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We describe the quantum mechanical rotation of a photon state, the Wigner rotation—a quantum effect that couples a transformation of a reference frame to a particle’s spin, to investigate geometric phases induced by Earth’s gravitational field for observers in various orbits. We find a potentially measurable quantum phase of the Wigner rotation angle in addition to the rotation of standard fame, the latter of which is computed and agrees well with the geodetic rotation. When an observer is in either a circular or a spiraling orbit containing non-zero angular momentum, the additional quantum phase contributes 10−6 degree to 10−4 degree respectively, depending on the altitude of the Earth orbit. In the former case, the additional quantum phase is dominant over the near-zero classical geodetic rotation. Our results show that the Wigner rotation represents a non-trivial semi-classical effect of quantum field theory on a background classical gravitational field.

We derive a general expression for the absorptive part of the one-loop photon polarization tensor in a strongly magnetized quark-gluon plasma at nonzero baryon chemical potential. To demonstrate the application of the main result in the context of heavy-ion collisions, we study the effect of a nonzero baryon chemical potential on the photon emission rate. The rate and the ellipticity of photon emission are studied numerically as a function the transverse momentum (energy) for several values of temperature and chemical potential. When the chemical potential is small compared to the temperature, the rates of the quark and antiquark splitting processes (i.e., $q\rightarrow q +\gamma$ and $\bar{q}\rightarrow \bar{q} +\gamma$, respectively) are approximately the same. However, the quark splitting gradually becomes the dominant process with increasing the chemical potential. We also find that increasing the chemical potential leads to a growing total photon production rate but has only a small effect on the ellipticity of photon emission. The quark-antiquark annihilation ($q+\bar{q}\rightarrow \gamma$) also contributes to the photon production, but its contribution remains relatively small for a wide range of temperatures and chemical potentials investigated., Comment: 17 pages, 6 multi-panel figures, published version

Quantum logic gates are the foundation of circuit-based quantum computation and quantum simulation. Multi-qubit quantum controlled gates are of vital importance when large-scale quantum circuits are concerned. Here, assisted by diamond nitrogen-vacancy (NV) centers inside single-sided optical cavities, we first introduce the single-photon quantum routing operation, with which a single-photon polarization qubit can be coherently routed to the superposition of two spatial modes according to the state of the other polarization qubit. On this basis, we propose two schemes for the implementation of universal three-qubit quantum Fredkin- and Toffoli-gate operations on single-photon polarization qubits. Moreover, we show that a general multi-qubit controlled-unitary gate can be realized using the same strategy. Compared to other NV-cavity-based schemes, our schemes utilize photonic polarization, spatial, and temporal degrees of freedom (DOFs) simultaneously, so that the quantum circuits are simplified and the required quantum resources are reduced, making our schemes more resource-efficient and highly scalable. Our calculations show that the schemes can posses near-deterministic efficiency and near-unity fidelity with current or near-future techniques, which guarantees the experimental feasibility. Our schemes can therefore provide a new recipe for the construction of multi-qubit quantum gates and for efficient processing of multi-photon states.

International audience; One of the key issue in machine learning is the characterization of the learnability of a problem. The regret is way to quantify learnability. The quantum tomography is a special case of machine learning where the training set is a set of quantum measurements and the ground truth the results of these measurements, but nothing is known about the hidden quantum system. We will show that in some case the quantum tomography is a hard problem to learn. We consider a problem related to optical fiber communication where information are encoded in photon polarizations. We will show that the learning regret cannot decay faster than 1/ √ T where T is the size of the training dataset, and that incremental gradient descents may converge worse.

Considering the demand for long spin communication distance in spintronics, graphene presents micrometer spin relaxation length at room temperature, making it one of the most promising two dimensional spintronic materials. However, achieving efficient spin injection (including pure spin current and spin polarized current) by reducing the spin dependent scattering between graphene and other materials like contact is still a core challenge. Here, we propose a novel approach to generate spin current in zigzag graphene nanoribbon (ZGNR) via photogalvanic (or photovoltaic) effect (PGE) from atomic first principle calculations. By designing ZGNR based device with spatial inversion symmetry, we find that the PGE induced pure spin current can be hiddenly generated without accompanying charge current. Furthermore, through applying a dual gate in the system, the generated pure spin current can be controlled when dual gate voltages have the opposite signs. Interestingly, when the signs of dual gate voltages are the same, the pure spin current can turn into the fully spin polarized current. More importantly, the generated spin current via PGE is independent of photon polarization and incident angles. Our investigations demonstrate ZGNR’s great potential application in noninvasive spin injection of the graphene based spintronic device.

