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arXiv:1412.7298 · Physical Review Letters

Coherent Storage of Microwave Excitations in Rare-Earth Nuclear Spins

Gary Wolfowicz, Hannes Maier-Flaig, Robert Marino, Alban Ferrier, Hervé Vezin, John J. L. Morton, Philippe Goldner

Published Apr 30, 2015• 87 citations

Abstract

Interfacing between various elements of a computer--from memory to processors to long range communication--will be as critical for quantum computers as it is for classical computers today. Paramagnetic rare-earth doped crystals, such as Nd(3+):Y2SiO5(YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory), and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and (145)Nd nuclear spin states in Nd(3+):YSO. We find nuclear spin coherence times can reach 9 ms at ∼5 K, about 2 orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage or retrieval operation between the electron and nuclear spin reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.

DOI: 10.1103/physrevlett.114.170503

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Interfacing between various elements of a computer--from memory to processors to long range communication--will be as critical for quantum computers as it is for classical computers today. Paramagnetic rare-earth doped crystals, such as Nd(3+):Y2SiO5(YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory), and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and (145)Nd nuclear spin states in Nd(3+):YSO. We find nuclear spin coherence times can reach 9 ms at ∼5 K, about 2 orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage or retrieval operation between the electron and nuclear spin reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.

Rare-earth doped crystals are promising systems for quantum information processing. In particular, paramagnetic rare earths could be used to build coherent interfaces with optical and microwave photons. In addition, isotopes with nonzero nuclear spins could provide long-lived states for quantum state storage and processing.

{\mathrm{Yb}}^{3+}

is particularly interesting in this respect since it is the only paramagnetic rare earth with a spin-

\frac{1}{2}

isotope, which corresponds to the simplest possible level structure. In this paper, we report on the optical and magnetic properties of

{\mathrm{Yb}}^{3+}

in the two sites of

{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}

, a commonly used crystal for quantum applications. We measured optical inhomogeneous linewidths, peak absorption coefficients, oscillator strengths, and excited-state lifetimes. The Zeeman tensors were also determined in the ground and excited states, as well as the ground-state hyperfine tensor for the

^{171}\mathrm{Yb}^{3+}

(

I=\frac{1}{2}

) isotope. These results suggest that

{\mathrm{Yb}}^{3+}

:

{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}

is a promising material for applications like solid-state optical and microwave quantum memories.

Solid-state impurity spins with optical control are currently investigated for quantum networks and repeaters. Among these, rare-earth-ion doped crystals are promising as quantum memories for light, with potentially long storage time, high multimode capacity, and high bandwidth. However, with spins there is often a tradeoff between bandwidth, which favors electronic spin, and memory time, which favors nuclear spins. Here, we present optical storage experiments using highly hybridized electron-nuclear hyperfine states in ^{171}Yb^{3+}:Y_{2}SiO_{5}, where the hybridization can potentially offer both long storage time and high bandwidth. We reach a storage time of 1.2 ms and an optical storage bandwidth of 10 MHz that is currently only limited by the Rabi frequency of the optical control pulses. The memory efficiency in this proof-of-principle demonstration was about 3%. The experiment constitutes the first optical storage using spin states in any rare-earth ion with electronic spin. These results pave the way for rare-earth based quantum memories with high bandwidth, long storage time, and high multimode capacity, a key resource for quantum repeaters.

Rare-earth-ion doped crystals are state-of-the-art materials for optical quantum memories and quantum transducers between optical and microwave photons. Here we describe our progress towards a nanophotonic quantum memory based on a rare-earth (Neodymium) doped yttrium orthosilicate (YSO) photonic crystal resonator. The Purcell-enhanced coupling of the 883 nm transitions of Neodymium (Nd3+) ions to the nano-resonator results in increased optical depth, which could in principle facilitate highly efficient photon storage via cavity impedance matching. The atomic frequency comb (AFC) memory protocol can be implemented in the Nd:YSO nano-resonator by efficient optical pumping into the long-lived Zeeman state. Coherent optical signals can be stored and retrieved from the AFC memory. We currently measure a storage efficiency on par with a bulk crystal Nd:YSO memory that is millimeters long. Our results will enable multiplexed on-chip quantum storage and thus quantum repeater devices using rare-earth-ions.

Rare earth doped crystals are a promising platform for long-distance quantum communications, where they can perform quantum light storage. Among different systems,

{}^{171}

Yb

{}^{3+}

-doped Y

{}_{2}

SiO

{}_{5}

(YSO) crystals stand out because of long optical and spin coherence times, and large hyperfine splitting, enabling long-time storage over large bandwidths. Here, the authors study the mechanisms affecting the optical decoherence in a broad range of temperatures (40 mK to 18 K) using high-resolution spectroscopic techniques, and identify ways to further improve the crystal's properties.

Rare-earth spins in optical crystals like yttrium orthosilicate (YSO) are of much interest for providing coherent interfaces with both microwave and optical photons. Here, the authors study the spin dynamics of a new system, Yb in YSO, which has favorable optical properties and strong hyperfine coupling to the

{}^{171}

Yb (spin-1/2) and

{}^{173}

Yb (spin-5/2) isotopes. The results demonstrate long coherence times for both the electron and nuclear spins (~1 ms). Employing a range of pulsed electron spin resonance measurements as well as comparison with relevant theory, the study elucidates a number of spin relaxation and decoherence mechanisms at work.