Fast and simple qubit-based synchronization for quantum key distribution

We propose Qubit4Sync, a synchronization method for Quantum Key Distribution (QKD) setups, based on the same qubits exchanged during the protocol and without requiring additional hardware other than the one necessary to prepare and measure the quantum states. Our approach introduces a new cross-correlation algorithm achieving the lowest computational complexity, to our knowledge, for high channel losses. We tested the robustness of our scheme in a real QKD implementation

All-fiber self-compensating polarization encoder for Quantum Key Distribution

Quantum Key Distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used in QKD implementations along free-space, optical fiber and satellite-based links. However, the polarization encoders used in such implementations are unstable, expensive, complex and can even exhibit side-channels that undermine the security of the implemented protocol. Here we propose a self-compensating polarization encoder based on a Lithium Niobate phase modulator inside a Sagnac interferometer and implement it using only standard telecommunication commercial off-the-shelves components (COTS). Our polarization encoder combines a simple design and high stability reaching an intrinsic quantum bit error rate as low as 0.2%. Since realization is possible from the 800 nm to the 1550 nm band by using COTS, our polarization modulator is a promising solution for free-space, fiber and satellite-based QKD.Comment: REVTeX, 5 pages, 4 figure

Stable, low-error and calibration-free polarization encoder for free-space quantum communication

Polarization-encoded free-space Quantum Communication requires a quantum state source featuring fast polarization modulation, long-term stability and a low intrinsic error rate. Here we present a source based on a Sagnac interferometer and composed of polarization maintaining fibers, a fiber polarization beam splitter and an electro-optic phase modulator. The system generates predetermined polarization states with a fixed reference frame in free-space that does not require calibration neither at the transmitter nor at the receiver. In this way we achieve long-term stability and low error rates. A proof-of-concept experiment is also reported, demonstrating a Quantum Bit Error Rate lower than 0.2% for several hours without any active recalibration of the devices.Comment: 6 pages, 2 figure

Extending Wheeler's delayed-choice experiment to Space

Gedankenexperiments have consistently played a major role in the development of quantum theory. A paradigmatic example is Wheeler's delayed-choice experiment, a wave-particle duality test that cannot be fully understood using only classical concepts. Here, we implement Wheeler's idea along a satellite-ground interferometer which extends for thousands of kilometers in Space. We exploit temporal and polarization degrees of freedom of photons reflected by a fast moving satellite equipped with retro-reflecting mirrors. We observed the complementary wave-like or particle-like behaviors at the ground station by choosing the measurement apparatus while the photons are propagating from the satellite to the ground. Our results confirm quantum mechanical predictions, demonstrating the need of the dual wave-particle interpretation, at this unprecedented scale. Our work paves the way for novel applications of quantum mechanics in Space links involving multiple photon degrees of freedom.Comment: 4 figure

Postselection-loophole-free Bell violation with genuine time-bin entanglement

Entanglement is an invaluable resource for fundamental tests of physics and the implementation of quantum information protocols such as device-independent secure communications. In particular, time-bin entanglement is widely exploited to reach these purposes both in free-space and optical fiber propagation, due to the robustness and simplicity of its implementation. However, all existing realizations of time-bin entanglement suffer from an intrinsic postselection loophole, which undermines their usefulness. Here, we report the first experimental violation of Bell's inequality with "genuine" time-bin entanglement, free of the postselection loophole. We introduced a novel function of the interferometers at the two measurement stations, that operate as fast synchronized optical switches. This scheme allowed to obtain a postselection-loophole-free Bell violation of more than nine standard deviations. Since our scheme is fully implementable using standard fiber-based components and is compatible with modern integrated photonics, our results pave the way for the distribution of genuine time-bin entanglement over long distances.Comment: RevTe

Simple Quantum Key Distribution with qubit-based synchronization and a self-compensating polarization encoder

