Alberto Carrasco-Casado

Satellite-to-ground quantum-limited communication using a 50-kg-class microsatellite

The feasibility of satellite-to-ground quantum communication is demonstrated by using a microsatellite in low-Earth orbit. The quantum states are discriminated by a ground receiver with four photon-counters with a quantum bit error rate below 5%.

LEO-to-ground polarization measurements aiming for space QKD using Small Optical TrAnsponder (SOTA)

Quantum communication, and more specifically Quantum Key Distribution (QKD), enables the transmission of information in a theoretically secure way, guaranteed by the laws of quantum physics. Although fiber-based QKD has been readily available since several years ago, a global quantum communication network will require the development of space links, which remains to be demonstrated. NICT launched a LEO satellite in 2014 carrying a lasercom terminal (SOTA), designed for in-orbit technological demonstrations. In this paper, we present the results of the campaign to measure the polarization characteristics of the SOTA laser sources after propagating from LEO to ground. The most-widely used property for encoding information in free-space QKD is the polarization, and especially the linear polarization. Therefore, studying its behavior in a realistic link is a fundamental step for proving the feasibility of space quantum communications. The results of the polarization preservation of two highly-polarized lasers are presented here, including the first-time measurement of a linearly-polarized source at λ = 976 nm and a circularly-polarized source at λ = 1549 nm from space using a realistic QKD-like receiver, installed in the Optical Ground Station at the NICT Headquarters, in Tokyo, Japan.

Space QUEST mission proposal: experimentally testing decoherence due to gravity

Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum correlations, such as entanglement, may exhibit different behavior to purely classical correlations in curved space. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph et al [5] and Ralph and Pienaar [1], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agencys Space QUEST (Space-Quantum Entanglement Space Test) mission, and study the feasibility of the mission scheme.

In-orbit verification of small optical transponder (SOTA): evaluation of satellite-to-ground laser communication links

Research and development of space optical communications is conducted in the National Institute of Information and Communications Technology (NICT). The NICT developed the Small Optical TrAnsponder (SOTA), which was embarked on a 50kg-class satellite and launched into a low earth orbit (LEO). The space-to-ground laser communication experiments have been conducted with the SOTA. Atmospheric turbulence causes signal fadings and becomes an issue to be solved in satellite-to-ground laser communication links. Therefore, as error-correcting functions, a Reed-Solomon (RS) code and a Low-Density Generator Matrix (LDGM) code are implemented in the communication system onboard the SOTA. In this paper, we present the in-orbit verification results of SOTA including the characteristic of the functions, the communication performance with the LDGM code via satellite-to-ground atmospheric paths, and the link budget analysis and the comparison between theoretical and experimental results.

Correction of Wavefront Tilt Caused by Atmospheric Turbulence Using Quadrant Detectors for Enabling Fast Free-Space Quantum Communications in Daylight

Low photon-level signals used in most free-space quantum communication systems require a narrow field of view in the receiver to minimize the amount of background noise coupled into the single photon detectors. This can be achieved through beam tracking techniques, which compensate atmospheric effects, such as beam wander, in the receiver, reducing the long-term beam area. However, reducing the diameter of this area below a few microns, typically necessary to achieve a low level of solar background noise and successful daylight quantum transmission, require fine tracking precision and diffraction-limited optics. We demonstrate that this can be done with standard voice-coil fast steering mirrors and cheap commercially-available quadrant detectors. Two correcting strategies (open and closed loop) are experimentally tested and analyzed for their applicability in metropolitan (~km range) free-space quantum communications. The area containing the random fluctuations of the beam centroid caused by atmospheric turbulence at the focal plane of the receiver was reduced by a factor of 4 with an open-loop configuration, and up to a factor of nine with a closed loop configuration. This is equivalent to a reduction in the quantum bit error rate caused by background solar noise of up to one order of magnitude, which, combined with spectral filtering techniques, enable the possibility of fast daylight quantum key distribution.

