Coherence Time Extension by Large-Scale Optical Spin Polarization in a Rare-Earth Doped Crystal
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-09-16 |
| Journal | Physical Review X |
| Authors | Sacha Welinski, Alexey Tiranov, Moritz Businger, Alban Ferrier, Mikael Afzelius |
| Institutions | Centre National de la Recherche Scientifique, Chimie ParisTech |
| Citations | 12 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research introduces Diffusion Enhanced Optical Pumping (DEOP) as a novel method to significantly extend optical coherence lifetimes in paramagnetic rare-earth doped crystals, specifically 171Yb3+:Y2SiO5 (YSO).
- Core Achievement: DEOP enables large-scale spin polarization (> 90%) across the entire laser-addressed ensemble by optically pumping a very small fraction (â 0.5%) of ions, relying on subsequent spin diffusion via flip-flop interactions.
- Performance Gain: The optical coherence lifetime (T2,o) was increased 2.5-fold, from 0.3 ms (thermal equilibrium) to 0.8 ms, by strongly decreasing detrimental spin-spin interactions.
- Record Coherence: The achieved T2,o of 800 ”s is the longest reported optical coherence time for any paramagnetic solid-state system operating at zero or very-low magnetic fields.
- Linewidth Reduction: The corresponding homogeneous linewidth (Îh,o) was reduced to 407 ± 15 Hz, the narrowest reported for a paramagnetic rare-earth ion at zero magnetic field.
- Operating Conditions: This high polarization and coherence enhancement were achieved at 2 K and zero external magnetic field, simplifying integration with devices like superconducting circuits.
- Versatility: DEOP allows for tailoring specific, highly out-of-equilibrium population distributions among hyperfine levels, a feature difficult to obtain using only magnetic fields or low temperatures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Host Material | Y2SiO5 (YSO) | Crystal | Monoclinic structure, Site 2 ions investigated. |
| Dopant Concentration | 10 | ppm 171Yb3+ | Isotopic purity 94%. |
| Operating Temperature | 2 | K | Liquid helium bath cryostat. |
| External Magnetic Field | Zero/Very-low | Field | Insensitive transitions used; ideal for superconducting interfaces. |
| Optical Transition Wavelength | 978.854 | nm (vac.) | 2F7/2 â 2F5/2 transition. |
| Maximum Spin Polarization | > 90% (up to 96 ± 1%) | % | Achieved via DEOP into the |1g> level. |
| Fraction of Ions Optically Pumped | â 0.5 | % | Small fraction required to polarize the entire ensemble. |
| Initial Optical T2,o (Thermal Eq.) | 278 ± 20 | ”s | Without DEOP. |
| Enhanced Optical T2,o (DEOP) | 782 ± 30 | ”s | 2.5-fold increase; longest T2,o for paramagnetic RE at zero field. |
| Enhanced Homogeneous Linewidth (Îh,o) | 407 ± 15 | Hz | Achieved under optimal DEOP conditions. |
| Excited State Lifetime (T1,1e) | 1.3 | ms | Radiative lifetime. |
| Ground State SLR Lifetime (R) | 1.4 x 10-2 | s-1 | Spin-Lattice Relaxation rate at 2 K (T1 â 72 s). |
| Polarization Rate (RP) | 3 ± 0.6 | s-1 | Highest rate observed (for C = 5.8 x 10-3). |
| Spin Coherence Time (T2,s) | 0.2 to 2.5 | ms | Measured on the |3g> â |4g> transition at 3 K, depending on polarization. |
Key Methodologies
Section titled âKey MethodologiesâThe Diffusion Enhanced Optical Pumping (DEOP) mechanism and coherence measurements were performed using the following steps:
- Sample Growth and Preparation: A single crystal of Y2SiO5 doped with 10 ppm of 171Yb3+ (94% isotopic purity) was grown using the Czochralski technique.
- Experimental Environment: Experiments were conducted in transmission mode with the sample placed in a liquid helium bath cryostat maintained at 2 K, operating at zero external magnetic field.
