We present an approach to engineer the photon correlations emerging from the interference between an input field and the field scattered by a single atom in free space. Nominally, the inefficient atom-light coupling causes the quantum correlations to be dominated by the input field alone. To overcome this issue, we propose the use of separate pump and probe beams, where the former increases the atomic emission to be comparable to the probe. Examining the second-order correlation function $g^{\left(2\right)}\left(\tau \right)$ of the total field in the probe direction, we find that the addition of the pump formally plays the same role as increasing the coupling efficiency, even though the physical atom-light coupling efficiency remains unchanged. We show that one can tune the correlation function $g^{\left(2\right)}\left(0\right)$ from zero (perfect antibunching) to infinite (extreme bunching) by a proper choice of pump amplitude. We further elucidate the origin of these correlations in terms of the transient atomic state following the detection of a photon, and show that these correlations can be observed under realistic conditions.

Quantum walks are counterparts of classical random walks. They spread faster, which can be exploited in information processing tasks, and constitute a versatile simulation platform for many quantum systems. Yet, some of their properties can be emulated with classical light. This raises a question: which aspects of the model are truly nonclassical? We address it by carrying out a photonic experiment based on a pre- and post-selection paradox. The paradox implies that if somebody could choose to ask either if the particle is at position $x=0$ at even time steps or at position $x=d$ $(d>1)$ at odd time steps, the answer would be positive, no matter the question asked. Therefore, the particle seems to undergo long distance oscillations despite the fact that the model allows it to jump one position at a time. We translate this paradox into a Bell-like inequality and then into a contextuality witness. Finally, we experimentally verify this witness up to eight standard deviations.

The monogamy of entanglement means that entanglement cannot be freely shared. In 2014, Oliveira et al. [T. R. de Oliveira, M. F. Cornelio, and F. F. Fanchini, Phys. Rev. A 89, 034303 (2014)] proposed a monogamy relation in the linear version and considered it in terms of entanglement of formation. Here we generalize the above version and consider a multilinear monogamy relation for a multiqubit system in terms of entanglement of formation and concurrence. Based on the above results, we present an entanglement criterion for genuine entangled states; also we consider the absolutely maximally entangled states and present what an absolutely maximally entangled state is for a three-qubit system. Last, we apply our results to a three-qubit pure state in terms of quantum discord.

We study the van der Waals (vdW) interaction between a polarizable particle and a grounded conducting corrugated surface. For sinusoidal corrugations, one knows that, under the action of the lateral vdW force, an isotropic particle is always attracted to the nearest corrugation peak, with such behavior called in the present paper as peak regime. Here, considering an anisotropic polarizable particle, and making analytical calculations valid beyond the proximity force approximation (PFA), we show that the attraction is not only toward the peaks, but, for certain particle orientations and distances from the surface, the lateral force attracts the particle to the nearest corrugation valley (valley regime), or even to an intermediate point between a peak and a valley (intermediate regime). We also show that in the configurations of transition between the peak and valley regimes the lateral vdW force vanishes, even in the presence of a corrugated surface. In addition, we find that these regimes occur in general, for periodic and nonperiodic corrugated surfaces. Moreover, we demonstrate that similar regimes arise in the classical interaction between a neutral polarized particle and a rough surface. The description of these valley and intermediate regimes, which are out of reach of the predictions based on the PFA, may be relevant for a better understanding of the interaction between anisotropic particles and corrugated surfaces in classical and quantum physics, with experimental verifications feasible in both domains.

An experimental procedure is reported, which provides the absolute triple differential cross sections (ATDCSs) for electron-impact ionization of large (bio)molecules. This type of measurements represents the most stringent tests for new or existing theoretical models. We will use this procedure to test the accuracy of the best currently available theoretical models for the problems of electron-impact (65 eV) ionization of the molecules water $\left({\mathrm{H}}_2\mathrm{O}\right)$, tetrahydrofuran $\left({\mathrm{C}}_4{\mathrm{H}}_8\mathrm{O}\right)$, and their hydrogen-bonded dimer ${\mathrm{H}}_2\mathrm{O}·{\mathrm{C}}_4{\mathrm{H}}_8\mathrm{O}$. The cross sections were calculated using the molecular three-body distorted-wave (M3DW) model, the multicenter three-distorted-wave (MCTDW) approach, and the multicenter three-distorted-wave using the Ward-Macek approximation (MCTDW-WM). When compared to the new experimental ATDCS results which cover almost the full solid angle of the ejected electron and a broad range of ejected electron energies and projectile scattering angles, it is found that the data for water are generally well reproduced by the M3DW model, while strong deviations in the absolute magnitude of the cross sections are found for the MCTDW. The MCTDW-WM model provides improved agreement over the MCTDW. These theoretical models, however, become less adequate for the ATDCS of ${\mathrm{C}}_4{\mathrm{H}}_8\mathrm{O}$, in particular concerning the absolute magnitude. Furthermore, we find that a water environment can play a noticeable role for the ionization dynamics in the case of hydrated molecules.

