NOON states play an important role in quantum information processing and quantum metrology, but the fidelities of these states generated previously are limited, typically by the practically unavoidable decoherence and operational imperfections. Here, we propose an efficient scheme to generate photonic NOON states alternatively using the rapid population passage technique via the shortcut to adiabaticity (STA), rather than the usual Rabi oscillations. Since the deterministic population passages based on the STAs are insensitive to the details of the operations and can be implemented as fast as the Rabi oscillations, the fidelity of the generated NOON state could be satisfactorily high. The feasibility of the proposal is demonstrated specifically with the experimental circuit QED systems by rapidly driving two artificial qutrits.

This work examines superradiance in initially inverted clouds of multilevel atoms. We develop a set of equations that can approximately calculate the temporal evolution of $N$ coupled atoms. This allows us to simulate clouds containing hundreds of multilevel atoms while eschewing the assumption and/or approximation of symmetric dipole-dipole interactions. This treatment is used to explore the effects that dephasing causes by elastic dipole-dipole interactions and that competition between multiple transitions has on superradiance. Both of these mechanisms place strong parametrical restrictions on a given transition's ability to superradiate. These results are likely important to recent experiments that probe superradiance in Rydberg atoms.

Radical-ion pairs and their reactions have triggered the study of quantum effects in biological systems. This is because they exhibit a number of effects best understood within quantum information science, and at the same time are central in understanding the avian magnetic compass and the spin transport dynamics in photosynthetic reaction centers. Here we address radical-pair reactions from the perspective of quantum metrology. Since the coherent spin motion of radical pairs is effected by an external magnetic field, these spin-dependent reactions essentially realize a biochemical magnetometer. Using the quantum Fisher information, we find the fundamental quantum limits to the magnetic sensitivity of radical-pair magnetometers, arriving at a sensitivity $\delta B=2\mathrm{pT}/\tau \left[1\mu \text{s}\right]\sqrt{{\nu }_0\left[{10}^{12}\right]}$, given in terms of radical-pair lifetime $\tau$ and number of radical pairs ${\nu }_0$. We then explore how well the usual measurement scheme considered in radical-pair reactions, the measurement of reaction yields, approaches the fundamental limits. In doing so, we find the optimal hyperfine interaction Hamiltonian that leads to the best magnetic sensitivity as obtained from reaction yields. This is still an order of magnitude smaller than the absolute quantum limit. Finally, we demonstrate that with a realistic quantum reaction control reminding one of Ramsey interferometry, here presented as a quantum circuit involving the spin-exchange interaction and a recently proposed molecular switch, we can approach the fundamental quantum limit within a factor of 2. This work opens the application of well-advanced quantum metrology methods to biological systems.

We introduce and analyze a task that we call symmetrization, in which a state of a quantum system, associated with a symmetry group, is transformed by a random unitary operation to a symmetric state. Each element of the unitary ensemble is required to be symmetry preserving, in the sense that it keeps the set of symmetric states invariant. We consider an asymptotic limit of infinitely many copies and vanishingly small error, and analyze the symmetrizing cost, that is the minimum cost of randomness per copy required for symmetrization. We prove that the symmetrizing cost of an arbitrary quantum state is equal to the relative entropy of frameness, thereby providing it with a direct operational meaning.

We propose a scheme to measure the parity of two distant qubits, while ensuring that losses on the quantum channel between them does not destroy coherences within the parity subspaces. This capability enables deterministic preparation of highly entangled qubit states whose fidelity is not limited by the transmission loss. The key observation is that, for a probe electromagnetic field in a particular quantum state; namely, a superposition of two coherent states of opposite phases, the transmission loss stochastically applies a near-unitary backaction on the probe state. This leads to a parity measurement protocol where the main effect of the transmission losses is a decrease in the measurement strength. By repeating the nondestructive (weak) parity measurement, one achieves a high-fidelity entanglement in spite of a significant transmission loss.

Photoionization using attosecond pulses can lead to the formation of coherent superpositions of the electronic states of the parent ion. However, ultrafast electron ejection triggers not only electronic but also nuclear dynamics—leading to electronic decoherence, which is typically neglected on time scales up to tens of femtoseconds. We propose a full quantum-dynamical treatment of nuclear motion in an adiabatic framework, where nuclear wave packets move on adiabatic potential energy surfaces expanded up to second order at the Franck-Condon point. We show that electronic decoherence is caused by the interplay of a large number of nuclear degrees of freedom and by the relative topology of the potential energy surfaces. Application to ${\mathrm{H}}_2\mathrm{O}$, paraxylene, and phenylalanine shows that an initially coherent state evolves to an electronically mixed state within just a few femtoseconds. In these examples the fast vibrations involving hydrogen atoms do not affect electronic coherence at short times. Conversely, vibrational modes involving the whole molecular skeleton, which are slow in the ground electronic state, quickly destroy it upon photoionization.