We study di-lepton production from a single photon in the presence of a strong constant magnetic field. By the use of the Ritus-basis formalism, we analytically evaluate the photon--to--di-lepton conversion vertex with fully taking into account the non-perturbative interactions between the produced fermions and the strong magnetic field. We show that the di-lepton spectrum becomes anisotropic with respect to the magnetic-field direction and depends on the photon polarization as a manifestation of the vacuum dichroism in a strong magnetic field. According to the energy conservation in the presence of the Landau quantization, not only the transverse momentum of the produced fermions but also the longitudinal momentum is discretized, and the di-lepton spectrum exhibits spike structures as functions of the incident photon energy and the magnetic field strength. We also show that the di-lepton production is strictly prohibited for massless fermions in the lowest Landau levels as an analogue of the so-called helicity suppression., Minor improvements, Refs added in ver. 2; Notations improved, extended discussions added in outlook, published version

Accretion disks around stellar-mass black holes (BHs) emit radiation peaking in the soft X-rays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disk symmetry axis, because of electron scattering in the disk. However, due to General Relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer's inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disk, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disk, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors show an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disk material by including more realistic albedo profiles for the disk surface. Our findings in this respect show that considering also the ionization state of the disk may deeply modify the behavior of polarization observables., Accepted for publication in MNRAS - 18 pages, 13 figures

Quantum key distribution (QKD) in a Space-Earth communication link is a difficult technical task. Aside from precise mutual pointing of the optical axes during the satellite QKD session, the polarization reference frame coincidence of the satellite and the receiving station is also required. Satellite motion causes a rotation of the polarization reference frame in respect to ground station measurements of quantum states, which contributes to the error rate in time. In order to reduce the quantum bit error rate, we designed and tested a polarization correction device for the receiving ground station that is included as a part of our data analysis and processing module. We have measured the polarization properties of the ground-based receiver and showed the evolution of four polarization states over time for a typical satellite passage. An average polarization extinction ratio is equal to 200:1 for the optical free-space receiver. We have calculated the maximum permitted deviation of the polarization reference frame at the performance of the compensation system, which is less than 5.8 degrees when bit error rate is equal to 1,5%., Квантовое распределение ключей по линии связи Космос-Земля является сложной технической задачей. Помимо точного взаимного наведения оптических осей во время сеанса квантовой связи со спутником, также требуется совпадение поляризационной системы отсчета космического аппарата и приемной наземной станции. В статье представлены результаты тестов системы коррекции поляризации для наземной станции, которая входит в состав приемного модуля анализа и обработки данных. Проведены измерения поляризационных свойств наземной приемной станции для четырех поляризационных состояний от времени в ходе симуляции типичного пролёта спутника. Усредненный коэффициент поляризационной экстинкции для оптического приемника составил 200:1. Максимально допустимое отклонение поляризационной системы отсчета при работе системы компенсации составляет менее 5,8 градусов при ограничении итоговой величин ошибки в 1,5%.

We present a method for multi-depth imaging that uses polarisation modulation to reconstruct multiple depths from photon-counting observations. The echo photon signals of the scene with multiple targets are modelled under the low-flux condition based on a confocal scanning system. We establish a computational method by constructing the relationship between the received photon rate after polarisation modulation and several variables described for multi-echo signals: the detected average number of photons before polarisation modulation and the flight time corresponding to phase shift based on polarisation modulation. The photon rate is directly calculated from the photon-counting value received by a photon-counting detector based on the Poisson negative log-likelihood function. We suggest solving the abovementioned relationship by changing the waveform of the operating voltage applied to the modulator to derive analytic forms of the detected average number of photons and the flight time. We experimentally demonstrate that the proposed method can accurately reconstruct the reflectivity and depth images of two targets in a 3D scene with a significant improvement beyond what the polarisation modulation imaging method for single-depth can achieve. Moreover, we demonstrate the effectiveness of the proposed method by varying the reflection properties of the targets, the number of echo photons (e.g. less than one photon in a pulse) and the background noise. We exhibit a fast imaging capability with the imaging frequency of one pixel at 8 kHz and the root mean-square error of depth smaller than 6 cm.

Coherence and steerability are two essential characteristics of quantum systems. For a two-qubit state, the first-order coherence and the maximal violation of linear steering inequality are used to operationally measure the degree of coherence and steerability, respectively. Recently, a complementary relation between first-order coherence and linear steerability has been proposed. In this paper, we report an experimental verification of the complementary relation by preparing biphoton polarization entangled states in an all-optical setup. We propose an operable method for experimental measurement of the first-order coherence and linear steerability and calculate the purity of the initial states by reconstructing the density matrices of them. The experimental results coincide with the theoretical predictions very well, which provides a valuable reference for the application of optical quantum technology.

