Generalized contextuality is a resource for a wide range of communication and information processing protocols. However, contextuality is not possible without coherence, and so can be destroyed by dephasing noise. Here, we explore the robustness of contextuality to partially dephasing noise in a scenario related to state discrimination (for which contextuality is a resource). We find that a vanishing amount of coherence is sufficient to demonstrate the failure of noncontextuality in this scenario, and we give a proof of contextuality that is robust to arbitrary amounts of partially dephasing noise. This is in stark contrast to partially depolarizing noise, which is always sufficient to destroy contextuality.

We present a resource theory to investigate the power of a multqubit system as a probe in the task of dephasing estimation. Our approach employs the quantum Fisher information about the dephasing parameter as the resource measure. Based on the monotonicity of quantum Fisher information, we propose two sets of free operations in our resource theory: the Hamming distance preserving operations and the selectively Hamming distance preserving operations. We derive a necessary condition for the state transformation under these free operations and demonstrate that uniform superposition states are the golden states in our resource theory. We further compare our resource theory with the resource theory of coherence and thoroughly investigate the relation between their free operations in both single-qubit and multiqubit cases. Additionally, for multiqubit systems, we discover the incompatibility between the resource theory of dephasing estimation and that of $\text{U}\left(1\right)$ asymmetry, which is responsible for phase estimation. The condition for enhancing the performance of a probe state in phase estimation while preserving its ability in dephasing estimation is also discussed. Our results provide insights into quantum parameter estimation by the resource-theoretic approach.

Weak value amplification permits small effects to be measured as observable changes at the sacrifice of power due to postselection. The power-recycling scheme has been proven to eliminate this inefficiency of the rare postselection, thus surpassing the limit of the shot noise and improving the precision of the measurement. However, the improvement is strictly limited by the system setup, especially the system loss. Here we introduce a dual-recycling model based on the interferometric weak-value-based deflection measurement. Two mirrors, the power-recycling mirror and signal-recycling mirror, are placed at the bright and dark port of the interferometer, respectively, creating a composite resonator. The results show that both the power and the signal-to-noise ratio (SNR) are greatly enhanced in a wider range of experimental parameters compared to the power-recycling scheme. This work considerably loosens the constraint of the system setup and further explores the real advantage of weak measurement over traditional schemes.

The goal of quantum tracking control is to identify shaped fields to steer observable expectation values along designated time-dependent tracks. The fields are determined via an iteration-free procedure, which is based on inverting the underlying dynamical equations governing the controlled observables. In this paper, we generalize the ideas in [Phys. Rev. A 98, 043429 (2018)] to the task of orienting symmetric top molecules in three dimensions. To this end, we derive equations for the control fields capable of directly tracking the expected value of the three-dimensional dipole orientation vector along a desired path in time. We show this framework can be utilized for tracking the orientation of linear molecules as well, and present numerical illustrations of these principles for symmetric-top tracking control problems.

We report a study of the quantum beats in two-color photoionization of argon. An attosecond extreme ultraviolet pulse train prepares an electronic wave packet of definite odd parity, with total angular momentum $J=1$, targeting the states between 14.0 and 14.5 eV from the ground state. Two-photon ionization of this wave packet with a tunable infrared probe pulse makes the constituent states interfere in both continuum channels, corresponding to the core angular momenta $j_c=1/2$ and $3/2$, respectively. We analyze photoelectron spectrograms as a function of the time delay of the probe pulse and identify oscillations due to several pairs of states through Fourier decomposition. We observe phase differences between the corresponding beat signals in the two spin-orbit split continua. Comparison of theoretical simulations with the experimental measurements allows us to interpret the amplitudes and phases of ionization signals. We express the observed phase differences in terms of the off-diagonal elements of the short-range scattering matrix and the dipole matrix elements to the continuum eigenchannels.

A free-space-to-chip pipeline is proposed for the efficient transportation of single atoms from a magneto-optical trap to an on-chip evanescent-field trap. Due to the reflection of the dipole laser on the chip surface, the conventional conveyor-belt approach can transport only atoms in close proximity to the chip surface, with a distance of about one wavelength. This limitation hinders the efficient interaction between the atom and the on-chip waveguide devices. Here, a two-layer photonic chip architecture is proposed to realize a free-space-to-chip atom pipeline. By employing a diffraction beam of the integrated grating with an incident angle of the Brewster angle, the reflection of the dipole laser is suppressed, allowing the atoms to be brought to the chip surface with a distance of only $100\mathrm{nm}$. The pipeline is verified numerically and provides a reliable atom source for a hybrid photonic-atom chip.

