A one-atom laser in a regime of strong coupling

Abstract

Although conventional lasers operate with a large number of intracavity atoms, the lasing properties of a single atom in a resonant cavity have been theoretically investigated for more than a decade [1,2,3,4,5,6,7,8,9,10,11]. In this Letter we report the experimental realization of such a one-atom laser operated in a regime of strong coupling. Our experiment exploits recent advances in cavity quantum electrodynamics that allow one atom to be isolated in an optical cavity in a regime for which one photon is sufficient to saturate the atomic transition [12]. In this regime the observed characteristics of the atom-cavity system are qualitatively different from those of the familiar many atom case. Specifically, we present measurements of intracavity photon number versus pump intensity that exhibit "thresholdless" behavior, and infer that the output flux from the cavity mode exceeds that from atomic fluorescence by more than tenfold. Observations of the second-order intensity correlation function g (2) (τ ) demonstrate that our one-atom laser generates manifestly quantum (i.e., nonclassical) light that exhibits both photon antibunching g (2) (0) < g (2) (τ ) and sub-Poissonian photon statistics g (2) (0) < 1.An important trend in modern science is to push macroscopic physical systems to ever smaller sizes, eventually into the microscopic realm. Lasers are one important example of this progression, having moved from table-top systems to microscopic devices that are ubiquitous in science and technology. However, even over this remarkable span of implementations, lasers are typically realized with large atom and photon numbers in a domain of weak coupling for which individual quanta have negligible impact on the system dynamics. Usual laser theories therefore rely on system-size expansions in inverse powers of critical atom and photon numbers (N 0 , n 0 ) ≫ 1, and arrive at a consistent form for the laser characteristics [13,14,15,16,17]. By contrast, over the past twenty years, technical advances on various fronts have pushed laser operation to regimes of ever smaller atom and photon number, pressing toward the limit of strong coupling for which (N 0 , n 0 ) ≪ 1 [18]. Significant milestones include the realization of one and two-photon micromasers [19,20,21], as well as novel microlasers in atomic and condensed matter systems [22,23,24].In this march toward ever smaller systems, an intriguing possibility is that a laser might be obtained with a single atom in an optical cavity, as was considered in the is provided by the field Ω3, while recycling of the lower level F = 4 is achieved by way of the field Ω4 (4 → 4 ′ ) and spontaneous decay back to F = 3. Decay (3 ′ , 4 ′ ) → (3, 4) is also included in our model. Relevant cavity parameters are length l0 = 42.2 µm, waist w0 = 23.6 µm, and finesse F = 4.2 × 10 5 at λD 2 = 852 nm.

Experimental realization of a one-atom laser in the regime of strong coupling

“…4 in our paper [1]. Significantly, the nonclassical nature of the light emitted from the atom-cavity system is directly exhibited by these data.…”

“…[1], a one-atom laser operated in a regime of strong coupling with (N 0 , n 0 ) 1 will evidence qualitatively different characteristics than those of more familiar conventional lasers. The question then arises as how best to identify a laser in this new regime, with diverse criteria suggested and analyzed in prior work on one-atom lasers [2][3][4][5][6][7][8][9][10][11][12].…”

“…4 in our manuscript [1], which we do in Section III. We also describe the procedure employed for obtaining the normalized intensity correlation function g (2) (τ ) from these data.…”

“…The input-output relationshipn versus I 3 has several key features to be compared with experimental results presented in Ref. [1], namely the immediate onset of emission ("thresholdless" behavior), and the saturation and eventual quenching of the output. The saturation can be attributed to the recycling process being limited by the atomic decay rates, whereas the output reduction at high I 3 is possibly due to the splitting of the pumped excited state F = 3 by the Autler-Townes effect, although this is still under investigation.…”

“…[1]. A principal purpose of this Supplementary Information is to present a brief description of the relationship of the operating regime of our experiment to that of more conventional laser systems.…”