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 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 SP 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.
Active optical clocks have been suggested as an excellent way to improve the short-time performance of optical clocks by removing the requirement for an ultra stable interrogation laser Chen2005 (); Chen2009 (); MeiserPRL2009 (). Current state-of-the-art neutral-atom optical lattice clocks with exceedingly high accuracy are typically limited in precision by the interrogation lasers and not by the atomic quantum projection noise limit. The high Q-value of the atomic transitions () exceeds that of the corresponding reference laser stability for the duration of the interrogation cycle Oelker (); McGrew (); Nicholson (); Ushijima (); Huntemann (). State-of-the-art reference lasers now perform below the level of fractional frequency stability at s Matei (); Zhang (); Robinson (). This recent advance in performance is enabled by using crystalline mirror coatings and spacers as well as employing cryogenic techniques. Nevertheless these lasers continue to be limited by the thermal noise induced in the reference cavity mirrors Numata (); Cole (). By using an active optical clock, the narrow spectral features of the atoms themselves produce the reference light needed, via direct generation of lasing in an optical cavity. Operation in the bad-cavity limit, where the cavity field decays much faster than the bare atomic transition, allows high suppression of the cavity noise in these lasers. It has been predicted that such systems could reach Q-factors in excess of the transition Q-factor due to narrowing caused by collective effects MeiserPRL2009 (); Debnath (). It was recently shown experimentally that an active optical atomic clock can perform at the fractional frequency stability level between s, and with a fractional frequency accuracy of by using a cyclically operated optical lattice with Sr atoms NorciaPRX2018 (). Other systems with atoms at room temperature have shown continuous lasing with performance at the level of fractional frequency stability PanTUFFC ().
These results demonstrate the potential of active atomic clocks, and motivates the present attempt to improve the understanding of atom dynamics in such a system. We concentrate on a cold atomic gas consisting of bosonic Sr cooled to a temperature of mK, and permitted to expand freely as a thermal gas while lasing. Superradiant lasing in such a system would substantially reduce the technological challenges of maintaining truly continuous operation compared to the case of atoms confined in an optical lattice trap NorciaPRX2016 (); Kazakov ().
We experimentally demonstrate pulsed lasing on a narrow transition in Sr and describe the characteristic dynamics of our system. Using a Tavis-Cummings model we simulate the full system consisting of up to individual atoms and give a qualitative explanation of the dynamics. Rich velocity- and position-dependent dynamics of the atoms are included in the description and become an important factor for understanding the lasing behavior. The model allows us to quantify these behaviors and set requirements on ensemble size and temperature to realize strongly driven or weakly driven superradiance respectively. Such requirements are useful for the future development of truly continuous lasing on ultra-narrow clock transitions in strontium, or other atomic species.
In section II we describe the physical characteristics of our system and the experimental routine. Section III describes our theoretical model for the cold-atom based laser, and the proposed lasing dynamics. In Section IV the lasing characteristics are presented and comparisons between experiment and simulations are made.
Ii Experimental system
The system consists of an ensemble of cold Sr atoms cooled and trapped in a 3D Magneto-Optical Trap (MOT) from a Zeeman-slowed atomic beam, using the SP transition. The atomic cloud partially overlaps with the mode of an optical cavity, and can be state manipulated by an off-axis pumping laser, see Fig. 1. The cavity can be tuned on resonance with the SP intercombination line, and has a linewidth of kHz. Our mirror configuration ensures a large cavity waist radius of m, which ensures a reasonably high intra-cavity atom number, of order . is estimated from fluorescence measurements and lasing pulses, giving of the full MOT atom number , where . Throughout this paper the atom number within the cavity is an effective number since all atoms have different velocities and positions, resulting in a coupling for the ’th atom to the cavity mode. While this is included in the theoretical simulations, we permit ourselves to assume a constant effective coupling, , and effective atom number when discussing regimes and general scaling behavior.
