A paradigmatic algorithm for online learning is the Hedge algorithm by Freund and Schapire. An allocation into different strategies is chosen for multiple rounds and each round incurs corresponding losses for each strategy. The algorithm obtains a favorable guarantee for the total losses even in an adversarial situation. This work presents quantum algorithms for such online learning in an oracular setting. For $T$ time steps and $N$ strategies, we exhibit run times of about $O\left(\mathrm{poly}\left(T\right)\sqrt{N}\right)$ for estimating the losses and for betting on individual strategies by sampling. In addition, we discuss a quantum analog of the Sparsitron, a machine learning algorithm based on the Hedge algorithm. The quantum algorithm inherits the provable learning guarantees from the classical algorithm and exhibits polynomial speedups. The speedups may find relevance in finance, for example, for hedging risks, and machine learning, for example, for learning generalized linear models or Ising models.

The study of topological properties by machine learning approaches has attracted considerable interest recently. Here we propose machine learning the topological invariants that are unique in non-Hermitian systems. Specifically, we train neural networks to predict the winding of eigenvalues of four prototypical non-Hermitian Hamiltonians on the complex energy plane with nearly $100\%$ accuracy. Our demonstrations in the non-Hermitian Hatano-Nelson model, Su-Schrieffer-Heeger model, and generalized Aubry-André-Harper model in one dimension and the two-dimensional Dirac fermion model with non-Hermitian terms show the capability of the neural networks to explore topological invariants and the associated topological phase transitions and topological phase diagrams in non-Hermitian systems. Moreover, the neural networks trained by a small data set in the phase diagram can successfully predict topological invariants in untouched phase regions. Thus, our work paves the way to revealing non-Hermitian topology with the machine learning toolbox.

The dissociative ionization of ${\text{H}}_2^{+}$ in linearly polarized, 400 nm laser pulses is simulated by solving a three-particle time-dependent Schrödinger equation in full dimensionality. The joint energy spectra (JES) are computed for ${cos}^8$ and flat-top envelopes using the time-dependent surface flux (tSurff) methods. In JES, the energy sharing of $N$ photons with frequency $\omega$ by nuclear kinetic energy release (KER) $E_N$ and electronic KER $E_e$ is well described by $E_N+E_e=N\omega -U_p+E_0$ for the ${cos}^8$ envelope, but satisfy $E_N+E_e=N\omega +E_0$ for the flat-top envelope, exposing a deviation of the ponderomotive energy $U_p$, where $E_0$ is the ground energy of ${\text{H}}_2^{+}$ and this observation has been observed in experiments. The analysis of the wave function for electrons and protons after the pulse are presented, where we find $U_p$ is absorbed by the Freeman resonances between two excited ungerade states of ${\text{H}}_2^{+}$.

We demonstrate a time-domain method to quantitatively retrieve the angular dependence of laser-induced electron rescattering from the measured rotational half-revival signals of molecules with a reaction microscope. The method is based on the measured channel-resolved ion yields of single ionization and electron-rescattering-induced double ionization and the in situ measured alignment distribution of molecules. From the measured data for ${\mathrm{CO}}_2$, we retrieve the angular dependence of electron rescattering in nonsequential double ionization for both nondissociative and dissociative cases. The results imply that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step. The method demonstrated here opens up the possibility to quantitatively retrieve from measured data which molecular orbitals are involved in the interaction of strong laser fields with molecules.

We analyze the properties of an impurity in a dilute Bose-Einstein condensate (BEC). The quasiparticle residue of a static impurity in an ideal BEC is known to vanish exponentially with increasing particle number, leading to a bosonic orthogonality catastrophe. Here we introduce a conceptually simple variational ansatz for mobile impurities which accurately describes their macroscopic dressing in the regime close to orthogonality, including back-action onto the BEC as well as boson-boson repulsion beyond the Bogoliubov approximation. This ansatz predicts that the orthogonality catastrophe also occurs in the mobile case, whenever the BEC becomes ideal. Finally, we show that our ansatz agrees well with recent experimental results.

We study weakly repulsive Bose-Bose mixtures in two and three dimensions at zero temperature using the functional renormalization group (FRG). We examine the RG flows and the role of density and spin fluctuations. We study the condition for phase separation and find that this occurs at the mean-field point within the range of parameters explored. Finally, we examine the energy per particle and condensation depletion. We obtain that our FRG calculations compare favorably with known results from perturbative approaches for macroscopic properties.

We describe a theoretical investigation of a four-wave mixing (FWM) scheme in a six-level atomic system driven by a field with orbital angular momentum and making use of two electromagnetically induced transparency (EIT) control fields. The obtained results allow us to control the helical phase of the output FWM field by varying the intensities of the two EIT control fields as well as the detuning of the probe field. More interestingly, we can achieve FWM output fields that are very stable under variation of the system parameters and allow the helical phase twist to be manipulated and suppressed, if desired, by adjusting the probe detuning. In this way, our scheme may open a tool to control the helical phase in a compact, efficient, and robust way.

