Narrow Atomic Transitions Research Articles

Quantum resonant optical bistability with a narrow atomic transition: bistability phase diagram in the bad cavity regime

We report on the observation of a novel manifestation of saturation-induced optical bistability in a resonantly pumped optical ring cavity interacting strongly with a cloud of atoms via a narrow atomic transition. The bistability emerges, above a critical pump rate, as an additional peak in the cavity’s normal mode spectrum close to atomic resonance. This third transmission peak is usually suppressed due to strong resonant absorption, but in our experiment it is visible because of the linewidth of the atomic transition being much smaller than that of the cavity, which sets the experiment into the bad-cavity regime. Relying on complete saturation of the transition, this bistability has a quantum origin and cannot be mimicked by a classical material presenting a nonlinear refraction index. The appearance of the central peak in addition to the normal modes is predicted by a semi-classical model derived from the Tavis–Cummings Hamiltonian from which we derive a bistability phase diagram that connects our observations with former work on optical bistability in the good cavity regime. The phase diagram reveals several so far unexplored bistable phases.

Ultrastable, ultraprecise, portable

AbstractIn quantum applications such as optical clocks ultrastable laser sources address extremely narrow atomic transitions without introducing noise to the system. This level of stability and precision has historically only been achieved under controlled laboratory conditions. Novel commercial solutions combined with new approaches are now meeting the high demands for field applications of emerging quantum technologies.

Continuous narrowband lasing with coherently driven V-level atoms

Simultaneous strong coherent pumping of the two transitions of a V-level atom with very different decay rates has been predicted to create almost perfect inversion on the narrower transition. Using the example of the blue and red transitions in Strontium we show that for suitable operating conditions the corresponding resonant gain can be used to continuously operate a laser on the narrow transition. In particular, for a strong detuning of the pump field with respect to the narrow transition, coherent laser emission occurs close to the bare atomic transition frequency exhibiting only a negligible contribution from coherent pump light scattered into the lasing mode. Calculations of the cavity output spectrum show that the resulting laser linewidth can get much smaller than the bandwidth of the pump light and even the natural linewidth of the narrow atomic transition. Its frequency is closely tied to the atomic transition frequency for properly chosen atom numbers. Simulations including atomic motion show Doppler cooling on the strong transition with minor motion heating on the lasing transition, so that continuous laser operation in the presence of a magneto-optical trap should be possible with current experimental technology.

Sub-Hertz resonance by weak measurement

Weak measurement (WM) with state pre- and post-selection can amplify otherwise undetectable small signals and thus has potential in precision measurement applications. Although frequency measurements offer the hitherto highest precision due to the stable narrow atomic transitions, it remains a long-standing interest to develop new schemes to further escalate their performance. Here, we demonstrate a WM-enhanced correlation spectroscopy technique capable of narrowing the resonance linewidth down to 0.1 Hz in a room-temperature atomic vapour cell. The potential of this technique for precision measurement is demonstrated through weak magnetic-field sensing. By judiciously pre- and post-selecting frequency-modulated input and output optical states in a nearly orthogonal manner, a sensitivity of 7 fT Hz−1/2 at a low frequency near DC is achieved using only one laser beam with 15 µW of power. Additionally, our results extend the WM framework to a non-Hermitian Hamiltonian and shed new light on metrology and bio-magnetic field sensing.

Lasing on a narrow transition in a cold thermal strontium ensemble

Highly stable laser sources based on narrow atomic transitions provide a promising platform for direct generation of stable and accurate optical frequencies. Here we investigate a simple system operating in the high-temperature regime of cold atoms. The interaction between a thermal ensemble of $^{88}$Sr at mK temperatures and a medium-finesse cavity produces strong collective coupling and facilitates high atomic coherence which causes lasing on the dipole forbidden $^1$S$_0 \leftrightarrow ^3$P$_1$ transition. We experimentally and theoretically characterize the lasing threshold and evolution of such a system, and investigate decoherence effects in an unconfined ensemble. We model the system using a Tavis-Cummings model, and characterize velocity-dependent dynamics of the atoms as well as the dependency on the cavity-detuning.

