Recently, some attention has been given to the so-called Page-Wootters mechanism of quantum clocks. Among the various proposals to explore the mechanism using more modern techniques, some have chosen to use a quantum information perspective, defining and using informational measures to quantify how well a quantum system can stand as a reference frame for other quantum system. In this work, we explore the proposal based on a resource theory of asymmetry, known as mutual or shared asymmetry, which actually is equivalent to the approach from coherence theory in the case of interest here: quantum reference frames described by the $\text{U}\left(1\right)$ compact group. We extend some previous results in the literature about shared asymmetry and the Page-Wootters mechanism to more general cases, culminating in the enunciation of a theorem relating shared asymmetry of a bipartite state ${\rho }_{SR}$ with the relative entropy of entanglement of internal states ${\rho }_M$ on the charge sectors of the Hilbert space ${\mathcal{H}}_S\otimes {\mathcal{H}}_R$. Using this result, we reinterpret the relation between the Page-Wootters mechanism and entanglement, and we also open some paths to further studies.

Spin systems such as silicon and nitrogen-vacancy centers in diamond, quantum dots, and quantum dot molecules coupled to optical cavities appear to be key elements for creating quantum networks by not only constituting the nodes of the network but also assisting the creation of photonic networks. Here we study the deterministic preparation of arbitrary-size $W$ states with spin systems. We present an efficient operation on three qubits, with two being the logical qubits and one being the ancillary qubit, where no interaction between the logical qubits is required. The proposed operation can create a $W$-type Einstein-Podolsky-Rosen (EPR) pair from two separable qubits and expand that EPR pair or an arbitrary-size $W$ state by one, creating a $W$-like state. Taking this operation as the fundamental building block, we show how to create a large-scale $W$ state out of separable qubits or double the size of a $W$ state. Based on this operation and focusing on nitrogen-vacancy centers in diamond as an exemplary spin system, we propose a setup for preparing $W$ states of circularly polarized photons, assisted by a single-spin qubit, where no photon-photon interactions are required. Next, we propose a setup for preparing $W$ states of spin qubits of spatially separated systems, assisted by a single photon. We also analyze the effects of possible imperfections in implementing the gates on the fidelity of the generated $W$ states. In our setups, neither postmeasurement nor postprocessing the states of spin or photonic qubit is required. Our setups can be implemented with current technology, and we anticipate that they will contribute to quantum science and technologies.

We consider a scenario where a party, say, Alice prepares a pure two-qubit (either maximally entangled or nonmaximally entangled) state and sends one half of this state to another distant party, say, Bob through a qubit (either unital or nonunital) channel. Finally, the shared state is used as a teleportation channel. In this scenario, we focus on characterizing the set of qubit channels with respect to the final state's efficacy as a resource of quantum teleportation (QT) in terms of maximal average fidelity and fidelity deviation (fluctuation in fidelity values over the input states). Importantly, we point out the existence of a subset of qubit channels for which the final state becomes useful for universal QT (having maximal average fidelity strictly greater than the classical bound and having zero fidelity deviation) when the initially prepared state is either useful for universal QT (i.e., for a maximally entangled state) or not useful for universal QT (i.e., for a subset of nonmaximally entangled pure states). Interestingly, in the latter case, we show that nonunital channels (dissipative interactions) are more effective than unital channels (nondissipative interactions) in producing useful states for universal QT from nonmaximally entangled pure states.

Quantum plasmonic systems suffer from significant decoherence due to the intrinsically large dissipative and radiative dampings. Based on our quantum simulations $via$ a quantum tensor network algorithm, we numerically demonstrate the mitigation of this restrictive drawback by hybridizing a plasmonic nanocavity with an emitter ensemble with inhomogeneously broadened transition frequencies. By burning two narrow spectral holes in the spectral density of the emitter ensemble, the coherent time of Rabi oscillation for the hybrid system is increased tenfold. With the suppressed decoherence, we move one step further in bringing plasmonic systems into practical quantum applications.

We aim to explore the scenario that a single Alice steers multiple Bobs by using weak measurements. The multiple violations of two different Einstein-Podolsky-Rosen steering inequalities were discussed. Based on unbiased inputs, a single Alice can steer two Bobs when the double violation of the Clauser-Horne-Shimony-Holt-like inequality is realized. Meanwhile, three Bobs can be steered by a single Alice when the triple violation of the three-settings linear steering inequality is observed. More importantly, we theoretically inferred the analytical formula of the maximal violation for the corresponding inequality, which applied an arbitrary two-qubit state.

