arXiv subjects

Antia Lamas-Linares

Publications and source records attributed to Antia Lamas-Linares.

At least 19 recordsLinked to original sources

arXiv2019

Asymmetric delay attack on an entanglement-based bidirectional clock synchronization protocol

We demonstrate an attack on a clock synchronization protocol that attempts to detect tampering of the synchronization channel using polarization-entangled photon pairs. The protocol relies on a symmetrical channel, where propagation delays do not depend on propagation direction, for correctly deducing the offset between clocks -- a condition that could be manipulated with optical circulators, which rely on static magnetic fields to break the reciprocity of propagating electromagnetic fields. Despite the polarization transformation induced within a set of circulators, our attack creates an error in time synchronization while successfully evading detection.

quant-ph↗

arXiv2018

Symmetrical clock synchronization with time-correlated photon pairs

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally exchanging photons produced in spontaneous parametric down conversion (SPDC). The technique exploits tight timing correlations between photon pairs to achieve a precision of 51ps in 100s with count rates of order 200s$^{-1}$. The protocol is distance independent, secure against symmetric delay attacks and provides a natural complement to techniques based on Global Navigation Satellite Systems (GNSS). The protocol works with mobile parties and can be augmented to provide authentication of the timing signal via a Bell inequality check.

quant-ph↗

arXiv2018

Detecting Optical Channel Non-Reciprocity with Non-Local Quantum Geometric Phase

Non-reciprocal devices are of increasing interest in quantum information technologies. This paper examines whether the presence of a non-reciprocal device in an optical channel is detectable by the communicating parties. We find that a non-reciprocal device such as a Faraday Rotator results in a measurable geometric phase for the light propagating through the channel and that, when using entangled photon pairs, the resulting phase is non-local and robust against malicious manipulation.

quant-ph↗

arXiv2018

Randomness extraction from Bell violation with continuous parametric down conversion

We present a violation of the CHSH inequality without the fair sampling assumption with a continuously pumped photon pair source combined with two high efficiency superconducting detectors. Due to the continuous nature of the source, the choice of the duration of each measurement round effectively controls the average number of photon pairs participating in the Bell test. We observe a maximum violation of S= 2.01602(32) with average number of pairs per round of ~0.32, compatible with our system overall detection efficiencies. Systems that violate a Bell inequality are guaranteed to generate private randomness, with the randomness extraction rate depending on the observed violation and on the repetition rate of the Bell test. For our realization, the optimal rate of randomness generation is a compromise between the observed violation and the duration of each measurement round, with the latter realistically limited by the detection time jitter. Using an extractor composably secure against quantum adversary with quantum side information, we calculate an asymptotic rate of ~1300 random bits/s. With an experimental run of 43 minutes, we generated 617,920 random bits, corresponding to ~240 random bits/s.

quant-ph↗

arXiv2012

Nanosecond-scale timing jitter in transition edge sensors at telecom and visible wavelengths

Transition edge sensors (TES) have the highest reported efficiencies (>98%) for detection of single photons in the visible and near infrared. Experiments in quantum information and foundations of physics that rely critically on this efficiency have started incorporating these detectors into con- ventional quantum optics setups. However, their range of applicability has been hindered by slow operation both in recovery time and timing jitter. We show here how a conventional tungsten-TES can be operated with jitter times of < 4 ns, well within the timing resolution necessary for MHz clocking of experiments, and providing an important practical simplification for experiments that rely on the simultaneous closing of both efficiency and locality loopholes.

quant-ph↗

arXiv2012

Full-field implementation of a perfect eavesdropper on a quantum cryptography system

Quantum key distribution (QKD) allows two remote parties to grow a shared secret key. Its security is founded on the principles of quantum mechanics, but in reality it significantly relies on the physical implementation. Technological imperfections of QKD systems have been previously explored, but no attack on an established QKD connection has been realized so far. Here we show the first full-field implementation of a complete attack on a running QKD connection. An installed eavesdropper obtains the entire 'secret' key, while none of the parameters monitored by the legitimate parties indicate a security breach. This confirms that non-idealities in physical implementations of QKD can be fully practically exploitable, and must be given increased scrutiny if quantum cryptography is to become highly secure.

quant-ph↗

arXiv2012

Thwarting the Photon Number Splitting Attack with Entanglement Enhanced BB84 Quantum Key Distribution

We develop an improvement to the weak laser pulse BB84 scheme for quantum key distribution, which utilizes entanglement to improve the security of the scheme and enhance its resilience to the photon-number-splitting attack. This protocol relies on the non-commutation of photon phase and number to detect an eavesdropper performing quantum non-demolition measurement on number. The potential advantages and disadvantages of this scheme are compared to the coherent decoy state protocol.

quant-ph↗

arXiv2011

Detection of Single Molecules Illuminated by a Light-Emitting Diode

Optical detection and spectroscopy of single molecules has become an indispensable tool in biological imaging and sensing. Its success is based on fluorescence of organic dye molecules under carefully engineered laser illumination. In this paper we demonstrate optical detection of single molecules on a wide-field microscope with an illumination based on a commercially available, green light-emitting diode. The results are directly compared with laser illumination in the same experimental configuration. The setup and the limiting factors, such as light transfer to the sample, spectral filtering and the resulting signal-to-noise ratio are discussed. A theoretical and an experimental approach to estimate these parameters are presented. The results can be adapted to other single emitter and illumination schemes.

