Efficient Implementation of Discrete-Time Quantum Walks on Quantum Computers.

Quantum walks have proven to be a universal model for quantum computation and to provide speed-up in certain quantum algorithms. The discrete-time quantum walk (DTQW) model, among others, is one of the most suitable candidates for circuit implementation due to its discrete nature. Current implementations, however, are usually characterized by quantum circuits of large size and depth, which leads to a higher computational cost and severely limits the number of time steps that can be reliably implemented on current quantum computers. In this work, we propose an efficient and scalable quantum circuit implementing the DTQW on the 2n-cycle based on the diagonalization of the conditional shift operator. For t time steps of the DTQW, the proposed circuit requires only O(n2+nt) two-qubit gates compared to the O(n2t) of the current most efficient implementation based on quantum Fourier transforms. We test the proposed circuit on an IBM quantum device for a Hadamard DTQW on the 4-cycle and 8-cycle characterized by periodic dynamics and by recurrent generation of maximally entangled single-particle states. Experimental results are meaningful well beyond the regime of few time steps, paving the way for reliable implementation and use on quantum computers.

Generation of Pseudo-Random Quantum States on Actual Quantum Processors.

The generation of a large amount of entanglement is a necessary condition for a quantum computer to achieve quantum advantage. In this paper, we propose a method to efficiently generate pseudo-random quantum states, for which the degree of multipartite entanglement is nearly maximal. We argue that the method is optimal, and use it to benchmark actual superconducting (IBM's ibm_lagos) and ion trap (IonQ's Harmony) quantum processors. Despite the fact that ibm_lagos has lower single-qubit and two-qubit error rates, the overall performance of Harmony is better thanks to its low error rate in state preparation and measurement and to the all-to-all connectivity of qubits. Our result highlights the relevance of the qubits network architecture to generate highly entangled states.

Correcting Coherent Errors by Random Operation on Actual Quantum Hardware.

Characterizing and mitigating errors in current noisy intermediate-scale devices is important to improve the performance of the next generation of quantum hardware. To investigate the importance of the different noise mechanisms affecting quantum computation, we performed a full quantum process tomography of single qubits in a real quantum processor in which echo experiments are implemented. In addition to the sources of error already included in the standard models, the obtained results show the dominant role of coherent errors, which we practically corrected by inserting random single-qubit unitaries in the quantum circuit, significantly increasing the circuit length over which quantum computations on actual quantum hardware produce reliable results.

Towards underwater quantum communication in the mesoscopic intensity regime.

The problem of secure underwater communication can take advantage of the exploitation of quantum resources and novel quantum technologies. At variance with the current experiments performed at the single photon level, here we propose a different scenario involving mesoscopic twin-beam states of light and two classes of commercial photon-number-resolving detectors. We prove that twin-beam states remain nonclassical even if the signal propagates in tubes filled with water, while the idler is transmitted in free space. We also demonstrate that from the study of the nonclassicality information about the loss and noise sources affecting the transmission channels can be successfully extracted.

Novel scheme for secure data transmission based on mesoscopic twin beams and photon-number-resolving detectors.

Quantum resources can improve the quality and security of data transmission. A novel communication protocol based on the use of mesoscopic twin-beam (TWB) states of light is proposed and discussed. The message sent by Alice to Bob is encoded in binary single-mode thermal states having two possible mean values, both smaller than the mean value of the TWB. Such thermal states are alternately superimposed to the portion of TWB sent to Bob. We demonstrate that in the presence of an eavesdropping attack that intercepts and substitutes part of the signal with a thermal noise, Bob can still successfully decrypt the message by evaluating the noise reduction factor for detected photons. The protocol opens new perspectives in the exploitation of quantum states of light for applications to Quantum Communication.

Feasibility of a Novel Quantum Communication Protocol in Jerlov Type I Water.

Underwater communication based on the use of optical quantum resources has attracted a lot of attention in the last five years due to the potential advantages offered by quantum states of light. In this context, we propose to operate in the mesoscopic intensity regime, where the optical states are well populated and the employed detectors have photon-number resolution. By exploiting these features, we demonstrate that a novel communication protocol based on the experimental quantification of nonclassicality of mesoscopic twin-beam states can be used to transmit binary signals encoded in two single-mode pseudothermal states with different mean values through a Jerlov type I water channel. The experimental results are in perfect agreement with the developed theoretical model, and the feasibility of the protocol is also investigated as a function of the data samples corresponding to each one of the two signals. The good quality of the results encourages a more realistic implementation of the protocol, also exploring the maximum distance at which the quantum states remain nonclassical and thus can be still properly discriminated.

Effect of noisy channels on the transmission of mesoscopic twin-beam states.

Quantum properties of light, which are crucial resources for quantum technologies, are quite fragile in nature and can be degraded and even concealed by the environment. We show, both theoretically and experimentally, that mesoscopic twin-beam states of light can preserve their nonclassicality even in the presence of major losses and different types of noise, thus suggesting their potential usefulness to encode information in quantum communication protocols. We develop a comprehensive general analytical model for a measurable nonclassicality criterion and find thresholds on noise and losses for the survival of entanglement in the twin beam.