- Optical Pumping (OP): A tunable single-mode diode laser (1 MHz linewidth) was used for excitation at 978.854 nm. The laser power during pumping was 7 mW, focused to a 1 mm diameter spot. Pumping durations (TP) ranged from 1 s up to 40 s.
- Absorption Spectroscopy: Absorption spectra were recorded by frequency scanning the laser (0.4 mW power) after a 10 ms delay (to allow excited state population decay). This data was fitted to determine the normalized ground state spin level populations (kig).
- DEOP Mechanism Confirmation: The large-scale polarization observed (e.g., 96% into the |1g> state) was attributed to spin diffusion, where the small fraction of optically pumped ions (A-spins) transfers polarization to the non-pumped ensemble (B-spins) via magnetic dipole-dipole flip-flop interactions.
- Spin-Lattice Relaxation (SLR) Measurement: The recovery rate (Rc) of the absorption spectrum after OP was stopped was measured at various temperatures (2 K to 4 K) to determine the SLR rate, confirming the low intrinsic relaxation rate of the system.
- Optical Coherence Measurement (T2,o): The optical coherence lifetime was measured using the standard Hahn photon echo sequence (Ï/2 - Ï - Ï - Ï - echo). A second AOM was used to gate the laser and prevent optical pumping during the echo sequence.
- Spin Coherence Measurement (T2,s): Spin coherence was measured using Raman heterodyne scattering (RHS) on the |4g> â |3g> transition (655 MHz) at 3 K to investigate the direct contribution of flip-flop rates to coherence time.
Commercial Applications
Section titled âCommercial ApplicationsâThe DEOP technique and the resulting highly coherent, concentrated spin ensembles are critical for several quantum technologies:
- Quantum Memories:
- Enables high-efficiency, broadband absorptive quantum memories by allowing high concentrations of active ions (10 ppm) while maintaining long coherence times (0.8 ms T2,o).
- The ability to tailor population distributions is essential for implementing protocols like the Atomic Frequency Comb (AFC) memory.
- Quantum Transducers and Interfacing:
- The achievement of long coherence times (T2,o â 800 ”s) at zero magnetic field is crucial for interfacing solid-state electronic spins (like 171Yb3+) with superconducting qubits and resonators, which operate in low-magnetic-noise environments.
- Quantum Processing and Computing:
- Provides a robust method for large-scale spin initialization (e.g., 96% polarization into a single level) prior to quantum processing operations or spectral tailoring.
- Quantum Sensing:
- Allows for precise engineering and control of the spin bath, reducing magnetic noise and enhancing the sensitivity of quantum sensors based on rare-earth ions or other concentrated spin systems (e.g., NV- centers in diamond).
- Solid-State Qubit Platforms:
- The technique is applicable to other paramagnetic rare-earth ions (like Er3+, Nd3+) and concentrated spin systems, paving the way for new designs of robust solid-state qubits.
View Original Abstract
Optically addressable spins are actively investigated in quantum\ncommunication, processing and sensing. Optical and spin coherence lifetimes,\nwhich determine quantum operation fidelity and storage time, are often limited\nby spin-spin interactions, which can be decreased by polarizing spins in their\nlower energy state using large magnetic fields and/or mK range temperatures.\nHere, we show that optical pumping of a small fraction of ions with a fixed\nfrequency laser, coupled with spin-spin interactions and spin diffusion, leads\nto substantial spin polarization in a paramagnetic rare earth doped crystal,\n$^{171}$Yb$^{3+}$:YSO. Indeed, up to more than 90 % spin polarizations have\nbeen achieved at 2 K and zero magnetic field. Using this spin polarization\nmechanism, we furthermore demonstrate an increase in optical coherence lifetime\nfrom 0.3 ms to 0.8 ms, due to a strong decrease in spin-spin interactions. This\neffect opens the way to new schemes for obtaining long optical and spin\ncoherence lifetimes in various solid-state systems such as ensembles of rare\nearth ions or color centers in diamond, which is of interest for a broad range\nof quantum technologies.\n