We study the stationary and excitation properties of a one-dimensional quantum droplet in a two-component Bose mixture trapped in a harmonic potential. By constructing the energy functional for the inhomogeneous mixture, we elaborate the extended Gross-Pitaevskii equation applicable to both symmetric and asymmetric mixtures into a universal form, and the equations in two different dimensionless schemes are in a duality relation; that is, the unique parameters that are left are the inverse of each other. The Bogoliubov equations for the trapped droplet are obtained by linearizing the small density fluctuation around the ground state, and the low-lying excitation modes are calculated numerically. It is found that the confinement trap easily changes the flat-top structure for large droplets and intensively alters the mean-square radius and the chemical potential. The breathing mode of the confined droplet connects the self-bound and ideal-gas limits, with the excitation in the weakly interacting Bose condensate for large particle numbers lying in between. We explicitly show how the continuum spectrum of the excitation is split into discrete modes and finally taken over by the harmonic trap. Two critical particle numbers are identified by the minimum size of the trapped droplet and the maximum breathing-mode energy, both of which are found to decrease exponentially with the trapping parameter.

We report on standoff detection of dc electric field by bidirectional cavity-free lasing emission of neutral nitrogen molecules excited by intense circularly polarized femtosecond laser pulses. We observed that both the backward and forward 337.4 nm coherent lasing emission present a monotonous dependence on the strength of a remotely applied dc field up to $\sim 1$ kV/cm field strength. Moreover, this method shows a dependence on the polarity of the external dc field, providing a sensitive method for remote characterization of the electric field amplitude and direction. We attribute the underlying mechanism of lasing signal modulation to the electric field induced electron acceleration and deceleration, which results in a variation of the kinetic energy of the free electrons and a modulation of the population inversion responsible for the nitrogen molecules’ lasing. The polarity-sensitive detection anisotropy is interpreted by the symmetry breaking of the electron motion in the plane perpendicular to the laser propagation due to the injection of a weak second harmonic laser field produced in the quarter-wave plate for a circularly polarized pump laser. Numerical simulations based on the two-dimensional time-dependent Schrödinger equation for electron kinetic energy support our interpretation. This study provides a proof-of-principle method for standoff detection of electric fields based on nitrogen lasing, which can be potentially useful for atmospheric and metrological applications.

Numerical transfer matrices have been widely used in the study of wave propagation and scattering. These may be viewed as discretizations of a recently introduced fundamental notion of transfer matrix which admits a representation in terms of the evolution operator for an effective nonunitary quantum system. We use the fundamental transfer matrix to develop a general method for the solution of the problem of radiation of an oscillating source in an arbitrary, possibly nonhomogeneous, anisotropic, and active or lossy linear medium. This allows us to obtain an analytic solution to this problem for an oscillating source located in the vicinity of a planar collection of possibly anisotropic and active or lossy point scatterers such as those modeling a two-dimensional photonic crystal.

The Pauli stabilizer formalism is perhaps the most thoroughly studied means of procuring quantum error-correcting codes, whereby the code is obtained through commutative Pauli operators and “stabilized” by them. In this work we will show that every quantum error-correcting code, including Pauli stabilizer codes and subsystem codes, has a similar structure, in that the code can be stabilized by commutative “Paulian” operators which share many features with Pauli operators and which form a Paulian stabilizer group. By facilitating a controlled gate we can measure these Paulian operators to acquire the error syndrome. Examples concerning codeword stabilized codes and bosonic codes will be presented; specifically, one of the examples has been demonstrated experimentally and the observable for detecting the error turns out to be Paulian, thereby showing the potential utility of this approach. This work provides a possible approach to implement error-correcting codes and to find new codes.

For electro-optical modeling in solids, the transition dipole phase and the related Berry connection carry important symmetry information about the system. This is especially important in the extremely nonlinear optics regime. One example is the even order harmonic intensity in high harmonic generation from solids. Semiconductor Bloch equations are widely used as theoretical model of this phenomenon. Recently, what has been denoted as the “random-phase problem” or “random gauge” in density-functional theory has gained increasing attention, since it complicates phase sensitive modeling within this framework. Here, we propose and evaluate a phase-sensitive approximation based on a triple dipole product as a solution to this problem. We further evaluate the approximation in a semiconductor Bloch equations model for high harmonic generation in GaAs and compare it to the twisted parallel transport gauge.

We provide a description of the space-time reflection operator ($PT$) acting on the two-dimensional polarization space of light represented by the linear algebra of Jones vectors and matrices. We establish the form of a $PT$-symmetric Jones matrix. We present two examples of laser resonators whose polarization eigenstates are described by $PT$-symmetric Jones matrices: one is based on the Faraday effect and a dichroic attenuation, while the other is made of twisted anisotropic mirrors. Both possess a control parameter that experimentally covers the exact and the broken $PT$-symmetry regions and their boundary, called an exceptional point, where the eigenstates of the resonator coalesce into a single state. The exact $PT$-symmetric region produces laser polarization modes emitting at the same frequency with different intracavity losses, while the broken $PT$-symmetric region features polarization modes emitting at distinct frequencies with the same intracavity losses. By applying unitary transformations, the concept of a $PT$-symmetric Jones matrix is extended to matrices that commute with any antiunitary operator, thereby opening the prospect of a larger family of resonator geometries that also feature real or complex-conjugate spectra.

