We investigate, both theoretically and experimentally, the mechanism behind the creation of a macroscopic magnetization in a gas of paramagnetic molecules with an optical centrifuge, reported in [Phys. Rev. Lett. 118, 243201 (2017)]. Our analysis shows that the centrifuged super-rotors and noncentrifuged molecules are polarized in opposite directions, while the net magnetic moment of the whole ensemble at the end of the interaction with the laser pulse remains close to zero. As the super-rotors are more stable against reorienting collisions, their spin polarization, which points along the centrifuge axis, decays more slowly than the oppositely oriented polarization of the noncentrifuged molecules. The latter lose their directional rotation much more quickly and with it the polarization of their electronic spin. We show numerically that owing to this difference in decay rates, a net magnetization in the direction of the centrifuge is generated. The proposed model is supported by experimental data.

We measure the sequential double ionization and the subsequent Coulomb explosion of molecular ${\mathrm{Cl}}_2$ in an intense femtosecond laser field by using the dc-sliced ion imaging technique. The results indicate that not only the highest occupied molecular orbital (HOMO) but also the next two lower-lying molecular orbitals are involved in three distinguished reaction pathways. Pathways ${\left(1,1\right)}^2$ and ${\left(1,1\right)}^3$ are both dissociated from ${\mathrm{Cl}}_2{}^{2+}\to {\mathrm{Cl}}^{+}+{\mathrm{Cl}}^{+}$, but with the two electrons removed from HOMO and HOMO-2 in reverse sequence, and populated on the ionic excited states ${}^3\mathrm{\Pi }_g$ and ${}^1\mathrm{\Pi }_g$, respectively. The kinetic energy release differences observed in these two channels are ascribed to the nuclear motion during the ionization process. For pathway ${\left(1,1\right)}^1$ (also dissociated from ${\mathrm{Cl}}_2{}^{2+}\to {\mathrm{Cl}}^{+}+{\mathrm{Cl}}^{+}$) populated on the ionic excited states ${}^1\mathrm{\Pi }_g$, the isotropic angular distribution of the fragment ions is attributed to the combination of the electron density distribution of HOMO-1, the vibrational electronic wave-packet evolution, and the field excitation. Our results propose a feasible method to manipulate the electronic dynamics which takes place in the attosecond time domain via accurately tuned laser parameters of a femtosecond laser field.

We study a quasi-two dimensional gas of bosonic dipoles, calculating the beyond-mean-field corrections to the ground-state energy and chemical potential neglecting the transverse mode structure. We show that the corrections are sensitive to the high-momentum part of the interaction and cannot be expressed solely in terms of the scattering length and the dipole strength. While nonuniversal, the correction is found to be negative, which provides an additional attractive term in the extended Gross-Pitaevskii equation, enhancing the roton instability.

Assemblies of interacting Rydberg atoms show promise for the quantum simulation of transport phenomena, quantum chemistry, and condensed-matter systems. Such schemes are typically limited by the finite lifetime of Rydberg states. Circular Rydberg states have the longest lifetimes among Rydberg states but lack the energetic isolation in the spectrum characteristic of low-angular-momentum states. The latter is required to obtain simple transport models with few electronic states per atom. Simple models can, however, even be realized with circular states by exploiting dipole-dipole selection rules or external fields. We show here that this approach can be particularly fruitful for scenarios where quantum transport is coupled to atomic motion, such as adiabatic excitation transport or quantum simulations of electron-phonon coupling in light harvesting. Additionally, we explore practical limitations of flexible Rydberg aggregates with circular states and to which extent interactions among circular Rydberg atoms can be described using classical models.

By applying the Born-Huang expansion, originally developed for coupled nucleus-electron systems, to the full nucleus-electron-photon Hamiltonian of nonrelativistic quantum electrodynamics (QED) in the long-wavelength approximation, we deduce an exact set of coupled equations for electrons on photonic energy surfaces and the nuclei on the resulting polaritonic energy surfaces. This theory describes seamlessly many-body interactions among nuclei, electrons, and photons including the quantum fluctuation of the electromagnetic field and provides a proper first-principle framework to describe QED-chemistry phenomena, namely polaritonic and cavity chemistry effects. Since the photonic surfaces and the corresponding nonadiabatic coupling elements can be solved analytically, the resulting expansion can be brought into a compact form, which allows us to analyze aspects of coupled nucleus-electron-photon systems in a simple and intuitive manner. Furthermore, we discuss structural differences between the exact quantum treatment and Floquet theory, show how existing implementations of Floquet theory can be adjusted to adhere to QED, and highlight how standard drawbacks of Floquet theory can be overcome. We then highlight, by assuming that the relevant photonic frequencies of a prototypical cavity QED experiment are in the energy range of the electrons, how from this generalized Born-Huang expansion an adapted Born-Oppenheimer approximation for nuclei on polaritonic surfaces can be deduced. This form allows a direct application of first-principle methods of quantum chemistry such as coupled-cluster or configuration interaction approaches to QED chemistry. By restricting the basis set of this generalized Born-Oppenheimer approximation, we furthermore bridge quantum chemistry and quantum optics by recovering simple models of coupled matter-photon systems employed in quantum optics and polaritonic chemistry. We finally highlight numerically that simple few-level models can lead to physically wrong predictions, even in weak-coupling regimes, and show how the presented derivations from first principles help to check and derive physically reliable simplified models.