QKD from a microsatellite: the SOTA experience

The transmission and reception of polarized quantum-limited signals from space is of capital interest for a variety of fundamental-physics experiments and quantum-communication protocols. Specifically, Quantum Key Distribution (QKD) deals with the problem of distributing unconditionally-secure cryptographic keys between two parties. Enabling this technology from space is a critical step for developing a truly-secure global communication network. The National Institute of Information and Communications Technology (NICT, Japan) performed the first successful measurement on the ground of a quantum-limited signal from a satellite in experiments carried out on early August in 2016. The SOTA (Small Optical TrAnsponder) lasercom terminal onboard the LEO satellite SOCRATES (Space Optical Communications Research Advanced Technology Satellite) was utilized for this purpose. Two non-orthogonally polarized signals in the ~800-nm band and modulated at 10 MHz were transmitted by SOTA and received in the single-photon regime by using a 1-m Cassegrain telescope on a ground station located in an urban area of Tokyo (Japan). In these experiments, after compensating the Doppler effect induced by the fast motion of the satellite, a QKD-enabling QBER (Quantum Bit Error Rate) below 5% was measured with estimated key rates in the order of several Kbit/s, proving the feasibility of quantum communications in a real scenario from space for the first time.

Design status of the development for a GEO-to-ground optical feeder link, HICALI

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some of them with capacities in excess of 100 Gbps. However, as the radio bandwidth resources become exhausted, high-speed optical communication can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has more than 20 years of experience in R&D of space laser communications, with important milestones like ETS-VI (Engineering Test Satellite VI), OICETS, and SOTA. We are currently developing a laser-communication terminal called “HICALI”, which goal is to achieve 10 Gbps-class space communications in the 1.5-μm band between Optical Ground Stations (OGSs) and a next generation high-throughput satellite (called ETS-IX) with a hybrid communication system using radio and optical frequencies, which will be launched into a geostationary orbit in 2021. The development of test and a breadboard model for HICALI has been conducted for several years and we are now carrying out an engineering model as well as designing the OGSs segment. In this paper, we describe concepts and current design status of the HICALI system.

LEO-to-ground optical communications using SOTA (Small Optical TrAnsponder) – Payload verification results and experiments on space quantum communications

Abstract Free-space optical communications have held the promise of revolutionizing space communications for a long time. The benefits of increasing the bitrate while reducing the volume, mass and energy of the space terminals have attracted the attention of many researchers for a long time. In the last few years, more and more technology demonstrations have been taking place with participants from both the public and the private sector. The National Institute of Information and Communications Technology (NICT) in Japan has a long experience in this field. SOTA (Small Optical TrAnsponder) was the last NICT space lasercom mission, designed to demonstrate the potential of this technology applied to microsatellites. Since the beginning of SOTA mission in 2014, NICT regularly established communication using the Optical Ground Stations (OGS) located in the Headquarters at Koganei (Tokyo) to receive the SOTA signals, with over one hundred successful links. All the goals of the SOTA mission were fulfilled, including up to 10-Mbit/s downlinks using two different wavelengths and apertures, coarse and fine tracking of the OGS beacon, space-to-ground transmission of the on-board-camera images, experiments with different error correcting codes, interoperability with other international OGS, and experiments on quantum communications. The SOTA mission ended on November 2016, more than doubling the designed lifetime of 1-year. In this paper, the SOTA characteristics and basic operation are explained, along with the most relevant technological demonstrations.

Ground-based gamma-ray telescopes as ground stations in deep-space lasercom

As the amount of information to be transmitted from deep-space rapidly increases, the radiofrequency technology bas become a bottleneck in space communications. RF is already limiting the scientific outcome of deep-space missions and could be a significant obstacle in the developing of manned missions. Lasercom holds the promise to solve this problem, as it will considerably increase the data rate while decreasing the energy, mass and volume of onboard communication systems. A key strategy to optimize the lasercom technology is to shift the complexity from the space systems towards the ground systems. In RF deep-space communications, where the received power is the main limitation, the traditional approach to boost the data throughput has been increasing the receivers aperture, e.g. the 70-m antennas in the NASAs Deep Space Network. Optical communications also can benefit from this strategy, thus 10-m class telescopes have typically been suggested to support future deep-space links. However, the cost of big telescopes increase exponentially with their aperture, and new ideas are needed to optimize this ratio. Here, the use of ground-based gamma-ray telescopes, known as Cherenkov telescopes, is suggested. These are optical telescopes designed to maximize the receivers aperture at a minimum cost with some relaxed requirements. As they are used in an array configuration and multiple identical units need to be built, each element of the telescope is designed to minimize its cost. Furthermore, the native array configuration would facilitate the joint operation of Cherenkov and lasercom telescopes. These telescopes offer very big apertures, ranging from several meters to almost 30 meters, which could greatly improve the performance of optical ground stations. The key elements of these telescopes have been studied applied to lasercom, reaching the conclusion that it could be an interesting strategy to include them in the future development of an optical deep-space network.