We propose an experimentally feasible scheme for generating genuinely tripartite entanglement in a hybrid quantum system which consists of topologically protected particle-like excitations (magnetic vortices), cavity microwave photons, and solid-state spins (NV centers in diamond). The magnetic vortices and solid-state spins are simultaneously coupled to the microwave cavity photons via magnetic dipole interaction. By introducing a microwave modulation to the collective spins and under appropriate parameter regimes, we find that the steady state of the system is a genuinely tripartite entangled state where vortices, cavity photons, and spins are entangled with each other. The effect of the introduced modulation on the produced entanglement is analyzed, and we also show that the entanglement is robust against dissipation. This work may offer a promising platform for studying macroscopic quantum effects and quantum information processing with the vortex–photon–spin system.

OBJECTIVE Cerenkov photons have distinctive features from scintillation photons. Among them is their polarization: their electric field is always perpendicular to the direction of propagation of light and parallel to the plane of incidence. Scintillation photons are instead considered unpolarized. APPROACH This study aims at understanding and optimizing the reflectance of polarized Cerenkov photons for optical Monte Carlo simulation of scintillation detectors with Geant4/GATE. First, the Cerenkov emission spectrum and polarization were implemented in the previously developed look-up-table Davis model of crystal reflectance. Next, we modified Geant4/GATE source code to account for scintillation and Cerenkov photons LUTs simultaneously. Then, we performed optical Monte Carlo simulations in BGO using GATE to show the effect of Cerenkov features on the photons' momentum at the photodetector face, using two surface finishes, with and without reflector. MAIN RESULTS In this work, we describe the new features added to the algorithm and GATE. We showed that Cerenkov characteristics affect their probability to be reflected/refracted and thus their travel path within a material. SIGNIFICANCE We showed the importance of accounting for accurate Cerenkov photons reflectance while performing advanced optical Monte Carlo simulations.

We compute corrections to the hard thermal (or dense) loop photon polarization tensor associated to a small mass $m$ of the fermions of an electromagnetic plasma at high temperature $T$ (or chemical potential $��$). To this aim we use the on-shell effective field theory, amended with mass corrections. We also carry out the computation using transport theory, reaching to the same result. Interemediate steps in the computations reveal the presence of potential infrared divergencies. We use dimensional regularization, as it is respectful with the gauge symmetry, and then show that all infrared divergencies cancel in the final result. We compare the mass corrections with both the power and two-loop corrections, and claim that they are equally important if the mass is soft, that is, of order $e T$ (or $e ��$), where $e$ is the gauge coupling constant, but are dominat if the mass obeys $ e T < m \ll T $ (or $e ��< m \ll ��)$., 16 pages, 1 figure

We theoretically demonstrate quantum interference in an anti-parity-time (anti-PT) symmetric system based on coupled waveguides. We calculate the coincidence probability in an input polarization-entangled two-photon state, which can be used to simulate different statistical particles. When the birefringence of the waveguides is negligible, our results indicate that the coincidence probabilities of the bosons and fermions decrease exponentially with the propagation distance in both the unbroken and broken anti-PT symmetry regions owing to the dissipation. Particularly, loss-induced transparency is observed for the bosons. Simultaneously, the statistical rule valid in the Hermitian system is violated and the antibunching of bosons is observed. When the birefringence of the waveguides is considered, the coincidence probability of the bosons and fermions is equalized at the exceptional point (EP), whereas that of the bosons is less(greater) than that of the fermions in the broken(unbroken) anti-PT symmetry region. Additionally, we observe the Hong-Ou-Mandel dip for bosons in the broken anti-PT phase. Our research provides a complementary technique for the manipulation of quantum interference compared with the PT symmetric system and may be applied in building quantum devices with anti-PT symmetric quantum mechanics.

Classical structured light with controlled polarization and orbital angular momentum (OAM) of electromagnetic waves has varied applications in optical trapping, bio-sensing, optical communications and quantum simulations. However, quantum noise and photon statistics of three-dimensional photonic angular momentum are relatively less explored. Here, we develop a quantum framework and put forth the concept of quantum structured light for space-time wavepackets at the single-photon level. Our work deals with three-dimensional angular momentum observables for twisted quantum pulses beyond scalar-field theory as well as the paraxial approximation. We show that the spin density generates modulated helical texture and exhibits distinct photon statistics for Fock-state vs. coherent-state twisted pulses. We introduce the quantum correlator of photon spin density to characterize nonlocal spin noise providing a rigorous parallel with electronic spin noise. Our work can lead to quantum spin-OAM physics in twisted single-photon pulses and opens explorations for phases of light with long-range spin order. Twisted light beams allow unique control of light-matter interaction, but classical models cannot describe this phenomenon at the single-photon level. Here, the quantum state of structured photons is derived instead from quantum field theory which captures the quantum uncertainty in its angular momentum and the non-local photonic spin density correlation.

