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arXiv:1605.02396 · Physical Review Letters

Properties of a Rare-Earth-Ion-Doped Waveguide at Sub-Kelvin Temperatures for Quantum Signal Processing

Neil E. Sinclair, Daniel Oblak, Charles W. Thiel, R. L. Cone, Wolfgang Tittel

Published Mar 7, 2017• 43 citations
arXivHTMLPDF

Abstract

We characterize the 795 nm ${^{3}\mathrm{H}}_{6}$ to ${^{3}\mathrm{H}}_{4}$ transition of ${\mathrm{Tm}}^{3+}$ in a ${\mathrm{Ti}}^{4+}:\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ waveguide at temperatures as low as 800 mK. Coherence and hyperfine population lifetimes---up to $117\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ and 2.5 h, respectively---exceed those at 3 K at least tenfold, and are equivalent to those observed in a bulk ${\mathrm{Tm}}^{3+}:\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ crystal under similar conditions. We also find a transition dipole moment that is equivalent to that of the bulk. Finally, we prepare a 0.5 GHz-bandwidth atomic frequency comb of finesse $>2$ on a vanishing background. These results demonstrate the suitability of rare-earth-ion-doped waveguides created using industry-standard Ti indiffusion in $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ for on-chip quantum applications.

DOI
10.1103/physrevlett.118.100504

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We characterize the 795 nm 3H6{^{3}\mathrm{H}}_{6}3H6​ to 3H4{^{3}\mathrm{H}}_{4}3H4​ transition of Tm3+{\mathrm{Tm}}^{3+}Tm3+ in a Ti4+:LiNbO3{\mathrm{Ti}}^{4+}:\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}Ti4+:LiNbO3​ waveguide at temperatures as low as 800 mK. Coherence and hyperfine population lifetimes---up to 117  \ensuremathμs117\text{ }\text{ }\ensuremath{\mu}\mathrm{s}117  \ensuremathμs and 2.5 h, respectively---exceed those at 3 K at least tenfold, and are equivalent to those observed in a bulk Tm3+:LiNbO3{\mathrm{Tm}}^{3+}:\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}Tm3+:LiNbO3​ crystal under similar conditions. We also find a transition dipole moment that is equivalent to that of the bulk. Finally, we prepare a 0.5 GHz-bandwidth atomic frequency comb of finesse >2>2>2 on a vanishing background. These results demonstrate the suitability of rare-earth-ion-doped waveguides created using industry-standard Ti indiffusion in LiNbO3\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}LiNbO3​ for on-chip quantum applications.
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We characterize the optical coherence and energy-level properties of the 795-nm 3H6^{3}\mathrm{H}_{6}3H6​ to 3H4^{3}\mathrm{H}_{4}3H4​ transition of Tm3+{\mathrm{Tm}}^{3+}Tm3+ in a Ti4+{\mathrm{Ti}}^{4+}Ti4+:LiNbO3\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}LiNbO3​ waveguide at temperatures as low as 0.65 K. Coherence properties are measured with varied temperature, magnetic field, optical excitation power and wavelength, and measurement timescale. We also investigate nuclear spin-induced hyperfine structure and population dynamics with varying magnetic field and laser excitation power. Except for accountable differences due to different Ti4+{\mathrm{Ti}}^{4+}Ti4+- and Tm3+{\mathrm{Tm}}^{3+}Tm3+-doping concentrations, we find that the properties of Tm3+{\mathrm{Tm}}^{3+}Tm3+:Ti4+{\mathrm{Ti}}^{4+}Ti4+:LiNbO3\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}LiNbO3​ produced by indiffusion doping are consistent with those of a bulk-doped Tm3+{\mathrm{Tm}}^{3+}Tm3+:LiNbO3\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}LiNbO3​ crystal measured under similar conditions. Our results, which complement previous work in a narrower parameter space, support using rare-earth ions for integrated optical and quantum signal processing.
Quantum memories are a key building block for optical quantum computers and quantum networks. Rare-earth ion-doped crystals are a promising material to achieve quantum memory using an atomic frequency comb protocol. However, current atomic frequency comb memories typically use bulk materials or waveguides with large cross sections or rely on fabrication techniques not easily adaptable to wafer scale processing. Here, we demonstrate a compact chip-integrated atomic frequency comb in rare-earth-doped thin-film lithium niobate. Our optical memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. The enhanced optical confinement in this device leads to three orders of magnitude reduction in optical power required for a coherent control as compared to ion-diffused waveguides. These compact atomic frequency comb memories pave the way toward scalable, highly efficient, electro-optically tunable quantum photonic systems that can store and manipulate light on a compact chip.