We study a dilute and ultracold Bose gas of interacting atoms by using an effective field theory which takes into account the finite-range effects of the interatomic potential. Within the formalism of functional integration from the grand canonical partition function, we derive beyond-mean-field analytical results which depend on both the scattering length and the effective range of the interaction. In particular, we calculate the equation of state of the bosonic system as a function of these interaction parameters both at zero and finite temperature including one-loop Gaussian fluctuation. In the case of zero-range effective interaction, we explicitly show that, due to quantum fluctuations, the bosonic system is thermodynamically stable only for very small values of the gas parameter. We find that a positive effective range above a critical threshold is necessary to remove the thermodynamical instability of the uniform configuration. Remarkably, also for relatively large values of the gas parameter, our finite-range results are in quite good agreement with recent zero-temperature Monte Carlo calculations obtained with hard-sphere bosons.

In this paper we study the properties of cold bosons in two-dimensional optical lattices where Bose condensation occurs at a momentum point $\mathbf{k}$ with nonzero $k$-space Berry curvature. By combining results from both analytic and numerical approaches, the bulk and edge excitation spectra of the system are derived. We show that the boson system carries nonuniversal, nontopological temperature-dependent equilibrium angular momentum and edge current at low temperatures. The differences between boson systems with real- and $k$-space Berry curvature are pointed out. In particular, using a simple variational wave-function approach, we show that for bosons in a harmonic trap, the stability of the Bose-condensed state towards vortex formation is rather different in the two cases because of the different angular momentum scalings in the ground states.

Weyl semimetal (WSM) is an exotic topological state in condensed-matter physics. In this paper, based on a two-band cubic lattice model, we study WSMs with a pair of tunable Weyl nodes. We point out that each Weyl node can be tuned freely, leading to three types of WSMs with different tilt strengths: types I, II, and 1.5. We study chiral modes, surface Fermi arcs, and quantum anomalous Hall effect to show the properties of a combination of independent nodes. In addition, we present an experimental setup to realize the different types of WSMs based on a timely technique.

Considering a two-dimensional (2D) Bose-Hubbard spinor lattice with weak nearest-neighbor interactions and no particle transfer between sites, we theoretically study the transport of energy from one initially excited dimer to the rest of the lattice. Beyond a critical interaction strength, low-energy on-site excitations are quickly dispersed throughout the array, while stronger excitations are self-trapped, resulting in localized energy breathers and solitons. These structures are quasiparticle analogs to the discrete 2D solitons in photonic lattices. Full many-body simulations additionally demonstrate the localization of one-particle entropy.

The laser coupled Landau-Zener avoided crossing has been investigated with an aim towards obtaining the laser source parameters for precise controlling of the state dynamics in a two-level quantum system. The conventional Landau-Zener equation is modified for including the interaction of the system with a laser field during a bias energy sweep and the obtained Hamiltonian is numerically solved for the investigation of the two-state occupation probabilities. We have shown that in the Landau-Zener process, using an additional laser source with controlled amplitude, frequency, and phase, the system dynamics could be arbitrarily engineered. This is while, by synchronous frequency sweeping of a chirped-pulse laser, the system could be guided into a resonance condition, which again gives the remarkable possibility for precise tuning and controlling of the quantum system dynamics.

Microresonator-based optical frequency combs (“microcombs”) have attracted lots of attention in the past few years thanks to their promising applications in telecommunications, spectroscopy, and optical clocks. The process of comb generation in microresonators can be modeled in the frequency domain using coupled mode equations and has recently been successfully described in the time domain using a nonlinear Schrödinger equation known as the Lugiato-Lefever equation. Time-domain approaches have brought many interesting insights for the understanding of microcombs. In this paper we present an intuitive frequency-domain model of microcombs that describes the overall structure of the spectra in terms of a few collective excitations of groups of neighboring comb lines, which we term “superoscillators.” This approach ties in nicely with the recently developed time-domain model based on soliton crystals and links the microcomb generation process with frequency response theory.

Einstein-Podolsky-Rosen (EPR) steering, as a new form of nonlocality, stands between entanglement and Bell nonlocality, implying promising applications for quantum information tasks. The problem of detecting EPR steering plays an important role in characterization of quantum nonlocality. Despite some significant progress, one still faces a practical issue: how to detect EPR steering in an experimentally friendly fashion. Resorting to an EPR steering inequality, one is required to apply a strategy as efficiently as possible for any selected measurement settings on the two subsystems, one of which may not be trusted. Inspired by the recent powerful linear criteria proposed by Saunders et al. [D. J. Saunders, S. J. Jones, H. M. Wiseman, and G. J. Pryde, Nat. Phys. 6, 845 (2010).], we present an optimized method of certifying steering for an arbitrary-dimensional state in a cost-effective manner. We provide a practical way to signify steering via only a few settings to optimally violate the steering inequality. Our method leads to steering detections in a highly efficient way, and can be performed with any number of settings, for an arbitrary bipartite mixed state, which can reduce experimental overheads significantly.

