Experimental relativistic zero-knowledge proofs.

Protecting secrets is a key challenge in our contemporary information-based era. In common situations, however, revealing secrets appears unavoidable; for instance, when identifying oneself in a bank to retrieve money. In turn, this may have highly undesirable consequences in the unlikely, yet not unrealistic, case where the bank's security gets compromised. This naturally raises the question of whether disclosing secrets is fundamentally necessary for identifying oneself, or more generally for proving a statement to be correct. Developments in computer science provide an elegant solution via the concept of zero-knowledge proofs: a prover can convince a verifier of the validity of a certain statement without facilitating the elaboration of a proof at all1. In this work, we report the experimental realization of such a zero-knowledge protocol involving two separated verifier-prover pairs2. Security is enforced via the physical principle of special relativity3, and no computational assumption (such as the existence of one-way functions) is required. Our implementation exclusively relies on off-the-shelf equipment and works at both short (60 m) and long distances (≥400 m) in about one second. This demonstrates the practical potential of multi-prover zero-knowledge protocols, promising for identification tasks and blockchain applications such as cryptocurrencies or smart contracts4.

Gigahertz Detection Rates and Dynamic Photon-Number Resolution with Superconducting Nanowire Arrays.

Superconducting nanowire single-photon detectors (SNSPDs) have enabled the realization of several quantum optics technologies thanks to their high system detection efficiency (SDE), low dark counts, and fast recovery time. However, the widespread use of linear optical quantum computing, quasi-deterministic single-photon sources, and quantum repeaters requires even faster detectors that can also distinguish between different photon-number states. Here, we present an SNSPD array composed of 14 independent pixels, achieving an SDE of 90% in the telecommunications band. By reading each pixel of the array independently, we show detection of telecommunication photons at 1.5 GHz with 45% absolute SDE. We exploit the dynamic photon-number resolution of the array to demonstrate accurate state reconstruction for a wide range of light inputs, including operation with long-duration light pulses, as obtained with some cavity-based sources. We show two-photon and three-photon fidelities of 74% and 57%, respectively, which represent state-of-the-art results for fiber-coupled SNSPDs.

Quantum technologies in space.

Recently, the European Commission supported by many European countries has announced large investments towards the commercialization of quantum technology (QT) to address and mitigate some of the biggest challenges facing today's digital era - e.g. secure communication and computing power. For more than two decades the QT community has been working on the development of QTs, which promise landmark breakthroughs leading to commercialization in various areas. The ambitious goals of the QT community and expectations of EU authorities cannot be met solely by individual initiatives of single countries, and therefore, require a combined European effort of large and unprecedented dimensions comparable only to the Galileo or Copernicus programs. Strong international competition calls for a coordinated European effort towards the development of QT in and for space, including research and development of technology in the areas of communication and sensing. Here, we aim at summarizing the state of the art in the development of quantum technologies which have an impact in the field of space applications. Our goal is to outline a complete framework for the design, development, implementation, and exploitation of quantum technology in space.

High-rate photon pairs and sequential Time-Bin entanglement with Si3N4 microring resonators.

Integrated photonics is increasing in importance for compact, robust, and scalable enabling quantum technologies. This is particularly interesting for developing quantum communication networks, where resources need to be deployed in the field. We exploit photonic chip-based Si3N4 microring resonators to realise a photon pair source with low-loss, high-noise suppression and coincidence rates of 80×103 s-1. A simple photonic noise characterisation technique is presented that distinguishes linear and nonlinear contributions useful for system design and optimisation. We then demonstrate an all-fiber 750 MHz clock-rate sequential Time-Bin entanglement scheme with raw interference visibilities > 98 %.

Correction: Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part I - self-quenched prodrugs.

Correction for 'Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part I - self-quenched prodrugs' by Jordan Bouilloux et al., Photochem. Photobiol. Sci., 2018, 17, 1728-1738.

Correction: Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part II - co-loading of pheophorbide A and black hole quencher.

Correction for 'Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part II - co-loading of pheophorbide A and black hole quencher' by Jordan Bouilloux et al., Photochem. Photobiol. Sci., 2018, 17, 1739-1748.

Secure Quantum Key Distribution over 421 km of Optical Fiber.

We present a quantum key distribution system with a 2.5 GHz repetition rate using a three-state time-bin protocol combined with a one-decoy approach. Taking advantage of superconducting single-photon detectors optimized for quantum key distribution and ultralow-loss fiber, we can distribute secret keys at a maximum distance of 421 km and obtain secret key rates of 6.5 bps over 405 km.

Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part I - self-quenched prodrugs.