This thesis presents a spectroscopic study of Pr3+ ions in a novel passive waveguide architecture. To make this structure, the high refractive glass TeO2 was selected as the thin film and it was deposited on a Pr3+:Y2SiO5 crystal. In this waveguide, the 3H4 to 1D2 transition of Pr3+ ions were probed by the optical evanescent field extending into the substrate. The main concern in assessing the suitability of this material for quantum information applications was ensuring that the coherence properties of rare-earth ions, making them suitable for quantum information purposes, are preserved in this architecture. After which, to make low loss devices with these waveguides, efficient coupling techniques had to be developed. To prove that the coherence properties of the rare-earth ion doped crystal were preserved, the critical parameters of inhomogeneous linewidth, the absorption of the ions, the coherence time and spin lifetimes of the Pr3+ ions were studied. The inhomogeneous linewidth of evanescent coupled ions was about 10.0 GHz, which was consistent with the linewidth of bulk samples with the same Pr3+ doping concentration (Hedges, 2011). The absorption due to the evanescent coupling was 9.38 dB, approximately 90% of what was expected with respect to the bulk crystal with the same doping concentration. Therefore, despite using the evanescent field, the absorption is high enough for quantum memory applications. An optical coherence time of about 121 microseconds was measured, which corresponded to a homogeneous linewidth of about 2.6 kHz. This is very close to bulk sample measurements of 111 microseconds, with the same temperature and doping concentration (R. W. Equall, 1995). The spin state lifetime observed was about 9.8 s, which is also very close to the bulk sample measurement of 8.67 s (Mieth, 2016). Initial Stark shifting experiments were performed to determine whether the active ions in the substrate of the passive waveguides could be electrically controlled by applying a small voltage to electrodes on the thin film. In these experiments with a voltage change of 100 mV, the measured holewidth broadening was increasing about 0.55 MHz, which was similar to the calculated values of 0.45 MHz. The Stark coefficient for site 1 was 51.6 kHzcm/V along the D2 axis of the crystal (site 1 will be explained in Section 4.3). (F.R. Graf, 1997). Prism coupling and grating coupling were used to couple light to the ions in the substrate. Prism coupling is an easy and quick method to couple light into a waveguide and observe the properties of the system. However, grating coupling is much more practical when moving towards building a device using this method. The measurements described above indicated that the properties of ions interacting with the evanescent tail of the waveguide mode were consistent with those of bulk ions. This investigation also showed that depositing a glass thin film on a rare-earth ion doped crystal was not affecting the good coherence properties of the substrate. These results establish the foundation for large, integrated, controllable and high performance rare earth ion quantum waveguide systems.
Long-lived sub-levels of the electronic ground-state manifold of rare-earth ions in crystals can be used as atomic population reservoirs for photon echo-based quantum memories. We measure the dynamics of the Zeeman sub-levels of erbium ions that are doped into a lithium niobate waveguide, finding population lifetimes at cryogenic temperatures down to 0.7 K as long as seconds. Then, using these levels, we prepare and characterize atomic frequency combs (AFCs), which can serve as a memory for quantum light at 1532 nm wavelength. The results allow predicting a 0.1% memory efficiency, limited mainly by unwanted background absorption that we believe to be caused by excitation-induced erbium spin flips and frequency shifting due to two-level systems or non-equilibrium phonons. Hence, while it should be possible to create an AFC-based quantum memory in E r 3 + : T i 4 + : L i N b O 3 , improved crystal growth together with optimized AFC preparation will be required to make it suitable for applications in quantum communication.
Abstract Atomic frequency combs memories that coherently store optical signals are a key building block for optical quantum computers1 and quantum networks2. Current atomic frequency combs typically use bulk materials or waveguides with large cross-sections3–5, or rely on fabrication techniques not easily adaptable to wafer scale processing6. Integrating such memories into compact and chip-scale devices is essential for scalable quantum technology. We demonstrate a compact chip-integrated atomic frequency comb in rare earth doped thin-film lithium niobate. Our optical memory exhibits a broad storage bandwidth exceeding 100 MHz, and optical storage time of over 250 ns. The enhanced optical confinement in this device leads to three orders of magnitude reduction in optical power required for coherent control as compared to ion-diffused waveguides 7. These compact atomic frequency comb memories pave the way towards scalable, highly efficient, electro-optically tunable quantum photonic systems that can store and manipulate light on a compact chip3.