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Laser-written waveguides in rare-earth-doped crystal for integrated optical memory applications (Conference Presentation)

Giacomo Corrielli, Alessandro Seri, Margherita Mazzera, Roberto Osellame, Hugues de Riedmatten

Published Apr 28, 2017• 0 citations
Publisher

Abstract

The reversible mapping of quantum states of light in cryogenically cooled rare earth doped crystals, represents one of the most promising routes towards the realization of efficient and high fidelity quantum memories. The miniaturization of these devices in robust and monolithic integrated-optics platforms would be beneficial both in terms of experimental scalability and of enhanced light-matter interaction, arising from the waveguide field confinement. Here, for the first time, we fabricate single mode channel waveguides for visible light at 606 nm in a Praseodymium-doped Yttrium Orthosilicate crystal, which is one of the most employed materials for light storage experiments, thanks to its excellent coherence properties. For the waveguide fabrication, we use the direct technique called femtosecond laser micromachining, in which a femtosecond laser beam is focused inside the crystal volume, and produces a permanent and very localized material modification. In particular, we fabricate the waveguide cladding by inscribing a pair of parallel damage tracks which confine light in the in-between region. With this approach, the waveguide core is not directly exposed to the laser irradiation and consequently its bulk properties result only marginally affected. Measurements of the optical coherence time in waveguide gave results comparable to those obtained in a bulk sample and this confirms that the fabrication procedure does not affect the coherence of the active ions. We performed the storage and the on-demand recall of bright coherent pulses in waveguide, using the atomic frequency comb (AFC) protocol extended to the ground state.

