and quantum sensing. Quantum communication is already quite advanced with several quantum key distribution (QKD) systems commercially available offering absolute security. Photonic quantum technologies are enabling breakthroughs in experimental tests of quantum physics and are close to practical applications of quantum technologies in imaging, secure communications, ultrasensitive metrology and quantum computing. There are several advantages to encoding a quantum bit onto a photon: the preparation and manipulation of single photons is easy, and the photonic qubit is a natural candidate for quantum imaging and communications. Furthermore, the qubit can be encoded in various degrees of freedom such as on the position, phase, time-bin, energy, angular momentum, polarization of the photon—or a combination of these. This flexibility leads to enormous opportunities for scientific exploration and in practical applications. This special issue on “Photonic Quantum Technologies” presents selected papers on photonic quantum systems from leading groups in the world. The articles represent a flavor of the cutting-edge research that is going on in this area. The studied material systems range from molecules to semiconductor quantum emitters. A major challenge is the design of telecom photon emitters with high efficiency, good yield, spectral purity, stability and the possibility of mass manufacture. We believe that this Special Issue will provide an overview of recent exciting developments in the field of photonic quantum technologies, serving to keep researchers up-to-date with the latest progress and spurring on new concepts in this field.
Light is composed of photons, but only recently has it become relatively common, albeit still challenging, to manipulate single or few photons. This special issue presents state-of-the-art theory and experiment concerning the production of few-photon states of light, processing the states, and performing highly sensitive low-noise detection at the single-photon level. In many cases, the connection between photon manipulation and quantum information science is apparent. Few-photon sources have applications for quantum cryptography and optical quantum computing. Photon processing can involve producing entangled states of light as resources for quantum information processing, performing optical gates for example by interferometry, and realizing quantum memory to store information and synchronize gate operations. Ultimately the photons are carriers of information, and this information is acquired by detectors, which have to be reliable, highly sensitive, fast and operate with low noise. Many of these photon generation, processing and detection issues are exemplified in the papers presented here. This special issue thus serves as a compendium of recent results in the area of few-photon optics, covering the development of new sources, the characterization and improvement of detection schemes, and the analysis and proposal of processing protocols. We trust that this issue will serve as a valuable resource for the community for years to come.
After the preceding four introductory chapters, we are finally ready to discuss optical quantum information processing. While in principle quantum communication can be implemented with atoms or electrons, in practice the implementation of choice for long-distance quantum communication will almost certainly be optical. In this chapter, we will develop some of the key topics in quantum communication with single photons, and we will discuss continuous-variable quantum communication in Chapter 8. In some ways, quantum communication is the most technologically advanced part of quantum information processing. Most protocols in this chapter, such as teleportation and cryptography, have been convincingly demonstrated in the lab, and there are already several commercial organizations that sell cryptographic systems based on quantum mechanical principles. In particular, the promise of secure communication with quantum cryptography is one of the driving forces behind the development of single-photon sources and detectors. In this chapter, we first construct several optical representations of the qubit, including polarized photons and time-bin encoding. Since the single-photon implementation of a qubit is central to optical quantum information processing, this constitutes a large part of the chapter. In Section 5.2 we discuss quantum teleportation and entanglement swapping with single photons. We also present a method for entanglement distillation using entanglement swapping. In Section 5.3 we introduce decoherence-free subspaces, which can be used to reduce noise in the transmission of quantum information, and even establish communication channels in the absence of shared reference frames.
Abstract As a foundational technology in quantum information science (QIS), the manipulation and detection of quantum spins enable precise control, opening new avenues for significant advancements. Early research focused on macroscopic ensemble quantum systems, where electron spin resonance is used to coherently address electron spins within different ensembles. However, the low initialization efficiency and readout sensitivity of such systems hinder their further development. Unlocking the full potential of quantum technology, it is essential to achieve single‐spin precision. Various quantum systems, including color centers, quantum dots, atoms, and molecules, have been demonstrated the capability for single‐spin manipulation. The successful control of spin quantum states in these systems through diverse techniques has paved the way for new ideas and boundless possibilities in QIS. This review provides a comprehensive overview of recent significant advances in the field and explores future development directions, aiming to shed light on the ongoing evolution of QIS.
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.