Session W49: Defect Qubits IV - Rare Earth Ions for Quantum Networking

Spin-Photon Entanglement of a Single Rare-Earth-Ion at the Telecom Band

Long-distance quantum networks require scalable telecom-band spin-photon interfaces. Among these, rare earth ions (REI) have recently garnered attention, with Er3+ standing out due to its telecom band optical transition. Our previous study demonstrated that integrating Er3+ into CaWO4 substantially suppresses spectral diffusion, leading to the observation of indistinguishable photons. Additionally, the low nuclear spin abundance and minimal background REI concentration in this system result in enhanced spin coherence [1]. In this work, we utilize the improved spin and optical coherence and employ a dynamical decoupling-based entanglement scheme to demonstrate spin-photon entanglement. This result marks a significant step towards REI-based long distance quantum communication.   

Development and defects control in the qubit hosts for quantum information processing

The energy levels of rare earth ions (REIs) provide an avenue to coherently convert the quantum information between microwave and telecom optical wavelengths. CaWO₄ and YVO₄ with a tetragonal crystal symmetry are promising host candidates for REIs and to create solid-state quantum systems. However, the charge fluctuation from paramagnetic defects and magnetic fluctuations from the impurities lead to inhomogeneous spectral broadening of REIs affecting the qubit coherence time and efficiency of photon conversion. Both CaWO₄ and YVO₄ have oxygen-related paramagnetic defects.

We present our results in the development and characterization of these and related host materials. In YVO₄, the V⁴⁺ ion is adjacent to an O vacancy and the V⁴⁺ leads to a yellow color. In CaWO₄ the yellow color is due to W⁵⁺. To avoid the O-related paramagnetic defects, we employ our state-of-the-art optical floating zone techniques to grow high-purity crystals, and both crystals were grown in high P_O2 environments. In YVO₄ growth to compensate for the evaporation of V, the crystals were grown from V-rich compositions. O-related defects were further improved by annealing in O₂ atmosphere. We also performed a lanthanide-doping in Ca_1-xLn_xWO₄ and we observed the creation of uncompensated charge-related centers, potentially W⁵⁺. The quality of crystals was assessed by x-ray diffraction and EPR. If time permits, other W⁵⁺ hosts will be discussed.   

Coherent Erbium Spin Defects in Colloidal Ceria Nanocrystals

Nanocrystals (NC), with its direct bottom-up synthesis capability and flexibility in device integration, have gained attention as viable quantum hosts for spin defects.  Here, we explore erbium (Er) doped ceria NC with an ensemble average level of a single erbium per NC and experimentally demonstrate spin coherence times approaching a microsecond. Ceria was chosen as a host material due to a low natural abundance of nuclear spins. As a result, we observe population lifetimes that approach a millisecond, suggesting the possibility of substantial improvements on the spin coherence. We postulate that the spin coherence is limited by the nuclear spin bath of hydrogen atoms from the oleic acid ligands on the surface of the nanocrystal that are used during synthesis. The observation of the Larmor frequency of hydrogen further suggests that the Er ions are sensitive to the nearby hydrogen nuclei. Scanning transmission electron microscopy combined with electron energy loss spectroscopy further show that Ce³⁺ and O vacancies are prevalent on the surface of the nanocrystal, likely playing a role in the observed spin coherence and lifetime. Nonetheless, the initial spin coherence times demonstrated suggests that spin defects in nanocrystals are a promising materials platform for quantum information processing.

Electronic structure and crystalline electric field splitting of trivalent erbium in MgO

Here, we present an effective ab initio method of calculating crystal field coefficients of erbium, a rare earth, ion hosted in magnesium oxide (MgO) with Oh point group symmetry. The calculated band gap of MgO is 7.65 eV at Γ, which closely agrees with the experimentally observed value of 7.5±0.5 eV. Using thus calculated crystal field coefficients, which satisfy the Oh point group symmetry, we solve an effective Hamiltonian and calculate crystal field splitting of Er³⁺ for the ground and excited states. The calculated energy transitions are in good agreement with available experimental values within 15 cm⁻¹ error between the ground and first excited state. The long optical transition about 6506 cm⁻¹ of environmentally shielded 4f shell of Er3+ in MgO can have a potential application in quantum networks.   

Spin-dependent optical properties of Ce-implanted MgAl2O4

Defect centers in diamond [1] and SiC [2,3] are common solid-state spin qubits with long coherence times T₂ [4,5] with a variety of potential applications in quantum information sciences. Based on the guidelines for developing a new optically accessible qubit system [2,5], and theoretical predictions of qubit host materials with long T₂ [6,7] we explore alternative qubit platforms. Here, we investigate the optical properties of Ce-implanted MgO and its mixed crystal with Al₂O₃.

 We implant Ce into three substrates: bare MgO substrate without cap layer (Sample A), MgO with a 10-nm-thick Al₂O₃ layer (Sample B), and bare MgAl₂O₄ substrate (Sample C). The implantation energy and the dose are 100 keV and 1.0×10¹⁴ atoms/cm², respectively. The substrates are annealed at 1000^oC in Ar atmosphere for 2 hours.

