Strong coupling on a forbidden transition in strontium and nondestructive atom counting

Abstract

We observe strong collective coupling between an optical cavity and the forbidden spin singlet to triplet optical transition 1 S0 to 3 P1 in an ensemble of 88 Sr. Despite the transition being 1000 times weaker than a typical dipole transition, we observe a well resolved vacuum Rabi splitting. We use the observed vacuum Rabi splitting to make non-destructive measurements of atomic population with the equivalent of projection-noise limited sensitivity and minimal heating (< 0.01 photon recoils/atom). This technique may be used to enhance the performance of optical lattice clocks by generating entangled states and reducing dead time.Two modes are strongly coupled when the frequency Ω at which they exchange excitations exceeds the rate of interaction with the environment. Strong coupling enables one to generate large amounts of entanglement [1, 2], achieve efficient quantum memories [3], cool the motion of atoms [4] and mesoscopic oscillators [5], and explore collective self-organization and synchronization phenomena such as superradiant lasing [6][7][8][9] and the Dicke phase transition [10,11].In this Letter, we observe strong coupling between an optical cavity and the collective excitation of up to N = 1.25 × 10 5 strontium atoms in a 1D optical lattice. The strong coupling is achieved using the forbidden electronic spin singlet to triplet optical transition 1 S 0 to 3 P 1 in 88 Sr. The excited state 3 P 1 couples to the environment via spontaneous emission at the relatively slow rate of γ = 2π × 7.5 kHz.Despite operating on a transition with 1000 times smaller squared matrix element than transitions typically used in optical cavity-QED experiments, we observe a highly-resolved splitting of the normal modes of the coupled atom-cavity system, known as a collective vacuum Rabi splitting [12,13]. This observation demonstrates that despite the relative feebleness of the transition, the tools of cavity-QED can now be applied to a system of extreme interest for quantum metrology [14]. Example technologies include optical lattice clocks [15][16][17] and ultra-narrow lasers [6][7][8][9], along with their associated broad range of potential applications such defining the second [14,18], quantum many-body simulations [19], measuring gravitational potentials [20] and gravity waves [21], and searches for physics beyond the standard model [22,23].One relevant application of this newly achieved regime is for state-selective, non-destructive counting of strontium atoms. Such counting has been used to generate highly spin-squeezed states [1,24] that surpass the standard quantum limit on phase estimation [25][26][27]. More simply, but quite importantly, non-destructive readout methods [28] can reduce the highly deleterious effects of local oscillator noise aliasing [29,30] in optical lattice clocks. Here, we utilize the observed vacuum Rabi splitting to non-destructively count atoms with the equivalent of sub-projection noise sensitivity. We verify that this sensitivity is achieved with as few as 0.01 photon recoils imparted to e...

Cavity-enhanced non-destructive detection of atoms for an optical lattice clock

“…Ultimately, the relatively modest single-atom cooperativity C = 4 g 2 /κΓ = 0.045 for the apparatus in this work will be a limiting factor for high detection efficiency at low atom number, as compared with previous demonstrations of high-fidelity cavity detection of Yb [32] and Sr [33]. However, quantum non-destructive detection is still feasible in our cavity above the critical atom number N crit = 28, with significant metrological gain possible in the N ≈ 10 4 range.…”

Magnetically Induced Optical Transparency on a Forbidden Transition in Strontium for Cavity-Enhanced Spectroscopy

“…1a and Refs. [16,17,28]). We tune a TEM00 resonance of the cavity at frequency ω c to be near resonance with the dipole-forbidden singlet to triplet optical transition 1 S 0 to 3 P 1 at frequency ω 0 or wavelength 689 nm (see Fig.…”

Coupling atoms to cavities using narrow linewidth optical transitions: applications to frequency metrology

“…7a). In ref [17], such a probing technique enabled the demonstration of cavity enhanced atom counting with levels of precision and scattering compatible with the generation of spin squeezed states. In this work, a mode of a high-finesse optical cavity (linewidth κ 160 kHz) was placed on resonance with the 7.5 kHz linewidth 1 S 0 to 3 P 1 transition in 88 Sr. For an atom number of N = 1.25 × 10 5 , the observed vacuum Rabi splitting was roughly 5 MHz, placing the system deep in the desirable strong collective coupling, bad cavity regime Ω κ γ.…”

“…In order to generate spin squeezed states using the nondestructive measurements presented in ref. [17], the measurements must be applied in conjunction with high-fidelity rotations on the optical clock transition. Thus, both of these methods require the ability to perform precise optical rotations on the clock transition, which requires tight spatial confinement of the atoms along the direction from which the clock laser is applied in order to eliminate the effects of Doppler shifts and photon recoil.…”