Quantum sensing explores protocols using the quantum resource of sensors to achieve highly sensitive measurement of physical quantities. The conventional schemes generally use unitary dynamics to encode quantities into sensor states. In order to measure the spectral density of a quantum reservoir, which plays a vital role in controlling the reservoir-caused decoherence to microscopic systems, we propose a nonunitary-encoding optical sensing scheme. Although the nonunitary dynamics for encoding in turn degrades the quantum resource, we surprisingly find a mechanism to make the encoding time a resource to improve the precision and to make the squeezing of the sensor a resource to surpass the shot-noise limit. Our result shows that it is due to the formation of a sensor-reservoir bound state. Enriching the family of quantum sensing, our scheme gives an efficient way to measure the quantum reservoir and might supply an insightful support to decoherence control.

Adiabatic gauge potential is the origin of nonadiabatic transitions. In counterdiabatic driving, which is a method of shortcuts to adiabaticity, adiabatic gauge potential can be used to realize identical dynamics to adiabatic time evolution without requiring slow change of parameters. We introduce an algebraic expression of adiabatic gauge potential. Then, we find that the explicit form of adiabatic gauge potential can be easily determined by some algebraic calculations. We demonstrate this method by using a single-spin system, a two-spin system, and the transverse Ising chain. Moreover, we derive a lower bound for fidelity to adiabatic time evolution based on the quantum speed limit. This bound enables us to know the worst case performance of approximate adiabatic gauge potential. We can also use this bound to find dominant terms in adiabatic gauge potential to suppress nonadiabatic transitions. We apply this bound to magnetization reversal of the two-spin system and to quantum annealing of the transverse Ising chain. Adiabatic gauge potential reflects structure of energy eigenstates, and thus we also discuss detection of quantum phase transitions by using adiabatic gauge potential. We find a signature of a quantum phase transition in the transverse Ising chain.

Quantum memory effects can be related to a bidirectional exchange of information between an open system and its environment, which in turn modifies the state and dynamical behavior of the last one. Nevertheless, non-Markovianity can also be induced by environments whose dynamics is not affected during the system evolution, implying the absence of any physical information exchange. An unsolved open problem in the formulation of quantum memory measures is the apparent impossibility of discerning between both paradigmatic cases. Here we present an operational scheme that, based on the outcomes of successive measurements processes performed over the system of interest, allows one to distinguish between both kinds of memory effects. The method accurately detects bidirectional information flows in diverse dissipative and dephasing non-Markovian open system dynamics.

Recently, several practical wavelength attacks on continuous-variable quantum key distribution (CVQKD) were proposed based on the wavelength-dependent property of a beam splitter in Bob's station. These attack schemes are limited to the fiber channel systems with a relatively stable transmittance. However, unlike the fiber channel, the transmittance of the atmospheric channel is complex and fluctuates dramatically in time, which may invalidate the attack equations that must be satisfied in the optical fiber channel. In this paper, we propose two wavelength attack schemes against complex atmospheric CVQKD systems and formulize the two schemes based on the parameter estimation method. To verify the feasibility and availability of this attack, we deduce the success criteria for the two attack schemes. By numerical analysis, our attack schemes are proved to be feasible in the permitted parameter regimes. The secret key rate and propagation distance of CVQKD can be further overestimated by selecting the condition parameters effectively. Moreover, we discuss the possible prevention countermeasures to resist these attack schemes. The proposed method will fill the gap of wavelength attack for the implementation of practical atmospheric CVQKD.

We present an analysis of modulational instability of diffractionless waves in a face-centered-square lattice of waveguides featuring non-Kerr nonlinearity, which are composed of a combination of positive and negative refractive indices. The unit cell of the lattice consists of three different waveguides with different optical properties. The dispersion curve of the lattice supports flatbands and thereby the base equations describing the model have particular solutions that correspond to the diffractionless waves propagating along the waveguides. We also observe a unique ramification of nonlinearities in controlling the flatbands optically. The diffractionless wave solutions are derived and the stability of these distributions is investigated concisely by adopting the standard linear stability approach.

Having both elastic and inelastic two-body processes that are characterized by a complex $s$-wave scattering length between $\uparrow$ and $\downarrow$ fermions in mind, here we apply the non-Hermitian extension of the mean-field theory to the BCS-BEC evolution at zero temperature. We construct the phase diagram of the system, where we find a reentrant superfluid (SF) transition that is intervened by a normal and/or a metastable phase as a function of increasing inelasticity. This transition occurs in a large parameter regime away from the unitarity, i.e., on both the BCS and BEC sides of the resonance, and it is mostly governed by the exceptional points. In addition, except for the strongly inelastic regime, we also show that the SF phase can be well described by the condensation of weakly interacting bosonic pairs in the two-body bound state with a complex binding energy.

