Stimulated Raman Transitions Research Articles

Precision atomic gravimeter based on Bragg diffraction

We present a precision gravimeter based on coherent Bragg diffraction of freely falling cold atoms. Traditionally, atomic gravimeters have used stimulated Raman transitions to separate clouds in momentum space by driving transitions between two internal atomic states. Bragg interferometers utilize only a single internal state, and can therefore be less susceptible to environmental perturbations. Here we show that atoms extracted from a magneto-optical trap using an accelerating optical lattice are a suitable source for a Bragg atom interferometer, allowing efficient beamsplitting and subsequent separation of momentum states for detection. Despite the inherently multi-state nature of atom diffraction, we are able to build a Mach–Zehnder interferometer using Bragg scattering which achieves a sensitivity to the gravitational acceleration of Δg/g = 2.7 × 10−9 with an integration time of 1000 s. The device can also be converted to a gravity gradiometer by a simple modification of the light pulse sequence.

Sensitive magnetometers based on dark states

24 In recent years interest has been growing into the fascinating properties of dark resonances. Most of the experiments have been carried out in alkali atoms due to their favourable level structure and the commercially available diode lasers, resulting in numerous applications. Teams from Siena and Sofia developed an all-optical magnetometer based on such dark resonances. The dark-state effect was discovered in 1976 by G. Alzetta, A. Gozzini, L. Moi and G. Orriols [1], while they were working on optical-pumping experiments with a multimode dye laser. They wanted to make the observation of multi-photon radio-frequency resonances in sodium atoms easy, i.e., observable by the naked eye. To this end, they introduced a magnetic field (MF) that was constant in time but spatially inhomogeneous along the laser beam, so as to produce Zeeman splitting of groundstate hyperfine levels (Fig.1a, left). In this way, an applied radio-frequency field was resonant only if it matched the Zeeman splitting. As a result, a bright radio-frequency resonance appeared (Fig.1a, right). During these experiments, besides the bright resonances, a new “dark” resonance was observed, which does not require any radio-frequency field. In the introductory illustration only the dark resonance is shown. The new dark state (DS) effect appears in a position along the laser beam where the frequency difference (ω10-ω20) between two optical frequencies is equal to the frequency splitting ω12 between two Zeeman sublevels (Fig.1b, left). This is the resonant condition for the stimulated Raman transitions between the two longliving ground levels. Theoretical studies described the dark resonance as a coherent superposition of two long-living ground levels prepared by the bi-chromatic laser field, and ‘atomic population trapping’ was introduced, leading to the term Coherent Population Trapping (CPT) [2]. l Luigi Moi1, Stefka Cartaleva2 DOI: 10.1051/epn/2012603

Quantum logic for the control and manipulation of molecular ions using a frequency comb

We propose a scheme for the preparation, manipulation and detection of quantum states of single molecular ions. In this scheme, molecular and atomic ions are confined in radio-frequency Paul trap and share common modes of motion. A frequency comb laser field is used to drive stimulated Raman transitions that couple internal states of the molecular ion with the motion of ions. State transfer from the molecular ion to the atomic ion via the common mode of motion results in efficient state detection for the molecule. The coupling of molecular states to the motion and the subsequent sideband cooling of the ions provide a way to prepare the molecular ion in a well-defined state.

Local gravity measurement with the combination of atom interferometry and Bloch oscillations

We present a local measurement of gravity combining Bloch oscillations and atom interferometry. With a falling distance of 0.8 mm, we achieve a sensitivity of 2x10-7 g with an integration time of 300 s. No bias associated with the Bloch oscillations has been measured. A contrast decay with Bloch oscillations has been observed and attributed to the spatial quality of the laser beams. A simple experimental configuration has been adopted where a single retro-reflected laser beam is performing atoms launch, stimulated Raman transitions and Bloch oscillations. The combination of Bloch oscillations and atom interferometry can thus be realized with an apparatus no more complex than a standard atomic gravimeter.