Long-lived isotopes of plutonium were studied using two complementary techniques, high-resolution resonance ionization spectroscopy (HR-RIS) and collinear laser spectroscopy (CLS). Isotope shifts have been measured on the $5f^{6}7s^2{}^7{F}_0\to 5f^{5}6d^{2}7s(J=1)$ and $5f^{6}7s^2{}^7{F}_1\to 5f^{6}7s7p(J=2)$ atomic transitions using the HR-RIS method and the hyperfine factors have been extracted for the odd mass nuclei  ${}^{239,241}\mathrm{Pu}$. CLS was performed on the $5f^{6}7s{}^8{F}_{1/2}\to J=1/2\left(27523.61{\text{cm}}^{-1}\right)$ ionic transition with the hyperfine $A$ factors measured for ${}^{239}\mathrm{Pu}$. Changes in mean-squared charge radii have been extracted and show a good agreement with previous nonoptical methods, with an uncertainty improvement by approximately one order of magnitude. Plutonium represents the heaviest element studied to date using collinear laser spectroscopy.

We study the formation of the RbCs molecule by an intense laser pulse using nonlinear dynamics. Under the Born-Oppenheimer approximation, the system is modeled by a two-degree-of-freedom rovibrational Hamiltonian, which includes the ground electronic potential energy curve of the diatomic molecule and the interaction of the molecular polarizability with the electric field of the laser. As the laser intensity increases, we observe that the formation probability first increases and then decreases after reaching a maximum. We show that the analysis can be simplified to the investigation of the long-range interaction between the two atoms. We conclude that the formation is due to a very small change in the radial momentum of the dimer induced by the laser pulse. From this observation, we build a reduced one-dimensional model which allows us to derive an approximate expression of the formation probability as a function of the laser intensity.

We study the fidelity decay and its freeze for an initial coherent state of two-mode Bose-Einstein condensates in the Fock regime considering a Bose-Hubbard model that includes two-particle tunneling terms. By using linear-response theory we find scaling properties of the fidelity as a function of the particle number that prove the existence of two-particle mode exchange when a nondegeneracy condition is fulfilled. Tuning the energy difference of the two modes serves to distinguish the presence of two-particle mode-exchange terms through the appearance of certain singularities. We present numerical calculations that illustrate our findings, and propose exploiting a Feshbach resonance to verify experimentally our predictions.

We present a method for calculating the dynamics of a bosonic mixture, the multicomponent correlated-basis-function (CBF) method. For a single component, CBF results for the excitation energies agree quite well with experimental results, even for highly correlated systems like ${}^4\mathrm{He}$, and recent systematic improvements of CBF achieve perfect agreement. We give a full derivation of multicomponent CBF, and apply the method to a dipolar Bose gas cut into two-dimensional layers by a deep optical lattice, with coupling between layers due to the long-ranged dipole-dipole interaction. We consider the case of strong coupling, leading to large positive interlayer correlations. We calculate the spectrum for a system of eight layers and show that the strong coupling can lead to a simpler spectrum than in the uncoupled case, with a single peak carrying most of the spectral weight.

We study the time evolution of a Bose-Einstein condensate with a vortex on it when it is released from a trap and expands freely in a spatially uncorrelated disordered media. As customary in such cases, we perform the evolution over different disorder realizations and average over the disorder to obtain the quantities of interest. The propagation of noninteracting quantum systems in disordered media is strongly linked to Anderson localization which can be understood as formation of islands of constant phase due to coherent backscattering. We find that the vortex superfluid localizes in such media and, moreover, the vortex is resilient to disorder effects. This is a single-particle effect. In the presence of interactions, no matter how small they are, the vortex rapidly decays into phase discontinuities although localization is still present. The study of dispersion of a bosonic condensate with vorticity in a disordered media bears similarities with the stability of topological excitations in two-dimensional $p$-wave fermionic superfluids where the ground state is a Majorana mode that arises in the form of a vortex in the order parameter.

We present a theory of cooperative light scattering valid in any dimension: connecting theories for an open line, open plane, and open space in the nonrelativistic regime. This theory includes near-field and dipole-orientation effects, highlighting how field-mode confinement controls the phenomena. We present an experimental implementation for planar collective effects.