The large cavity waist ensures a negligible decoherence induced from transit time broadening of kHz. While the SP transition used for operation has a linewidth of kHz, the ensemble temperature causes a Doppler broadening of the order of MHz. The bare atoms are thus deep in the bad-cavity regime, , whereas the total inhomogeneously broadened ensemble is just below the bad-cavity threshold. Here, the cavity field decay rate and total atomic decoherence rate is of similar size , and ensemble preparation can heavily affect the lasing process and the attainable suppression of cavity noise. We characterize the system by its collective cooperativity , and the single-atom cooperativity is given by . This leads us to the definition of a collective coupling rate . While the spectral linewidth of the emitted light is controlled by MeiserPRL2009 (); Kuppens (), the coherence build-up, and thus lasing power, is determined by . Increasing the coupling rate , and thus the single-atom cooperativity, will then improve interaction at the expense of the ultimately attainable lasing linewidth.
After preparing the MOT, the cooling light is switched off, and an off-axis pumping beam is used to excite the atoms to the P state. By pumping the atoms on resonance for ns a peak excitation of approximately is obtained for the atoms within the cavity mode. Inhomogeneous Doppler detuning caused by the thermal distribution of the atoms, and the large spatial distribution of the full atomic ensemble with respect to the pumping field result in varying Rabi frequencies for different atoms. The collective atomic Bloch vector will thus contain some level of atomic coherence from the imperfect pump pulse. Due to the excitation angle of the pump phase periodicity along the cavity axis suppresses any forced coherence between the atoms and cavity field. Additionally, the spatial coherence of the fast atoms will wash out as they propagate inhomogeneously along the cavity axis. In order to remove any remaining phase-coherence we apply light at nm detuned about MHz from the SP for 500 ns after pumping. This results in an average of scattering events per atom, and ensures atoms are either in the excited or ground state. We see no quantifiable change in the lasing behavior by doing this.
Once excited, the atoms will build up coherence mediated through the cavity field and emit a coherent pulse of light into the cavity mode. Light leaking through one of the cavity mirrors is then detected on a photodiode. We couple a reference field, , into a cavity mode tuned far off resonance, MHz, with the SP transition, allowing us to lock the cavity length on resonance with the atoms.
Fig. 2 shows an example of a lasing pulse and the associated beat signal when the cavity is on resonance (blue) and detuned kHz (orange) from the atomic resonance respectively. The pumping pulse ends at s and after some delay, (on the order of few s), a lasing pulse is emitted into the cavity mode. The lasing pulses are followed by a number of ringings. This is a fingerprint of the coherent evolution of the cavity-atom system, where light emitted into the cavity mode is reabsorbed by the atoms, and is then re-emitted into the cavity mode. The lasing pulse shown in Fig. 2 (a) is detected superimposed on a nW background. This background is a reference field used for locking the length of the cavity, and provides the opportunity for a heterodyne beat with the lasing pulse. At zero cavity-atom detuning () the ringings are barely visible due to reference field intensity noise. When detecting the beat signal, however, the signal scales linearly rather than quadratically with the lasing E-field , and here multiple ringings are visible in both the resonant and off-resonant cases.
The beat signal is down-sampled and given by . The envelope thus gives us a less noisy information on the amplitude of the lasing pulse. allows us to retrieve the phase information. In Fig. 2 (b) we see the phase during the initial lasing pulse differ between the two datasets. This is a result of the randomized phase of the atomic coherence for each experimental realization. A rapid-fire pulse laser is thus limited by the lack of memory between subsequent lasing pulses.
Iii Semi-classical model
In order to gain an improved understanding of the dynamics involved in our system, we simulate the behavior with a Monte-Carlo approach. We model the system using a Tavis-Cummings hamiltonian for an N-atom system. The description includes a classical pumping field at frequency . In the Schrödinger picture the full expression becomes:
Here is the angular frequency of the cavity mode, is the corresponding lowering operator. The involved electronic energy states and correspond to the ground and excited atomic states, with a transition frequency of , as seen in Fig. 1 (b). The interaction between cavity field and the j’th atom is governed by the coupling factor and the pump beam has a semi-classical interaction term with Rabi frequency . The position of the atom is given by , and is the pump beam wavevector.