Dichroic Two-Photon Rubidium Frequency Standard

Frequency standards find wide use in navigation and timing for industry, civilian, and defense applications. Unfortunately, the very best frequency standards are bulky, power-hungry, and confined to well-controlled laboratory environments. Here the authors aim to deliver a compact, high-performance standard, the key challenge being the need to produce a strong, narrow atomic transition. They produce a superstrong signal by driving a two-photon transition in ${}^{87}$Rb vapor that exploits a fortuitous third atomic level to greatly enhance the effective strength of the atom-light interaction, allowing short-term clock performance comparable to today's best commercial frequency standards.

Towards passive and active laser stabilization using cavity-enhanced atomic interaction

Ultra stable frequency references such as the ones used in optical atomic clocks and for quantum metrology may be obtained by stabilizing a laser to an optical cavity that is stable over time. State-of-the-art frequency references are constructed in this way, but their stabilities are currently limited by thermally induced length fluctuations in the reference cavity. Several alternative approaches using the potential for frequency discriminating of highly forbidden narrow atomic transitions have been proposed in, e.g., [1] and [2]. In this proceeding we will present some of the ongoing experimental efforts derived from these proposals, to use cavity-enhanced interaction with atomic 88Sr samples as a frequency reference for laser stabilization. Such systems can be realized using both passive and active approaches where either the atomic phase response is used as an error signal, or the narrow atomic transition itself is used as a source for a spectrally pure laser. Both approaches shows the promise of being able to compete with the current state of the art in stable lasers and have similar limitations on their ultimately achievable linewidths [1, 2].

Superradiant laser: Effect of long-range dipole-dipole interaction

We theoretically investigate the effect of long-range dipole-dipole interaction (LRDDI) on a superradiant laser (SL). This effect is induced from the atom-photon interaction in the dissipation process. In the bad-cavity limit usually performed to initiate SL, we demonstrate that cavity photon number oscillates as an interparticle distance of the atoms varies. Similarly the atom-atom coherence alternates with signs, showing critical transitions alternatively in SL operations. This suggests a complexity of the collective effect emerging in a large ensemble of atoms. Therefore this effect in a SL cannot be simply interpreted by only a part of the whole ensemble. We numerically solve for a steady-state SL including the spatially dependent LRDDI, and locate the maximal cavity photon number and the minimal spectral linewidth respectively at the optimal atomic separations in the setting of an equidistant atomic array. The scaling of a finite number of atoms shows that a steady-state SL outperforms the one without LRDDI, which allows for probing narrow atomic transitions and is potentially useful for precision measurements and next-generation optical clocks.

Detuning Effect Between the Cavity Resonance and Atomic Transition Frequencies on Noise Flux Profiles of Class-C Lasers

The class-C lasers are well recognized to have very narrow atomic transition bandwidths. Therefore, a minor mismatch between the cavity resonance and atomic transition frequencies (CAF) is normally led to the large modifications in the gain and noise profiles. The motion of class-C lasers is described by the three variables of cavity electric field, atomic population inversion, and dipole moment of atoms that are coupled to each other by the optical Maxwell-Bloch equations. The fluctuating features of these three variables are completely elaborated by solving their equations of motion in the presence of the cavity Langevin force. The results peculiarly indicate that the noise profile of the cavity electric field exactly mimics the amplification profile of an input signal by the laser in both below and above threshold states. The both gain and noise profiles simultaneously tend to infinity at the normalized pumping rates and CAF mismatches determined by the laser stability theory. The noise flux profiles are finally confirmed by illustrating the flux conservation.

