Atomic Spin Systems Research Articles

Cavity QED systems for steady-state sources of Wigner-negative light

We present a theoretical investigation of optical cavity quantum electrodynamics (cavity QED) systems, as described by the driven, open Jaynes–Cummings model and some of its variants, as potential sources of steady-state Wigner-negative light. We consider temporal modes in the continuous output field from the cavity and demonstrate pronounced negativity in their Wigner distributions for experimentally relevant parameter regimes. We consider models of both single and collective atomic spin systems, and find a rich structure of Wigner-distribution negativity as the spin size is varied. We also demonstrate an effective realization of all of the models considered using just a single 87Rb atom and based upon combinations of laser- and laser-plus-cavity-driven Raman transitions between magnetic sublevels in a single ground hyperfine state.

Cavity-driven Rabi oscillations between Rydberg states of atoms trapped on a superconducting atom chip

Hybrid quantum systems involving cold atoms and microwave resonators can enable cavity-mediated infinite-range interactions between atomic spin systems and realize atomic quantum memories and transducers for microwave-to-optical conversion. To achieve strong coupling of atoms to on-chip microwave resonators, it was suggested to use atomic Rydberg states with strong electric-dipole transitions. Here we report on the experimental realization of coherent coupling of a Rydberg transition of ultracold atoms, trapped on an integrated superconducting atom chip, to the microwave field of an on-chip coplanar waveguide resonator. Close to the chip surface, stray electric fields lead to inhomogeneous Rydberg level shifts, which would hinder coherent interactions of atoms in a cloud with the Rydberg excitation lasers and the microwave cavity field. We, however, use optical excitation of a Rydberg state of atoms in a narrow layer with nearly constant electric field, which allows for resonant coupling of the atoms to the cavity microwave field on an appropriate Rydberg transition. We also achieve state-selective detection of the energetically close Rydberg states. We then observe and characterize cavity-driven Rabi oscillations between a pair of atomic Rydberg states. The residual damping of the oscillations is dominated by the spread of Rabi frequencies of atoms in different positions of the resonant cavity mode, as revealed by our model. Despite the technical challenges, our studies demonstrate the feasibility of coherent-state manipulation of Rydberg atoms interacting with superconducting circuits, paving the way for realization of capable hybrid quantum systems.

Free coherent evolution of a coupled atomic spin system initialized by electron scattering.

Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins. By making use of magnetic interaction with the probe tip, we are able to tune the relative precession of the spins. We show that only if their Larmor frequencies match, the two spins can entangle, causing angular momentum to be swapped back and forth. These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice.

Single-photon generation of entangled triplet state in an atomic spin dimer

We show that the entanglement between two atomic spins, coupled via XY interaction, can be achieved with the aid of a single photon in a controllable manner. Assuming a constant spin-photon coupling, two distinct cases are considered — excitation with a constant and with a linear time-dependent photon frequency. We demonstrate that these problems reduce to the study of the well known Jaynes-Cummings and Jaynes-Cummings – Landau-Zener models, respectively, where the two-level system is formed by the zero-momentum mode states of the dimer. Owing to the exact solubility of both models, by tuning the relevant parameters, one can design and coherently control the dynamics of the excitation process. For instance, one can adiabatically switch from an atomic spin system initialized in a state with both spins down to the maximally entangled triplet state.

Role of the probe beam in a radio-frequency atomic magnetometer.

We explore the role of the linearly polarized probe beam in the radio-frequency atomic magnetometer. Two regimes of coupling of the linearly polarized light to the atomic ensemble, near and off-resonant, are demonstrated and discussed. Our studies cover three types of interaction between the atomic spin system and the linearly polarized beam, i.e., absorptive production of spin polarization, dispersive spin state readout, and nonlinear spin couplings. The work is performed with the outlook of optimization and simplification of magnetometer operation. Operation of a magnetometer with a single beam, generating and probing the atomic orientation, is presented. This single-beam configuration, which combines optical pumping, nonlinear spin couplings, and spin-exchange collisions, creates the option for a portable device with simple instrumentation.

Information-theoretic approach to atomic spin nonclassicality

Nonclassicality of light has been studied, both extensively and intensively, in terms of optical coherent states, and various quantifiers for nonclassicality of light have been introduced in recent decades. In contrast, the corresponding quantification issue for atomic spin systems is relatively less studied, though there has been some remarkable progress. In this paper, by virtue of the Wigner-Yanase skew information and working in the framework of atomic coherent states, we introduce a quantifier for nonclassicality of atomic spin states from an information-theoretic perspective and reveal its fundamental properties. This quantifier leads to a simple yet powerful criterion for detecting nonclassicality via convexity of the skew information. This criterion is sufficient, but not necessary. The concept is illustrated through several paradigms. We further elucidate some intrinsic relations between nonclassicality and entanglement and indicate that the nonclassicality quantifier may be transferred to an entanglement quantifier.