Double-stranded flanking ends affect the folding kinetics and conformational equilibrium of G-quadruplexes forming sequences within the promoter of KIT oncogene.

G-quadruplexes embedded within promoters play a crucial role in regulating the gene expression. KIT is a widely studied oncogene, whose promoter contains three G-quadruplex forming sequences, c-kit1, c-kit2 and c-kit*. For these sequences available studies cover ensemble and single-molecule analyses, although for kit* the latter were limited to a study on a promoter domain comprising all of them. Recently, c-kit2 has been reported to fold according to a multi-step process involving folding intermediates. Here, by exploiting fluorescence resonance energy transfer, both in ensemble and at the single molecule level, we investigated the folding of expressly designed constructs in which, alike in the physiological context, either c-kit2 or c-kit* are flanked by double stranded DNA segments. To assess whether the presence of flanking ends at the borders of the G-quadruplex affects the folding, we studied under the same protocols oligonucleotides corresponding to the minimal G-quadruplex forming sequences. Data suggest that addition of flanking ends results in biasing both the final equilibrium state and the folding kinetics. A previously unconsidered aspect is thereby unravelled, which ought to be taken into account to achieve a deeper insight of the complex relationships underlying the fine tuning of the gene-regulatory properties of these fascinating DNA structures.

Dynamical Localization Simulated on Actual Quantum Hardware.

Quantum computers are invaluable tools to explore the properties of complex quantum systems. We show that dynamical localization of the quantum sawtooth map, a highly sensitive quantum coherent phenomenon, can be simulated on actual, small-scale quantum processors. Our results demonstrate that quantum computing of dynamical localization may become a convenient tool for evaluating advances in quantum hardware performances.

Optimizing Silicon photomultipliers for Quantum Optics.

Silicon Photomultipliers are potentially ideal detectors for Quantum Optics and Quantum Information studies based on mesoscopic states of light. However, their non-idealities hampered their use so far. An optimal mode of operation has been developed and it is presented here, proving that this class of sensors can actually be exploited for the characterization of both classical and quantum properties of light.

Measuring nonclassicality with silicon photomultipliers.

Detector stochastic deviations from an ideal response can hamper the measurement of quantum properties of light, especially in the mesoscopic regime where photon-number resolution is required. We demonstrate that, by proper analysis of the output signal, nonclassicality of twin-beam states can be detected and exploited with commercial and cost-effective silicon-based photon-number-resolving detectors.

Time-Resolved Photoluminescence Microscopy Combined with X-ray Analyses and Raman Spectroscopy Sheds Light on the Imperfect Synthesis of Historical Cadmium Pigments.

Recent studies have shown that modern pigments produced after the Second Industrial Revolution are complex systems characterized by a high level of heterogeneities. Therefore, it is fundamental to adopt a multianalytical approach and highly sensitive methods to characterize the impurities present within pigments. In this work we propose time-resolved and spectrally resolved photoluminescence (PL) microscopy for the mapping of luminescent crystal defects and impurities in historical cadmium-based pigments. PL analysis is complemented by X-ray diffraction, X-ray fluorescence and Raman spectroscopies, and by scanning electron microscopy to determine the chemical composition and crystal structure of samples. The study highlights the heterogeneous and complex nature of historical samples that can be associated with the imperfect manufacturing processes tested during the period between the 1850s and 1950s. The results also allow us to speculate on a range of synthesis processes. Since it is recognized that the stability of paints can be related to pigments synthesis, this research paves the way to a wider study on the relationship between synthesis methods and deterioration of cadmium pigments and paints. This rapid and immediate approach using PL can be applied to other semiconductor pigments and real case studies.

Antibunching-like behavior of mesoscopic light.

We present the implementation of a compact setup for the generation of sub-Poissonian states of light exhibiting the analogous of antibunching behavior in the so-called mesoscopic intensity domain. In the scheme, the idler arm of a pulsed multi-mode twin-beam state is directly measured by a photon-number-resolving detector, whereas the signal arm is divided at a balanced beam splitter, at whose outputs other two photon-number-resolving detectors measure the number of photons. The three detectors measure synchronous with each laser pulse. Due to the nonclassical correlations in the twin beam, when a given value of photons is measured in the idler arm, the conditional states obtained in post processing at the two beam-splitter outputs are nonclassical, showing lower-than-one values of the Fano factor and of the photon autocorrelation coefficient. The possibility to engineer sub-Poissonian states nearly approaching the Fock state with one photon is also addressed.

Homodyne-like detection for coherent state-discrimination in the presence of phase noise.

We propose a homodyne-like detection scheme involving photon-number-resolving detectors to discriminate between two coherent states affected by either uniform or gaussian phase noise. A proof-of-principle experiment is performed employing two hybrid photodetectors, whose outputs are used in post processing to calculate the shot-by-shot photon-number differences. The performance of the strategy is quantified in terms of the error probability in discriminating the noisy coherent signals as a function of the characteristic noise parameters.

