T. Ferreira da Silva

Proof-of-principle demonstration of measurement-device-independent quantum key distribution using polarization qubits

We perform a proof-of-principle demonstration of the measurement-device-independent quantum key distribution (MDI-QKD) protocol using weak coherent states and polarization-encoded qubits over two optical fiber links of 8.5 km each. Each link was independently stabilized against polarization drifts using a full-polarization control system employing two wavelength-multiplexed control channels. A linear-optics-based polarization Bell-state analyzer was built into the intermediate station, Charlie, which is connected to both Alice and Bob via the optical fiber links. Using decoy-states, a lower bound for the secret-key generation rate of 1.04x10^-6 bits/pulse is computed.

High-dimensional decoy-state quantum key distribution over multicore telecommunication fibers

Author(s): Canas, G; Vera, N; Carine, J; Gonzalez, P; Cardenas, J; Connolly, PWR; Przysiezna, A; Gomez, ES; Figueroa, M; Vallone, G; Villoresi, P; da Silva, T Ferreira; Xavier, GB; Lima, G | Abstract: Multiplexing is a strategy to augment the transmission capacity of a communication system. It consists of combining multiple signals over the same data channel and it has been very successful in classical communications. However, the use of enhanced channels has only reached limited practicality in quantum communications (QC) as it requires the complex manipulation of quantum systems of higher dimensions. Considerable effort is being made towards QC using high-dimensional quantum systems encoded into the transverse momentum of single photons but, so far, no approach has been proven to be fully compatible with the existing telecommunication infrastructure. Here, we overcome such a technological challenge and demonstrate a stable and secure high-dimensional decoy-state quantum key distribution session over a 0.3 km long multicore optical fiber. The high-dimensional quantum states are defined in terms of the multiple core modes available for the photon transmission over the fiber, and the decoy-state analysis demonstrates that our technique enables a positive secret key generation rate up to 25 km of fiber propagation. Finally, we show how our results build up towards a high-dimensional quantum network composed of free-space and fiber based links

Quantum key distribution with untrusted detectors

Side-channel attacks currently constitute the main challenge for quantum key distribution (QKD) to bridge theory with practice. So far two main approaches have been introduced to address this problem, (full) device-independent QKD and measurement-device-independent QKD. Here we present a third solution that might exceed the performance and practicality of the previous two in circumventing detector side-channel attacks, which arguably is the most hazardous part of QKD implementations. Our proposal has, however, one main requirement: the legitimate users of the system need to ensure that their labs do not leak any unwanted information to the outside. The security in the low-loss regime is guaranteed, while in the high-loss regime we already prove its robustness against some eavesdropping strategies.

Few-photon heterodyne spectroscopy.

We perform a high-resolution Fourier transform spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram, between test and reference sources, it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques.

Quantum random number generation enhanced by weak-coherent states interference.

We propose and demonstrate a technique for quantum random number generation based on the random population of the output spatial modes of a beam splitter when both inputs are simultaneously fed with indistinguishable weak coherent states. We simulate and experimentally validate the probability of generation of random bits as a function of the average photon number per input, and compare it to the traditional approach of a single weak coherent state transmitted through a beam-splitter, showing an improvement of up to 32%. The ensuing interference phenomenon reduces the probability of coincident counts between the detectors associated with bits 0 and 1, thus increasing the probability of occurrence of a valid output. A long bit string is assessed by a standard randomness test suite with good confidence. Our proposal can be easily implemented and opens attractive performance gains without a significant trade-off.

Polarization-dependent loss characterization method based on optical frequency beat.

Characterization of the polarization-dependent loss (PDL) of optical components is fundamental for the reliable operation of fiber-optic communication systems. Here we present a method for determining the PDL of optical devices based on optical frequency beating and spectral analysis. Depending on the beat note between components of two orthogonally polarized probe signals modulated at different frequencies, the PDL value and its axis can be determined from a single sweep of an optical spectrum analyzer. Our proposal represents an alternative high-speed option for PDL characterization.

A heralded single-photon source for quantum communications compatible with long-distance optical fibers

This paper reports the implementation of a heralded single-photon source for quantum communications. Pump photons are parametric down-converted into pairs of photons at 810 and 1550 nm inside a non-linear crystal. The former acts as a heralding signal to the telecom wavelength photon. The temporal correlation between the photons is measured, as well as the compromise between source visibility and maximum achievable transmission distance.

Design and characterization of a multiple-Bragg-gratings Fabry-Perot filter

In this paper we present the design and characterization of single and multiple Fabry-Perot filters (SFPF and MFPF, respectively). Through the correct design, which means a careful project of the fiber Bragg-gratings (FBG) and an accurate sizing of the parameters of the Fabry-Perots, it is possible to enhance the finesse of the filters. Once a good finesse has been achieved, such filters can be employed to generate a narrow microwave carrier, one of the most important requirements of modern radio-over-fiber (ROF) links.

Comparative analysis of interferometric measurements of PMD on optical fibers

Polarization modes dispersion (PMD) is one of the major factors that impose restrictions in the speed of optical communication links and some efforts must be done in order to properly quantify this effect. In this way we develop studies on the behavior of such a phenomenon, critical when light is transmitted through long optical fibers. The focus in this paper is to discuss the different behavior of PMD over different wavelengths ranges. Results indicate that PMD value varies, depending on the spectral region covered by the optical source. Measurements were performed using the interferometric method on three different types of optical fiber with three broadband optical sources covering the infrared spectral bands O, S, C and L. The evaluation of mean PMD is also discussed in the metrology concepts.

Two-Way Quantum Communication in a Single Optical Fiber with Active Polarization Compensation

We experimentally demonstrate a two-way stable transmission of polarization encoded qubits over 23 km of spooled dispersion-shifted fiber with active polarization control in both directions, while simultaneously exchanging classical data. Two classical reference channels (one containing a telecom 10 Gb/s data stream), wavelength-multiplexed with the quantum signal, are used as feedback. The feasibility of quantum communication is demonstrated in the two opposite directions over 6 hours of continuous operation, as well as a classical error rate better than 1.0 x 10− 9.