The propagation of light-induced thermo-reorientational solitary waves in nematic liquid crystals is studied using numerical solutions of the full governing equations and variational approximations. These thermo-reorientational solitary waves form as the nonlocal refractive index response to extraordinarily polarized light beams is both self-focusing via the induced rotation of the constituent molecules and self-defocusing owing to the temperature increase through optical absorption. These competing nonlinearities can lead to the formation of one- and two-dimensional multihumped solitary and ring-shaped waves at high enough optical powers with a volcano profile on the plane transverse to propagation. The variational solutions for these self-localized structured beams are in remarkably good agreement with full numerical solutions of the governing equations.

Intuition dictates that a very long, very thin cavity (e.g., a fiber optic cable) could perhaps be modeled as an approximately one-dimensional system. In this paper we rigorously explore the validity of such intuition from the perspective of a localized probe coupling to a quantum field inside a cavity (e.g., an atom or an Unruh-DeWitt particle detector in a fiber optic cable). To do so, we introduce the notion of subfield decomposition in which a ($D+1$)-dimensional quantum field in an axially symmetric cavity can be reduced to an infinite collection of uncoupled, massive ($1+1$)-dimensional fields. We show that the ability to approximate a higher-dimensional scenario by a ($1+1$)-dimensional model is equivalent to making a certain change of the probe's shape in the higher-dimensional space. The approximation is justified whenever this change of shape is “small enough.” In this light, we identify the dynamically relevant norm by which the magnitude of these changes in probe shape ought to be judged. Finally, we explore this approximation in particular setups corresponding to quantum optics and superconducting circuits.

It is not possible to obtain information about the observable properties of a quantum system without a physical interaction between the system and an external meter. This physical interaction is described by a unitary transformation of the joint quantum state of the system and the meter, which means that the information transfer from the system to the meter depends on the initial quantum coherence of the meter. In the present paper, we analyze the measurement interaction in terms of the changes of the meter state caused by the interaction with the system. The sensitivity of the meter can then be defined by evaluating the distinguishability of meter states for different values of the target observable. It is shown that the sensitivity of the meter requires quantum coherences in the generator observable that determines the magnitude of the backaction of the meter on the system. The tradeoff between measurement resolution and backaction is decided by the relation between sensitivity and quantum coherent uncertainty in the external meter. No matter how macroscopic the device used as a meter is, it must necessarily be quantum coherent in the degrees of freedom that interact with the system being measured.

The problem of discriminating between many quantum channels with certainty is analyzed under the assumption of prior knowledge of algebraic relations among possible channels. It is shown, by explicit construction of a novel family of quantum algorithms, that when the set of possible channels faithfully represents a finite subgroup of SU(2) (e.g., $C_n, D_{2n}, A_4, S_4, A_5$) the recently developed techniques of quantum signal processing can be modified to constitute subroutines for quantum hypothesis testing. These algorithms, for group quantum hypothesis testing, intuitively encode discrete properties of the channel set in SU(2) and improve query complexity at least quadratically in $n$, the size of the channel set and group, compared to naïve repetition of binary hypothesis testing. Intriguingly, performance is completely defined by explicit group homomorphisms; these in turn inform simple constraints on polynomials embedded in unitary matrices. These constructions demonstrate a flexible technique for mapping questions in quantum inference to the well-understood subfields of functional approximation and discrete algebra. Extensions to larger groups and noisy settings are discussed, as well as paths by which improved protocols for quantum hypothesis testing against structured channel sets have application in the transmission of reference frames, proofs of security in quantum cryptography, and algorithms for property testing.