Raman Sideband Cooling of Two-Valence-Electron Fermionic Atoms

We propose a method for laser cooling two-valence-electron fermionic atoms. Our protocol employs resolved-sideband cooling on the stimulated Raman transition between the two magnetic sublevels (m = F and m = F − 1) of the ground state with total angular momentum F. The optical pumping from m = F − 1 to 1P1 are used to decouple atoms in the m = F − 1 state. We calculate the Raman coupling generated by an engineered optical lattice. The result shows that it is possible to laser cool the two-valence-electron fermionic atoms to the ground state. The atoms in the ground state provide a new system for quantum optics.

Magnetic field measurement based on a stimulated two-photon Raman transition**Project supported by the National Basic Research Program of China (Grant No. 2005CB724506), the National Natural Science Foundation of China (Grant Nos. 10604058 and 10974215), and by the Open Research Fund of State Key Laboratory of Precision Spectroscope.

The magnetic field in the microwave interaction zone of the fountain atomic clock was measured by stimulated Raman transitions. By measuring the two-photon transition frequency between the Zeeman levels of the two ground states, we achieved a magnetic field measurement accuracy of the order of 0.28 nT. This method is immune to the Doppler shift and the AC Stark shift. The second order Zeeman shift of the fountain clock is 170.7 × 10−15, with the uncertainty of 7.2 × 10−16.

Light pulse atom interferometry at short interrogation times

The use of cold atoms in any sensor operating in a dynamic environment requires that the measurement cycle be conducted before the atom cloud escapes the interaction region. Under multiple-g accelerations it is desirable to complete measurements in millisecond time scales, particularly when laser beams are used to interrogate the atoms. In this paper, we demonstrate high-contrast atom interferometry in a vapor cell using stimulated Raman transitions at millisecond interrogation times. Laser-cooled cesium atoms are interrogated with a sequence of three Raman pulses and the interferometer phase is read out in the same region in which the atoms are trapped. Our system achieved over 70% contrast with a Doppler insensitive interferometer and over 30% contrast with a Doppler sensitive interferometer, in an environment normally considered adverse to high-contrast atom interferometry (e.g., no retroreflector stabilization and no magnetic shielding). Demonstration of an inertially sensitive atom interferometer in this environment supports the feasibility of a high-bandwidth inertial sensor using light pulse atom interferometry. Finally, we show that Raman pulse population transfer efficiency in our system is primarily limited by nonuniformity of the Raman laser intensity across the atom cloud.

Generation of Raman light source with an anti-reflection-coated edge-emitting laser diode in an external cavity

Phase locked two laser beams with a tunable and controllable frequency difference in a range of several GHz play a major role in the stimulated Raman transition, coherent population trapping, quantum states preparation and other quantum manipulation researches. We demonstrate such leaser beams with a tunable frequency difference in a range of 6.09.3GHz. In particular, the frequency differences of 6.835GHz and 9.192GHz corresponding to the ground state's hyperfine splitting in Rb87 and Cs133 respectively are realized experimentally. The power of the modulated beam is measured to be 6.87mW. With an antireflection-coated edge-emitting diode placed in an external cavity, we can suppress the carrier completely when the modulation frequency is lower than 4.0GHz by adjusting the external cavity length, the temperature, the current of the diode, and the power of the modulation. When the modulation is higher than 4.0GHz we cannot fully suppress the carrier, but we can also obtain the laser beams each with a high modulation depth due to modulation enhancement by external cavity resonance.

Measurement of Local Gravity via a Cold Atom Interferometer

We demonstrate a precision measurement of local gravity acceleration g in Wuhan by a compact cold atom interferometer. The atom interferometer is in vertical Mach—Zehnder configuration realized using a π/2 - π - π/2 Raman pulse sequence. Cold atoms were prepared in a magneto-optical trap, launched upward to form an atom fountain, and then coherently manipulated to interfere by stimulated Raman transition. Population signal vs Raman laser phase was recorded as interference fringes, and the local gravity was deduced from the interference signal. We have obtained a resolution of 7 × 10−9 g after an integration time of 236s under the best vibrational environment conditions. The absolute g value was derived from the chirp rate with a difference of 1.5 × 10−7 g compared to the gravity reference value. The tidal phenomenon was observed by continuously monitoring the local gravity over 123 h.