We present a theoretical investigation on the quantum memory of photonic polarization qubits via electromagnetically induced transparency (EIT). The system we consider is a tripod-shaped four-level atomic system working under the condition of a double EIT, by which the storage and retrieval of a single-photon polarization qubit are implemented through the switching off and on of a control laser field, and the storage efficiency and the quantum-state fidelity for qubit memory are both calculated. We show that the optimal optical depth for acquiring the maximum efficiency and maximum fidelity of the qubit memory can be obtained simultaneously, which can be further improved by suppressing the optical absorption and dispersion via the choice of the time duration of the input qubit pulse and the amplitude of the control field. We also carry out a calculation on the quantum memory of a single-photon qudit by considering a multipod-shaped atomic-level configuration. The results reported here are useful for understanding the quantum transmission property of slow lights with multiple components and helpful for experimental realizations of high-quality memory of photonic qubits and qudits.

We study a direct-current-driven maser device enabled by spin-photon coupling, where coherent magnetic self-oscillation can be realized in a large-area ferromagnetic thin film under the excitation of spin-orbit torques. We show via both micromagnetic simulation and analytical derivation that above a critical value of the coupling strength between the spin-torque oscillator and the microwave resonator, magnetic oscillation develops macroscopic phase coherence, a narrow linewidth, and becomes phase-locked with the photon mode. The threshold coupling strength for synchronizing individual spins reduces as the sample dimension increases, suggesting that the spin-torque-oscillator maser can be readily realized using large-area thin-film ferromagnets without relying on dimension confinement. Moreover, the photon mode can directly provide microwave emission, which exhibits enhanced power and a reduced linewidth with an increasing number of spins, leading to a useful approach for developing highly coherent on-chip microwave sources.

This paper analyzes the algebraic and physical properties of the spin and orbital angular momenta of light in the quantum mechanical framework. The consequences of the fact that these are not angular momenta in the quantum mechanical sense are worked out in mathematical detail. It turns out that the spin part of the angular momentum has continuous eigenvalues. Particular attention is given to the paraxial limit, and to the definition of Laguerre--Gaussian modes for photons as well as classical light fields taking full account of the polarization degree of freedom., Comment: 7 pages, Latex

A measurement of the photon polarization in radiative penguin $B$ decays provides a test of the Standard Model and a probe for New Physics, that can lead to a deviation from the Standard Model prediction of left-handed photons in $b\to s \gamma$. We propose a new method to measure the photon polarization using the baryonic decay $B^- \to \Lambda\bar{p} \gamma$. The $P$-violating $\Lambda$-hyperon decay allows a measurement of the $\Lambda$ helicity to be performed, which can be uniquely related to the photon polarization in a model-independent way. The $B^- \to \Lambda\bar{p} \gamma$ decay was recently measured to have a large branching fraction providing a possibility to get meaningful results with the data already available at LHC and B-factory experiments. An increase of the $B$-meson sample at high luminosity LHC experiments and Belle II should provide a really stringent test by using this method already in the near future., Comment: 4 pages, 2 figures

We present an economical entanglement purification protocol for the multi-photon polarization-entangled system in a Greenberger–Horne–Zeilinger (GHZ) state. We use one pair of two-photon Bell state or M-photon GHZ state ( $$M

Real-time measurement of the biochemical reaction process has important application scenarios. Due to the chirality of a large number of life-sustaining molecules, many parameters of the reaction kinetics involving these chiral molecules, such as the reaction rate and the reagents concentrations, could be tracked by monitoring the optical activity of the substrate and/or product molecules. However, the optical activity of photosensitive biomolecules does not allow traditional laser-based real-time measurement due to the vulnerability of their biochemical properties under high-intensity light regimes. Here we introduce a real-time tracking technique of the sucrose hydrolysis reaction based on two-photon coincidence measurements. The two-photon source is generated based on a spontaneous parametric down-conversion process. During the reaction, the kinetic parameters are obtained by the real-time measurement of the change of the polarization of the photons when operating at extremely low-light regimes. Compared with single-photon counting measurements, two-photon coincidence measurements have higher signal-to-noise ratios and better robustness, which demonstrates the potential value in monitoring the photosensitive biochemical reaction processes.

Universal quantum gates play a critical role in quantum information processing. Here, based on the scattering property of photons off single emitters in one-dimensional waveguides, we present some heralded protocols for realizing controlled-not, Toffoli, and Fredkin gates on hybrid systems. The control qubit of our gates is encoded on a flying photon, and the target qubits are encoded in the degenerate ground states of the emitter. In our schemes, the faulty scattering processes between emitters and photons caused by system imperfections can be transformed into the detection of photon polarization. That is, our three quantum gates are accomplished in a heralded way. Moreover, the quantum circuits for the three gates are versatile and compact, and no additional qubits are required. With current technique on manipulating the emitter-waveguide system, our protocols may be experimentally feasible. This work provides an effective avenue for implementing quantum logic gates.