Sensitivity, quantum limits, and quantum enhancement of noise spectroscopies

We study the fundamental limits of noise spectroscopy using estimation theory, Faraday rotation probing of an atomic spin system, and squeezed light. We find a simple and general expression for the Fisher information, which quantifies the sensitivity to spectral parameters such as resonance frequency and linewidth. For optically-detected spin noise spectroscopy, we find that shot noise imposes "local" standard quantum limits for any given probe power and atom number, and also "global" standard quantum limits when probe power and atom number are taken as free parameters. We confirm these estimation theory results using non-destructive Faraday rotation probing of hot Rb vapor, observing the predicted optima and finding good quantitative agreement with a first-principles calculation of the spin noise spectra. Finally, we show sensitivity beyond the atom- and photon-number-optimized global standard quantum limit using squeezed light.

Manipulation of magnetic skyrmions with a scanning tunneling microscope

The dynamics of a single magnetic Skyrmion in an atomic spin system under the influence of Scanning Tunneling Microscope is investigated by computer simulations solving the Landau-Lifshitz-Gilbert equation. Two possible scenarios are described: manipulation with aid of a spin-polarized tunneling current and by an electric field created by the scanning tunneling microscope. The dynamics during the creation and annihilation process is studied and the possibility to move single Skyrmions is showed.

Anisotropy energy, spin‐atomic vibration interaction, and spin‐flip Hamiltonian of a single atomic spin system: application to iron ion

AbstractWe derive an anisotropy energy VA, a spin‐atomic vibration interaction VSA, and a spin‐flip Hamiltonian VSF of a single atomic spin system, “Fe2+ (3d6) in a crystal field of tetragonal symmetry”. The perturbation theory is here applied to the system with the spin‐orbit interaction and the kinetic and potential energies, in which difference of vibration displacement between a nucleus and electrons is taken into account. We first find conditions to enhance a coefficient |D | of VA = – |D |S2Z, with SZ being the Z component of a spin operator. Second, the presence or absence of VSA and VSF is shown for any parameter sets. Also, some coefficients of VSA are proportional to 1/√f, with f being a vibration frequency, although a coefficient of the conventional spin‐phonon interaction has √f (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-passage readout of atomic quantum memory

A scheme for retrieving quantum information stored in collective atomic spin systems onto optical pulses is presented. Two off-resonant light pulses cross the atomic medium in two orthogonal directions and are interferometrically recombined in such a way that one of the outputs carries most of the information stored in the medium. In contrast to previous schemes our approach requires neither multiple passes through the medium nor feedback on the light after passing the sample which makes the scheme very efficient. The price for that is some added noise which is however small enough for the method to beat the classical limits.

Quantum limits encountered in atomic spin measurements

The research program investigating the limits imposed by quantum mechanics on the precision with which a collective atomic spin can be measured is reviewed in this paper. We consider both the cases, when the interaction between atoms and light is weak and strong. In the first case, the precision of the measurement is mainly limited by the quantum noise of light, and the atomic spin noise plays a minor role. In the latter case, we must consider the quantum mechanical measurement induced back action feeding quantum noise of light into the atomic spin system. Here, we show that the precision is limited by atomic quantum spin noise as well as the optical quantum noise. Although it is disturbing for polarization spectroscopy, the measurement induced back action may prove to be useful from a quantum informatics viewpoint, since this enables storage of the polarization state of light in the atomic spin.

Coherence transfer between atomic transitions of different g-factor by modulated optical excitation

Modulated optical excitation of alkali atoms transversally to a magnetic field enables a coherence transfer between different atomic species if the pumping conditions are properly chosen. We analyze the influence of pump-light intensity and modulation sequence on the transfer efficiency and present experimental results for K Rb and Cs Rb mixtures. For the first time experimental evidence of a strong correlation between the free transient evolutions of two alkali atomic spin systems with different g-factors without the use of radio-frequency fields is given.

Electrical Resistivity of Metals and Alloys Containing Localized Magnetic Moments

In metals and alloys containing localized magnetic moments, an important contribution to the electrical resistivity can be associated with disorder of the atomic spin system. This contribution is ascribed to a scalar interaction between the spins of the conduction electrons and the atomic spins. Assuming a simple form for this interaction, the experimental and theoretical situation concerning the spin-disorder resistivity is reviewed for pure metals, dilute alloys, and concentrated binary alloys.