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arXiv:1501.03980 · Physical Review Letters

Solid State Spin-Wave Quantum Memory for Time-Bin Qubits

Mustafa Gündoğan, Patrick M. Ledingham, Kutlu Kutluer, Margherita Mazzera, Hugues de Riedmatten

Published Jun 12, 2015• 213 citations
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Abstract

We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a ${\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio $>10$. Narrow-band spectral filtering based on spectral hole burning in a second ${\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of $(2.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.

DOI
10.1103/physrevlett.114.230501

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We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a Pr3+\ensuremath:Y2SiO5{\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Pr3+\ensuremath:Y2​SiO5​ crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio >10>10>10. Narrow-band spectral filtering based on spectral hole burning in a second Pr3+\ensuremath:Y2SiO5{\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Pr3+\ensuremath:Y2​SiO5​ crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of (2.0\ifmmode±\else\textpm\fi0.3)\ifmmode×\else\texttimes\fi10\ensuremath−3(2.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}(2.0\ifmmode±\else\textpm\fi0.3)\ifmmode×\else\texttimes\fi10\ensuremath−3 photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.
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We report on the realization of an efficient solid-state spin-wave quantum memory, with on-demand readout, using the full atomic frequency comb (AFC) scheme in a Pr^{3+}:Y_{2}SiO_{5} crystal embedded in an impedance-matched cavity. We demonstrate operation at the single-photon level by storing weak coherent states with an efficiency up to (40±2)% and a signal-to-noise ratio of 14 for an input photon number of 0.42 photons per pulse. We also investigated the enhancement of the incoherent noise due to the impedance-matched cavity and characterized the quantum memory performance, showing a two-way transfer from excited to spin states and back of up to 83%. Finally, we confirmed the quantum nature of our memory by storing nonclassical states of light, i.e., a heralded single photon from a nondegenerate spontaneous parametric down-conversion source, and achieved nonclassical correlations between the heralding and the stored-and-retrieved photon. These results demonstrate that impedance-matched AFC spin-wave quantum memories with on-demand readout can be used for experiments involving the storage of photonic quantum states. They also open the door to solid-state on-demand quantum memories with very high efficiencies, serving as a key resource for quantum networks and quantum repeaters.
We demonstrate a solid-state spin-wave optical memory based on stopped light in a spectral hole. A long-lived narrow spectral hole is created by optical pumping in the inhomogeneous absorption profile of a Pr3+:Y2SiO5{\mathrm{Pr}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Pr3+:Y2​SiO5​ crystal. Optical pulses sent through the spectral hole experience a strong reduction of their group velocity and are spatially compressed in the crystal. A short Raman pulse transfers the optical excitation to the spin state before the light pulse exits the crystal, effectively stopping the light. After a controllable delay, a second Raman pulse is sent, which leads to the emission of the stored photons. We reach storage and retrieval efficiencies for bright pulses of up to 3939%39 in a 5-mm-long crystal. We also show that our device works at the single-photon level by storing and retrieving $3\ensuremath{-}\ensuremath{\mu}\mathrm{s}$-long weak coherent pulses with efficiencies up to 3131%31, demonstrating the most efficient spin-wave solid-state optical memory at the single-photon level so far. We reach an unconditional noise level of $(9\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ photons per pulse in a detection window of $4\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$, leading to a signal-to-noise ratio of $33\ifmmode\pm\else\textpm\fi{}4$ for an average input photon number of 1, making our device promising for long-lived storage of nonclassical light.
Photonic quantum memory is the core element in quantum information processing (QIP). For the scalable and convenient practical applications, great efforts have been devoted to the integrated quantum memory based on various waveguides fabricated in solids. However, on-demand storage of qubits, which is an essential requirement for QIP, is still challenging to be implemented using such integrated quantum memory. Here we report the on-demand storage of time-bin qubits in an on-chip waveguide memory fabricated on the surface of a ^{151}Eu^{3+}:Y_{2}SiO_{5} crystal, utilizing the Stark-modulated atomic frequency comb protocol. A qubit storage fidelity of 99.3%±0.2% is obtained with single-photon-level coherent pulses, far beyond the highest fidelity achievable using the classical measure-and-prepare strategy. The developed integrated quantum memory with the on-demand retrieval capability represents an important step toward practical applications of integrated quantum nodes in quantum networks.
Long-lived storage of single photons is a fundamental requirement for enabling quantum communication and foundational tests of quantum physics over extended distances. While the implementation of a global-scale quantum network requires quantum storage times on the order of seconds to minutes, existing photonic quantum memories have so far been limited to subsecond lifetimes. Although 151^{151}151Eu3+^{3+}3+:Y2_22​SiO5_55​ crystals exhibit substantially extended spin coherence times at the `magic' magnetic field, the concomitant weak optical absorption has until now prevented single-photon storage. Here, we overcome this challenge by integrating a noiseless photon echo protocol--which makes full use of the crystal's natural absorption for photonic storage--with a universally robust dynamical decoupling sequence incorporating adiabatic pulses to efficiently protect delocalized spin-wave excitation, enabling long-lived quantum storage at the `magic' magnetic field. At a storage time of 5.6 s, we achieve a time-bin qubit storage fidelity of 88.0 ±\pm± 2.1%, surpassing the maximum fidelity attainable via classical strategies. Our device reaches a 1/e1/e1/e storage lifetime of 27.6 ±\pm± 0.5 s, enabling single-photon-level storage for 42 s with a signal-to-noise ratio greater than unity. This work establishes photonic quantum memory in the minute-scale regime, laying a solid foundation for global-scale quantum network and deep-space quantum experiments.