By entering an interaction picture, and using the rotating wave approximation, the time evolution of the system operators can be obtained. Here we make the semi-classical approximation of factorizing the expectation values for operator products, which results in linear scaling of the number of differential equations with the number of atoms. This approximation results in the neglect of all quantum noise in the system, and by consequence any emerging entanglement Gonzales-Tudela (); Wolfe (); Barberena (). We motivate this assumption by the very large number of atoms in the system, whose individual behaviors are taken into account with separate coupling factors . The quantum noise is then expected to be negligible compared to the single-operator mean values. The operator mean values are described by three distinct forms of evolution:
Here is the detuning of the n’th field with respect to the m’th field. Because of the semi-classical approximation the hermitian conjugate of these operators are simply described by the complex conjugate of their mean values, while . This leaves us with a total of coupled differential equations where is on the order of .
The Monte-Carlo simulation assumes the atoms are initially in the ground state, and subsequently pumped into the excited state and left to evolve with time. Atomic positions and velocities are sampled randomly from a 3D Gaussian and a thermal Maxwell-Boltzmann distribution respectively. The positions are distributed with standard deviations of mm, and the velocities according to a temperature of mK, both found from experimental assessment of our system. Atomic motion is treated classically and without collisions. The atoms interact only via the cavity field, and any spontaneous emission into the cavity mode is neglected. The lasing process is initiated by an initial nonzero total coherence , resulting from the pumping pulse. This replaces the role of quantum noise in the system, and without it the system would couple only to the reservoir.
Our simulations indicate that the lasing occurs in three characteristic regimes determined by the ratios of the atomic decay rate , the cavity decay rate and the collective atomic coupling factor , see Fig. 3. We consider only since for the spontaneous atomic decay to the reservoir will dominate. In the bad cavity regime where a low coupling strength will allow the output power to scale quadratically with atom number, whereas high coupling strength will result in a linear power scaling and multiple Rabi oscillations in the atomic population during a single lasing pulse. Broadening effects can lead to higher effective decoherence rates which must be considered.
Iv System characterization
We characterize the lasing properties and pulse dynamics of the system by varying cavity-atom detuning and atom number. We compare the measurements with simulated experiments to verify the understanding in the numerical model. This will then allow us to draw out some behaviors from the model that are inaccessible experimentally.
iv.1 Lasing Threshold
By varying the total number of atoms in the trap, we find the lasing threshold of the system. This gives an atom number dependency of the pulse power and the associated delay, see Fig. 4 (a). We plot the peak power of the emitted lasing pulse as a function of the atom number in the full atomic cloud for experimental runs. Each point is about binned experimental runs, with the standard deviation indicated. For low atom numbers (white region), , excited atoms decay with their natural lifetime, s. In an intermediate regime of (red region) the peak power appears to scale quadratically with the atom number, as the onset of lasing occurs. This is what we would expect according to the parameter regimes illustrated in Fig. 3. Finally, for higher atom numbers (blue region), , the peak power becomes linearly dependent on the atom number, as the collective coupling becomes much larger than . We show a red and blue curve fitted to their corresponding regions with an - and -scaling respectively. The green points show the results from simulation, and appear to agree well with experiment. The curves are fitted to the raw data, within the respective regions before any binning, and the lasing threshold is determined from the quadratic fit. While the lasing process does not initiate for low atom numbers due to the requirement that , for high the cavity field builds up sufficiently that the slowest atoms become strongly driven. At high atom numbers () the cavity output seems to saturate as the assumption of linear scaling of the cavity atom number () with respect to the MOT atom number () breaks down, and these points are not included in the linear fit. The nonzero value of the data below threshold is caused by random noise.
