Raman Sideband Research Articles

Nonlinear mixing of optical vortices with fractional topological charge in Raman sideband generation

We studied the nonlinear parametric interaction of femtosecond fractionally-charged optical vortices in a Raman-active medium. Propagation of such beams was described using the Kirchhoff–Fresnel integrals by embedding a non-integer 2π phase step in a Gaussian beam profile. When using fractionally-charged pump or Stokes beams, we observed the production of new topological charge and phase discontinuities in the Raman field. These newly generated fractionally-charged Raman vortex beams were found to follow the same orbital angular momentum algebra derived by Strohaber et al (2012 Opt. Lett. 37 3411) for integer vortex beams.

Recent progress on the 27Al+ ion optical clock

An aluminium ion optical clock is under development at Huazhong University of Science and Technology. The 25Mg+ ion is chosen as logic ion to sympathetically cool an Al+ ion and to detect its states. The 25Mg+ ion is cooled to the motional ground state through Raman sideband cooling as the first step for quantum logic spectroscopy. Ultra-stable lasers for the interrogation of the clock transition are developed. The instability of the laser beat frequency is 1.2 x 10-15 at 1 s, which is close to the thermal noise limit of the reference cavity.

Raman sideband cooling of aBa+138ion using a Zeeman interval

Motional ground state cooling and internal state preparation are important elements for quantum logic spectroscopy (QLS), a class of quantum information processing. Since QLS does not require the high gate fidelities usually associated with quantum computation and quantum simulation, it is possible to make simplifying choices in ion species and quantum protocols at the expense of some fidelity. Here, we report sideband cooling and motional state detection protocols for $^{138}$Ba$^+$ of sufficient fidelity for QLS without an extremely narrowband laser or the use of a species with hyperfine structure. We use the two S$_{1/2}$ Zeeman sublevels of $^{138}$Ba$^+$ to Raman sideband cool a single ion to the motional ground state. Because of the small Zeeman splitting, near-resonant Raman sideband cooling of $^{138}$Ba$^+$ requires only the Doppler cooling lasers and two additional AOMs. Observing the near-resonant Raman optical pumping fluorescence, we estimate a final average motional quantum number $\bar{n}\approx0.17$. We additionally employ a second, far-off-resonant laser driving Raman $\pi$-pulses between the two Zeeman sublevels to provide motional state detection for QLS and to confirm the sideband cooling efficiency, measuring a final $\bar{n} = 0.15(6)$.

Dark-resonance Doppler cooling and high fluorescence in trapped Ca-43 ions at intermediate magnetic field

We demonstrate simple and robust methods for Doppler cooling and obtaining high fluorescence from trapped 43Ca+ ions at a magnetic field of 146 Gauss. This field gives access to a magnetic-field-independent ‘atomic clock’ qubit transition within the ground level hyperfine structure of the ion, but also causes the complex internal structure of the 64 states relevant to Doppler cooling to be spread over many times the atomic transition line-width. Using a time-dependent optical Bloch equation simulation of the system we develop a simple scheme to Doppler-cool the ion on a two-photon dark resonance, which is robust to typical experimental variations in laser intensities, detunings and polarizations. We experimentally demonstrate cooling to a temperature of 0.3 mK, slightly below the Doppler limit for the corresponding two-level system, and then use Raman sideband laser cooling to cool further to the ground states of the ion’s radial motional modes. These methods will enable two-qubit entangling gates with this ion, which is one of the most promising qubits so far developed.

Inverstigation on loading of the Dimple optical trap based on a magnetically levitated large-volume crossed optical dipole trap

