Quantum cryptography is arguably the fastest growing area in quantum
information science. Novel theoretical protocols are designed on a regular
basis, security proofs are constantly improving, and experiments are
gradually moving from proof-of-principle lab demonstrations to in-field
implementations and technological prototypes. In this paper, we provide
both a general introduction and a state-of-the-art description of the
recent advances in the field, both theoretical and experimental. We start
by reviewing protocols of quantum key distribution based on discrete
variable systems. Next we consider aspects of device independence,
satellite challenges, and protocols based on continuous-variable systems.
We will then discuss the ultimate limits of point-to-point private
communications and how quantum repeaters and networks may overcome these
restrictions. Finally, we will discuss some aspects of quantum
cryptography beyond standard quantum key distribution, including quantum
random number generators and quantum digital signatures.

/pdf/advances-in-quantum-cryptography-47uxe1y6u2.pdf

Advances in quantum cryptography

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 cryptography is arguably the fastest growing area in quantum information science. Novel theoretical protocols are designed on a regular basis, security proofs are constantly improving, and experiments are gradually moving from proof-of-principle lab demonstrations to in-field implementations and technological prototypes. In this review, we provide both a general introduction and a state of the art description of the recent advances in the field, both theoretically and experimentally. We start by reviewing protocols of quantum key distribution based on discrete variable systems. Next we consider aspects of device independence, satellite challenges, and high rate protocols based on continuous variable systems. We will then discuss the ultimate limits of point-to-point private communications and how quantum repeaters and networks may overcome these restrictions. Finally, we will discuss some aspects of quantum cryptography beyond standard quantum key distribution, including quantum data locking and quantum digital signatures.

/pdf/advances-in-quantum-cryptography-2uyo6whymh.pdf

Advances in Quantum Cryptography

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

Quantum teleportation of single-photon qubits from a ground observatory to a satellite in low-Earth orbit via an uplink channel is achieved with a fidelity that is well above the classical limit. The laws of quantum physics give rise to protocols for ultra-secure cryptography and quantum communications. However, to be useful in a global network, these protocols will have to function with satellites. Extending existing protocols to such long distances poses a tremendous experimental challenge. Researchers led by Jian-Wei Pan present a pair of papers in this issue that take steps toward a global quantum network, using the low-Earth-orbit satellite Micius. They demonstrate satellite-to-ground quantum key distribution, an integral part of quantum cryptosystems, at kilohertz rates over 1,200 kilometres, and report quantum teleportation of a single-photon qubit over 1,400 kilometres. Quantum teleportation is the transfer of the exact state of a quantum object from one place to another, without physical travelling of the object itself, and is a central process in many quantum communication protocols. These two experiments suggest that Micius could become the first component in a global quantum internet. An arbitrary unknown quantum state cannot be measured precisely or replicated perfectly1. However, quantum teleportation enables unknown quantum states to be transferred reliably from one object to another over long distances2, without physical travelling of the object itself. Long-distance teleportation is a fundamental element of protocols such as large-scale quantum networks3,4 and distributed quantum computation5,6. But the distances over which transmission was achieved in previous teleportation experiments, which used optical fibres and terrestrial free-space channels7,8,9,10,11,12, were limited to about 100 kilometres, owing to the photon loss of these channels. To realize a global-scale ‘quantum internet’13 the range of quantum teleportation needs to be greatly extended. A promising way of doing so involves using satellite platforms and space-based links, which can connect two remote points on Earth with greatly reduced channel loss because most of the propagation path of the photons is in empty space. Here we report quantum teleportation of independent single-photon qubits from a ground observatory to a low-Earth-orbit satellite, through an uplink channel, over distances of up to 1,400 kilometres. To optimize the efficiency of the link and to counter the atmospheric turbulence in the uplink, we use a compact ultra-bright source of entangled photons, a narrow beam divergence and high-bandwidth and high-accuracy acquiring, pointing and tracking. We demonstrate successful quantum teleportation of six input states in mutually unbiased bases with an average fidelity of 0.80 ± 0.01, well above the optimal state-estimation fidelity on a single copy of a qubit (the classical limit)14. Our demonstration of a ground-to-satellite uplink for reliable and ultra-long-distance quantum teleportation is an essential step towards a global-scale quantum internet.

Ground-to-satellite quantum teleportation

The utility model discloses a device with a self-calibration function for measuring parallelism of a laser receiving and transmitting optical axis, comprising a simulation echo source, a two-dimension light beam directing scanning unit, a system self-checking optical path assembly, a receiving optical axis offset adjusting unit and a light beam supervision and data processing unit The self-calibration type device of the utility model not only can satisfy measurement of a coaxial type or a co-optical-path type laser receiving and transmitting coaxiality, but also can perform measurement for the parallelisms of non coaxial optical axises of different offsets The self-calibration type device of the utility model has the beneficial effects that: the device has the self-calibration function and can monitor states of the device at any time; and the device also has an optical axis offset adjusting function and can satisfy optical axis parallelism testing of the different offsets Therefore, the device of the utility model is especially suitable for calibration and testing of a laser active photoelectric instrument

Self-calibration type device for measuring parallelism of laser receiving and transmitting optical axis

The invention discloses a self-calibration device and method for measuring the parallelism of laser receiving and transmitting optical axes. The device comprises an analog echo source, a two-dimensional light beam pointing scanning unit, a system self-detection light path component, a receiving optical axis offset regulating unit and a light beam monitoring and data processing unit. The device and the method can meet measurement of coaxial or common-path laser receiving and transmitting coaxiality and can be used for measuring the parallelism of non-coaxial optical axes of different offsets. The device is mainly characterized by having a self-calibration function so that the state of the device can be monitored at any time and an optical axis offset adjusting function so that the parallelism test of optical axels of different offsets can be met, thereby being particularly suitable for calibrating and testing a laser positive photoelectric instrument.

Self-calibration device and method for measuring parallelism of laser receiving and transmitting optical axes

Coupling of scattering light in space into a single-mode fiber is a key technology in the process of developing the Laser Doppler Velocity Radar. In order to make sure that the radar can get the longest detect distance, we have discussed the method of determining the key parameters of the receiver/transmitter common-path optical system based on the singlemode fiber coupling, from the points of laser radar detect distance equation, principle of single-mode fiber coupling efficiency reach maximum, and considerations of laser transmitter. In the engineering development process of an laser Doppler velocity radar, we have designed the optical system which fulfills the requirement of detect distance bigger than 3Km, then given out simulation results of single-mode fiber coupling efficiency versus lens spherical aberration, fiber defocus and fiber tilt. The tolerance analysis result indicates that the coupling efficiency will bigger than 52% under the usual levels of optical manufacture and assembly. At last we designed specialized equipment for testing the single-mode fiber coupling efficiency of the system we developed, the results showed that after mechanical vibration and thermal experiments, the single-mode coupling efficiency is 55.9%, and in the operating temperature range of 20±3°C, the lowest coupling efficiency is 45%, which is still bigger than 35% as the system required to make sure the detect distance bigger than 3Km.

Yaowu Kuang

Papers

Ground-to-satellite quantum teleportation

Self-calibration type device for measuring parallelism of laser receiving and transmitting optical axis

Self-calibration device and method for measuring parallelism of laser receiving and transmitting optical axes

Design and engineering development of single-mode fiber coupling system for Laser Doppler Velocity Radar