We address the discrimination of structured baths at different temperatures by dephasing quantum probes. We derive the exact reduced dynamics and evaluate the minimum error probability achievable by three different kinds of quantum probes, namely, a qubit, a qutrit, and a quantum register made of two qubits. Our results indicate that dephasing quantum probes are useful in discriminating low values of temperature and that lower probabilities of error are achieved for intermediate values of the interaction time, where the minimum probability of error scales as $1/2N$, with $N$ the number of energy levels of the probe. A qutrit probe outperforms a qubit one in the discrimination task, whereas entangled probes show smaller optimal times.

We introduce a measure of non-Markovianity based on the minimal amount of extra Markovian noise we have to add to the process via incoherent mixing, in order to make the resulting transformation Markovian too at all times. We show how to evaluate this measure by considering the set of depolarizing evolutions in arbitrary dimension and the set of dephasing evolutions for qubits.

Although time is one of our most intuitive physical concepts, its understanding at the fundamental level is still an open question in physics. For instance, time in quantum mechanics and general relativity are two distinct and incompatible entities. While relativity deals with events (points in spacetime), with time being observer dependent and dynamical, quantum mechanics describes physical systems by treating time as an independent parameter. To resolve this conflict, in this work, we extend the classical concept of an event to the quantum domain by defining an event as a transfer of information between physical systems. Then, by describing the universe from the perspective of a certain observer, we introduce quantum states of events with space-time-symmetric wave functions that predict the joint probability distribution of a measurement (observation) at $(t,\vec{x})$. Under these circumstances, without assuming collapse, we propose that a well-defined instant of time, like any other observable, arises from a single event, thus being an observer-dependent property. In this manner, we obtain a stationary quantum state written as a sum of a sequence of normalized states of events with increasing memories. As a consequence, a counterfactual asymmetry along this sequence of events gives rise to the flow of time as being successive “snapshots” from the observer's perspective. In this proposal, which contrasts strikingly to the view in which time is an illusion, it is the many distinguishable states in which the observer stores information that makes the existence of time possible.

The paper studies unambiguous discrimination of fermionic states through local operations and classical communication (LOCC). In the task of unambiguous discrimination, no error is tolerated but an inconclusive result is allowed. We show that contrary to the quantum case, it is not always possible to distinguish two fermionic states through LOCC unambiguously with the same success probability as if global measurements were allowed. Furthermore, we prove that we can overcome such a limit through an ancillary system made of two fermionic modes, independently of the dimension of the system, prepared in a maximally entangled state: in this case, LOCC protocols achieve the optimal success probability.

We applied a relativistic configuration-interaction (CI) framework to the stabilization method as an approach for obtaining the autoionization resonance structure of heliumlike ions. In this method, the ion is confined within an impenetrable spherical cavity, the size of which determines the radial space available for electron wave functions and electron-electron interactions. By varying the size of the cavity, one can obtain the autoinization resonance position and width. The applicability of this method is tested on the resonances of He atom while comparing with benchmark data available in the literature. The present method is further applied on the determination of the resonance structure of heliumlike uranium ion, where a relativistic framework is mandatory. In the strong-confinement region, the present method can be useful to simulate the properties of an atom or ion under extreme pressure. An exemplary application of the present method to determine the structure of ions embedded in dense plasma environment is briefly discussed.

For conventional ion traps, the trapping potential is close to independent of the electronic state, providing confinement for ions dependent primarily on their charge-to-mass ratio $Q/m$. In contrast, storing ions within an optical dipole trap results in state-dependent confinement. Here we experimentally study optical dipole potentials for ${}^{138}\mathrm{Ba}^{+}$ ions stored within two distinctive traps operating at 532 and 1064 nm. We prepare the ions in either the electronic ground ($6S_{1/2}$) or one of the metastable excited states ($5D_{3/2}$ or $5D_{5/2}$) and probe the relative strength and polarity of the potential. On the one hand, we apply our findings to selectively remove ions from a Coulomb crystal, despite all ions sharing the same $Q/m$. On the other hand, we deterministically purify the trapping volume from parasitic ions in higher-energy orbits, resulting in reliable isolation of Coulomb crystals down to a single ion within a radio-frequency trap.

We study the effective spin models for a Mott insulating Bose-Hubbard model in the presence of gauge fields. We first introduce a simple method based on adiabatic elimination to determine the low-energy effective Hamiltonian. Demonstration of a two-component one-dimensional model with synthetic magnetic field is shown. We further consider a generalized Bose-Hubbard model which covers the cases where spin-orbit coupling and synthetic flux are present. The resulting XYZ model contains Dzyaloshinskii-Moriya interactions and symmetric anisotropic couplings. Under certain conditions, this model reduces to various interesting magnetic Hamiltonians. Phase diagrams for two typical situations are obtained. We investigate magnetic phase transitions by focusing on order parameters and spin-spin correlations. Results are validated by numerical calculations using tensor network algorithms.