Controlling single rare earth ion emission in an electro-optical nanocavity

Quantum networks require solid-state spins for quantum memory and coherent transduction. Addressing single atomic defects is crucial for solid-state qubits, spin-photon entanglement, and single-photon sources. Rare earth ions (REIs), particularly erbium (Er) with its telecom-band emission, are promising due to their narrow, coherent transitions. However, addressing weakly coupled 4f-4f optical transitions of single REIs is challenging. Purcell enhancement of emission rate in optical cavities offers a solution, with recent successes in coupling micro-cavities with bulk host crystals. For improved control and scalability, integrated photonic resonators with separate and instant tunability are highly desirable. Dynamic control of ion-cavity coupling has been achieved with Er-doped nanoparticles and Yb-doped lithium niobate resonators. Real-time modulation of emission rate is valuable for designing tailored single-photon sources and high-efficiency spin-photon interfaces. Lithium niobate (LN), with its electro-optic, piezoelectric, and nonlinear properties, is a suitable host crystal for REIs, particularly in thin-film lithium niobate on insulator (LNOI) integrated photonics. Existing works primarily focus on ion ensembles; single Er emitters in LN require further investigation. This work aims to demonstrate detection and control of single ion emission in smart-cut erbium-doped lithium niobate (ErLN) thin films using electro-optically tunable photonic crystal nanobeam cavities.

Previous research has demonstrated progress in controlling and manipulating single rare-earth ions for quantum information processing. Studies using silicon nanocavities stamped on YSO and cryogenic Fabry-Perot resonators have achieved optical probing of single Er ions. Similarly, single Nd ions have been resolved in nanophotonic cavities fabricated on YVO crystals. Dynamic control of ion-cavity coupling has been demonstrated using piezoelectrically tunable Fabry-Perot microcavities with Er-doped nanoparticles and Yb-doped lithium niobate microdisk resonators. These advancements highlight the potential of integrated photonic resonators with electro-optic tunability for creating controllable single-photon sources and efficient spin-photon interfaces. The integration of rare-earth ions into LNOI platforms has been explored through techniques like ion implantation, flip-chip bonding, and smart-cut methods. However, most studies have focused on ion ensembles, leaving the investigation of single Er emitters in LN largely unexplored. This study addresses this gap by employing smart-cut ErLN thin films and electro-optically tunable photonic crystal nanocavities.

The researchers fabricated devices using 300 nm z-cut ErLN (100 ppm doped) thin films. Photonic crystal nanobeam cavities with electro-optic tunability were designed with a half-etched ridge waveguide and periodic holes forming a photonic crystal mirror. The SiO2 layer beneath was removed to improve mode confinement. Gold electrodes were deposited for frequency tuning. The cavity was formed by tapering the lattice constant in the middle, creating a defect mode. The fabrication process involved sweeping the cavity resonance to match the Er emission wavelength by varying the defect lattice constant. Cryogenic electrical and optical access was achieved through wire bonding and fiber glue techniques. Devices were measured at 1 K in a dilution refrigerator. The reflection spectrum was measured to determine the quality factor and extinction ratio. Electro-optic frequency tuning was calibrated by applying voltage. Fluorescence from Er ions was collected using a superconducting nanowire single photon detector (SNSPD). A pulse measurement setup allowed synchronized control of the optical pump, SNSPD bias, and tuning voltage. Population lifetime measurements were performed in both the waveguide and the cavity to determine the Purcell factor. Single ion detection was performed by tuning the cavity to the tail of the inhomogeneous broadening, with verification via second-order autocorrelation measurement (g(2)(0)). Electro-optic control of Er emission was demonstrated using pulsed tuning voltage, and storage and retrieval of single ion excitation was achieved by detuning and realigning the cavity. The emission spectrum was monitored to ensure no perturbation by this process.

The fabricated photonic crystal nanobeam cavities achieved a high quality factor (Q) of 158 k and an extinction ratio close to 10 dB. The electro-optic tuning rate was 1.6 pm/V, with a total tuning range of approximately 1.5 nm using bipolar voltage. Lifetime measurements revealed a cavity enhancement of Er emission, reducing the lifetime from approximately 2.5 ms in the waveguide to 14 µs in the cavity, yielding a Purcell factor of approximately 177. This is consistent with simulations. By tuning the cavity resonance to the tail of the inhomogeneous broadening, single Er ion emission was spectrally resolved. Second-order autocorrelation measurements confirmed single-ion emission with g(2)(0) = 0.38 ± 0.08, indicating that the majority of collected photons originated from a single ion. Pulsed electro-optic tuning successfully tailored the waveform of Er emission. Furthermore, the researchers demonstrated the storage and retrieval of single ion excitation by detuning and then realigning the cavity resonance. The emission spectrum remained largely unperturbed during this process. The storage times were found to be extended with different applied voltages, with lifetimes matching the waveguide values when the cavity was detuned.

The results demonstrate successful control of single Er ion emission in an electro-optically tunable nanophotonic cavity. The high Purcell factor and electro-optic tunability are crucial for achieving single-ion detection and manipulation. The ability to dynamically control the emission rate allows for tailoring single-photon sources and creating efficient spin-photon interfaces. The findings address the challenge of addressing weakly coupled 4f-4f optical transitions in single REIs. The use of thin-film lithium niobate offers advantages in terms of scalability and integration with other on-chip components. The demonstrated storage and retrieval capabilities are important for quantum memory applications. The relatively long coherence times achievable with external magnetic fields and lower doping concentrations further enhance the potential of this system for quantum information processing.

This work demonstrates the control and manipulation of single rare-earth ion emission using an electro-optically tunable photonic crystal cavity. High Purcell enhancement enabled single ion detection and the electro-optic tuning facilitated dynamic control of the emission rate, including storage and retrieval of single ion excitation. This platform offers a promising approach for developing controllable single-photon sources and efficient spin-photon interfaces. Future research could focus on improving coherence times, integrating on-chip microwave resonators for spin manipulation, exploring the coupling between single REIs and mechanical modes, and developing multi-ion systems for frequency multiplexing.

The current study utilizes a relatively high doping concentration of erbium ions in lithium niobate. This could lead to energy transfer and interactions between Er ions, potentially affecting coherence times. Lower doping concentrations could improve coherence, but may reduce the probability of single-ion detection. The observed spectral diffusion in ErLN, while mitigated by external magnetic fields, might still impact the stability and reproducibility of measurements. The study's focus on the Y1-Z1 transition of erbium limits the investigation of other transitions that could have different properties. Further investigations into different host materials or rare-earth ions could help explore a wider range of applications.