High-bandwidth quantum memory protocol for storing single photons in rare-earth doped crystals

We present a detailed analysis of a high-bandwidth quantum memory protocol for storing single photons in a rare-earth-ion doped crystal. The basic idea is to benefit from a coherent free-induced decay type re-emission which occurs naturally when a photon with a broadband spectrum is absorbed by a narrow atomic transition in an optically dense ensemble. This allows for a high-bandwidth memory for realistic material parameters. Long storage time and on-demand readout are obtained by means of spin states in a lambda-type configuration, through the transfer of the optical coherence to a spin coherence (so-called spin-wave storage). We give explicit formulae and show numerical results which make it possible to gain insight into the dependence of the memory efficiency on the optical depth and on the width and the shape of stored photons. We present a feasibility study in rare-earth doped crystals and show that high efficiencies and high bandwidth can be obtained with realistic parameters. High-bandwidth memories using spin-wave storage offers the possibility of very high time-bandwidth products, which is important for experiments where high repetition rates are needed.

Spectrum of Light in a Quantum Fluctuating Periodic Structure

We address the general problem of the excitation spectrum for light coupled to scatterers having quantum fluctuating positions around the sites of a periodic lattice. In addition to providing an imaginary part to the spectrum, we show that these quantum fluctuations affect the real part of the spectrum, in a way that we determine analytically. Our predictions may be observed with ultracold atoms in an optical lattice, on a J = 0 --> J;{'} = 1 narrow atomic transition. As a side result, we resolve a controversy for the occurrence of a spectral gap in a fcc lattice.

A simple feedback-loop for a frequency lock on a narrow atomic transition

In this paper we consider an atomic system with a narrow transition probed with a user-controlled electromagnetic radiation, the basis of atomic clocks. We propose a simple feedback-loop in order to lock automatically the probe frequency on the atomic transition, allowing continuous clock operation without any preparation steps. The proof of the convergence of the feedback-loop is based on averaging arguments, with approximations compatible with realistic physical parameters. Numerical simulations illustrate the robustness of the proposed feedback-loop versus measurement noise and bias.

Multi-constituents detection in contaminated aerosol clouds using remote-filament-induced breakdown spectroscopy

Remote-filament-induced breakdown spectroscopy (R-FIBS) was used for probing a cloud of aqueous aerosols containing a mixture of PbCl2, CuCl2, FeCl2 and NaCl. We demonstrated experimentally that it can be used as a sensitive sensing technique to remotely retrieve the composition of microdroplets in clouds located at a distance. In fact, fluorescence from all the metallic ions dissolved was observed. Moreover, these spectrally narrow atomic transitions excited by the low-density plasma did not show any signal overlap. These characteristic spectra demonstrate that R-FIBS can be used to simultaneously recognize and distinguish every single metallic constituent dissolved inside such a cloud.

Narrow atomic transitions with enhanced sensitivity to variation of the fine structure constant

To study the possible variation of the fine structure constant, α, in laboratory experiments, we propose using narrow lines that correspond to atomic transitions between close lying, long-lived atomic states of different configurations. The small transition frequency, coupled with differences in the electron structure ensures a strong enhancement of the relative frequency change compared to a possible change in α. The small linewidth makes it possible to perform very accurate measurements. These two features, when combined, make the measurements very sensitive to any variation of α.

Minimization of ion micromotion in a Paul trap

Micromotion of ions in Paul traps has several adverse effects, including alterations of atomic transition line shapes, significant second-order Doppler shifts in high-accuracy studies, and limited confinement time in the absence of cooling. The ac electric field that causes the micromotion may also induce significant Stark shifts in atomic transitions. We describe three methods of detecting micromotion. The first relies on the change of the average ion position as the trap potentials are changed. The second monitors the amplitude of the sidebands of a narrow atomic transition, caused by the first-order Doppler shift due to the micromotion. The last technique detects the Doppler shift induced modulation of the fluorescence rate of a broad atomic transition. We discuss the detection sensitivity of each method to Doppler and Stark shifts, and show experimental results using the last technique.