 The photoluminescence (PL) spectroscopy signal is measured using a spectrometer with an excitation wavelength of 325 nm. All samples show a broad peak at around 450 nm, consistent with the 4f-5d transition of the Ce³⁺ center. Sample C shows 14 (8) times larger intensity than Sample A (Sample B). This increase of the PL intensity suggests an increase in the formation of the luminescent Ce³⁺ center [8]. Next, we investigate the polarized optical properties of Ce-implanted MgAl₂O₄. The degree of circular polarization (DOCP) reaches ~2% under 500 mT at 4K. This behavior suggests spin-dependent PL [9,10], which is inevitable in optical initialization and readout of the spin qubit state.   

New Color Centers in Diamond for Quantum Applications

Color centers in diamond are promising platforms for quantum communication and quantum sensing applications. All the defects in diamond reported so far emit photons in the visible to near-infrared region, which experience high losses in fiber optics and scattering media such as biological samples. Furthermore, no color centers observed thus far exhibit coherence times comparable to nitrogen vacancies at elevated temperatures. Here, we report the experimental observation of two new color centers in diamond that emit in the telecom band. The first color center emits in the telecom O-band [1]. From absorption, photoluminescence, and transient absorption spectroscopy, we identify a zero-phonon line at 1221 nm with phonon replicas separated by 42 meV and an excited state lifetime of ~270 ps. The second is substitutional Er³⁺ incorporated in diamond, which emits in the telecom C-band. Finally, we deploy our spectroscopy and materials characterization pipeline to investigate other new defects in diamond as potential candidates for quantum sensing by characterizing spin dynamics using pulsed electron spin resonance.

 

[1] S. Mukherjee et al., Nano Lett. 2023, 23, 7, 2557–2562.   

Understanding the dopant-driven impact on electronic and crystal structure of erbium-doped oxides for quantum memory

Quantum communication networks rely on qubits, such as photons, for secure long-distance communication. High-fidelity rare-earth ion (REI) memory systems, especially erbium (Er³⁺) in oxide hosts with C-band emission properties, are crucial for synchronizing entanglement for signal amplification in quantum networks [1,2]. Oxides offer growth simplicity, CMOS compatibility, and long coherence times. However, embedding Er³⁺ introduces defects, disrupting the host lattice and resulting in photoluminescence linewidth and lifetime variations in Er-doped oxide films with unclear causes [3,4]. In this work, we employed synchrotron-based X-ray tools, including X-ray absorption spectroscopy and diffraction, to investigate Er-doped titanium oxides electronic and crystal structures as doping levels vary. This information is crucial for controlling the tunability of excited state lifetimes and rare-earth defect linewidths to mitigate decoherence.   

Material and optical differences between three different growth methods for Er:TiO2/Si(100) films

Erbium ions (Er_­­³⁺) are an attractive defect for quantum communication applications due to their well-shielded 4f-4f transition, which emits at telecom wavelengths (~1.5 μm). Developing a scalable solid-state platform with intentionally doped and optically controlled trivalent erbium and its long-lived electron spin states is key to realizing repeater-based quantum networks. Titanium dioxide (TiO­_2­) thin films are an attractive host material for Er due to its CMOS compatibility.¹ Here, we report on the exploration of differences between three modes of growth for Er:TiO₂ films on Si(100): traditional molecular beam epitaxy (MBE) using a solid titanium source, metal-organic MBE (MOMBE) using a titanium precursor,^1,2 and atomic layer deposition (ALD).³ Material characteristics such as phase transition temperature, X-ray diffraction linewidths, and phase mapping are reported. Optical inhomogeneous and spectral diffusion linewidths are also measured and compared between modes of growth and annealing conditions.

(1) A. Dibos, et al. Nano Letters. 6530–6536 (2022)

(2) M.K. Singh, et al. arXiv:2202.05376 (2022)

(3) C. Ji, et al. arXiv:2309.13490 (2023)   

Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 1/3

Electron spin resonance (ESR) spectroscopy is the method of choice for characterizing paramagnetic

impurities, with applications ranging from chemistry to quantum computing, but it gives only

access to ensemble-averaged quantities due to its limited signal-to-noise ratio. The sensitivity needed

to detect single electron spins has been reached so far using spin- dependent photoluminescence,

transport measurements, or scanning probes. These techniques are system-specific or sensitive only

in a small detection volume, so that practical single spin detection remains an open challenge.

Using single-electron-spin-resonance techniques recently demonstrated [3] we characterize the

magnetic environment of the single electron probe. The technique consists in measuring the spin

fluorescence signal at microwave frequencies [1, 2] using a microwave photon counter based on a

superconducting transmon qubit [3]. In our experiment, individual paramagnetic erbium ions in a

scheelite crystal of CaWO4 are magnetically coupled to a small-mode-volume, high-quality factor

superconducting microwave resonator to enhance their radiative decay rate [4]. The method applies

to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large

detection volumes ( ∼ 10μm3) ; as such, it may find applications in magnetic resonance and quantum

computing.