We give a microscopic description of the optical bistability, where the transmission coefficient has two different values as a function of input light intensity, and the system exhibits a discontinuous jump with a hysteresis loop. We developed an efficient numerical algorithm to treat the quantum master equation for hybridized systems of many photons and a large number of two-level atoms. By using this method, we characterize the bistability from the viewpoint of eigenmodes and eigenvalues of the time-evolution operator of the quantum master equation. We classify types of optical bistability according to the photon number density in the cavity. In contrast to previous studies of optical bistability in the high-photon-density regime where the photons can be treated as a classical electromagnetic field and the resonance spectrum has a single-peak structure, we study the nature of optical bistability in the low-photon-density regime where the hybridization of photon and atom degrees of freedom occurs and the resonance spectrum has a double-peak structure. Unraveling the nature of the optical bistability in the latter regime may be important for the manipulation of quantum systems. Concretely, we discuss the steady-state properties of the optical bistability: dependencies of the photon number density on the intensity and the double-peak structure of the photon number distribution inside the bistable region. As for the dynamical properties, we find that the relaxation timescale shows an exponential growth with the system size and reveal how the hysteresis loop of the optical bistability depends on the size of the system and the sweeping rate of the driving amplitude. Finally, by investigating the effects of detuning frequency of the input field, we clarify the characteristic properties of the present optical bistability within the low-photon-density regime, which are qualitatively different from the standard optical bistable phenomena.

We consider the role of detection noise in quantum-enhanced metrology in collective spin systems and derive a fundamental bound for the maximum obtainable sensitivity for a given level of added detection noise. We then present an interaction-based readout utilizing the commonly used one-axis twisting scheme that approaches this bound for states generated via several commonly considered methods of generating quantum enhancement, such as one-axis twisting, two-axis countertwisting, twist-and-turn squeezing, quantum nondemolition measurements, and adiabatically scanning through a quantum phase transition. We demonstrate that our method performs significantly better than other recently proposed interaction-based readouts. These results may help provide improved sensitivity for quantum-sensing devices in the presence of unavoidable detection noise.

Using equations of motion with the anisotropic dissipative term for quantum particle and quantum-mechanical commutation rules, the general Maxwell-type differential equations are derived. The direct modifications of the well-known Maxwell equations due to the medium effects (openness of the system) are discussed.

We introduce a concept, called the mutual uncertainty between two observables in a given quantum state, which enjoys features similar to those of the mutual information for two random variables. Further, we define conditional uncertainty as well as conditional variance and show that conditioning on more observables reduces the uncertainty. Given three observables, we prove a “strong subadditivity” relation for the conditional uncertainty under certain conditions. As an application, we show that by using the conditional variance one can detect bipartite higher dimensional entangled states. The efficacy of our detection method lies in the fact that it gives better detection criteria than most of the existing criteria based on geometry of the states. Interestingly, we find that for $N$-qubit product states, the mutual uncertainty is exactly equal to $N-\sqrt{N}$, and if it is other than this value, the state is entangled. We also show that using the mutual uncertainty between two observables, one can detect non-Gaussian steering where Reid's criterion fails to detect it. Our results may open a direction of exploration in quantum theory and quantum information using mutual uncertainty, conditional uncertainty, and the strong subadditivity for multiple observables.

We present a quantum-classical algorithm to study the dynamics of the two-spatial-site Schwinger model on IBM's quantum computers. Using rotational symmetries, total charge, and parity, the number of qubits needed to perform computation is reduced by a factor of $\sim 5$, removing exponentially large unphysical sectors from the Hilbert space. Our work opens an avenue for exploration of other lattice quantum field theories, such as quantum chromodynamics, where classical computation is used to find symmetry sectors in which the quantum computer evaluates the dynamics of quantum fluctuations.

Two-way Gaussian protocols have the potential to increase quantum key distribution (QKD) protocols' secret-key rates by orders of magnitudes [Phys. Rev. A 94, 012322 (2016)]. Security proofs for two-way protocols, however, are underdeveloped at present. In this paper, we establish a security proof framework for the general coherent attack on two-way Gaussian protocols in the asymptotic regime. We first prove that coherent-attack security can be reduced to collective-attack security for all two-way QKD protocols. Next, we identify two different constraints that each provide intrusion parameters which bound an eavesdropper's coherent-attack information gain for any two-way Gaussian QKD protocol. Finally, we apply our results to two such protocols.

