High dimensional measurement device independent quantum key distribution on two dimensional subspaces

Quantum key distribution (QKD) provides ultimate cryptographic security based on the laws of quantum mechanics. For point-to-point QKD protocols, the security of the generated key is compromised by detector side channel attacks. This problem can be solved with measurement device independent QKD (mdi-QKD). However, mdi-QKD has shown limited performances in terms of the secret key generation rate, due to post-selection in the Bell measurements. We show that high dimensional (Hi-D) encoding (qudits) improves the performance of current mdi-QKD implementations. The scheme is proven to be unconditionally secure even for weak coherent pulses with decoy states, while the secret key rate is derived in the single photon case. Our analysis includes phase errors, imperfect sources and dark counts to mimic real systems. Compared to the standard bidimensional case, we show an improvement in the key generation rate.Comment: 6 pages, 3 figure

Experimental quantum key distribution with finite-key security analysis for noisy channels

In quantum key distribution implementations, each session is typically chosen long enough so that the secret key rate approaches its asymptotic limit. However, this choice may be constrained by the physical scenario, as in the perspective use with satellites, where the passage of one terminal over the other is restricted to a few minutes. Here we demonstrate experimentally the extraction of secure keys leveraging an optimal design of the prepare-and-measure scheme, according to recent finite-key theoretical tight-bounds. The experiment is performed in different channel conditions, and assuming two distinct attack models: individual attacks, or general quantum attacks. The request on the number of exchanged qubits is then obtained as a function of the key size and of the ambient quantum bit error rate. The results indicate that viable conditions for effective symmetric, and even one-time-pad, cryptography are achievable.Comment: 20 pages, 4 figure

Space division multiplexing chip-to-chip quantum key distribution

Quantum cryptography is set to become a key technology for future secure communications. However, to get maximum benefit in communication networks, transmission links will need to be shared among several quantum keys for several independent users. Such links will enable switching in quantum network nodes of the quantum keys to their respective destinations. In this paper we present an experimental demonstration of a photonic integrated silicon chip quantum key distribution protocols based on space division multiplexing (SDM), through multicore fiber technology. Parallel and independent quantum keys are obtained, which are useful in crypto-systems and future quantum network

Quantum communications between Earth and Space

In this society people are always connected, and everyday they manage a lot of personal data also risking to be eavesdropped. Quantum science is one the most promising field of the next years, from quantum computing to quantum communications and above all quantum cryptography. Quantum cryptography is the first commercial application of quantum physics and moreover it results one of the most reliable solution for security problem. Using Quantum Physic law's it is possible to establish secure communications between two users, guaranteeing unconditionally security in the transmission of data. Unfortunately due to the intrinsic losses inside optical fibers, it is not possible to establish a quantum link over $300$ km until quantum repeater will be achievable. The natural extension of terrestrial quantum links are space communications, where however the problems due to environment, temperature and pressure are totally new for quantum devices. The study investigated the possibility of sending quantum signals through atmosphere, in particular trying to realize quantum communications between Earth and Space. In this perspective we used Laser Ranging corner-cubes mounted into satellites to recreate a space quantum link. It was possible to prove that even with high losses, variable attenuation, and high backgorund a quantum key distribution system works, and an unconditionally secure key, needful for encryption, can be generated. With this experiments we demonstrate that not only free-space quantum key distribution is a ready technology, but also that quantum satellite communications is nowadays possible and realizable. Moreover these results open the way to look towards a global space quantum network, where optical station (OGS) could talk with satellite and vice-versa. This work was supported by the Strategic Project QUANTUMFUTURE of University of Padova, by ESAGNSS program and realized in Luxor laboratories in Padova. The principal tests were made at Telespazio (Matera) using the Matera Laser Ranging Observatory and into Thales Alenia Space (Torino)

Experimental Satellite Quantum Communications

Quantum Communications on planetary scale require complementary channels including ground and satellite links. The former have progressed up to commercial stage using fiber-cables, while for satellite links, the absence of terminals in orbit has impaired theirs development. However, the demonstration of the feasibility of such links is crucial for designing space payloads and to eventually enable the realization of protocols such as quantum-key-distribution (QKD) and quantum teleportation along satellite-to-ground or intersatellite links. We demonstrated the faithful transmission of qubits from space to ground by exploiting satellite corner cube retroreflectors acting as transmitter in orbit, obtaining a low error rate suitable for QKD. We also propose a two-way QKD protocol exploiting modulated retroreflectors that necessitates a minimal payload on satellite, thus facilitating the expansion of Space Quantum Communications

