Nanophotonic coherent light-matter interfaces based on rare-earth-doped crystals

Abstract

Quantum light-matter interfaces connecting stationary qubits to photons will enable optical networks for quantum communications, precise global time keeping, photon switching and studies of fundamental physics. Rare-earth-ion-doped crystals are state-of-the-art materials for optical quantum memories and quantum transducers between optical photons, microwave photons and spin waves. Here we demonstrate coupling of an ensemble of neodymium rare-earth-ions to photonic nanocavities fabricated in the yttrium orthosilicate host crystal. Cavity quantum electrodynamics effects including Purcell enhancement (F=42) and dipole-induced transparency are observed on the highly coherent (4)I(9/2)-(4)F(3/2) optical transition. Fluctuations in the cavity transmission due to statistical fine structure of the atomic density are measured, indicating operation at the quantum level. Coherent optical control of cavity-coupled rare-earth ions is performed via photon echoes. Long optical coherence times (T2∼100 μs) and small inhomogeneous broadening are measured for the cavity-coupled rare-earth ions, thus demonstrating their potential for on-chip scalable quantum light-matter interfaces.

Figure 1. Photonic crystal nanobeam resonator fabricated in Nd:YSO.

(a) Scanning electron microscope image of the device. Scale bar, 2 μm. The red inset is a zoomed-in view of the 45° angle-cut coupler that allows vertical coupling of light from a microscope objective. The blue inset shows the grooves forming the photonic crystal. (b) Schematics of the nanobeam resonator with simulated electric field (E along D₁) profiles of the fundamental TE resonance mode. The TE polarization aligns with the D₁ axis of the YSO crystal. A magnetic field of 500 mT is applied in the D₁–D₂ plane at an angle of α=135° with respect to the D₁ axis. (c) Broadband cavity transmission spectrum showing the cavity resonance with quality factor Q=4,400.

Figure 2. Purcell-enhanced coupling of Nd³⁺ ions to the YSO cavity mode.

(a) Schematic of energy levels for Nd³⁺ in YSO. Optical excitation at 810 nm results in PL at several wavelengths with only the 883-nm transition enhanced by the cavity. (b) Spectrogram showing the Nd³⁺ PL, while the cavity is tuned across resonance using gas condensation. The dashed line is a guide to the eye indicating the central wavelength of the cavity resonance. The cavity resonance is not visible, because there is no background luminescence to populate the cavity mode. PL spectra in the uncoupled (c) and coupled (d) cases. The cavity resonance was drawn to indicate the cavity location. (e) Lifetime measurements for coupled (τ^c=87 μs, Δλ=0) and uncoupled (τ⁰=254 μs, Δλ=0.3 nm) cases. (f) Change in lifetime as a function of the cavity detuning, which fits well with the calculation (red curve) using quality factor Q=4,400, 4.5% branching ratio and field intensity averaged over the mode volume.

Figure 3. Photon echo measurements from an ensemble of Nd³⁺ ions in the cavity.

(a) Two-pulse photon echo sequence (π/2−π) used to measure T₂. (b) Two-pulse photon echo decays measured in both the cavity (red) and the bulk (black) samples with two different doping concentrations. The inset plots the echo decays measured with a 0.2% doped sample. (c) Oscillation of echo intensity with increasing width of the π rephasing pulse. The periodic signal reveals the ensemble averaged Rabi frequency of the coupled ions. The ideal π pulse duration for the input power was 0.4 μs. (d) Enhanced photon echo intensity (by ∼12-fold) when the cavity is coupled, compared with the uncoupled case (cavity detuned by Δλ=15 nm so that the transition is outside the photonic bandgap).

Figure 4. Control of cavity transmission and observation of SFS.

(a) Broadband transmission spectra as the cavity is tuned to the 883 nm Nd transition. A dip is observed when the two are on resonance. The negligible dip at far detunings confirms that this effect is not due to absorption, but quantum interference between the intracavity field and the ions. (b) High-resolution transmission spectrum (red curve) obtained by scanning a narrow linewidth (∼20 kHz) Ti:Sapphire laser over the inhomogeneous line. The green curve is the fit using parameters: , Γ_h=2π × 100 kHz, Γ_inhom=16.0 GHz, and assuming a Gaussian ion density distribution. The green shaded region is the estimated fluctuation in the transmitted laser intensity caused by statistical variations of the ion density. Large fluctuations are expected, because the density N is low (few tens), which agrees with the measurement. The fluctuation within the inhomogeneous linewidth is noticeably larger than that at far detunings (>25 GHz) and the technical background noise (grey area), confirming that they are caused by SFS of the ion spectral density. (c) Two traces of the transmitted intensities over the same 100 MHz bandwidth near zero detuning at different times. The high degree of correlation confirms the static and repeatable nature of SFS.