Strongly interacting one-dimensional fermions form an effective spin chain in the absence of an external lattice potential. We show that the exchange coefficients of such a chain may be locally tuned by properly tailoring the transversal confinement. In particular, in the vicinity of a confinement-induced resonance (CIR) the exchange coefficients may have simultaneously opposite ferromagnetic and antiferromagnetic characters at different locations along the trap axis. Moreover, the local exchanges may be engineered to induce avoided crossings between spin states at the CIR, and hence a ramp across the resonance may be employed to create different spin states and to induce spin dynamics in the chain. We show that such unusual spin chains have already been realized in the experiment of Murmann et al. [Phys. Rev. Lett. 115, 215301 (2015)].

Mutual information is the reciprocal information that is common to or shared by two or more parties. Quantum mutual information for bipartite quantum systems is non-negative, and bears the interpretation of total correlation between the two subsystems. This may, however, no longer be true for three or more party quantum systems. In this paper, we propose an alternative definition of multipartite information, taking into account the shared information between two and more parties. It is non-negative, observes monotonicity under partial trace as well as completely positive maps, and equals the multipartite information measure in literature for pure states. We then define multiparty quantum discord, and give some examples. Interestingly, we observe that quantum discord increases when a measurement is performed on a large number of subsystems. Consequently, the symmetric quantum discord, which involves a measurement on all parties, reveals the maximal quantumness. This raises a question on the interpretation of measured mutual information as a classical correlation.

The non-Abelian geometric phases appear only in a degenerate subspace and the most familiar model with such a subspace is the $N$-pod system. Here we propose an alternative system to realize a non-Abelian gauge structure. We demonstrate that a three-level system with a triangular form can have two degenerate eigenstates which can induce non-Abelian geometric phases, and notably they are the lowest eigenstates of the system. As an application of the degenerate subspace, we propose an experimentally feasible scheme to realize a universal set of quantum gates based on the adiabatic non-Abelian geometric phases. To shorten the evolution time constrained by the adiabatic condition, the shortcut to adiabaticity is adopted to speed up the adiabatic geometric quantum gates. The combination of the intrinsic geometric characteristic and shortcut to adiabaticity makes the quantum operations fast and robust against certain control errors.

Due to the exponential complexity of the resources required by quantum state tomography (QST), people are interested in approaches towards identifying quantum states which require less effort and time. In this paper, we provide a tailored and efficient method for reconstructing mixed quantum states up to 12 (or even more) qubits from an incomplete set of observables subject to noises. Our method is applicable to any pure or nearly pure state $\rho$ and can be extended to many states of interest in quantum information processing, such as a multiparticle entangled $W$ state, Greenberger-Horne-Zeilinger states, and cluster states that are matrix product operators of low dimensions. The method applies the quantum density matrix constraints to a quantum compressive sensing optimization problem and exploits a modified quantum alternating direction multiplier method (quantum-ADMM) to accelerate the convergence. Our algorithm takes $8,35$, and 226 seconds, respectively, to reconstruct superposition state density matrices of $10,11,\text{and}12$ qubits with acceptable fidelity using less than $1\%$ of measurements of expectation. To our knowledge it is the fastest realization that people can achieve using a normal desktop. We further discuss applications of this method using experimental data of mixed states obtained in an ion trap experiment of up to 8 qubits.

We report a comparative study of low-lying valence electronic states of carbon dioxide by high-resolution inelastic x-ray and electron scattering. Momentum-transfer-dependent inelastic squared form factors for the two states ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ and ${}^1{\mathrm{\Pi }}_{\mathrm{u}}$ and generalized oscillator strength for the 9 eV feature from the ground state $X^1{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$ have been derived from the inelastic x-ray scattering method at an impact photon energy around 10 keV, and the electron energy-loss spectra measured at an incident electron energy of 1500 eV. It is found from the comparison between the present results and the previous outcomes that the recent calculations taking the vibronic effects into consideration satisfactorily reproduce the inelastic squared form-factor profile for the ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ transition and the generalized oscillator strength profile for the 9 eV feature. However, the vibronic effects seem to play no role in the ${}^1{\mathrm{\Pi }}_{\mathrm{u}}$ transition. The difference existing between the inelastic x-ray scattering and electron energy-loss spectroscopy results in the larger momentum-transfer squared region may be attributed to the increasing role of the higher-order Born terms. Furthermore, the controversy concerning the designations of electronic states around 11 eV is solved by assigning the two peaks centered at 10.98 and 11.05 eV to the vibrational progression ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{{+}^{\prime }}$ and ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$, and the peak centered at 11.16 eV to the forbidden transition $2^1{\mathrm{\Delta }}_{\mathrm{u}}$ based on the present results.

