Band theory for partially coherent light is introduced by using the formalism of second-order classical coherence theory. It is demonstrated that the cross-spectral density function can have bands and gaps and form a correlation band structure. The propagation of a partially coherent beam in non-Hermitian periodic structures is considered to elucidate the interplay between the degree of coherence and the gain or loss present in the lattice. We apply the formalism to study partially coherent Bloch oscillations in lattices having parity-time symmetry.

The notion of classicality of quantum evolution of light is an object of both conceptual and practical importance. The main goal of this work is to derive the exact conditions for the classicality of quantum Gaussian evolution, i.e., the evolution of Gaussian states of light and their convex combinations, a model which is of great significance in quantum optics and information. According to our findings, quantum Gaussian evolution should be considered classical if the Hamiltonian and Lindblad operators generating it correspond to passive optical transformations. This is illustrated with several explicit examples, ranging from Gaussian thermal operations to entanglement-maximizing dissipative engineering. Our results are obtained using the recently introduced mesoscopic formalism of the reduced state of the field, which was originally devised as as a description of macroscopic quantum fields. Here, to make the framework suitable for our goal, we redevelop it as a tool for probing classicality, which constitutes our second main contribution.

We obtain the accuracy limit for estimating the expectation value of the position of a relativistic particle for an observer moving in one direction at a constant velocity. We use a specific model of a relativistic spin-1/2 particle described by a Gaussian wave function with a spin down in the rest frame. To derive the state vector of the particle for the moving observer, we use the Wigner rotation that entangles the spin and the momentum of the particle. Based on this wave function for the moving frame, we obtain the symmetric logarithmic derivative (SLD) Cramér-Rao bound that sets the estimation accuracy limit for an arbitrary moving observer. It is shown that estimation accuracy decreases monotonically in the velocity of the observer when the moving observer does not measure the spin degree of freedom. This implies that the estimation accuracy limit worsens with the observer's increasing velocity, but it is finite even in the relativistic limit. We derive the amount of this information loss by the exact calculation of the SLD Fisher information matrix in an arbitrary moving frame.

Quantum phase transition in the $XY$ spin model with three-spin interaction is investigated using magic resource (non-stabilizerness), which is crucial in universal fault-tolerant quantum computation. The magic quantifier we employ here is defined straightforwardly via characteristic functions of quantum states, which are well defined for all dimensional quantum systems (in sharp contrast to those defined by discrete Wigner functions) and can be easily calculated. We show that the magic quantifier of both the reduced single-site spins and two-site spins of the system ground state increase to their maximum around the critical points for quantum phase transition. This indicates that the magic resource can be used to detect the critical phenomena in the $XY$ spin model and reveals a connection between quantum phase transition in many-body systems and quantum resource in stabilizer quantum computation.

Heavy fluxonium at zero external flux has a long-lived state when coupled capacitively to any other system. We analyze it by projecting all the fluxonium-relevant operators into the qutrit subspace, as this long-lived configuration corresponds to the second excited fluxonium level. This state becomes a bound state in the continuum (BIC) when the coupling occurs to an extended system supporting a continuum of modes. In the case without noise, we find BIC lifetimes $T_1$ that can be much larger than seconds when the fluxonium is coupled to a superconducting waveguide, while typical device frequencies are on the order of gigahertz. We have performed a detailed study of the different sources of decoherence in a realistic experiment, finding that upwards transitions caused by a finite temperature in the waveguide and decay induced by $1/f$ flux noise are the most dangerous ones. Even in their presence, BIC decay times could reach the range of $T_1\sim {10}^{-1}\mathrm{ms},$ while preparation times are of the order of ${10}^2\mathrm{ns}$.

We analyze three quantum communication protocols that have been proposed in the literature and compare how well they communicate single-rail entanglement. We use specific metrics for output-state purity and probability of success and include the presence of imperfect photon source and detection components. We find that a distributed noiseless linear amplification (NLA) protocol with a relay point placed halfway between Alice and Bob outperforms NLA at Bob's end and a recently proposed purification protocol under most conditions, unless the distance is very small or the photon source component is very good.

The second law of thermodynamics prohibits spontaneous heat from a cold to a hot body. However, it has been theoretically and experimentally shown that energy can flow from a cold to a hot body if the bodies are initially correlated. We investigated the anomalous energy exchange between dissipation-less quantum systems that are initially entangled. Then, we extended this model to include dissipation demonstrating anomalous heat from a cold to a hot body. Based on these models, we constructed a heat pump driven entirely by quantum correlation as fuel and investigated its performance with numerical simulations. Using the recently proposed definition of efficiency based on mutual information, the performance of the pump is found to be consistent with the second law of thermodynamics.