In Fig. 4 (b) we plot the delay between the end of the pumping pulse and the associated peak in laser pulse emission. The delay has high uncertainties in the red region where we expect an scaling Gross (). For high we expect a trend Kumarakrishnan () (blue), and we choose to fit this to the entire dataset. When going to low atom numbers background noise becomes increasingly important until no emission peak is visible, and the effective delay goes to infinity. Once again we show the simulation with green dots. Notice that there is a clear tendency towards longer delays in the simulation. We believe this to be caused by the fact that the model does not include spontaneous emission into the cavity mode.
As the number of atoms decreases, so does the collective cooperativity, and thus the ensemble coherence build-up. The time it takes for the ensemble to phase-synchronize increases, leading to the increased delay time and associated decrease in peak intensity. The total number of photons emitted during a pulse is not constant, but scales with the collective cooperativity , just as the peak output power in Fig. 4.
iv.2 Pulse evolution
We map out the time evolution as a function of atom number and cavity-atom detuning respectively. To ease interpretation we align all datasets to the peak intensity at s in figures 5 and 6. The delay in the system can be seen as the distance between this maximal value at time zero and the green dots indicating the end of the pumping pulse.
iv.2.1 Atom number variation
In Fig. 5 (a) we simulate the behavior of the lasing pulse when the atom number is changed. We set the cavity-atom detuning to zero () and vary the atom number in the MOT while keeping the density profile constant. The green points indicate the end of the pumping pulse, and are binned in order to ease interpretation. The ringing frequency decreases as the atom number is reduced, and at low atom numbers only a single lasing pulse is visible.
On Fig. 5 (b) experimental results are shown. We vary again the atom number, allowing comparison to the simulations. A clear primary peak can be seen for atom numbers , and a subsequent ringing in the light emission is observed. The reference light used for cavity locking shows up as a noisy background in the experimental data, and a constant offset corresponding to the mean value of the background signal has been subtracted in both figures 5 (b) and 6 (b). The data was recorded in sets of 50 with a randomly varying . This means that slowly varying experimental conditions show up as slices of skewed data, but avoids unintended biasing of the results.
The ringing behavior is well explained by following the evolution of the atomic inversion. For high atom numbers the collective coupling in the system becomes strong enough that the photons emitted into the cavity mode are not lost before significant reabsorption takes place. This leads to an ensemble that is more than excited by the end of the primary lasing pulse, and subsequent laser emission (ringing) will thus follow. In this regime where the collective coupling rate is much larger than the cavity decay rate the system output is expected to behave as the central plot of Fig. 3. At low atom numbers we get and light emitted by the atoms is lost from the cavity mode too fast to be reabsorbed by the atoms. As a result the primary lasing pulse creates a significant reduction in the ensemble excitation so that no further collective decay occurs, and the atoms subsequently decay only through spontaneous emission. In this regime the output power is expected to scale as , illustrated by the rightmost graph of Fig. 3. This is the behavior expected from an ideal superradiant system Gross (); Dicke (); Kumarakrishnan ().
iv.2.2 Cavity detuning
With a constant atom number of , we now vary the cavity-atom detuning in Fig. 6. Here a broad range of cavity-atom detunings, up to about MHz is seen to facilitate lasing. At zero detuning the primary lasing feature is maximal and subsequent ringings are suppressed. The lasing pulse delay is seen to scale as , and is thus linearly insensitive to fluctuations close to .
For non-zero detuning the oscillatory behavior of the lasing intensity is seen to increase in frequency, scaling with the generalized Rabi frequency of the coupled atom-cavity system Bochmann (); Brecha (). A noticeable effect is the apparent suppression of ringings for any detunings kHz.