We present a joint experimental and theoretical study of the VUV-induced dynamics of ${\mathrm{H}}_2\mathrm{O}$ and its deuterated isotopologues in the first excited state ($\tilde{A}{}^1{B}_1$) utilizing a VUV-pump VUV-probe scheme combined with $ab$ initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation.

We propose a method to generate path-entangled NOON-state photons from quantum dots and coupled nanocavities. In the systems we considered, cavity mode frequencies are tuned close to the biexciton two-photon resonance. Under appropriate conditions, the system can have the target NOON state in the energy eigenstate, as a consequence of destructive quantum interference. The NOON state can be generated by a resonant laser excitation. This method, first introduced for a two-photon NOON state ($N=2$), can be extended toward a higher NOON state ($N>2$) based on our recipe, which is applied to the case of $N=4$ as an example.

Nanomechanical oscillators as well as Rydberg-atomic waveguides hosted on microfabricated chip surfaces hold promise to become pillars of future quantum technologies. In a hybrid platform with both, we show that beams of Rydberg atoms in waveguides can quantum coherently interrogate and manipulate nanomechanical elements, allowing full quantum state tomography. Central to the tomography are quantum nondemolition measurements using the Rydberg atoms as probes. Quantum coherent displacement of the oscillator is also made possible by driving the atoms with external fields while they interact with the oscillator. We numerically demonstrate the feasibility of this fully integrated on-chip control and read-out suite for quantum nanomechanics, taking into account noise and error sources.

Stimulated by suggestions of quantum effects in energy transport in photosynthesis, the fundamental principles responsible for the near-unit efficiency of the conversion of solar to chemical energy became active again in recent years. Under natural conditions, the formation of stable charge-separation states in bacteria and plant reaction centers is strongly affected by the coupling of electronic degrees of freedom to a wide range of vibrational motions. These inspire and motivate us to explore the effects of the environment on the operation of such complexes. In this paper, we apply the polaron master equation, which offers the possibilities to interpolate between weak and strong system-bath coupling, to study how system-bath couplings affect the exciton-transfer processes in the Photosystem II reaction center described by a quantum heat engine (QHE) model over a wide parameter range. The effects of bath correlation and temperature, together with the combined effects of these factors are also discussed in detail. We interpret these results in terms of noise-assisted transport effect and dynamical localization, which correspond to two mechanisms underpinning the transfer process in photosynthetic complexes: One is resonance energy transfer and the other is the dynamical localization effect captured by the polaron master equation. The effects of system-bath coupling and bath correlation are incorporated in the effective system-bath coupling strength determining whether noise-assisted transport effect or dynamical localization dominates the dynamics and temperature modulates the balance of the two mechanisms. Furthermore, these two mechanisms can be attributed to one physical origin: bath-induced fluctuations. The two mechanisms are manifestations of the dual role played by bath-induced fluctuations depending on the range of parameters. The origin and role of coherence are also discussed. It is the constructive interplay between noise and coherent dynamics, rather than the mere presence or absence of coherence or noise, that is responsible for the optimal heat engine performance. In addition, we find that the effective voltage of QHE exhibits superior robustness against the bath noise as long as the system-bath coupling is not very strong.

We study the loss of quantumness caused by time dilation [I. Pikovski, M. Zych, F. Costa, and Č. Brukner, Nat. Phys. 11, 668 (2015)] for a Schrödinger cat state. We give a holistic view of the quantum to classical transition by comparing the dynamics of several nonclassicality indicators, such as the Wigner function interference fringe, the negativity of the Wigner function, the nonclassical depth, the Vogel criterion, and the Klyshko criterion. Our results show that only two of these indicators depend critically on the size of the cat, namely, on how macroscopic the superposition is. Finally we compare the gravitation-induced decoherence times to the typical decoherence times due to classical noise originating from the unavoidable statistical fluctuations in the characteristic parameters of the system [J. Trapani, M. Bina, S. Maniscalco, and M. G. A. Paris, Phys. Rev. A 91, 022113 (2015)]. We show that the experimental observation of decoherence due to time dilation imposes severe limitations on the allowed levels of classical noise in the experiments.

We analyze the entanglement measure $C_4$ for specific mixed states in general and for the ground state of the transverse $XY$ spin-1/2 chain. We find that its factorizing property for pure states does not easily extend to mixed states. For cases where the density matrix is a tensor product, $C_4$ is definitely upper bounded by the product of the corresponding concurrences. In transverse $XY$ chains, we find that for large distances this condition goes conform with the working hypotheses of a factorizing property of density matrices in this limit. Additionally, we find that $C_4$ together with the genuine multipartite negativity makes it impossible to decide—at the present state of knowledge—which type of entanglement prevails in the system. In particular, this is true for all entanglement measures that detect SL-invariant genuine $n$-partite entanglement for different $n$. Further measures of SL-invariant genuine multipartite entanglement have to be considered here. $C_4$ is, however, of the same order of magnitude as the genuine multipartite negativity in Phys. Rev. B 89, 134101 (2014) and shows the same functional behavior, which we read as a hint towards the Greenberger-Horne-Zeilinger (GHZ) type of entanglement. Furthermore, we observe an interesting feature in the $C_4$ values that resembles a destructive interference with the underlying concurrence.