Precise control of state-selective excitation in stimulated Raman scattering

Multiphoton transitions can be manipulated by tailoring the ultrashort laser pulse. In this paper, we propose two schemes for achieving precise control of the selective excitation between two excited states in stimulated Raman-scattering process. We theoretically demonstrate that by properly designing the spectral phase distribution, the stimulated Raman transition probability for one excited state is kept at zero or a maximal value, while that for the other excited state can be continuously tuned over a wide range. Furthermore, the influence of the spectral bandwidth on the tunable range by the two schemes is discussed. We conclude that these schemes have significant application to the selective excitation of femtosecond coherent anti-Stokes Raman scattering.

Ramsey Spectroscopy and Geometric Operations on Sodium Bose–Einstein Condensates Using Two-Photon Stimulated Raman Transitions

Ramsey spectroscopy and geometric operations on the released sodium Bose–Einstein condensates consisting of 10 7 atoms were demonstrated using copropagating two-photon stimulated Raman pulses, which were resonant to the clock transition between the F =1, m F =0 and F =2, m F =0 states of sodium atoms. The Ramsey fringes with a visibility close to 1 were obtained for two Raman pulses separated by 10 ms. A relative frequency resolution of Ramsey resonance was 6×10 -10 and a phase uncertainty of fringes was 0.06 rad for single measurement. Using resonant laser-controlled Raman pulses, geometric operations for rotations of the two-level Bose–Einstein condensates around axes 2 and 3 on the Bloch sphere were also demonstrated with a relative phase uncertainty of 4%.

Squeezing properties of light field in the process preparing Raman atom laser

In this paper, the squeezing properties of the probe light field in the process preparing Raman-type atom laser via stimulated Raman transition of the sodium atom Bose–Einstein condensate (BEC) interacting with two light beam of a weaker squeezed coherent probe light and a stronger classical pump light are studied. The results show that the probe light can be periodically squeezed. The squeezing period is related to the mean number of atoms in trap, the strength of interaction between squeezed light and BEC atoms, and the detuning of the light, and the squeezing depth is determined by the initial squeezing factor of the probe light.

Low noise amplification of an optically carried microwave signal: application to atom interferometry

In this paper, we report a new scheme to amplify a microwave signal carried on a laser light at λ=852 nm. The amplification is done via a semiconductor tapered amplifier and this scheme is used to drive stimulated Raman transitions in an atom interferometer. Sideband generation in the amplifier, due to self-phase and amplitude modulation, is investigated and characterized. We also demonstrate that the amplifier does not induce any significant phase-noise on the beating signal. Finally, the degradation of the performances of the interferometer due to the amplification process is shown to be negligible.

Stimulated Raman transitions via multiple atomic levels

We consider the stimulated Raman transition between two long-lived states via multiple intermediate states, such as between hyperfine ground states in the alkali-metal atoms. We present a concise treatment of the general, multilevel, off-resonant case, and we show how the lightshift emerges naturally in this approach. We illustrate our results by application to alkali-metal atoms and we make specific reference to cesium. We comment on some artifacts, due solely to the geometrical overlap of states, which are relevant to existing experiments.

Quantum gate using qubit states separated by terahertz

A two-qubit quantum gate is realized using electronic excited states in a single ion with an energy separation on the order of a terahertz times the Planck constant as a qubit. Two phase locked lasers are used to excite a stimulated Raman transition between two metastable states $D_{3/2}$ and $D_{5/2}$ separated by 1.82 THz in a single trapped $^{40}$Ca$^+$ ion to construct a qubit, which is used as the target bit for the Cirac-Zoller two-qubit controlled NOT gate. Quantum dynamics conditioned on a motional qubit is clearly observed as a fringe reversal in Ramsey interferometry.