We investigate Leggett-Garg inequalities for neutrino oscillations in the quantum field theoretical setting. We derive an exact flavor-mass uncertainty relation and prove that this uncertainty product yields an upper bound to the violation of the inequalities. The relation between temporal nonclassicality and quantum uncertainty realizes the Lüders upper bound to the violation of the Leggett-Garg inequalities in quantum field theory, analogous to the Tsirelson upper bound to the violation of the Bell inequalities. By studying the problem both in the exact field-theoretical setting and in the limiting quantum mechanical approximation, we show that on average the inequalities are violated more often and more strongly in quantum field theory than in quantum mechanics.

Observational entropy is interpreted as the uncertainty an observer making measurements associates with a system. So far, properties that make such an interpretation possible rely on the assumption of ideal projective measurements. We show that the same properties hold even when considering generalized measurements. Thus, the interpretation still holds: Observational entropy is a well-defined quantifier determining how influential a given series of measurements is in information extraction. This generalized framework allows for the study of the performance of indirect measurement schemes, which are those using a probe. Using this framework, we first analyze the limitations of a finite-dimensional probe. Then we study several scenarios of the von Neumann measurement scheme, in which the probe is a classical particle characterized by its position. Finally, we discuss observational entropy as a tool for quantum-state inference. Further developed, this framework could find applications in quantum information processing. For example, it could help in determining the best read-out procedures from quantum memories and providing adaptive measurement strategies alternative to quantum-state tomography.

One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this paper, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure with communication at room temperature up to distances of around 200 m, limited by the total coupled noise photon number. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelengths and imperfect weather conditions.

The formal correspondences between pseudo-spin-1/2 systems in optics and in atomic physics have provided fertile ground for exploring polarization in atom optics. Previous experimental results demonstrated atomic polarimetry techniques for two-level wave functions and explored wave-field singularities in the multicomponent wave function of a spinor Bose-Einstein condensate using visualization techniques developed in optics [J. T. Schultz, A. Hansen, and N. P. Bigelow, Opt. Lett. 39, 4271 (2014); A. Hansen, J. T. Schultz, and N. P. Bigelow, Optica 3, 355 (2016)]. Here we further this discussion, reexamining the atomic Stokes parameters and Stokes polarimetry in the context of spin-1 systems where the tensor moments of the higher-spin system enrich the physics considerably. We show that our atomic polarimetry methods provide tools to engineer the multipole tensor moments of the higher-spin atomic wave function, and to realize nontrivial couplings between these multipole moments and an atomic center-of-mass orbital angular momentum. The different forms of coupling between internal tensor moments and external angular momenta can be used to realize a variety of topological structures in the atomic wave field. We identify these $\pi$- and $2\pi$-symmetric features in the streamlines of the atomic Stokes fields constructed in analogy with singular optics.

Zero-index materials (ZIMs) have many peculiar properties such as supercoupling, directional radiation, and large optical nonlinearity because of the near-zero electromagnetic parameters. Additionally, the index contrast between the cavity and the ZIM can approach infinity, resulting in exotic properties such as resonance nesting and mode degeneracy. However, the mode characteristics of bilayer spherical dielectric cavities within ZIMs have not been studied thoroughly. Here, we investigate the resonance nesting and mode degeneracy effect in bilayer spherical dielectric cavities embedded in ZIMs. First, we analytically deduce the Mie resonance condition for each eigenmode of the bilayer spherical dielectric cavities. Based on the resonance conditions, we find the resonance nesting effect, in which a set of cavities with different outer radii can possess the same type of resonance at the same wavelength. We also reveal the mode degeneracy effect, wherein the modes with different angular mode numbers such as $2^l-\mathrm{TM}$ and $2^{l+1}-\mathrm{TE}$ can be degenerated. Superior to monolayer cavities, the electric field distribution in bilayer cavities can be manipulated by changing the inner radius. The mode degeneracy in the bilayer cavity only happens with some specific radii, which is different from the case in the monolayer cavity where the mode degeneracy always exists. These results can not only improve our understanding of cavities within ZIMs, but can also have potential applications in deformable devices and quantum information processes.

Recent experiments have revealed ultrastrong coupling between light and matter as a promising avenue for modifying material properties, such as electrical transport, chemical reaction rates, and even superconductivity. Here, we explore (ultra)strong coupling as a means for manipulating the optical response of metamaterials based on ensembles of constituent units individually in the ultrastrong-coupling regime. We develop a framework based on linear response for quantum electrodynamical systems to study how light-matter coupling affects the optical response. We begin by applying this framework to find the optical response of a two-level emitter coupled to a single cavity mode, which could be seen as a “meta-atom” of a metamaterial built from repeated units of this system. We find optical behavior ranging from that of a simple two-level system (Lorentz-oscillator) to effectively transparent, as the coupling goes from the weak to deep strong-coupling regimes. We explore a one-dimensional chain of these meta-atoms, demonstrating the tunability of its optical behavior. Our scheme may ultimately provide a framework for designing metamaterials with low-loss, highly confined modes, as well as tunable (single-photon) nonlinearities.