Optical trapping techniques and the ability to tune the atomic interactions both have made the unprecedented progress in the quantum gas research field. The major advantage of the optical trap is that the atoms are likely to be trapped at various sub-levels of the electronic ground state and the interaction strength can be controlled by Feshbach resonance. Optical trapping methods in combination with magnetic tuning of the scattering properties directly lead to the experimental achievements of Bose-Einstein condensation (BEC) of Cesium, which at first failed by using magnetic trapping approaches due to the large inelastic collision rate. The rapid loss of cesium atoms due to the inelastic two-body collisions greatly suppresses the efficient evaporative cooling to obtain a condensate. For optical production of cesium atomic BEC, it is necessary to prepare a large number of Cs atoms at specified state in an optical trap for condensation, especially for an efficient forced evaporation cooling. In this paper, we demonstrate our research on enhancing the loading rate of the atoms by using a dimple trap combined with a large-volume optical dipole trap (reservoir trap). In our work, the cold cesium atoms are prepared by a three-dimensional degenerated Raman sideband cooling, and then loaded into a large-volume crossed dipole trap by using the magnetic levitation technique. Effective load of the dimple optical trap is realized by superposing the small-volume dimple trap on the center of the largevolume optical trap. The theoretical analyses are performed for the magnetically levitated large-volume crossed dipole trap in variable magnetic field gradients and uniform bias fields. Optimal experimental values are acquired accordingly. The combined potential curve of the dimple trap, which is superimposed on the magnetically levitated large-volume dipole trap, is also given. The loading of precooled atoms from Raman sideband cooling into the magnetically levitated large-volume optical trap is measured in variable magnetic field gradients and uniform bias fields. Different loading results of the dimple trap are investigated, including direct loading after Raman sideband cooling, the large-volume optical trap and the magnetically levitated large-volume dipole trap without anti-trapping potential. Comparatively, the atomic number density is enhanced by a factor of ~15 by loading the atomic sample from the magnetically levitated large-volume dipole trap into the dimple optical trap. The experimental results lay a sound basis for the further cooling and densifying the atomic cloud through the evaporating cooling stage. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for atomic species with large atom mass.

Cascade Raman sideband generation and orbital angular momentum relations for paraxial beam modes

In this work, the nonlinear parametric interaction of optical radiation in various transverse modes in a Raman-active medium is investigated both experimentally and theoretically. Verification of the orbital angular momentum algebra (OAM-algebra) [Strohaber et al.,Opt. Lett.37,3411 (2012)] was performed for high-order Laguerre Gaussian modes ℓ>1. It was found that this same algebra also describes the coherent transfer of OAM when Ince-Gaussian modes were used. New theoretical considerations extend the OAM-algebra to even and odd Laguerre Gaussian, and Hermite Gaussian beam modes through a change of basis. The results of this work provide details in the spatiotemporal synthesis of custom broadband pulses of radiation from Raman sideband generation.

Site-resolved imaging of fermionic ^{6}Li in an optical lattice.

We demonstrate site-resolved imaging of individual fermionic ^{6}Li atoms in a single layer of a 3D optical lattice. To preserve the density distribution during fluorescence imaging, we simultaneously cool the atoms with 3D Raman sideband cooling. This laser cooling technique, demonstrated here for the first time for ^{6}Li atoms, also provides a pathway to rapid low-entropy filling of an optical lattice. We are able to determine the occupation of individual lattice sites with a fidelity >95%, enabling direct, local measurement of particle correlations in Fermi lattice systems. This ability will be instrumental for creating and investigating low-temperature phases of the Fermi-Hubbard model, including antiferromagnets and d-wave superfluidity.

Quantum-gas microscope for fermionic atoms.

We realize a quantum-gas microscope for fermionic ^{40}K atoms trapped in an optical lattice, which allows one to probe strongly correlated fermions at the single-atom level. We combine 3D Raman sideband cooling with high-resolution optics to simultaneously cool and image individual atoms with single-lattice-site resolution at a detection fidelity above 95%. The imaging process leaves the atoms predominantly in the 3D motional ground state of their respective lattice sites, inviting the implementation of a Maxwell's demon to assemble low-entropy many-body states. Single-site-resolved imaging of fermions enables the direct observation of magnetic order, time-resolved measurements of the spread of particle correlations, and the detection of many-fermion entanglement.