Two-dimensional femtosecond stimulated Raman spectroscopy: Observation of cascading Raman signals in acetonitrile

A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)). The resultant signal can be generated by two alternative mechanisms: a fifth-order Raman process that would directly probe anharmonic coupling between the two modes, or a third-order cascade in which a third-order coherent Raman process produces a field that goes on to participate in a third-order stimulated Raman transition. The third-order cascade is shown to dominate the 2D-FSRS spectrum as determined by comparison with the predicted magnitude of the two signals, the 2D spectrum of a mixed isotope experiment, and the concentration dependence of the signal. In acetonitrile, theoretical calculations of the vibrational anharmonicity indicate that the third-order cascade signal should be 10(4) times larger than the fifth-order Raman signal. 2D-FSRS signals are observed between acetonitrile's CCN bend, of E symmetry, and several different A(1) modes but are forbidden by symmetry in the fifth-order pathway. A 2D-FSRS spectrum of a 50:50 mixture of acetonitrile and d(3)-acetonitrile shows equivalent intensity for intramolecular coupling peaks and intermolecular coupling peaks, indicating that the observed signal cannot be probing molecular anharmonicity. Finally, the magnitudes of the 2D-FSRS peaks are observed to be proportional to the square of the number density, supporting the cascade mechanism.

Measurement and compensation of optical Stark shifts for manipulating the terahertz-separated qubit inC40a+

We describe the measurement and compensation of optical Stark shifts induced by an off-resonant pulse for manipulating the terahertz-separated states $3\text{ }{^{2}D}_{3/2}--3\text{ }{^{2}D}_{5/2}$, in a single trapped ${^{40}\text{C}\text{a}}^{+}$ ion. These states can be used as a quantum bit and are coupled by a two-photon stimulated Raman transition with phase-locked light sources bridged by an optical comb generator. The induced optical Stark shift is measured by Ramsey interferometry. We estimate the power ratio of Raman beams that cancel the level shift with an uncertainty of 170 Hz. A shift of the same amount as this uncertainty is estimated to give rise to an infidelity of approximately $7.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ during a $\ensuremath{\pi}/2$ rotation operation.

A proposal for laser cooling of OH molecules

We describe a possible method for laser cooling OH molecules. The method is based on exciting the ΔJ = 2 stimulated Raman transition in the ground electronic and vibronic state. The molecules are assumed to already be trapped with a temperature of ∼10 mK or less in a spatially varying magnetic field. By exciting the molecule in a region of high magnetic field, the subsequent spontaneous photon emission at lower magnetic field removes centre-of-mass energy from the molecule. The vacuum decay rate from the J = 7/2, F = 4, MF = 4 to J = 5/2, F = 3, MF = 3 is roughly 0.31 Hz and the next cascade to J = 3/2, F = 2, MF = 2 is roughly 0.15 Hz. A cavity tuned to stimulate both transitions in a small region of space can enhance the decay. Simulations of the radiative transitions indicate that up to 20% of a population of OH molecules with an average energy of 10 mK will finish with energies less than 200 µK.

Cold atom interferometers and their applications in precision measurements

Experimental realization of cold (85)Rb atom interferometers and their applications in precision measurements are reported in this paper. Mach-Zehnder and Ramsey-Borde type interferometers were demonstrated. Detailed descriptions of the interferometers are given including manipulation of cold atoms, Rabi oscillation, stimulated Raman transitions, and optical pumping. As an example of using atom interferometers in precision measurements, the quadratic Zeeman shift of hyperfine sublevels of (85)Rb was determined.

Generating of squeezed atom laser via stimulated Raman transition

We present a scheme to generate a squeezed atom laser via stimulated Raman transition of the atoms in Bose–Einstein condensate (BEC) interacting with two light beams, including a weaker squeezed coherent probe light and a stronger classical pump light. The results show that the quantum fluctuation of this atom laser can be periodically squeezed. The squeezing depth of such atom laser is determined by the initial squeezing factor of the probe light, and the squeezing period of that is related to the mean number of atoms in the trap, the strength of interaction between squeezing light and BEC atoms, and the detuning of the light.