Herein, we report the synthesis of a new prodrug system consisting of regioselectively addressable functionalized templates bearing multiple pheophorbide A moieties for use in photodynamic therapy. These coupling reactions were achieved using copper-free "click" chemistry, namely a strain-promoted azide-alkyne cycloaddition. This new design was used to obtain well-defined quenched photosensitizer prodrugs with perfect knowledge of the number and position of loaded photosensitizers, providing structures bearing up to six photosentitizers and two PEG chains. These conjugates are ideally quenched in their native state regarding their fluorescence emission (up to 155 ± 28 times less fluorescent for an hexasubstituted conjugate than a monosubstituted non-quenched reference compound) or singlet oxygen production (decreased 8.7-fold in the best case) when excited. After 2 h of proteolytic activation, the fluorescence emission of a tetrasubstituted conjugate was increased 17-fold compared with the initial fluorescence emission.

Cyclopeptidic photosensitizer prodrugs as proteolytically triggered drug delivery systems of pheophorbide A: part II - co-loading of pheophorbide A and black hole quencher.

Previously, we have shown that the use of a cyclopeptidic carrier could be of great interest for the design of fully characterized prodrugs for further use in photodynamic therapy. In order to further optimize the design, we decided to modify the highly quenched conjugate uPA-cPPP4/5 by co-loading a long-distance fluorescence quencher. For this purpose we tethered two black hole quenchers (BHQ3) together with two pheophorbide A moities onto the same PEGylated backbone and assessed the modified photophysical properties. In addition, to prove the reliability of our concept, we designed two analogues, uPA-cPPQ2+2/5 and CathB-cPPQ2+2/5, by using two different peptidic linkers as substrates for uPA and cathepsin B, respectively. These two conjugates proved to be much more water-soluble than their analogues bearing only Phas. These conjugates are not only highly quenched in their native state with regard to their fluorescence emission (up to 850 ± 287 times less fluorescent for CathB-cPPQ2+2/5 as compared to the unquenched monosubstituted reference uPA-cPPP1/5), but also prevent singlet oxygen production (with a total quenching of the emission when the quenchers are co-loaded with photosensitizers) when the photosentistizers are excited. After proteolytic activation, these conjugates recover their photophysical properties in the same way as occurred for uPA-cPPP4/5, with up to a 120-fold increase in fluorescence emission for uPA-cPPQ2+2/5 after two hours of incubation with uPA.

Heralded amplification of photonic qubits.

We demonstrate postselection free heralded qubit amplification for Time-Bin qubits and single photon states in an all-fibre, telecom-wavelength, scheme that highlights the simplicity, stability and potential for fully integrated photonic solutions. Exploiting high-efficiency superconducting detectors, the gain, fidelity and the performance of the amplifier are studied as a function of loss. We also demonstrate the first heralded single photon amplifier with independent sources. This provides a significant advance towards demonstrating device-independent quantum key distribution as well as fundamental tests of quantum mechanics over extended distances.

Temporal jitter in free-running InGaAs/InP single-photon avalanche detectors.

Negative-feedback avalanche diodes (NFADs) provide a practical solution for different single-photon counting applications requiring free-running mode operation with low afterpulsing probability. Unfortunately, the timing jitter has never been as good as for gated InGaAs/InP single-photon avalanche diodes. Here we report on the timing jitter characterization of InGaAs/InP based NFADs with particular focus on the temperature dependence and the effect of carrier transport between the absorption and multiplication regions. Values as low as 52 ps full-width at half-maximum were obtained at an excess bias voltage of 3.5 V and an operating temperature of around -100°C.

24-Hour Relativistic Bit Commitment.

Bit commitment is a fundamental cryptographic primitive in which a party wishes to commit a secret bit to another party. Perfect security between mistrustful parties is unfortunately impossible to achieve through the asynchronous exchange of classical and quantum messages. Perfect security can nonetheless be achieved if each party splits into two agents exchanging classical information at times and locations satisfying strict relativistic constraints. A relativistic multiround protocol to achieve this was previously proposed and used to implement a 2-millisecond commitment time. Much longer durations were initially thought to be insecure, but recent theoretical progress showed that this is not so. In this Letter, we report on the implementation of a 24-hour bit commitment solely based on timed high-speed optical communication and fast data processing, with all agents located within the city of Geneva. This duration is more than 6 orders of magnitude longer than before, and we argue that it could be extended to one year and allow much more flexibility on the locations of the agents. Our implementation offers a practical and viable solution for use in applications such as digital signatures, secure voting and honesty-preserving auctions.

Probability density function of the electric field in diffuse correlation spectroscopy of human bone in vivo.

Diffuse correlation spectroscopy (DCS) is the technique of choice for non-invasive assessments of human bone blood flow. However, DCS classical algorithms are based on the fundamental assumption that the electric field of the light reaching the DCS photodetector is a zero-mean complex Gaussian variable. The non-validity of this hypothesis might produce inaccurate blood flow estimations. It is shown that for the human tibia, the "Gaussian hypothesis" holds for interoptode distances ≥20 mm. This lower boundary seems to depend on the type of investigated tissue.

Self-testing quantum random number generator.