Magnetic levitation for effective loading of cold cesium atoms in a crossed dipole trap

We report a detailed study of effective magnetically levitated loading of cold atoms in a crossed dipole trap: an appropriate magnetic field gradient precisely compensates for the destructive gravitational force of the atoms and an additional bias field simultaneously eliminates the antitrapping potential induced by the magnetic field gradient. The magnetic levitation is required for a large-volume crossed dipole trap to form a shallow but very effective loading potential, making it a promising method for loading and trapping more cold atoms. For cold cesium atoms in the $F=3, mF=3$ state prepared by three-dimensional degenerated Raman sideband cooling, a large number of atoms $\ensuremath{\sim}3.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ have been loaded into a large-volume crossed dipole trap with the help of the magnetic levitation technique. The dependence of the number of atoms loaded and trapped in the dipole trap on the magnetic field gradient and bias field, respectively, is in good agreement with the theoretical analysis. The optimum magnetic field gradient of 31.13 G/cm matches the theoretical value of 31.3 G/cm well. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for many atomic species in high-field seeking states.

Efficient sympathetic motional-ground-state cooling of a molecular ion

experiment. We optimize the cooling rate of ground state cooling schemes for single 25 Mg + ions and sympathetic ground state cooling of 24 MgH + . In particular, we show that robust cooling is achieved by combining pulsed Raman sideband cooling with continuous quench cooling. Furthermore, we experimentally demonstrate an efficient strategy for ground state cooling outside the Lamb-Dicke regime.

Efficient sympathetic motional-ground-state cooling of a molecular ion

Cold molecular ions are promising candidates in various fields ranging from precision spectroscopy and test of fundamental physics to ultra-cold chemistry. Control of internal and external degrees of freedom is a prerequisite for many of these applications. Motional ground state cooling represents the starting point for quantum logic-assisted internal state preparation, detection, and spectroscopy protocols. Robust and fast cooling is crucial to maximize the fraction of time available for the actual experiment. We optimize the cooling rate of ground state cooling schemes for single $^{25}\mathrm{Mg}^{+}$ ions and sympathetic ground state cooling of $^{24}\mathrm{MgH}^{+}$. In particular, we show that robust cooling is achieved by combining pulsed Raman sideband cooling with continuous quench cooling. Furthermore, we experimentally demonstrate an efficient strategy for ground state cooling outside the Lamb-Dicke regime.

Enhanced Raman sideband cooling of caesium atoms in a vapour-loaded magneto-optical trap

We report enhanced three-dimensional degenerated Raman sideband cooling (3D DRSC) of caesium (Cs) atoms in a standard single-cell vapour-loaded magneto-optical trap. Our improved scheme involves using a separate repumping laser and optimized lattice detuning. We load 1.5 × 107 atoms into the Raman lattice with a detuning of −15.5 GHz (to the ground F = 3 state). Enhanced 3D DRSC is used to cool them from 60 µK to 1.7 µK within 12 ms and the number of obtained atoms is about 1.2 × 107. A theoretical model is proposed to simulate the measured number of trapped atoms. The result shows good agreement with the experimental data. The technique paves the way for loading a large number of ultracold Cs atoms into a crossed dipole trap and efficient evaporative cooling in a single-cell system.

Increased radiative lifetime of Tm3+:3F4 → 3H6 transition in oxyfluoride tellurite glasses

The 1.8μm emission properties of Tm3+-doped zinc tellurite glasses modified by the substitution of ZnF2 are investigated in this paper. The thermal stability, Raman and phonon sideband spectra, transmission and absorption spectra, emission spectra and decay curves are discussed. It is found that substitution of fluoride ions into the zinc tellurite matrix produces dramatic increase in the emission lifetime of Tm3+ 1.8μm emission. Absorption, Raman and phonon sideband spectra are used to estimate the local structure of Tm3+ ions. These analyses indicate structural change around Tm3+ ions caused by substitution of fluoride ions monitors the increased intrinsic radiative lifetimes. An increase in the measured radiative lifetimes of the Tm3+:3F4→3H6 transition is observed. The origin has been discussed and the reduction of OH− absorption, decrease of maximum phonon energy and phonon density are considered to be dominant in all of the nonradiative relaxations.

