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Solid-state light-matter interfaces on the quantum test bench

Christoph Clausen

Published Jan 1, 2013• 0 citations
Publisher

Abstract

Solid-state light-matter interfaces based on crystals doped with rare-earth ions have shown great progress in recent years, heading towards quantum technology applications. In this thesis we present three experiments testing the suitability of an atomic frequency comb light-matter interface, implemented in an Nd:YSO crystal, for the storage of quantum information. A dedicated source of entangled photons has been developed, and the storage and retrieval of quantum correlations and entanglement been studied. We show the successful transfer of photonic entanglement to the crystal, and the generation of a matter-matter entangled state between two crystals. These are two of the main steps in a quantum repeater protocol. We also demonstrate faithful storage and retrieval of quantum information encoded in the polarization of heralded single photons. These experiments prove the strong potential of solid-state light-matter interfaces based on atomic frequency combs for quantum communication.

DOI
10.13097/archive-ouverte/unige:27706

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Solid-state light-matter interfaces based on crystals doped with rare-earth ions have shown great progress in recent years, heading towards quantum technology applications. In this thesis we present three experiments testing the suitability of an atomic frequency comb light-matter interface, implemented in an Nd:YSO crystal, for the storage of quantum information. A dedicated source of entangled photons has been developed, and the storage and retrieval of quantum correlations and entanglement been studied. We show the successful transfer of photonic entanglement to the crystal, and the generation of a matter-matter entangled state between two crystals. These are two of the main steps in a quantum repeater protocol. We also demonstrate faithful storage and retrieval of quantum information encoded in the polarization of heralded single photons. These experiments prove the strong potential of solid-state light-matter interfaces based on atomic frequency combs for quantum communication.
The present thesis includes a series of experiments exploring the potential of using rare-earth ion-doped crystals as a light-matter quantum interface using different quantum states of light as a probe resource. We start with the realization of an elementary quantum repeater block, implementing quantum teleportation of a polarization qubit into a solid-state quantum memory based on a yttrium-orthosilicate crystal doped with neodymium ions. The storage of an hyperentangled state of light compatible with long-distance quantum communication was demonstrated to show the potential of using this quantum memory for quantum purification techniques. To explore the temporal multiplexing capability of atomic frequency combs in atomic ensembles we performed temporal multimode storage of multi-photon entangled state of light. Such a temporal multimode capacity together with the developed novel theoretical techniques allowed us to show the storage of more than one bit of entanglement (ebit) using multi-photon and multi-dimensional entangled photon states.
(English) Future networks for quantum communication will require a new form of technology in order to operate over long distances. The no-cloning theorem precludes the use of amplification, so quantum repeater schemes have been proposed to achieve long-distance quantum communication through the distribution of entanglement. A key device for this setup is a quantum memory, which is required in order to store entanglement and synchronise its distribution. Among the many physical systems than can be used as a quantum memory, rare-earth ion-doped crystals stand out due to their long coherence times and potential for multiplexing and integration. This thesis reports the use of one such system, Pr3+:Y2SiO5, as a quantum memory using the atomic frequency comb (AFC) protocol. Building on previous work, we improved the performance of the memory to be able to demonstrate light-matter entanglement with a telecommunications wavelength photon and the quantum memory, for both a bulk memory and an integrated version. To measure light-matter entanglement, we use photon pairs generated through cavity-enhanced spontaneous parametric down conversion, where one photon (the signal) is stored in the memory and the other (the idler) is at a telecommunications wavelength. These photon pairs exhibit energy-time entanglement, which we analyse in the time basis using fibre- and AFC-based Mach-Zehnder interferometers. In the first experiment, we demonstrate light-matter entanglement where the bulk quantum memory is used with on-demand retrieval via storage in a spin-wave. We measure interference fringes with average visibilities of 89(2)% for AFC storage, and 70(3)% for spin-wave storage, sufficient to demonstrate entanglement. This is the first such demonstration using an on-demand multimode solid-state memory and telecom photon. We show that entanglement is maintained for up to 47.7 µs of storage. These results were made possible through improvements in the AFC efficiency and the noise filtering. Furthermore, we perform on-demand storage and retrieval of up to 30 temporal modes. We take the next step towards implementing our system in a real world scenario by measuring entanglement over a distance. To do so, we send the idler photons through both optical fibre spools and deployed fibre in the metropolitan area of Barcelona, Spain, while the photon detection still occurs in the same lab. The resulting visibilities (80% or higher) demonstrate that the photonic qubit does not decohere during transmission through the optical fibre. We then decouple the photon creation and detection by measuring non-classical correlations between signal photons detected in the lab, and idler photons detected 16 km away (44 km in fibre) in a different location. The final experiment uses a fibre-integrated memory for light-matter entanglement. We use a Pr3+:Y2SiO5 crystal containing a Type I laser-written waveguide, interfaced directly with optical fibre. The fibre-integration allows for improved transmission and AFC efficiency compared to a free-space coupled waveguide memory. To analyse the performance of the memory, we demonstrate non-classical correlations for storage times up to 28 µs. We perform qubit tomography in the time basis for storage times of 3 and 10 µs, measuring two-qubit fidelities of 86(2)% and 86(4)%, respectively, demonstrating storage of entanglement. These experiments demonstrate the potential of our system for use as part of a quantum repeater, with many opportunities for further improvement to the storage time, efficiency, and multiplexing capability.
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.
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.
Quantum networks of the future will be required to span the globe and interconnect many different real world systems employed by different commercial, academic, and government users. Thus, an important measure of such a network's capacity will be its ability to interact with and distribute entanglement among different mediums and devices with potentially disparate properties. Few experiments have to date shown this sort of interconnectivity between different systems with different atomic species. Here I will present our demonstration of a system in which we store two correlated photons in solid state quantum memories implemented in different rare earth ion-doped materials using the atomic frequency comb protocol. The photon pairs are generated through a spontaneous parametric down-conversion process with their wavelengths chosen to be compatible with quantum repeater implementations. One member, with a wavelength at 795 nm, is stored in an Tm:LiNbO3 crystal while, simultaneously, the other member, generated at 1535 nm wavelength, is stored in an amorphous erbium doped optical fiber. The measured cross-correlation function between the recalled photons is g(2) = 16 +/- 2, which verifies that the non-classical correlations are preserved and were present during the storage of both photons. The two storage mediums are diverse not only in terms of their different photon acceptance wavelengths, but also with regards to the material properties as hosts for the dopant rare-earth ions. Our results show that quantum nodes and networks based on fundamentally different systems can be interfaced to distribute entanglement.
Faithfully storing an unknown quantum light state is essential to advanced quantum communication and distributed quantum computation applications. The required quantum memory must have high fidelity to improve the performance of a quantum network. Here we report the reversible transfer of photonic polarization states into collective atomic excitation in a compact solid-state device. The quantum memory is based on an atomic frequency comb (AFC) in rare-earth ion-doped crystals. We obtain up to 0.999 process fidelity for the storage and retrieval process of single-photon-level coherent pulse. This reliable quantum memory is a crucial step toward quantum networks based on solid-state devices.