DOI
10.1117/12.2251441

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The reversible mapping of quantum states of light in cryogenically cooled rare earth doped crystals, represents one of the most promising routes towards the realization of efficient and high fidelity quantum memories. The miniaturization of these devices in robust and monolithic integrated-optics platforms would be beneficial both in terms of experimental scalability and of enhanced light-matter interaction, arising from the waveguide field confinement. Here, for the first time, we fabricate single mode channel waveguides for visible light at 606 nm in a Praseodymium-doped Yttrium Orthosilicate crystal, which is one of the most employed materials for light storage experiments, thanks to its excellent coherence properties. For the waveguide fabrication, we use the direct technique called femtosecond laser micromachining, in which a femtosecond laser beam is focused inside the crystal volume, and produces a permanent and very localized material modification. In particular, we fabricate the waveguide cladding by inscribing a pair of parallel damage tracks which confine light in the in-between region. With this approach, the waveguide core is not directly exposed to the laser irradiation and consequently its bulk properties result only marginally affected. Measurements of the optical coherence time in waveguide gave results comparable to those obtained in a bulk sample and this confirms that the fabrication procedure does not affect the coherence of the active ions. We performed the storage and the on-demand recall of bright coherent pulses in waveguide, using the atomic frequency comb (AFC) protocol extended to the ground state.
Efficient and long-lived multimode quantum memories are crucial devices in the development of quantum technolgies. The reversible mapping of quantum states of light in rare earth doped crystals represents one of the most promising routes towards the realization of this goal. Such systems are also compatible with the miniaturization of quantum memories in integrated optics platforms, which offer unique features in terms of experimental scalability and enhanced light-matter interaction. Here, we fabricate single mode channel waveguides for 606 nm light in a praseodymium-doped yttrium orthosilicate crystal (Pr3+:Y2SiO5), that, thanks to its excellent coherence properties, is a widely studied material for light storage experiments. Waveguides are inscribed by femtosecond laser writing, adopting the so-called Type I configuration, where the core is directly obtained at the irradiated area. Remarkably, fabricating this kind of waveguides in crystals is a difficult task, as it requires to operate in a very narrow processing parameters window, if existing. We then use these waveguides for performing the storage and retrieval of single photons, implementing the atomic frequency comb protocol. We achieve a storage time of 5,5 µs, which is almost 2 orders of magnitude longer than previous realizations of quantum light storage in a waveguide. In addition, we investigate the potential information multiplexing capabilities of our system by performing the quantum storage of single photons delocalized over 14 different spectral modes. Our results show that laser written waveguides in rare earth-doped solid state systems are very promising for the development of efficient and long-lived multimode quantum memories.
151 E u 3 + -doped yttrium silicate ( 151 E u 3 + : Y 2 S i O 5 ) crystal is a unique material that possesses hyperfine states with coherence time up to 6 h. Many efforts have been devoted to the development of this material as optical quantum memories based on bulk crystals, but integrable structures (such as optical waveguides) that can promote 151 E u 3 + : Y 2 S i O 5 -based quantum memories to practical applications have not been demonstrated so far. Here we report the fabrication of type II waveguides in a 151 E u 3 + : Y 2 S i O 5 crystal using femtosecond-laser micromachining. The resulting waveguides are compatible with single-mode fibers and have the smallest insertion loss of 4.95 dB. On-demand light storage is demonstrated in a waveguide by employing the spin-wave atomic frequency comb (AFC) scheme and the revival of silenced echo (ROSE) scheme. We implement a series of interference experiments based on these two schemes to characterize the storage fidelity. Interference visibility of the readout pulse is 0.99 ± 0.03 for the spin-wave AFC scheme and 0.97 ± 0.02 for the ROSE scheme, demonstrating the reliability of the integrated optical memory.
Quantum memory is an essential building block for applications in quantum information science, where scalability and integrability are needed for progress toward large-scale architectures. The authors present a platform for integrated photonic memory, based on laser-written waveguides in rare-earth-doped crystals. The coherence properties of the optically active ions persist after micromachining, enabling proof-of-principle light storage using the full atomic-frequency-comb protocol. This is a pioneering demonstration of on-demand optical memory on a chip.
Quantum light-matter interfaces that reversibly map the quantum state of photons onto the quantum states of atoms, are essential components in the quantum engineering toolbox with applications in quantum communication, computing, and quantum-enabled sensing. In this talk I present our progress towards developing on-chip quantum light-matter interfaces based on nanophotonic resonators fabricated in rare-earth-doped crystals known to exhibit the longest optical and spin coherence times in the solid state. We recently demonstrated coherent control of neodymium (Nd3+) ions coupled to yttrium orthosilicate Y2SiO5 (YSO) photonic crystal nano-beam resonator. The coupling of the Nd3+ 883 nm 4I9/2-4F3/2 transition to the nano-resonator results in a 40 fold enhancement of the transition rate (Purcell effect), and increased optical absorption (~80%) - adequate for realizing efficient optical quantum memories via cavity impedance matching. Optical coherence times T2 up to 100 μs with low spectral diffusion were measured for ions embedded in photonic crystals, which are comparable to those observed in unprocessed bulk samples. This indicates that the remarkable coherence properties of REIs are preserved during nanofabrication process. Multi-temporal mode photon storage using stimulated photon echo and atomic frequency comb (AFC) protocols were implemented in these nano-resonators. Our current technology can be readily transferred to Erbium (Er) doped YSO devices, therefore opening the possibility of efficient on-chip optical quantum memory at 1.5 μm telecom wavelength. Integration with superconducting qubits can lead to devices for reversible quantum conversion of optical photons to microwave photons.
Optical quantum memories will enable technologies including long distance quantum communication and modular quantum computing. Rare earth ion doped crystals provide an excellent solid state platform for optical quantum memories. Among rare earths, erbium is particularly appealing due to its long-lived telecom-wavelength resonance, allowing integration with silicon photonics and with existing optical communication technology and infrastructure. We present an on-chip all-optical quantum memory at telecom wavelengths using a nanobeam photonic crystal cavity fabricated directly in erbium-167 doped yttrium orthosilicate. Using an atomic frequency comb protocol, we store coherent pulses for memory times as long as 10 µs, albeit with low efficiency. For shorter memory times, we achieve a memory efficiency of 0.4%, which is limited by the coupling rate between the resonator and the ensemble of ions. By working at dilution refrigerator temperatures, we are able to access a regime where the ions have long optical coherence times and good spectral holeburning properties using only a moderate magnetic field applied with permanent magnets. We characterize the multimode properties and fidelity of the quantum memory in this device, and outline a path toward higher efficiency.
Quantum memory is the core device for the construction of large-scale quantum networks. For scalable and convenient practical applications, integrated optical memories, especially on-chip optical memories, are crucial requirements because they can be easily integrated with other on-chip devices. Here, we report the coherent optical memory based on a type-IV waveguide fabricated on the surface of a rare-earth ion-doped crystal (i.e., Eu3+\mathrm{E}{\mathrm{u}}^{3+}Eu3+:Y2SiO5{\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}Y2​SiO5​). The properties of the optical transition (${}^{7}{\mathrm{F}}_{0}\ensuremath{\rightarrow}{}^{5}{\mathrm{D}}_{0}$) of the Eu3+{\mathrm{Eu}}^{3+}Eu3+ ions inside the surface waveguide are well preserved compared to those of the bulk crystal. Spin-wave atomic frequency comb storage is demonstrated inside the type-IV waveguide. The reliability of this device is confirmed by the high interference visibility of $97\ifmmode\pm\else\textpm\fi{}1\mathrm{%}$ between the retrieval pulse and the reference pulse. The developed on-chip optical memory paves the way towards integrated quantum nodes.