We propose key-sifting algorithms for a continuous-variable quantum key distribution (CV-QKD) protocol using a machine-learning framework based on the isolation forest and a traditional digital signal-processing scheme based on the Wiener filter. In previous works, the key-sifting procedure performs by discarding bits on different measurement basis, but a few useless portions are still retained, so that the transmission distance of CV-QKD systems is limited by excess noise. Here, we develop sifting schemes based on anomaly detection, which provide better feasibility in noisy environments. We compare the detection results for both approaches and estimate the excess noise to confirm the utility of our proposal in real-life applications. Results obtained over a 100-km fiber link indicate that the key-sifting algorithms can ensure low excess noise. Moreover, some of the techniques we develop are applicable to other QKD protocols.

We simulate the photoemission from an electronically excited system by computing the escape of electron density in real space using time-dependent density functional theory in real time. We show that for a one-electron system, the angular resolved photoemission after an initial excitation can be interpreted as the mapping of a previously unoccupied orbital. For the molecule perylene-3,4,9,10-tetracarboxylic dianhydride, the angular resolved photoemission (ARPES) calculated after a preceding pump pulse reveals signatures of the many-particle character of the first electronic excitation: The photoemission results from more than one time-dependent orbital, and comparing the ARPES pattern to a particle-hole analysis of the first electronic excitation confirms that the excitation does not just correspond to one electron having been moved into a previously empty orbital, but is a superposition of several single-particle excitations.

We propose and experimentally demonstrate a noninterferometric method to quantitatively determine the topological charge and complete phase structure of a vortex beam scrambled by a random scattering medium. In the proposed technique, the vortex beam with an arbitrary topological charge coaxially enters into the scattering medium with another nonvortex beam with an orthogonal polarization state. This couples the spatial and polarization modes of the coherent light prior to entering the scattering medium. Correlation of the orthogonal polarization states of a random field encodes the signature of the twisted mode of the vortex beam. Recovery of the complex polarization correlation functions and hence twisted modes of the beam are retrieved from the Stokes parameters of a randomly scattered field. Two-point correlations of the Stokes parameters form a 4 × 4 matrix with a total of 16 elements. Out of these 16 elements, only four elements of the matrix are used to retrieve real and imaginary parts of the complex polarization correlation function and are subsequently applied in the reconstruction of twisted wavefronts. A complete theoretical basis is developed to retrieve the twisted wavefront of propagating light through random scattering media and is also supported by numerical simulation and experimental demonstration. As an application of the proposed technique, recovery of the complete phase maps of the different vortex beams from the random light is experimentally demonstrated and the twisted modes of the incident light are quantitatively analyzed using orthogonal projections. The experimental results are in good agreement with the theoretical basis and numerical results.

The Rabi Hamiltonian, describing the interaction between a two-level atomic system and a single-cavity mode of the electromagnetic field, is one of the fundamental models in quantum optics. The model becomes exactly solvable by considering an atom without permanent dipole moments, whose excitation energy is quasiresonant with the cavity photon energy, and by neglecting the nonresonant (counter-rotating) terms. In this case, after including the decay of either the atom or the cavity mode to a continuum, one can derive the well-known phenomenology of quasiresonant transitions, including the fluorescence triplets. In this work we consider the most general Rabi model, incorporating the effects of permanent atomic electric dipole moments. Based on a perturbative analysis, we compare the intensities of emission lines induced by rotating terms, counter-rotating terms, and parity-symmetry-breaking terms in order to identify the parameter regimes in which these different contributions play a significant role. The analysis reveals that the emission strength related to the existence of permanent dipoles may surpass the one due to the counter-rotating interaction terms but is usually much weaker than the emission due to the main, resonant coupling. This ratio can be modified in systems with a reduced dimensionality or by engineering the energy spectral density of the continuum.

We study energetic exchanges and fluctuations in two-stroke quantum thermodynamic engines where the working fluid is represented by two multilevel quantum systems, i.e., qudits, the heat flow is allowed by relaxation with two thermal reservoirs at different temperatures, and the work exchange is operated by a partial-swap unitary interaction. We identify three regimes of operation (heat engine, refrigerator, and thermal accelerator), present the thermodynamic uncertainty relations between the entropy production and the signal-to-noise ratio of work and heat, and derive the full joint probability of the stochastic work and heat. Our results bridge the gap between two-qubit and two-mode bosonic swap engines, and show which properties are maintained (e.g., a nonfluctuating Otto efficiency) and which are lost for increasing dimension (e.g., small violations of the standard thermodynamic uncertainty relations or the possibility of beating the Curzon-Ahlborn efficiency).