The dissociation of the molecular ion ${\mathrm{HD}}^{+}$ in moderate-intensity laser pulses is studied using the time-dependent quantum wave-packet method. Simulations of the time-of-flight (TOF) neutral atom spectra produced from dissociation of the $v=10$ vibrational state at a laser intensity of ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and from $v=0$ at the intensity of $5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ are calculated. Good agreement is found with previous experimental results [P. A. Orr et al., Phys. Rev. Lett. 98, 163001 (2007) and J. D. Alexander et al., J. Phys. B 42, 154027 (2009)]. Furthermore, the kinetic energy distribution of the dissociated fragments of molecular ion ${\mathrm{HD}}^{+}$ is studied within the moderate laser-intensity region from ${10}^{12}$ to $5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ by considering initial vibrational states from $v=0$ to 15. It is found that the above-threshold dissociation could be more obviously observed from the TOF spectra when the initial vibrational state is set as $v=6$ for a moderate-intensity femtosecond laser field.

We report on the observation and numerical calculation of ionization spectra of highly Stark-shifted Rydberg states of rubidium beyond the classical ionization threshold. In the numerical calculations, a complex absorbing potential (CAP) allows us to predict the energy levels and ionization rates of Rydberg states in this regime. Our approach of adjusting the CAP to the external electric field reduces the number of free parameters from one per resonance to a single one. Furthermore, we have measured the ionization spectra of magneto-optically trapped rubidium atoms which are excited to principal quantum numbers of 43 and 70 at various electric fields. The emerging ions are detected using an ion optics. We find good agreement between the numerically and experimentally obtained spectra.

We study, by quantum Monte Carlo simulations, the ground state of a harmonically confined dipolar Bose gas with aligned dipole moments and with the inclusion of a repulsive two-body potential of varying range. Two different limits can clearly be identified, namely, a classical one in which the attractive part of the dipolar interaction dominates and the system forms an ordered array of parallel filaments and a quantum-mechanical one, wherein filaments are destabilized by zero-point motion, and eventually the ground state becomes a uniform cloud. The physical character of the system smoothly evolves from classical to quantum mechanical as the range of the repulsive two-body potential increases. An intermediate regime is observed in which ordered filaments are still present, albeit forming different structures from the ones predicted classically; quantum-mechanical exchanges of indistinguishable particles across different filaments allow phase coherence to be established, underlying a global superfluid response.

Motivated by the recent experimental realization of the Haldane model by ultracold fermions in an optical lattice, we investigate phase diagrams of the hard-core Bose-Hubbard model on a honeycomb lattice. This model is closely related with a spin-1/2 antiferromagnetic (AF) quantum spin model. Nearest-neighbor (NN) hopping amplitude is positive and it prefers an AF configuration of phases of Bose-Einstein condensates. On the other hand, an amplitude of the next-NN hopping depends on an angle variable as in the Haldane model. Phase diagrams are obtained by means of an extended path-integral Monte Carlo simulation. Besides the AF state, a ${120}^{\circ }$-order state, there appear other phases including a Bose metal in which no long-range orders exist.

By using an analogy with axioniclike systems, we study light propagation in a periodic photonic topological insulator (PTI). The main result of this paper is an explicit expression for the PTI band structure. More specifically, it was found that for nonzero values of the topological phase difference $\gamma ={\theta }_2-{\theta }_1$ a finite gap $\delta \propto {\gamma }^2$ opens in the spectrum which is equivalent to the appearance of nonzero effective photon mass $m^{*}\left(\delta \right)\propto \frac{\sqrt{\delta }}{\delta +2}$.

We study the impact of Rydberg molecule formation on the storage and retrieval of Rydberg polaritons in an ultracold atomic medium. We observe coherent revivals appearing in the storage and retrieval efficiency of stored photons that originate from simultaneous excitation of Rydberg atoms and Rydberg molecules in the system with subsequent interference between the possible storage paths. We show that over a large range of principal quantum numbers the observed results can be described by a two-state model including only the atomic Rydberg state and the Rydberg dimer molecule state. At higher principal quantum numbers the influence of polyatomic molecules becomes relevant and the dynamics of the system undergoes a transition from coherent evolution of a few-state system to an effective dephasing into a continuum of molecular states.