We investigate a total thermodynamic entropy production rate of an isolated quantum system. In particular, we consider a quantum model of coupled harmonic oscillators in a star configuration, where a central harmonic oscillator (system) is coupled to a finite number of surrounding harmonic oscillators (bath). In this model, when the initial state of the total system is given by the tensor product of the Gibbs states of the system and the bath, every harmonic oscillator is always in a Gibbs state with a time-dependent temperature. This enables us to define time-dependent thermodynamic entropy for each harmonic oscillator and total nonequilibrium thermodynamic entropy as the summation of them. We analytically confirm that the total thermodynamic entropy satisfies the third law of thermodynamics. Our numerical solutions show that, even when the dynamics of the system is well approximated by the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL)-type Markovian master equation, the total thermodynamic entropy production rate can be negative, while the total thermodynamic entropy satisfies the second law of thermodynamics. This result is a counterexample to the common belief that the total entropy production rate is non-negative when the system is under the GKSL-type Markovian dynamics.

We provide a rigorous construction of Markovian master equations for a wide class of quantum systems that encompass quadratic models of finite size, linearly coupled to an environment modeled by a set of independent thermal baths. Our theory can be applied for both fermionic and bosonic models in any number of physical dimensions and does not require any particular spatial symmetry of the global system. We show that, for nondegenerate systems under a full secular approximation, the effective Lindblad operators are the normal modes of the system, with coupling constants that explicitly depend on the transformation matrices that diagonalize the Hamiltonian. Both the dynamics and the steady-state (guaranteed to be unique) properties can be obtained with a polynomial amount of resources in the system size. We also address the particle and energy current flowing through the system in a minimal two-bath scheme and find that they hold the structure of Landauer's formula, being thermodynamically consistent.

We propose a quantum algorithm to solve systems of nonlinear differential equations. Using a quantum feature map encoding, we define functions as expectation values of parametrized quantum circuits. We use automatic differentiation to represent function derivatives in an analytical form as differentiable quantum circuits (DQCs), thus avoiding inaccurate finite difference procedures for calculating gradients. We describe a hybrid quantum-classical workflow where DQCs are trained to satisfy differential equations and specified boundary conditions. As a particular example setting, we show how this approach can implement a spectral method for solving differential equations in a high-dimensional feature space. From a technical perspective, we design a Chebyshev quantum feature map that offers a powerful basis set of fitting polynomials and possesses rich expressivity. We simulate the algorithm to solve an instance of Navier-Stokes equations and compute density, temperature, and velocity profiles for the fluid flow in a convergent-divergent nozzle.

We study the particle-entanglement dynamics witnessed by the quantum Fisher information (QFI) of a trapped Bose-Einstein condensate governed by the kicked rotor Hamiltonian. The dynamics is investigated with a beyond-mean-field approach. We link the time scales of the validity of this approximation in both classical regular and chaotic regions, with the maximum Lyapunov exponents of the classical system. This establishes an effective connection between the classical chaos and the QFI. We finally study the critical point of a quantum phase transition using the beyond-mean-field approximation by considering a two-mode bosonic Josephson junction with attractive interparticle interaction.

Quantum resource theories provide a diverse and powerful framework for extensively studying the phenomena in quantum physics. Quantum coherence, a quantum resource, is the basic ingredient in many quantum information tasks. It is a subject of broad and current interest in quantum information, and many new concepts have been introduced and generalized since its establishment. Here we show that the block coherence can be transformed into entanglement via a block-incoherent operation. Moreover, we find that the coherence based on positive-operator-valued measures (POVMs) associated with block coherence through the Naimark extension acts as a potential resource from the perspective of generating entanglement. Finally, we discuss avenues of creating entanglement from POVM-based coherence, present strategies that require embedding channels and auxiliary systems, give some examples, and generalize them.

Quantum illumination (QI) is a quantum sensing protocol mainly for target detection which uses entangled signal-idler photon pairs to enhance the detection efficiency of low-reflectivity objects that are immersed in thermal noisy environments. Due to the naturally occurring background radiation, the photon emitted toward potential targets more appropriately lies in the microwave region. Here, we propose a hybrid quantum source based on cavity magnonics for microwave QI, where the medium that bridges the optical and the microwave modes is magnon, the quanta of spin wave. Within experimentally accessible parameters, significant microwave-optical quantum resources of interest can be generated, leading to orders of magnitude lower detecting error probability compared with the electro-optomechanical prototype quantum radar and any classical microwave radar with equal transmitted energy.

In quantum illumination, the signal mode of light, entangled with an idler mode, is dispatched towards a suspected object bathed in thermal noise, and the returning mode, along with the stored idler mode, is measured to determine the presence or absence of the object. In this process, entanglement is destroyed but its benefits in the form of classical correlations and enlarged Hilbert space survive. Here, we propose the use of a probe state hyperentangled in two degrees of freedom, polarization and frequency, to achieve a significant 12-dB performance improvement in the error probability exponent over the best-known quantum illumination procedure in the low-noise regime. We present a simple receiver model using four optical parametric amplifiers (OPAs) that exploits hyperentanglement in the probe state to match the performance of the feed-forward sum-frequency generator (FF-SFG) receiver in the high-noise regime. By replacing each OPA in the proposed model with a FF-SFG receiver, a further 3-dB improvement in the performance of a lone FF-SFG receiver can be seen.