physics.chem-ph↗

arXiv2009

Eliminating Spectral Distinguishability in Ultrafast Spontaneous Parametric Down-conversion

Generation of polarization-entangled photon pairs with a precise timing through down-conversion of femtosecond pulses is often faced with a degraded polarization entanglement quality. In a previous experiment we have shown that this degradation is induced by spectral distinguishability between the two decay paths, in accordance with theoretical predictions. Here, we present an experimental study of the spectral compensation scheme proposed and first implemented by Kim et al. in 2002. By measuring the joint spectral properties of the polarization correlations of the photon pairs, we show that the spectral distinguishability between the down-converted components is eliminated. This scheme results in a visibility of 97.9+/-0.5% in the polarization basis without any spectral filtering.

quant-ph↗

arXiv2009

Testing quantum correlations versus single-particle properties within Leggett's model and beyond

Quantum theory predicts and experiments confirm that nature can produce correlations between distant events that are nonlocal in the sense of violating a Bell inequality. Nevertheless, Bell's strong sentence {\it Correlations cry out for explanations} remains relevant. The maturing of quantum information science and the discovery of the power of nonlocal correlations, e.g. for cryptographic key distribution beyond the standard Quantum Key Distribution schemes, strengthen Bell's wish and make it even more timely. In 2003, Leggett proposed an alternative model for nonlocal correlations [Found. Phys. 33, 1469 (2003)], that he proved to be incompatible with quantum predictions. We present here a new approach to this model, along with new inequalities for testing it. Remarkably these inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions; they are also stronger than previously published Leggett-type inequalities. The simplest of these inequalities is experimentally violated. Then we go beyond Leggett's model, and show that one cannot ascribe even partially defined individual properties to the components of a maximally entangled pair.

quant-ph↗

arXiv2009

Clock synchronization by remote detection of correlated photon pairs

We present an algorithm to detect the time and frequency difference of independent clocks based on observation of time-correlated photon pairs. This enables remote coincidence identification in entanglement-based quantum key distribution schemes without dedicated coincidence hardware, pulsed sources with a timing structure or very stable reference clocks. We discuss the method for typical operating conditions, and show that the requirement in reference clock accuracy can be relaxed by about 5 orders of magnitude in comparison with previous schemes.

quant-ph↗

arXiv2008

Daylight operation of a free space, entanglement-based quantum key distribution system

Many quantum key distribution (QKD) implementations using a free space transmission path are restricted to operation at night time in order to distinguish the signal photons used for a secure key establishment from background light. Here, we present a lean entanglement-based QKD system overcoming that imitation. By implementing spectral, spatial and temporal filtering techniques, we were able to establish a secure key continuously over several days under varying light and weather conditions.

quant-ph↗

arXiv2008

Accuracy of minimal and optimal qubit tomography for finite-length experiments

Practical quantum state tomography is usually performed by carrying out repeated measurements on many copies of a given state. The accuracy of the reconstruction depends strongly on the dimensionality of the system and the number of copies used for the measurements. We investigate the accuracy of an experimental implementation of a minimal and optimal tomography scheme for one- and two-qubit states encoded in the polarization of photons. A suitable statistical model for the attainable accuracy is introduced.

quant-ph↗

arXiv2008

Experimental quantum key distribution based on a Bell test

We report on a complete free-space field implementation of a modified Ekert91 protocol for quantum key distribution using entangled photon pairs. For each photon pair we perform a random choice between key generation and a Bell inequality. The amount of violation is used to determine the possible knowledge of an eavesdropper to ensure security of the distributed final key.

quant-ph↗

arXiv2008

Absolute emission rates of Spontaneous Parametric Down Conversion into single transverse Gaussian modes

We provide an estimate on the absolute values of the emission rate of photon pairs produced by spontaneous parametric down conversion in a bulk crystal when all interacting fields are in single transverse Gaussian modes. Both collinear and non-collinear configurations are covered, and we arrive at a fully analytical expression for the collinear case. Our results agree reasonably well with values found in typical experiments, which allows this model to be used for understanding the dependency on the relevant experimental parameters.

quant-ph↗

arXiv2008

Sagnac-loop phase shifter with polarization-independent operation

A bulk-optics, polarization-independent phase shifter for photonic quantum information applications is demonstrated. A transverse mode electro-optic modulator using lithium niobate is placed in a Sagnac-like loop containing Faraday rotators. This direction-dependent polarization rotation in the loop is combined with a Mach-Zehnder interferometer to form an optical switch with an on/off contrast of 96% and a switching time of 1.6 ns.

quant-ph↗

arXiv2007

Experimental Falsification of Leggett's Non-Local Variable Model

Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also some models based on non-local variables, if subject to apparently "reasonable" constraints, may fail to reproduce quantum physics. In this paper, we introduce a family of inequalities, which allow testing Leggett's non-local model versus quantum physics, and which can be tested in an experiment without additional assumptions. Our experimental data falsify Leggett's model and are in agreement with quantum predictions.

quant-ph↗

arXiv2007

Breaking a quantum key distribution system through a timing side channel

The security of quantum key distribution relies on the validity of quantum mechanics as a description of nature and on the non-existence of leaky degrees of freedom in the practical implementations. We experimentally demonstrate how, in some implementations, timing information revealed during public discussion between the communicating parties can be used by an eavesdropper to undetectably access a significant portion of the ``secret'' key.

quant-ph↗