ECAMP 15

Huckans J.†², Laburthe-Tolra B.¹, Pasquiou B.¹, Pargoire Y.¹, Robert-de-Saint-Vincent M.¹
¹Université Sorbonne Paris Nord, LPL, France
² Commonwealth University of Pennsylvania, Bloomsburg, PA USA

† jhuckans@commonwealthu.edu

We are building a continuous superradiant laser on the narrow (7.5kHz) intercombination line of ⁸⁸Sr using a cold, fast (10-100 m/s) atomic beam passing through the mode of a “bad” optical cavity. One of our goals is to study correlations that should arise between atoms, especially near the superradiant threshold.

Superradiant lasing is an emerging technology, which has been extensively studied both theoretically and experimentally. This light source can be used as a frequency reference. Fundamental quantum fluctuations of the collective atomic dipole set the ultimate laser linewidth, allowing it to be lower than the natural width of the transition and with a frequency only slightly dependent on fluctuations in the positions of the cavity mirrors. A continuous superradiant laser can therefore overcome many of the current limitations of atomic clocks.

In the case of a pulsed superradiant laser, the linewidth is defined as in (1, left). By contrast, the linewidth of a continuous- wave superradiant laser is defined as in (1, right). With our setup, designed to be a proof of principle of a continuous-wave superradiant laser in the bad-cavity regime, we expect to reach a linewidth of ~700Hz (based on the 7.5kHz natural width). For this purpose, we choose a low (C<1) single-atom cooperativity defined as C=g²/(κγ), (C = 0.211 in our setup, with g = 2π × 31.54kHz the single-atom-cavity coupling term, γ = 2π × 7.5kHz the natural linewidth of the ¹S0 −> ³ P1 transition, κ = 2π × 630kHz the cavity loss rate) and need a high collective cooperativity (NC»1) with N the atom number inside the cavity.

ΔwPulsed_SR = Ng²/ κ ΔwContinuous_Wave_SR =4g²/κ (1)

The dynamics of superradiance is described by the quantity Ng²/ κ. For superradiance to occur one needs a superradiance dynamics faster than the spontaneous emission , leading to Ng²/ κ >> γ. The continuous-wave regime also requires a superradiance dynamics faster than the refreshing rate Γ_R of the cavity, leading to Ng²/ κ >> Γ_R.

These conditions fix the minimum atom number to reach the superradiant emission threshold. For an atomic beam with a velocity of 100 m/s, this threshold is 80 atoms, a criterion we fulfill in our setup.

To achieve, control and characterize superradiance, we use a Zeeman slower to tune the atom velocity, thereby controlling the flux and the refreshing rate. Out of the Zeeman slower, we deflect the atoms toward the cavity. For this purpose, we use a moving molasses and have started to characterize its efficiency depending on the atoms' forward velocity. Then, we use SWAP (Sawtooth Wave Adiabatic Passage) Cooling² on the ¹S0 −> ³ P1 transition to reach μK temperatures along the cavity axis. Finally, we have devised an adiabatic excitation scheme to be implemented just before the atoms reach the cavity, which we are currently testing.

Once our laser is emitting, we will characterize it with a beat note measurement and make photon quantum-correlation measurements. We will use the flexibility of our architecture to test different regimes such as having a distribution of velocities along the cavity axis that could lead to different collective dipoles³ and we will modify the cavity to decrease the mirror reflectivity in order to reach a decreased laser linewidth.

1Bruno Laburthe-Tolra, Ziyad Amodjee, Benjamin Pasquiou and Martin Robert-de-Saint-Vincent, SciPost Phys Core 6, 015 (2023).

2John P. Bartolotta, Matthew A. Norcia, Julia R. K. Cline, James K. Thompson and Murray J. Holland, Phys. Rev. A 98, 023404 (2018).

3Simon B. Jäger, Haonan Liu, Athreya Shankar, John Cooper, Murray J. Holland, Phys. Rev. A 103, 013720 (2021).