Quantum Key Distribution (QKD) relies on quantum communication to allow distant parties to share a secure cryptographic key. Widespread adoption of QKD in current telecommunication networks will require the development of simple, low cost and stable systems. However, current QKD implementations usually include additional hardware that perform auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a polarization-based QKD system operating at 1550 nm that performs synchronization and polarization compensation by exploiting only the hardware already needed for the quantum communication task. Polarization encoding is performed by a self-compensating Sagnac loop modulator which exhibits high temporal stability and the lowest intrinsic quantum bit error rate reported so far.The QKD system was tested over a fiber-optic link, demonstrating tolerance up to about 40 dB of channel losses. Thanks to its reduced hardware requirements and the quality of the source, this work represents an important step towards technologically mature QKD systems.Comment: 8 pages, 4 figure

Resource-effective Quantum Key Distribution: a field-trial in Padua city center

Field-trials are of key importance for novel technologies seeking commercialization and wide-spread adoption. This is certainly also the case for Quantum Key Distribution (QKD), which allows distant parties to distill a secret key with unconditional security. Typically, QKD demonstrations over urban infrastructures require complex stabilization and synchronization systems to maintain a low Quantum Bit Error (QBER) and high secret key rates over time. Here we present a field-trial which exploits a low-complexity self-stabilized hardware and a novel synchronization technique, to perform QKD over optical fibers deployed in the city center of Padua, Italy. In particular, two techniques recently introduced by our research group are evaluated in a real-world environment: the iPOGNAC polarization encoder was used for the preparation of the quantum states, while the temporal synchronization was performed using the Qubit4Sync algorithm. The results here presented demonstrate the validity and robustness of our resource-effective QKD system, that can be easily and rapidly installed in an existing telecommunication infrastructure, thus representing an important step towards mature, efficient and low-cost QKD systems.Comment: 5 pages, 3 figure

Advances in Quantum Communications for Fundamental Studies and Applications

Quantum Communication is a groundbreaking technology that promises to revolutionize our current telecommunication systems. However, being a relatively new field, many challenges are yet to be confronted. This Doctoral thesis is dedicated to the advancement of Quantum Communications for fundamental studies and applications. Here, satellite-based, free-space and optical-fiber links are exploited to test and develop different Quantum Communication protocols. Different technologies, in particular free-space optics, fiber-based components and silicon photonic integrated circuits, are adopted to manipulate the quantum states of light. Different types of studies are performed ranging from fundamental tests of physics, to technological advancements, up till the development of fully-functioning Quantum Key Distribution systems. The work described in this thesis, I hope, represents a substantial contribution in the effort to transition Quantum Communications from the laboratories to a wide-spread deployment in future telecommunication infrastructures.La Comunicazione Quantistica è una tecnologia all'avanguardia che promette di rivoluzionare i nostri sistemi di telecomunicazione. Ciononostante, essendo un campo di ricerca relativamente nuovo, molte sfide devono essere ancora affrontate. Codesta tesi è dedicata allo sviluppo delle Comunicazioni Quantistiche per studi fondamentali e applicazioni. Vengono sfruttati collegamenti satellitari, a spazio libero e in fibra ottica per collaudare e sviluppare protocolli di Comunicazione Quantistica. Diverse tecnologie, in particolare, ottica a spazio libero, componenti in fibra ottica e circuiti di fotonica integrata in silicio, vengono adoperati per manipolare gli stati quantistici della luce. Sono stati condotti diverse tipologie di studi, da test fondamentali di fisica, a sviluppi tecnologici, fino alla creazione di sistemi completamente funzionanti di Scambio Quantistico di Chiave. Certamente, un obiettivo futuro consiste nel proiettare le Comunicazioni Quantistiche dai banchi ottici dei laboratori a una larga adozione nelle infrastrutture di telecomunicazione. Spero, ergo, che i lavori descritti in questa tesi ne possano rappresentare un contributo sostanzioso