Nondestructive imaging of an ultracold lattice gas

We demonstrate the nondestructive imaging of a lattice gas of ultracold bosons. Atomic fluorescence is induced in the simultaneous presence of degenerate Raman sideband cooling. The combined influence of these processes controllably cycles an atom between a dark state and a fluorescing state while eliminating heating and loss. Through spatially resolved sideband spectroscopy following the imaging sequence, we demonstrate the efficacy of this imaging technique in various regimes of lattice depth and fluorescence acquisition rate. Our work provides an important extension of quantum gas imaging to the nondestructive detection, control and manipulation of atoms in optical lattices. In addition, our technique can also be extended to atomic species that are less amenable to molasses-based lattice imaging.

Raman cooling imaging: Detecting single atoms near their ground state of motion

We demonstrate imaging of neutral atoms via the light scattered during continuous Raman sideband cooling. We detect single atoms trapped in optical tweezers while maintaining a significant motional ground-state fraction. The techniques presented provide a framework for single-atom resolved imaging of a broad class of atomic species.

Coherent broadband light generation with a double-path configuration.

We generate broadband light by focusing two femtosecond pulses into a Raman-active crystal. By reflecting Raman sideband beams together with the two driving beams back to the same crystal (with a slight spatial offset), we generate sidebands covering a broader spectral range, compared to a single pass. In this novel double-path configuration, multiple Raman sideband beams interact with each other since the phase-matching condition is automatically fulfilled. This scheme enables an enhanced cascaded coherent anti-Stokes scattering process and also doubles the interaction length, thus it allows one to use relatively weak energy pump pulses and thereby avoid optical damage.

Preparing single ultra-cold antihydrogen atoms for free-fall in GBAR

We discuss an experimental approach allowing to prepare antihydrogen atoms for the GBAR experiment. We study the feasibility of all necessary experimental steps: The capture of incoming [Formula: see text] ions at keV energies in a deep linear RF trap, sympathetic cooling by laser cooled Be + ions, transfer to a miniaturized trap and Raman sideband cooling of an ion pair to the motional ground state, and further reducing the momentum of the wavepacket by adiabatic opening of the trap. For each step, we point out the experimental challenges and discuss the efficiency and characteristic times, showing that capture and cooling are possible within a few seconds.

Suppression of Ion Transport due to Long-Lived Subwavelength Localization by an Optical Lattice

We report the localization of an ion by a one-dimensional optical lattice in the presence of an applied external force. The ion is confined radially by a radio frequency trap and axially by a combined electrostatic and optical-lattice potential. Using a resolved Raman sideband technique, one or several ions are cooled to a mean vibrational number <n>=(0.1±0.1) along the optical lattice. We measure the average position of a periodically driven ion with a resolution down to λ/40, and demonstrate localization to a single lattice site for up to 10 ms. This opens new possibilities for studying many-body systems with long-range interactions in periodic potentials, as well as fundamental models of friction.

Micro-fabricated stylus ion trap

An electroformed, three-dimensional stylus Paul trap was designed to confine a single atomic ion for use as a sensor to probe the electric-field noise of proximate surfaces. The trap was microfabricated with the UV-LIGA technique to reduce the distance of the ion from the surface of interest. We detail the fabrication process used to produce a 150 μm tall stylus trap with feature sizes of 40 μm. We confined single, laser-cooled, (25)Mg(+) ions with lifetimes greater than 2 h above the stylus trap in an ultra-high-vacuum environment. After cooling a motional mode of the ion at 4 MHz close to its ground state (<n> = 0.34 ± 0.07), the heating rate of the trap was measured with Raman sideband spectroscopy to be 387 ± 15 quanta/s at an ion height of 62 μm above the stylus electrodes.

Ground-State Cooling of a Single Atom at the Center of an Optical Cavity

A single neutral atom is trapped in a three-dimensional optical lattice at the center of a high-finesse optical resonator. Using fluorescence imaging and a shiftable standing-wave trap, the atom is deterministically loaded into the maximum of the intracavity field where the atom-cavity coupling is strong. After 5 ms of Raman sideband cooling, the three-dimensional motional ground state is populated with a probability of (89±2)%. Our system is the first to simultaneously achieve quantum control over all degrees of freedom of a single atom: its position and momentum, its internal state, and its coupling to light.