The coherence properties of optical emitters in crystals are critical for quantum technologies and optical frequency metrology. Cooling to sub-kelvin temperatures can significantly enhance their coherence, making it essential to identify the key parameters governing emitter and host crystal behavior in this ultra cold regime. We investigate a Czochralski-grown europium doped yttrium orthosilicate crystal, and we report measurements of the heat capacity, a parameter fundamental to evaluating thermal noise limits in metrology schemes based on spectral hole stabilization in such samples. In parallel, we characterize optical coherence via photon echo measurements as a function of temperature. Below 1 K, where phonon contributions diminish, two-level systems (TLS) associated with crystal imperfections may emerge as a limiting factor. A linear-in-temperature term in the heat capacity serves as a signature of TLS, and from our data, we establish an upper bound on this contribution. This, combined with the optical homogeneous linewidth from photon-echo measurements being constant in the interval from 300 mK to 2 K demonstrates a minimal TLSrelated effects in our sample. These findings highlight the promise of ultralow-temperature operation for enhancing the performance of optical quantum devices based on doped crystals.
The coherence properties of optical emitters in crystals are critical for quantum technologies and optical frequency metrology. Cooling to sub-kelvin temperatures can significantly enhance their coherence, making it essential to identify the key parameters governing emitter and host crystal behavior in this ultra cold regime. We investigate a Czochralski-grown europium doped yttrium orthosilicate crystal, and we report measurements of the heat capacity, a parameter fundamental to evaluating thermal noise limits in metrology schemes based on spectral hole stabilization in such samples. In parallel, we characterize optical coherence via photon echo measurements as a function of temperature. Below 1 K, where phonon contributions diminish, two-level systems (TLS) associated with crystal imperfections may emerge as a limiting factor. A linear-in-temperature term in the heat capacity serves as a signature of TLS, and from our data, we establish an upper bound on this contribution. This, combined with the optical homogeneous linewidth from photon-echo measurements being constant in the interval from 300 mK to 2 K demonstrates a minimal TLSrelated effects in our sample. These findings highlight the promise of ultralow-temperature operation for enhancing the performance of optical quantum devices based on doped crystals.
The coherence properties of optical emitters in crystals are crucial for quantum technologies and optical frequency metrology. Cooling to sub-kelvin temperatures can markedly enhance coherence, making it important to identify the parameters governing emitter and host crystal behavior in this regime. We investigate a Czochralski-grown europium-doped yttrium orthosilicate crystal, reporting measurements of its heat capacity and optical coherence. Heat capacity not only informs thermal noise limits in metrology schemes but can also reveal two-level systems (TLS) arising from crystal imperfections via a linear-in-temperature term. Below 1 K, where phonon contributions are suppressed, TLS can drive decoherence, leading to a linear broadening of the homogeneous linewidth. From our data, we place an upper bound on the TLS contribution. This, together with constant optical linewidths between 300 mK and 2 K measured via photon-echo lifetimes, is consistent with a minimal TLS effects in our sample. A low level of TLS is particularly important for the performance of optical quantum devices based on doped crystals, since their presence could otherwise limit further improvements in coherence at sub-kelvin temperatures.
We explore the properties of ultranarrow spectral holes in ensembles of solid-state emitters in crystals over a range of sub-kelvin temperatures, as a new step toward leveraging their exceptional coherence properties more effectively. As an example, we consider the potential gain observed in their application in frequency stabilization schemes. We investigate how the parameters used to burn the spectral hole impact its shape, and how these factors determine the minimum achievable linewidth. In addition to the stability of the hole's center frequency, the linewidth and contrast play a crucial role in frequency locking. At sub-kelvin temperatures, the temperature-dependent T7 broadening from two-phonon Raman scattering is expected to be negligible, and the spectral hole's linewidth should therefore remain constant in this interval. We observe however a linear broadening with increasing temperature, highlighting the need for further investigation into the mechanisms governing the linewidth at ultra-low temperatures.
We explore the properties of ultra-narrow spectral holes in ensembles of solid-state emitters in crystals over a range of sub-kelvin temperatures, with a focus on their potential application in frequency stabilization schemes as an alternative to ultrastable cavities. We investigate how the parameters used to burn the spectral hole impact its shape, and how these factors determine the minimum achievable linewidth. In addition to the stability of the hole's center frequency, the linewidth and contrast play a crucial role in frequency locking. At sub-kelvin temperatures, the temperaturedependent T^7 broadening from two-phonon Raman scattering is expected to be negligible, and the spectral hole's linewidth should therefore remain constant in this interval. We observe however a linear broadening with increasing temperature, highlighting the need for further investigation into the mechanisms governing the linewidth at ultra-low temperatures.
Optical coherence and spin population dynamics in 171Yb3+:Y2SiO5 single crystals
171Yb3+-doped Y2SiO5 crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in 171Yb long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are however not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two 171 Yb:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above 6 K the homogeneous linewidth is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, the homogeneous linewidth reaching only 25 kHz at 10 K. At lower temperatures, interactions with 89Yb nuclear spin-flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal, are likely the main limiting factor to the homogeneous linewidth. In particular, we conclude that the direct effect of spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range studied. Our results indicate possible paths and regimes for further decreasing the homogeneous linewidths or maintaining narrow lines at higher 171Yb concentration.
Quantum state lifetimes T_{2}, or equivalently homogeneous linewidths Γ_{h}=1/πT_{2}, are a key parameter for understanding decoherence processes in quantum systems and assessing their potential for applications in quantum technologies. The most common tool for measuring narrow optical homogeneous linewidths, i.e., long T_{2}, is the measurement of coherent photon echo emissions, which however gives very weak signal when the number of emitters is small. This strongly hampers the development of nanomaterials, such as those based on rare earth ions, for quantum communication and processing. In this work, we propose, and demonstrate in an erbium doped crystal, a measurement of photon echoes based on incoherent fluorescence detection and its variance analysis. It gives access to T_{2} through a much larger signal than direct photon echo detection, and, importantly, with a laser which is incoherent over the measurement timescale, on the order of a few T_{2}. Our results thus open the way to efficiently assess the properties of a broad range of emitters and materials for applications in quantum nanophotonics.
Erbium-doped solids are promising candidates for fiber-based quantum networks due to their emission wavelength, which aligns with the telecom band over which optical fibers exhibit minimal loss. Among these, erbium-doped silica fiber (EDF) stands out for its availability, ease of use, and seamless integration with existing fiber-optic infrastructure. In this work, using the two-pulse photon-echo (2PPE) technique, we measured the homogeneous linewidth of the 4 I 13/2 ↔ 4 I 15/2 optical transition under varying magnetic fields and temperatures. We observed an effective homogeneous linewidth of approximately 8 kHz at ∼7mK at an optimal magnetic field of 0.09 T, representing over two orders of magnitude improvement compared to earlier reports measured at T ≈700mK. We also present a comprehensive model for the combined magnetic field and temperature dependency of the effective homogeneous linewidth. Additionally, we employed three-pulse photon-echo (3PPE) measurements to investigate spectral diffusion and decoherence processes and conclude that Two-Level System (TLS) effects are significantly suppressed at sufficiently low temperatures, below ∼100mK.