The generation of random numbers is a task of paramount importance in modern science. A central problem for both classical and quantum randomness generation is to estimate the entropy of the data generated by a given device. Here we present a protocol for self-testing quantum random number generation, in which the user can monitor the entropy in real time. Based on a few general assumptions, our protocol guarantees continuous generation of high quality randomness, without the need for a detailed characterization of the devices. Using a fully optical setup, we implement our protocol and illustrate its self-testing capacity. Our work thus provides a practical approach to quantum randomness generation in a scenario of trusted but error-prone devices.

Absolute calibration of fiber-coupled single-photon detector.

We show a setup for characterising the efficiency of a single-photon-detector absolutely and with a precision better than 1%. Since the setup does not rely on calibrated devices and can be implemented with standard-optic components, it can be realised in any laboratory. Our approach is based on an Erbium-Doped-Fiber-Amplifier (EDFA) radiometer as a primary measurement standard for optical power, and on an ultra-stable source of spontaneous emission. As a proof of principle, we characterise the efficiency of an InGaAs/InP single-photon detector. We verified the correctness of the characterisation with independent measurements. In particular, the measurement of the optical power made with the EDFA radiometer has been compared to that of the Federal Institute of Metrology using a transfer power meter. Our approach is suitable for frequent characterisations of high-efficient single-photon detectors.

Testing the speed of 'spooky action at a distance'.

Correlations are generally described by one of two mechanisms: either a first event influences a second one by sending information encoded in bosons or other physical carriers, or the correlated events have some common causes in their shared history. Quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This reveals itself in correlations that violate Bell inequalities (implying that they cannot be described by common causes) between space-like separated events (implying that they cannot be described by classical communication). Many Bell tests have been performed, and loopholes related to locality and detection have been closed in several independent experiments. It is still possible that a first event could influence a second, but the speed of this hypothetical influence (Einstein's 'spooky action at a distance') would need to be defined in some universal privileged reference frame and be greater than the speed of light. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test over more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed two-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of the influence. For example, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10(-3) times that of the speed of light, then the speed of the influence would have to exceed that of light by at least four orders of magnitude.

A high-speed multi-protocol quantum key distribution transmitter based on a dual-drive modulator.

We propose a novel source based on a dual-drive modulator that is adaptable and allows Alice to choose between various practical quantum key distribution (QKD) protocols depending on what receiver she is communicating with. Experimental results show that the proposed transmitter is suitable for implementation of the Bennett and Brassard 1984 (BB84), coherent one-way (COW) and differential phase shift (DPS) protocols with stable and low quantum bit error rate. This could become a useful component in network QKD, where multi-protocol capability is highly desirable.

Fast single-photon detectors and real-time key distillation enable high secret-key-rate quantum key distribution systems.

Quantum key distribution has emerged as the most viable scheme to guarantee information security in the presence of large-scale quantum computers and, thanks to the continuous progress made in the past 20 years, it is now commercially available. However, the secret key rates remain limited to just over 10 Mbps due to several bottlenecks on the receiver side. Here we present a custom multipixel superconducting nanowire single-photon detector that is designed to guarantee high count rates and precise timing discrimination. Leveraging the performance of the detector and coupling it to fast acquisition and real-time key distillation electronics, we remove two major roadblocks and achieve a considerable increase of the secret key rates with respect to the state of the art. In combination with a simple 2.5-GHz clocked time-bin quantum key distribution system, we can generate secret keys at a rate of 64 Mbps over a distance of 10.0 km and at a rate of 3.0 Mbps over a distance of 102.4 km with real-time key distillation.

MHz rate and efficient synchronous heralding of single photons at telecom wavelengths.

We report on the realization of a synchronous source of heralded single photons at telecom wavelengths with MHz heralding rates and high heralding efficiency. This source is based on the generation of photon pairs at 810 and 1550 nm via Spontaneous Parametric Down Conversion (SPDC) in a 1 cm periodically poled lithium niobate (PPLN) crystal pumped by a 532 nm pulsed laser. As high rates are fundamental for multi-photon experiments, we show that single telecom photons can be announced at 4.4 MHz rate with 45% heralding efficiency. When we focus only on the optimization of the coupling of the heralded photon, the heralding efficiency can be increased up to 80%. Furthermore, we experimentally observe that group velocity mismatch inside long crystals pumped in a pulsed mode affects the spectrum of the emitted photons and their fibre coupling efficiency. The length of the crystal in this source has been chosen as a trade off between high brightness and high coupling efficiency.

Security of distributed-phase-reference quantum key distribution.

Distributed-phase-reference quantum key distribution stands out for its easy implementation with present day technology. For many years, a full security proof of these schemes in a realistic setting has been elusive. We solve this long-standing problem and present a generic method to prove the security of such protocols against general attacks. To illustrate our result, we provide lower bounds on the key generation rate of a variant of the coherent-one-way quantum key distribution protocol. In contrast to standard predictions, it appears to scale quadratically with the system transmittance.