In the first part of this talk, we will focus on a Single Microwave Photon Detector (SMPD), which

working principle is based on Circuit QED. A previous version of

this design, which enabled single electron spin detection for the first time, has reached a sensitivity

of 10−22W/√Hz [2]. We present here an improved version of this design, reaching a sensitivity of

10−23W/√Hz. This improvement has been achieved by enhancing the two figures of merit of the

SMPD : The dark count rate (rate of false-positive detection events) and the detection efficiency (probability of successful detection).

[1] Albertinale, E. et al. Nature 600, 434– 438 (2021).

[2] L. Balembois, et al. arXiv :2307.03614.

[3] Z. Wang, et al. Nature 619, 276–281 (2023).

[4] R. Lescanne et al. Phys. Rev. X 10, 021038 (2020).

[5] A. Bienfait et al. Nature 531, 74 (2016).   

Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 2/3

Electron spin resonance (ESR) spectroscopy is the method of choice for characterizing parama-

gnetic impurities, with applications ranging from chemistry to quantum computing, but it gives only

access to ensemble-averaged quantities due to its limited signal-to-noise ratio. The sensitivity nee-

ded to detect single electron spins has been reached so far using spin-dependent photoluminescence,

transport measurements, or scanning probes. These techniques are system-specific or sensitive only

in a small detection volume, so that practical single spin detection remains an open challenge.

Using single-electron-spin-resonance techniques recently demonstrated [3] we characterize the

magnetic environment of the single electron probe. The technique consists in measuring the spin

fluorescence signal at microwave frequencies [1, 2] using a microwave photon counter based on a

superconducting transmon qubit [3]. In our experiment, individual paramagnetic erbium ions in a

scheelite crystal of CaWO4 are magnetically coupled to a small-mode-volume, high-quality factor

superconducting microwave resonator to enhance their radiative decay rate [4]. The method applies

to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large

detection volumes ( ∼ 10μm3) ; as such, it may find applications in magnetic resonance and quantum

computing.

In this second part, I will present the spectroscopy of individual Erbium ions by microwave photon counting with an improved detector sensitivity.

[1] Albertinale, E. et al. Detecting spins by their fluorescence with a

microwave photon counter. Nature 600, 434– 438 (2021).

[2] L. Balembois, et al. Practical Single Microwave Photon Counter

with 10−22 W/√Hz sensitivity. arXiv :2307.03614.

[3] Z. Wang, et al. Single-electron spin resonance detection by mi-

crowave photon counting. Nature 619, 276–281 (2023).

[4] R. Lescanne et al. Irreversible Qubit-Photon Coupling for the De-

tection of Itinerant Microwave Photons. Phys. Rev. X 10, 021038

(2020).

[5] A. Bienfait et al. Controlling spin relaxation with a cavity. Nature

531, 74 (2016).   

Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 3/3

Electron spin resonance (ESR) spectroscopy is the method of choice for characterizing paramagnetic impurities, with applications ranging from chemistry to quantum computing, but it gives only, access to ensemble-averaged quantities due to its limited signal-to-noise ratio. The sensitivity needed to detect single electron spins has been reached so far using spin- dependent photoluminescence,transport measurements, or scanning probes. These techniques are system-specific or sensitive only in a small detection volume, so that practical single spin detection remains an open challenge.

Using single-electron-spin-resonance techniques recently demonstrated [3] we characterize the magnetic environment of the single electron probe. The technique consists in measuring the spin fluorescence signal at microwave frequencies [1, 2] using a microwave photon counter based on a superconducting transmon qubit [3]. In our experiment, individual paramagnetic erbium ions in a scheelite crystal of CaWO4 are magnetically coupled to a small-mode-volume, high-quality factor superconducting microwave resonator to enhance their radiative decay rate [4]. The method applies to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large detection volumes ( ∼ 10μm3) ; as such,  it may find applications in magnetic resonance and quantum computing.

The third part of this talk will present the mechanisms by which the electron spin couples to the magnetic environment and discuss the techniques used to detect and characterize said environment.

[1] Albertinale, E. et al. Detecting spins by their fluorescence with am microwave photon counter. Nature 600, 434– 438 (2021).

[2] L. Balembois, et al. Practical Single Microwave Photon Counter with 10−22 W/√Hz sensitivity. arXiv :2307.03614.

[3] Z. Wang, et al. Single-electron spin resonance detection by microwave photon counting. Nature 619, 276–281 (2023).

[4] R. Lescanne et al. Irreversible Qubit-Photon Coupling for the Detection of Itinerant Microwave Photons. Phys. Rev. X 10, 021038 (2020).

[5] A. Bienfait et al. Controlling spin relaxation with a cavity. Nature 531, 74 (2016).