We investigate the properties of two anti-parity-time (anti-$\mathcal{PT}$)-symmetric four-site scattering centers. The anti-$\mathcal{PT}$-symmetric scattering center may have imaginary couplings, real couplings, and real on-site potentials. The only difference between the two scattering centers is the coupling between two central sites of the scattering center, which plays a crucial role in determining the parity of anti-$\mathcal{PT}$ symmetry and significantly affects the scattering properties. For the imaginary coupling, the even-parity anti-$\mathcal{PT}$-symmetric scattering center possesses nonunitary scattering and the difference between the reflection and transmission is unity; for the real coupling, the odd-parity anti-$\mathcal{PT}$-symmetric scattering center possesses unitary scattering and the sum of the reflection and transmission is unity. The coupling-induced different scattering behaviors are verified in the numerical simulations. Our findings reveal that a significant difference in the dynamics can be caused by a slight difference between two similar anti-$\mathcal{PT}$-symmetric non-Hermitian scattering centers.

Strong nonlocality based on local distinguishability is a stronger form of quantum nonlocality recently introduced in multipartite quantum systems: an orthogonal set of multipartite quantum states is said to have the property of strong nonlocality if it is locally irreducible for every bipartition of the subsystems. Most of the known results are limited to sets with product states. Shi et al. presented the first result of strongly nonlocal sets with entanglement [Phys. Rev. A 102, 042202 (2020)], and they questioned the existence of a strongly nonlocal set with genuine entanglement. In this work, we relate the strong nonlocality of some special set of genuine entanglement to the connectivities of some graphs. Using this relation, we successfully construct sets of genuinely entangled states with strong nonlocality. As a consequence, our constructions give a negative answer to Shi et al.'s question and also provide an answer to the open problem raised by Halder et al. [Phys. Rev. Lett. 122, 040403 (2019)]. This work associates a physical quantity named strong nonlocality with a mathematical quantity called graph connectivity.

The quantum level population behavior of two coupled flux qubits depending on the external driving field characteristics is studied. Explicit expressions for the multiphoton transition probabilities at an arbitrary control field amplitude are obtained for the case of small tunnel splitting energies. We describe the controllable features of their formation and thereby the creation or destruction of entanglement on the direct interlevel transition and during the transition through intermediate states by system bias tuning. We found a feature of the qubit population inversion that results in the independence of the resonance positions from the qubit coupling strength. Using the Floquet-Markov equation, we numerically demonstrate that the positions of multiphoton resonances are stable to dissipative processes.

Quantum computation has seen rapid progress in the past years. Due to implementation complexity and cost, cloud-based quantum computing is strongly believed to be the most feasible framework for individual users. Universal blind quantum computing (UBQC) provides the protocol for secure delegation of arbitrary quantum computations and has received significant attention. However, one of the major challenges in UBQC is a secure and reliable transmission of a quantum state over a long distance. Herein, we improve the efficiency of UBQC using alternative photon sources. Our analysis shows that a heralded single-photon source can significantly improve the transmitted distance of UBQC and a modified coherent source can reduce the required transmitted number of pulses. Furthermore, we study the finite-data-size effect, which is a crucial for real-world applications. Our proposed scheme can be a promising alternative for the future development of UBQC.

Neutral atoms are promising for large-scale quantum computing, but accurate neutral-atom entanglement depends on large Rydberg interactions which strongly limit the interatomic distances. Via a phase accumulation in detuned Rabi cycles enabled by a Rydberg interaction of similar magnitude to the Rydberg Rabi frequency, we study a controlled-phase gate with an arbitrary phase and extend it to the controlled-not gate. The gates need only three steps for coupling one Rydberg state, depend on an easily accessible van der Waals interaction that naturally arises between distant atoms, and have no rotation error in the weak interaction regime. Importantly, they can work with very weak interactions so that well-separated qubits can be entangled. The gates are sensitive to the irremovable fluctuation of Rydberg interactions, but can still have a fidelity over 98% with realistic position fluctuation of qubits separated over 20 $\mu \mathrm{m}$.