Widespread adoption of Quantum Key Distribution (QKD) in current telecommunication networks will require the development of simple, low cost and stable systems. Current QKD implementations usually include separate sub-systems to implement auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a QKD system with polarization encoding that performs synchronization, polarization compensation and QKD with the same optical setup without requiring any changes or any additional hardware. Polarization encoding is performed by a self-compensating Sagnac loop modulator which exhibits high stability and the lowest intrinsic QBER ever reported by an active polarization source fully implemented using only commercial offthe-shelf components. We tested our QKD system over a fiber-optic channel, tolerating up to 43 dB of total losses and representing an important step towards technologically mature QKD systems. * These authors contributed equally to this work.

New techniques based on weak measurements have recently been introduced to the field of quantum state reconstruction. Some of them allow the direct measurement of each matrix element of an unknown density operator and need only O(d) different operations, compared to d 2 linearly independent projectors in the case of standard quantum state tomography, for the reconstruction of an arbitrary mixed state. However, due to the weakness of these couplings, these protocols are approximated and prone to large statistical errors. We propose a method which is similar to the weak measurement protocols but works regardless of the coupling strength: our protocol is not approximated and thus improves the accuracy and precision of the results with respect to weak measurement schemes. We experimentally apply it to the polarization state of single photons and compare the results to those of preexisting methods for different values of the coupling strength. Our results show that our method outperforms previous proposals in terms of accuracy and statistical errors.

Entanglement is a fundamental resource for quantum information science. However, bipartite entanglement is destroyed when one particle is sharply measured, which occurs in most applications.Here we experimentally show that, if instead of sharp measurements, one performs many sequential unsharp measurements on one particle which are suitably chosen depending on the previous outcomes, then entanglement is preserved and can reveal quantum correlations through measurements on the second particle. Specifically, we observe that pairs of photons entangled in polarization maintain their entanglement and their ability to violate Bell inequalities when one particle undergoes three sequential measurements. This proof-of-principle experiment demonstrates the possibility of repeatedly harnessing a quantum resource.

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The future envisaged global-scale quantum-communication network will comprise various nodes interconnected via optical fibers or free-space channels, depending on the link distance. The free-space segment of such a network should guarantee certain key requirements, such as daytime operation and the compatibility with the complementary telecom-based fiber infrastructure. In addition, space-to-ground links will require the capability of designing light and compact quantum devices to be placed in orbit. For these reasons, investigating available solutions matching all the above requirements is still necessary. Here we present a full prototype for daylight quantum key distribution at 1550 nm exploiting an integrated silicon-photonics chip as state encoder. We tested our prototype in the urban area of Padua (Italy) over a 145 m-long free-space link, obtaining a quantum bit error rate around 0.5% and an averaged secret key rate of 30 kbps during a whole sunny day (from 11:00 to 20:00). The developed chip represents a cost-effective solution for portable free-space transmitters and a promising resource to design quantum optical payloads for future satellite missions.

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.

Quantum nonlocality offers a secure way to produce random numbers: Their unpredictability is intrinsic and can be certified just by observing the statistic of the measurement outcomes, without assumptions on how they are produced. To do this, entangled pairs are generated and measured to violate a Bell inequality with the outcome statistics. However, after a projective quantum measurement, entanglement is entirely destroyed and cannot be used again. This fact poses an upper bound to the amount of randomness that can be produced from each quantum state when projective measurements are employed. Instead, by using weak measurements, some entanglement can be maintained and reutilized, and a sequence of weak measurements can extract an unbounded amount of randomness from a single state as predicted in Phys. Rev. A 95, 020102(R) (2017). We study the feasibility of these weak measurements, analyze the robustness to imperfections in the quantum state they are applied to, and then test them using an optical setup based on polarization-entangled photon pairs. We show that the weak measurements are realizable, but can improve the performance of randomness generation only in close-to-ideal conditions.

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