We propose a nuclear-spin-polarization protocol in a general evolution-and-measurement framework. The protocol works in a spin-star configuration, where the central spin is coupled to the surrounding bath (nuclear) spins by flip-flop interaction of equal strength and is subject to a sequence of projective measurements on its ground state. Then a nondeterministic nuclear spin polarization could be implemented by entropy reduction through measurement. The optimized measurement-interval ${\tau }_{\mathrm{opt}}$ is analytically obtained in the near-resonant condition, which is relevant to the nuclear spins' polarization degree of the last-round measurement, the number of nuclear spins, and the coupling strength between the central spin and nuclear spins. Hundreds and even thousands of randomly aligned nuclear spins at the thermal state could be almost fully polarized with an optimized sequence of less than 20 unequal-time-spacing measurements. In comparison to the conventional approaches, our protocol is not sensitive to the magnetic-field intensity, and it is robust against the extra counterrotating interaction in the near-resonant situation.

The imaging process and its corresponding state estimation are treated as parallel mode channels and two estimation subproblems, respectively. In the two subproblems, the displacement vector and covariance matrix of the object field are estimated independently from the measured ones of the aperture field by minimizing their respective cost functions defined quantum mechanically. The measurement scheme is also briefly discussed. As an application, the location estimation of a point feature of a disk object is studied analytically and numerically. The results show that the estimation that uses only the displacement vector does not enhance the resolution of the amplitude image to a large extent. On the contrary, the quantum Cramér-Rao bound of spatial resolution can be pushed down toward the uncertainty relation. Furthermore, the high quantum fidelities verify the effectiveness of the estimation and the broad applicability to arbitrary multimode Gaussian states.

Precision spectroscopy of molecular hydrogen can be used to test the quantum electrodynamics theory and determine the proton-to-electron mass ratio in a four-body system. Saturated absorption spectroscopy of the R(1) line in the (2-0) band of HD is measured at 76 K. An asymmetric line profile is observed and analyzed, and the transition frequency is predicted to be $217105182284{\left(11\right)}_{\mathrm{stat}}{\left(27\right)}_{\mathrm{sys}}$ kHz. The value is nine times more accurate than our previous result obtained at room temperature and differs by $6.6\sigma$ from the recent result derived with a different line-shape model.

The nonlinear response of an atomic system interacting with short extreme ultraviolet (XUV) and infrared (IR) pulses is considered within the adiabatic approach (for an IR field) and the perturbation theory (for an XUV pulse). The second-order perturbation theory in the XUV field is developed for the IR-dressed atom, and the laser-induced generation of a pulse with a doubled carrier XUV frequency is analyzed. We study the yield of harmonics in the frequency band of the generated doubled frequency XUV pulse as a function of the time delay between initial XUV and IR pulses. Our analytical and numerical results show that this dependence carries the time dependence of the intense IR pulse and can be utilized to retrieve the waveform of the IR pulse.

We investigate theoretically the collective radiance characteristics of an atomic ensemble with the simultaneous decay of two atoms. We show that the two-atom decay can significantly suppress the steady-state collective radiance of the atoms, expanding the region of subradiance. In the steady-state subradiance regime, the system is in an entangled state, and the mean populations of the system in the excited state and ground state of the atoms are almost equal. The processes of the two-atom decay can be demonstrated by the population distribution of the system state on the Dicke ladder. Moreover, we show the correlation property of the emitted light from the atomic ensemble, where the correlation function is rewritten in the presence of the two-atom decay. We find that the emitted photons of steady state only show bunching in the case of two-atom decay. This work broadens the realm of collective radiance, with potential applications for quantum information processing.

The space-time connection between the Mollow cascaded transitions with time orderings and the spatially directional correlated emissions of Mollow photons is analyzed theoretically in a strong laser-dressed subwavelength two-atom antenna, which supports only two-level electric dipole transitions. It can be regarded as the most prototypical collective quantum radiator to exploit the space-time quantum characteristics of the Mollow physics of resonance fluorescence. By constructing the photon filtering dynamics from the dressed two-atom antenna to a pair of quantum sensors, the frequency-resolved spatial two-point two-time correlations of different Mollow spectral lines are examined analytically in terms of the quantum sensor modes. It is shown that a rich significance of spatial directionality lies in the time orderings of cascaded emissions between different Mollow spectral lines. This feature enables the time orderings of different Mollow spectral lines to be tailored geometrically and thus prepared deterministically to produce highly directional temporal bunching effects of the Mollow photons with giant nonclassicality only for a specific two-photon detection order. The dressed two-atom system is therefore able to operate as a highly directional atomic antenna that may provide versatile applications in the fields of atom-photon-based quantum precise measurement physics and Mollow spectroscopy.