Gauge invariance is one of the more important concepts in physics. We discuss this concept in connection with the unitary evolution of discrete-time quantum walks in one and two spatial dimensions, when they include the interaction with synthetic, external electromagnetic fields. One introduces this interaction as additional phases that play the role of gauge fields. Here, we present a way to incorporate those phases, which differs from previous works. Our proposal allows the discrete derivatives, that appear under a gauge transformation, to treat time and space on the same footing, in a way which is similar to standard lattice gauge theories. By considering two steps of the evolution, we define a density current which is gauge invariant and conserved. In the continuum limit, the dynamics of the particle, under a suitable choice of the parameters, becomes the Dirac equation and the conserved current satisfies the corresponding conservation equation.

We present a theoretical proposal of a tunable charge qubit, hosted in triple quantum dots. The manipulation is solely performed by changing the heights of the two potential barriers between the three dots, while the energies of all three dots are fixed. We have found that when the relative heights of the two barriers are changed, the direction of the axis of rotation in performing single-qubit gates can be varied. On the other hand, the corresponding rotation speed can be tuned by raising or lowering the two barriers at the same time. Our proposal therefore allows for tunability of both the rotation axis and rotating speed for a charge qubit via all-electrical control, which may facilitate realization of quantum algorithms in these devices.

Optical telecommunication is at the heart of today's internet and is currently enabled by the transmission of intense optical signals between remote locations. As we look to the future of telecommunication, quantum mechanics promise new ways to be able to transmit and process that information. Demonstrations of quantum key distribution and quantum teleportation using multiphoton states have been performed, but only over ranges limited to a few hundred kilometers. To go beyond this, we need repeaters that are compatible with these quantum multiphoton continuous-variable pulses. Here we present a design for continuous-variable quantum repeaters that can distribute entangled and pure two-mode squeezed states over arbitrarily long distances with a success probability that scales only polynomially with distance. The proposed quantum repeater is composed from several basic known building blocks such as non-Gaussian operations for entanglement distillation and an iterative Gaussification protocol (for retaining the Gaussian character of the final state), but complemented with a heralded non-Gaussian entanglement swapping protocol, which allows us to avoid extensive iterations of quantum Gaussification. We characterize the performance of this scheme in terms of key rates for quantum key distribution and show a secure key can be generated over thousands of kilometers.

We examine a range of effects arising from ac magnetic fields in high-precision metrology. These results are directly relevant to high-precision measurements and accuracy assessments for state-of-the-art optical clocks. Strategies to characterize these effects are discussed and a simple technique to accurately determine trap-induced ac magnetic fields in a linear Paul trap is demonstrated by using ${}^{171}\mathrm{Yb}^{+}$.

We present a systematic study of the $\mathrm{\Lambda }$-enhanced gray molasses cooling ($\mathrm{\Lambda }$-GMC) of cesium atoms on the $D_2$ transition. Due to the large splitting in the excited hyperfine transitions of the cesium $D_2$ line, it is relatively simple to implement the $\mathrm{\Lambda }$-GMC with a suitable frequency control of the trapping and repumping lasers of the magneto-optical trap (MOT). We achieve a temperature of $1.7\pm 0.2\mu \mathrm{K}$ with $3.2\times {10}^8$ atoms. The phase space density of atoms is $1.43\times {10}^{-4}$. Compared to the condition with a bare MOT or a MOT with gray-molasses cooling without $\mathrm{\Lambda }$ enhancement, there is an increase in the phase space density by a factor of more than ${10}^3$ and 10, respectively.

The Josephson relation is generalized for conserved charges in multicomponent bosons. With linear response theory, a formula for derivation of generalized superfluid density is given. When there are several conserved charges, the superfluid density is generally a second-order tensor in internal spin space. When the rank of the Green's function matrix is one, the Josephson relation is given explicitly with a phase operator method. For two-component bosons, a generalized Hugenholtz-Pines relation holds and there exist two gapless phonon excitations. When the interactions are $\text{U}\left(2\right)$ invariant, we show there is a gapless quadratic dispersion excitation no matter how strong the interactions are. The corresponding generalized Josephson relation is expressed with Green's function matrix elements.

We systematically investigate the properties of three-dimensional dipolar binary Bose mixture at low temperatures. A set of coupled self-consistent equations of motion are derived for the two condensates. In the homogeneous case, useful analytical formulas for the condensate depletion, the anomalous density, the ground-state energy, and the equation of state are obtained. The theory is extended to the inhomogeneous case and the importance of the inhomogeneity is highlighted. Our results open up an avenue for studying dipolar mixture droplets. Impacts of the dipole-dipole interaction on the stability, density profiles, and the size of the self-bound droplet are deeply discussed. The finite-temperature behavior of such a state is also examined.

We study the atom-vapor-based photon-number-resolving detection from first principles, including quantum-mechanical treatment of the electromagnetic field. We study a photon detector model that combines coherently controlled absorption of light and resonance fluorescence to achieve photon counting at room temperature. In particular we identify the fundamental limits to this particular scheme of photon detection. We show that there exists a time-energy uncertainty between the incident pulse strength and the time period of the incident pulse. We verify the role of a large ensemble of atoms to boost the efficiency of such a detector.