Experimental single photon exchange along a space link of 7000 km

Extending the single photon transmission distance is a basic requirement for the implementation of quantum communication on a global scale. In this work we report the single photon exchange from a medium Earth orbit satellite (MEO) at more than 7000 km of slanted distance to the ground station at the Matera Laser Ranging Observatory. The single photon transmitter was realized by exploiting the corner cube retro-reflectors mounted on the LAGEOS-2 satellite. Long duration of data collection is possible with such altitude, up to 43 minutes in a single passage. The mean number of photons per pulse ({\mu}sat) has been limited to 1 for 200 seconds, resulting in an average detection rate of 3.0 cps and a signal to noise ratio of 1.5. The feasibility of single photon exchange from MEO satellites paves the way to tests of Quantum Mechanics in moving frames and to global Quantum Information.Comment: 5 pages, updated versio

Experimental demonstration of the DPTS QKD protocol over a 170 km fiber link

Quantum key distribution (QKD) is a promising technology aiming at solving the security problem arising from the advent of quantum computers. While the main theoretical aspects are well developed today, limited performances, in terms of achievable link distance and secret key rate, are preventing the deployment of this technology on a large scale. More recent QKD protocols, which use multiple degrees of freedom for the encoding of the quantum states, allow an enhancement of the system performances. Here, we present the experimental demonstration of the differential phase-time shifting protocol (DPTS) up to 170 km of fiber link. We compare its performance with the well-known coherent one-way (COW) and the differential phase shifting (DPS) protocols, demonstrating a higher secret key rate up to 100 km. Moreover, we propagate a classical signal in the same fiber, proving the compatibility of quantum and classical light.Comment: 5 pages, 3 figures, journal pape

High-Dimensional Quantum Key Distribution based on Multicore Fiber using Silicon Photonic Integrated Circuits

Quantum Key Distribution (QKD) provides an efficient means to exchange information in an unconditionally secure way. Historically, QKD protocols have been based on binary signal formats, such as two polarisation states, and the transmitted information efficiency of the quantum key is intrinsically limited to 1 bit/photon. Here we propose and experimentally demonstrate, for the first time, a high-dimensional QKD protocol based on space division multiplexing in multicore fiber using silicon photonic integrated lightwave circuits. We successfully realized three mutually unbiased bases in a four-dimensional Hilbert space, and achieved low and stable quantum bit error rate well below both coherent attack and individual attack limits. Compared to previous demonstrations, the use of a multicore fiber in our protocol provides a much more efficient way to create high-dimensional quantum states, and enables breaking the information efficiency limit of traditional QKD protocols. In addition, the silicon photonic circuits used in our work integrate variable optical attenuators, highly efficient multicore fiber couplers, and Mach-Zehnder interferometers, enabling manipulating high-dimensional quantum states in a compact and stable means. Our demonstration pave the way to utilize state-of-the-art multicore fibers for long distance high-dimensional QKD, and boost silicon photonics for high information efficiency quantum communications.Comment: Please see the complementary work arXiv:1610.01682 (2016

Adaptive real time selection for quantum key distribution in lossy and turbulent free-space channels

The unconditional security in the creation of cryptographic keys obtained by quantum key distribution (QKD) protocols will induce a quantum leap in free-space communication privacy in the same way that we are beginning to realize secure optical fiber connections. However, free-space channels, in particular those with long links and the presence of atmospheric turbulence, are affected by losses, fluctuating transmissivity, and background light that impair the conditions for secure QKD. Here we introduce a method to contrast the atmospheric turbulence in QKD experiments. Our adaptive real time selection (ARTS) technique at the receiver is based on the selection of the intervals with higher channel transmissivity. We demonstrate, using data from the Canary Island 143-km free-space link, that conditions with unacceptable average quantum bit error rate which would prevent the generation of a secure key can be used once parsed according to the instantaneous scintillation using the ARTS technique