Distinct from Bell's approach, Wigner derived a form of local realist (LR) inequality which is quantum-mechanically violated for a bipartite maximally entangled state. Subsequently, this approach was generalized to obtain a multipartite LR inequality. However, the violation of such a generalized Wigner inequality does not guarantee nonlocality between all possible different bipartitions of the multipartite system. In the present paper, this limitation has been overcome by formulating a further generalization of Wigner's approach through the derivation of a set of LR inequalities with respect to all different bipartitions of a $N$-partite system. Quantum-mechanical violations of all individual LR inequalities belonging to such a set would rigorously certify multipartite nonlocality by also providing a finer characterization of the nature of multipartite nonlocality in the following sense. The quantum-mechanical violation of any given inequality of our complete set of LR inequalities would enable identification of the corresponding bipartition which exhibits nonlocality. This is in contrast to other multipartite LR inequalities such as the Svetlichny inequality or its generalization that cannot be used to detect whether there is any particular bipartition which is nonlocally correlated. The efficacy of the scheme developed in this paper is illustrated for the tripartite and quadripartite states.

The Kitaev honeycomb model is a system allowing for experimentally realizable quantum computation with topological protection of quantum information. Practical implementation of quantum information processing typically relies on adiabatic, i.e., slow dynamics. Here we show that the restriction to adiabatic dynamics can be overcome with optimal control theory, enabled by an extension of the fermionization of the Kitaev honeycomb model to the time-dependent case. Moreover, we present a quantum control method that is applicable to large lattice models due to subexponential scaling.

Nonadiabatic holonomic quantum computation is a method used to implement high-speed quantum gates with non-Abelian geometric phases associated with paths in state space. Due to their noise tolerance, these phases can be used to construct error resilient quantum gates. We extend the holonomic dark path qubit scheme in [M.-Z. Ai et al., Fundam. Res. 2, 661 (2022)] to qudits. Specifically, we demonstrate one- and two-qudit universality by using the dark path technique. Explicit qutrit ($d=3$) gates are demonstrated and the scaling of the number of loops with the dimension $d$ is addressed. This scaling is linear and we show how any diagonal qudit gate can be implemented efficiently in any dimension.

We introduce a proposal to prepare spin-obit maximally discordant mixed states by a linear optical circuit, with quantum bits (qubits) encoded in the polarization and transverse mode degrees of freedom of photons. In particular, we discuss how to prepare nonbalanced spin-orbit entangled states, applying this technique to obtain maximally discordant mixed states. We present a simulation of the optical circuit by using the Jones matrix formalism. We performed a study of entanglement, classical, and quantum correlations. The results show excellent agreement with the underlying theory and open an alternative experimental approach for addressing quantum correlations in optical setups.

Insight into enthalpic and entropic contributions to the activation free energy of noncovalent bonds is crucial for understanding various phenomena in molecular systems, from formation of different molecular conformers to complex biophysical and biochemical processes. Standard experimental and computational methods provide only the average strength of noncovalent bonds. In this paper, we present a simple, formal, computational framework to determine both energy contributions for a single bond. Our approach is based on steered molecular dynamics simulations, the dynamic force spectroscopy method, and a modified theoretical model of force-induced bond rupture. To demonstrate the methodology, the enthalpic and entropic components for a single intramolecular hydrogen bond of a peptide helix were determined. The sum of the contributions is in agreement with the previously reported Gibbs energy for intramolecular hydrogen bonds. The proposed methodological framework is universal and can be applied to other noncovalent interactions in various molecular systems.

YbF has been identified as a molecule that can be used to investigate charge-parity symmetry violations that are beyond the standard model of particle physics. Cooling to sub-milli-Kelvin is advantageous for experiments that probe manifestations of these symmetry violations. One approach involves laser cooling of YbF via the $A^2{\mathrm{\Pi }}_{1/2}-X^2{\mathrm{\Sigma }}^{+}$ transition. However, it appears that cooling by means of this transition may be limited by the radiative loss of population from the cooling cycle. YbF has low-energy states that arise from the ${\mathrm{Yb}}^{+}\left(4f^{13}6s^2\right){\mathrm{F}}^{-}$ configuration. Recent theoretical calculations [C. Zhang et al., J. Mol. Spectrosc. 386, 111625 (2022)] predict that radiative decay from $A^2{\mathrm{\Pi }}_{1/2}$ to the $4f^{13}6s^2$ states occurs with a branching fraction of approximately ${10}^{-3}$. In the present study we have used dispersed laser induced fluorescence spectroscopy to observe the lowest energy $4f^{13}6s^2$ states. These measurements were carried out using excitation of previously unobserved YbF transitions in the near UV spectral range. An accurate ionization energy (IE) of $48703\pm 5\mathrm{c}{\mathrm{m}}^{-1}$ for YbF is also reported. A two-color photoionization technique was used to determine the IE and observe the $v^{+}=0\text{−}3$ vibrational levels of $\mathrm{Yb}{\mathrm{F}}^{+}{X}^1{\mathrm{\Sigma }}^{+}$.