Photonic Quantum Information Processing: a Review

Reports on Progress in Physics REVIEW Photonic quantum information processing: a review To cite this article: Fulvio Flamini et al 2019 Rep. Prog. Phys. 82 016001 View the article online for updates and enhancements. This content was downloaded from IP address 142.132.1.147 on 13/11/2018 at 10:24 IOP Reports on Progress in Physics Reports on Progress in Physics Rep. Prog. Phys. Rep. Prog. Phys. 82 (2019) 016001 (32pp) https://doi.org/10.1088/1361-6633/aad5b2 82 Review 2019 Photonic quantum information processing: © 2018 IOP Publishing Ltd a review RPPHAG Fulvio Flamini , Nicolò Spagnolo and Fabio Sciarrino1 016001 Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy F Flamini et al E-mail: [email protected] Received 1 June 2017, revised 10 July 2018 Photonic quantum information processing: a review Accepted for publication 25 July 2018 Published 13 November 2018 Printed in the UK Corresponding Editor Professor Maciej Lewenstein Abstract ROP Photonic quantum technologies represent a promising platform for several applications, ranging from long-distance communications to the simulation of complex phenomena. 10.1088/1361-6633/aad5b2 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 1361-6633 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 1 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. Keywords: quantum information, quantum technologies, photonic quantum computing and simulation, quantum cryptography, quantum communication, integrated photonics (Some figures may appear in colour only in the online journal) Introduction process information: they propagate fast, do not interact with the environment and can be easily manipulated. Moreover, Nearly thirty years ago, a short period in the history of modern future advances will benefit from the technological infra- physics, quantum algorithms were presented to the scientific structures and know-how already developed in the classical community as ingenious strategies to realize tasks thought to context, thus further encouraging to proceed in this direc- be hard with classical approaches. Significant advances in this tion. While this aspect is particularly relevant for instance for sense sprouted within only a few years from different areas of quantum communication, where the availability of fiber net- research in cryptography [1], algorithms [2], simulation [3] works can play a key role, milestone achievements have been and communication [4], ever since placing a quantum- before reached also in the scope of information processing, raising the name of each field. This shift of paradigm required think- a bridge between the linear-optical platform and information ing out of the box and starting to consider quantum mechan- processing [7]. ics not only as a stage to investigate but, interestingly, as a Encouraged by these considerations, we will review the resource for practical purposes. state of the art in photonic quantum information to provide the Various platforms have been investigated for implementing reader with a broad perspective in a unified framework. This quantum tasks [5] (see table 1). Among them, photons natu- review article is structured in three chapters, where we attempt rally fit this quantum revolution [6] as an effective system to to present the numerous works in a convenient classification, even though most of them easily overlap. Chapter 1 focuses on 1 Author to whom any correspondence should be addressed. the single-photon encoding schemes and on the technological 1361-6633/19/016001+32$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK Rep. Prog. Phys. 82 (2019) 016001 Review state of the art for experimental implementations, namely 1.1.1. Encoding in angular momentum. The angular momen- single-photon sources, integrated circuits and single-photon tum of light concerns the rotation of the electromagnetic field detectors. Chapter 2 delves into the field of quantum com- vector, a dynamic quantity that influences light-matter inter- munication, describing various theoretical schemes developed action. Two forms of angular momentum exist: spin angular to this purpose, overviewing the developments of quantum momentum (SAM), associated to its circular polarization, and repeaters and distributed blind quantum computing protocols orbital angular momentum (OAM), associated to the spatial for photonic quantum networks. This chapter also presents structure of the wavefront. While the two mechanisms cannot recent significant achievements in long-distance communica- be separated in the general case of focused or divergent light tion and quantum key distribution. Finally, Chapter 3 presents beams, their nature becomes manifest for sufficiently colli- the latest achievements in photonic quantum simulation, dis- mated ones [17]. In the latter case, the total angular momen- cussing the potentialities of single- and multi-photon quantum tum J takes the simpler form walks for quantum computing and simulation in the domain of quantum chemistry. For the sake of clarity and completeness, we will also refer the interested reader to more specialized 3 2 2 ∂Ej J d r EL ER ıEj∗ (1) literature and review articles on each topic. ∝ | | −| | − ∂φ j:x,y,z where the first term, the SAM contribution, corresponds 1. Implementing quantum information with single to the familiar left (L) and right (R) circular polarizations photons while the second, associated to the OAM, describes wave- fronts with the typical helical profiles and is thus related to Quantum information can be encoded in a variety of physi- light spatial distribution. The paraxial approximation is the cal systems [5], ranging from photonic states, solid state natural framework for light manipulation and encoding [18], devices [8], atomic [9] or nuclear [10] spin systems to thus in the following sections we will focus on this regime. electrons [11], Josephson junctions and superconducting For specialized reviews, we refer the reader to [19, 20]. devices [12] (see table 1). Photonic states present several advantages with respect to other platforms, due to the lack Polarization. Polarization qubits represent a common and of interaction with the external environment that thus cor- practical way to encode quantum information for most appli- responds to long decoherence times. As we discuss in the cations, thanks to the ease of generation (see section 1.2.1) next sections, this feature is particularly relevant in appli- and to the availability of effective tools for manipulation. cations such as long-distance quantum communications. Indeed, in the limit of paraxial waves of equation (1), the In this section we will review the encoding of quantum SAM component interacts with anisotropic transparent sys- information in single photons with a focus on discrete- tems such as birefringent crystals, which are consequently variable approach in the visible domain. For an overview of widely employed for polarization manipulation. quantum information processing in the microwave regime, A qubit encoded in polarization is usually written as the interested reader can refer to [13 15] for specialized, – Ψ = α H + β V , where H and V stand for horizontal and comprehensive reviews on the field, while for continuous- |vertical polarization,| | respectively, and variable quantum information we refer the reader to [16]. Section 1.1 reviews the main degrees of freedom employed ∞ ıω t p = dk f (k) e− k aˆ†(k, p) 0 (2) to encode qubits or qudits in this physical platform, while | | section 1.2 summarizes the main recently-developed tech- −∞ nological platforms to generate, manipulate and detect sin- where p =(H, V), f (.) is a wave packet mode function and gle-photon states. aˆ†(k, p) is the creation operator for a photon with momentum k and polarization p [18]. Polarization qubits can be as well expressed in the diagonal basis = 1 ( H V ) or with 1.1. Single-photon encoding |± √2 | ±| left and right circular polarizations as L/ = 1 ( H ı V ). R √2 | ± | Photon-based quantum information uses the degrees of Together, the three pairs of states form a set of mutually unbi- freedom of light (see figure 1), suitable quantities related ased bases encoded in polarization [21], at the core of several to propagation directions (path encoding), to spin angular applications that we will overview in the following sections. momentum (polarization encoding), to light spatial distri- Polarization encoding has always played an important role bution (orbital angular momentum encoding) and to time in a significant number of investigations in quantum informa- (time-bin and time-frequency encoding). All encoding strate- tion, ranging from quantum simulation [22, 23] to quant um gies present their own advantages and weaknesses, and can computation [24–28] and communication [29, 30]. Its ubiq- be combined in a hybrid configuration. In this section we uitous presence in quantum information processing has been will review each of them

Load more