We investigate the manipulation of electron dynamics in high-order harmonic generation from semiconductor via resonant electron injection channels, which are formed by adding ultraviolet light to the midinfrared driving laser. In the presence of these channels, valence electrons far from the top of the valence band can be first preaccelerated to the channels and then effectively injected into the conduction band, leading to a dramatic extension of the cutoff of high harmonics even with a small vector potential of the driving field. The role of the channels in the electron dynamics is unambiguously demonstrated from the time-frequency analysis and time-dependent population imaging. By adjusting the frequency of the ultraviolet field, the harmonic cutoff can be tuned in a wide range. Besides, as compared to trivial semiconductors, one can achieve the cutoff extension with a lower frequency of the ultraviolet light for topological semiconductors owing to the emergences of extra energy levels in the middle of the band gap.

We have theoretically and experimentally investigated the laser-detected magnetic resonance induced by radio-frequency two-photon processes in $F_g=4$ of $D_1$ line of cesium atoms. The effective Hamiltonian for the interaction between the two radio-frequency magnetic fields and sublevels at the ground state of $F_g=4$ is derived. By appropriately selecting the frequencies of the two radio-frequency fields, the Raman two-photon process and the cascade two-photon process can occur, which induce the magnetic resonance detected through laser transmission spectra. The theoretical calculation results fit well with the experimental data. Our results maybe apply for measuring the high bandwidth radio-frequency magnetic fields in space and studying the magnetic-induction tomography.

We have investigated the performance of classical models in the description of the single and double ionization of argon induced by 4-fs, 750-nm linearly polarized laser pulses by comparing the calculated quantities with existing experimental data, primarily the results of the systematic measurements of Kübel at al. [Phys. Rev. A 93, 053422 (2016)], in the intensity range (1–10)$\times {10}^{14}{\mathrm{Wcm}}^{-2}$. The comparison involved different one- and two-electron models based on the classical ensemble approximation. The analysis covered single- and double-ionization probabilities and their ratios, longitudinal momentum distributions of the ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{2+}$ recoiled ions, and the carrier-envelope-phase-dependent asymmetry of the yield of the ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{2+}$ ions.

The annihilation of vortex-antivortex pairs is a key event in two-dimensional Bose-Einstein condensates (BECs). It is known that dissipation or a catalyst vortex is required for the annihilation of the pairs in one-component BECs. We numerically confirmed in two-component BECs that the pairs can be annihilated even without any dissipation or catalyst vortices when the intercomponent interaction is strong. In addition, the pair is recreated alternately in two components under certain conditions, which we call the recurrence.

Optical oligomers are clusters consisting of several nanoparticles that support multipolar resonances. Each multipole couples to all multipoles in every other particle, leading to formation of a set of oligomer quasinormal modes. Here we present an “optical downfolding” method for semianalytical study of the optical oligomer quasinormal modes in the spectral region of the individual nanoparticle dipole resonance. We considered a dimer and a linear trimer of dielectric nanopillars for demonstration. Our studies revealed the physical nature of the Fabry-Pérot–like modes which manifest themselves as fringes in dimer scattering spectra at large separations between the particles. We have also demonstrated these modes to coalesce at an exceptional point. Finally, our studies uncovered which quasinormal modes interact resulting in the appearance of a Fano resonance in the scattering spectrum of a linear nanopillar trimer. Our results were verified by finite-element-method simulations.

Circular dichroism is the differential rate of absorption of right- and left-handed circularly polarized light by chiral particles. Optical vortices which convey orbital angular momentum (OAM) possess a chirality associated with the clockwise or anticlockwise twisting of their wave front. Here it is highlighted that both oriented and randomly oriented chiral particles absorb photons from twisted beams at different rates depending on whether the vortex twists to the right or the left through a dipole coupling scheme. This is in contrast to previous studies that investigated dipole couplings with vortex modes in the paraxial approximation and showed no such chiral sensitivity to the vortex handedness: only in oriented media where electric quadrupole coupling contributes to optical activity effects due to absorption does such a mechanism exist for paraxial vortices. The distinct difference in the scheme highlighted in this work is that longitudinal fields are taken into account. Due to the vortex dichroism persisting in randomly oriented collections of chiral particles, the mechanism has a distinct advantage in its potential applicability in chemical and biochemical applications where the systems under study are invariably in the liquid phase. Additionally, the result is put into context in terms of the quantifiable optical chirality, highlighting that optical OAM can in fact increase the optical chirality density of an electromagnetic field.