Entanglement and nonclassicality in four-mode Gaussian states generated via parametric down-conversion and frequency up-conversion.

Multipartite entanglement and nonclassicality of four-mode Gaussian states generated in two simultaneous nonlinear processes involving parametric down-conversion and frequency up-conversion are analyzed assuming the vacuum as the initial state. Suitable conditions for the generation of highly entangled states are found. Transfer of the entanglement from the down-converted modes into the up-converted ones is also suggested. The analysis of the whole set of states reveals that sub-shot-noise intensity correlations between the equally-populated down-converted modes, as well as the equally-populated up-converted modes, uniquely identify entangled states. They represent a powerful entanglement identifier also in other cases with arbitrarily populated modes.

Phase-reference monitoring in coherent-state discrimination assisted by a photon-number resolving detector.

Phase estimation represents a crucial challenge in many fields of Physics, ranging from Quantum Metrology to Quantum Information Processing. This task is usually pursued by means of interferometric schemes, in which the choice of the input states and of the detection apparatus is aimed at minimizing the uncertainty in the estimation of the relative phase between the inputs. State discrimination protocols in communication channels with coherent states also require the monitoring of the optical phase. Therefore, the problem of phase estimation is relevant to face the issue of coherent states discrimination. Here we consider a quasi-optimal Kennedy-like receiver, based on the interference of two coherent signals, to be discriminated, with a reference local oscillator. By means of the Bayesian processing of a small amount of data drawn from the outputs of the shot-by-shot discrimination protocol, we demonstrate the achievement of the minimum uncertainty in phase estimation, also in the presence of uniform phase noise. Moreover, we show that the use of photon-number resolving detectors in the receiver improves the phase-estimation strategy, especially with respect to the usually employed on/off detectors. From the experimental point of view, this comparison is realized by employing hybrid photodetectors.

Internal dynamics of intense twin beams and their coherence.

The dynamics of intense twin beams in pump-depleted parametric down-conversion is studied. A generalized parametric approximation is suggested to solve the quantum model. Its comparison with a semiclassical model valid for larger twin-beam intensities confirms its applicability. The experimentally observed maxima in the spectral and spatial intensity auto- and cross- correlation functions depending on pump power are explained in terms of different speeds of the (back-) flow of energy between the individual down-converted modes and the corresponding pump modes. This effect is also responsible for the gradual replacement of the initial exponential growth of the down-converted fields by the linear one. Furthermore, it forms a minimum in the curve giving the effective number of twin-beam modes. These effects manifest a tight relation between the twin-beam coherence and its internal structure, as clearly visible in the model. Multiple maxima in the intensity correlation functions originating in the oscillations of energy flow between the pump and down-converted modes are theoretically predicted.

Effects of non-CpG site methylation on DNA thermal stability: a fluorescence study.

Cytosine methylation is a widespread epigenetic regulation mechanism. In healthy mature cells, methylation occurs at CpG dinucleotides within promoters, where it primarily silences gene expression by modifying the binding affinity of transcription factors to the promoters. Conversely, a recent study showed that in stem cells and cancer cell precursors, methylation also occurs at non-CpG pairs and involves introns and even gene bodies. The epigenetic role of such methylations and the molecular mechanisms by which they induce gene regulation remain elusive. The topology of both physiological and aberrant non-CpG methylation patterns still has to be detailed and could be revealed by using the differential stability of the duplexes formed between site-specific oligonucleotide probes and the corresponding methylated regions of genomic DNA. Here, we present a systematic study of the thermal stability of a DNA oligonucleotide sequence as a function of the number and position of non-CpG methylation sites. The melting temperatures were determined by monitoring the fluorescence of donor-acceptor dual-labelled oligonucleotides at various temperatures. An empirical model that estimates the methylation-induced variations in the standard values of hybridization entropy and enthalpy was developed.

Spatial and spectral coherence in propagating high-intensity twin beams.

Spatial and spectral coherence of high-intensity twin-beam states propagating from the near-field to the far-field configurations is experimentally investigated by measuring intensity auto- and cross-correlation functions. The experimental setup includes a moving crystal and an iCCD camera placed at the output plane of an imaging spectrometer. Evolution from the tight near-field spatial position cross-correlations to the far-field momentum cross-correlations, accompanied by changeless spectral cross-correlations, is observed. Intensity autocorrelation functions and beam profiles are also monitored as they provide the number of degrees of freedom constituting the down-converted beams. The strength of intensity cross-correlations as an alternative quantity for the determination of the number of degrees of freedom is also measured. The relation between the beam coherence and the number of degrees of freedom is discussed.

Direct detection of super-thermal photon-number statistics in second-harmonic generation.

Changes in the statistical properties of light undergoing second-harmonic generation are investigated in the photon-number-resolving domain. We theoretically demonstrate that when a portion of multimode thermal light produced by parametric down-conversion is up-converted, both the second-harmonics and the residual beam at the fundamental wavelength are endowed with super-thermal photon-number distributions. The experimental results, which were obtained by exploiting the photo-number-resolving capability of hybrid photodetectors, are in excellent agreement with the theoretical expectations.