The cavity-atom detuning range for which a significant lasing pulse is produced can be quite broad compared to the natural transition linewidth . The effective coupling between the atomic ensemble and the cavity field scales with the overlap of the cavity resonance and the atomic ensemble linewidth. The inhomogeneous broadening of this atomic linewidth caused by build-up of optical power in the cavity thus increases this range significantly. The pulse can be initiated by only a few photons in the cavity field. As the intensity in the cavity mode builds up, power broadening acts to increase the effective mode overlap between the field and individual atoms. This increases the effective gain in the system, both at finite and zero detuning, allowing more atoms to participate and more energy to be extracted than would have otherwise been the case. For the case of a much colder atomic ensemble (K) the Doppler broadening of the atomic transition is no longer significant, but further simulations indicate that the width of the cavity-atom detuning region that supports lasing remains wide.
iv.2.3 Velocity-dependent dynamics
During the lasing process, the Rabi frequency of each atom will vary in time due to the changing cavity field intensity, while the atomic motion along the cavity mode leads to velocity-dependent dynamics. A typical thermal atom may move a distance of a few wavelengths during the lasing process in this temperature regime. Our simulation shows how atoms affect the lasing process differently, depending on their velocities along the cavity axis, see Fig. 7. With an angle of 45 between the cavity axis and the pump pulse beam, the slow atoms along the cavity axis are preferentially excited during pumping. These atoms initiate the lasing process, while faster atoms may suppress it by absorbing light. Different velocity classes dominate emission or absorption of the cavity photons at different times during the initial pulse and subsequent ringings. The theoretical description of the velocity-dependent behavior shown in Fig. 7 provides a qualitative understanding of the effect of having thermal atoms in the system. As atoms are cooled further, their behavior becomes increasingly homogeneous, and the asynchronous behavior of hot atoms no longer destroys the ensemble coherence.
Fig. 7 (a) illustrates the velocity dynamics for the case of a resonant cavity, . For a range of different velocity groups, this shows the rate of change of the atomic ground state population due to interactions with the cavity field. Significantly more ringings after the primary pulse are visible here than in the emitted power on Fig. 5. Most of these ringings see an approximately equal amount of emission and absorption, causing the energy to remain in the atomic excitations rather than being lost from decay of the cavity mode. They can, however, be seen in the phase response of the system as illustrated in Fig. 2 (b). Eventually loss from spontaneous emission into the reservoir becomes an important decay channel. Concentrating on the slowest atoms, we see emission during the full length of the primary lasing pulse. For the subsequent pulses these atoms alternate between absorbing or emitting light. If we could isolate the light from the slowest atoms, we would thus only see every second oscillation in the output power. For atoms with larger velocities, there will sometimes be both emission and absorption during any single pulse, and we even see the tendency of some velocity groups to consistently emit ( m/s) or absorb ( m/s) throughout the full process (red dashed lines on Fig. 7). This indicates that even for the case of a resonant cavity the velocity groups contributing most to the emitted light during the pulse ringings are not the resonant ones. In the case of a detuned cavity mode, Fig. 7 (b), the initial behavior is very similar. The atoms whose Doppler detuning brings them on resonance with the detuned cavity, emit throughout the primary lasing pulse, whereas others will begin to absorb. Once again some atoms ( m/s) appear to emit light throughout the pulse ringings. The periods of zero emission or absorption between ringings (gray) are no longer visible, as some light is always emitted and absorbed by the atoms. The minima in the emitted power is thus no longer caused by zero emission, but rather by the cancellation between different velocity classes. This behavior corresponds well to the results of Fig. 6 where ringings are much more pronounced in the case of large cavity-atom detuning. Future studies of the spectral properties of superradiant light in cold-atom systems, could elucidate the dependency of emitted light frequency on the finite temperature of the atoms.
In this paper we have investigated the behavior of an ensemble of cold atoms excited on a narrow transition and coupled to the mode of an optical resonator. The enhanced interaction provided by the cavity facilitates synchronization of the atomic dipoles, and results in the emission of a lasing pulse into the cavity mode. This realizes the fundamental operating principle for an active optical clock, where superradiant emission of laser light can be used as a narrow-linewidth and highly stable oscillator.
We mapped out the emitted laser power as a function of atom number in order to identify the threshold of about atoms inside the cavity mode. Two different scalings of laser output power in the bad cavity regime are identified, and though our system is at the limit of the bad cavity regime, both regimes are realized by varying the atom number.
In an attempt to quantify the decoherence effects resulting from finite atomic temperature, a Tavis-Cummings model was developed. By using detailed parameters of the pumping sequence, atomic spatial distribution and orientation, the model is seen to reproduce the experimental results to a high degree. The emitted energy from the atoms is seen to exhibit temporal Rabi oscillations as it undulates between atomic and cavity excitation. This behavior is elucidated via the numerical simulation by investigating the change in atomic excitation as a function of atomic speed throughout the lasing pulse sequence. We see that different velocity groups behave anti-symmetrically, with respect to each other. Surprisingly the velocity group mainly contributing to emission rapidly changes from resonant atoms to atoms that are more detuned with respect to the cavity. This is caused by a faster initial loss of excitation for resonant atoms. A similar effect is seen in the case of a detuned cavity. Here the atomic behavior is much more uniform across different atomic speeds, as the relation between atom number and cavity coupling becomes more homogeneous.
This system relies on rapid-fire pulses of lasing from independent ensembles of atoms, which limits the pulse-to-pulse phase coherence. The phase coherence remains intact between pulses if we could ensure atoms are always present in the cavity as a memory, e.g., in a continuous system. The prospect of a continuously lasing atom-cavity system based on unconfined cold atoms is intriguing because of the severe reduction in engineering requirements compared to a system based on, e.g., sequential loading of atoms into an optical lattice Kazakov (). The velocity-dependent dynamics are important in order to understand what kind of equilibrium one can expect in such a system. An investigation of the spectral characteristics of stationary atomic systems have been shown in NorciaPRX2018 (); Debnath (), and are promising for the transition we have used here. Investigation of the spectral properties in an unconfined ensemble will be presented in future work.
Acknowledgements.SAS and MT contributed equally to the scientific work. The authors would like to thank J. H. Müller, A. S. Sørensen, J. Ye, J. K. Thompson, and M. Norcia for helpful discussions. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820404 (iqClock project), the USOQS project (17FUN03) under the EMPIR initiative, and the Q-Clocks project under the European Comission’s QuantERA initiative. JWT and BTRC were additionally supported by a research grant 17558 from Villum fonden.
Appendix A Simulation parameters
Simulations of the system are based on numerical integration of Eqs. III MTthesis (). The system is initiated with all atoms in the ground state and no coherence. The atoms are randomly distributed, assuming a Gaussian density profile in each dimension, and randomly generated thermal velocities for a temperature of mK. These velocities are assumed constant due to negligible collision rates. The pumping is simulated by turning on the Rabi frequency , which is calculated for each atom based on their coupling to the running-wave pump pulse and its intensity. The spatial intensity distribution is estimated based on measurements of the pump beam with a CCD camera and an optical power meter. After spatial smoothing to even out noise, the data from the CCD camera is used directly in the simulations, and correspond approximately to a slightly non-Gaussian elliptic profile with waists of mm and mm. The minor axis of the ellipse is rotated by with respect to the magnetic symmetry axis, and the intensity used is mW. The simulated time evolution of the pumping pulse ignores the ramp-up and ramp-down of the AOM, and assumes a square pulse for ns. Furthermore, the MOT coils impose a quantization axis for the transition. The pump pulse is polarized along the MOT coil axis, and as a result, atoms near the center of the MOT field are driven less strongly by the pump pulse. In the model, this is accounted for by introducing an effective intensity driving the transition, given by , where y is the MOT coil axis. In the simulations the MOT cloud center is offset by mm with respect to based on measurements of the energy splittings of the magnetic states. The pumping leaves the ensemble inhomogeneously excited, with the excitation being highest for atoms slightly away from the beam axis and for the slowest atoms along the beam axis. On average of atoms are excited within the cavity waist at the end of the pump pulse. Atomic spontaneous decay at a rate and leak of cavity photons through the mirrors at a rate are accounted for by Liouvillian terms. Throughout the simulation each atom interacts with the cavity mode with different coupling rates depending on their positions relative to the Gaussian cavity mode waist ( mm) and the standing wave structure. We calculate the cavity output power from one mirror (comparable to our experimental observations) by .
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