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Entanglement and nonlocality between disparate solid-state quantum memories mediated by photons

Marcel.li Grimau Puigibert, Mohsen Falamarzi Askarani, Jacob H. Davidson, Varun Boehm Verma, Matthew D. Shaw, Sae Woo Nam, Thomas A. Lutz, Gustavo Castro do Amaral, Daniel Oblak, Wolfgang Tittel

Published Jan 13, 2020• 28 citations
Publisher

Abstract

Entangling quantum systems with different characteristics through the exchange of photons is a prerequisite for building future quantum networks. Proving the presence of entanglement between quantum memories for light working at different wavelengths furthers this goal. Here, we report on a series of experiments with a thulium-doped crystal, serving as a quantum memory for 794-nm photons, an erbium-doped fiber, serving as a quantum memory for telecommunication-wavelength photons at 1535 nm, and a source of photon pairs created via spontaneous parametric down-conversion. Characterizing the photons after re-emission from the two memories, we find nonclassical correlations with a cross-correlation coefficient of g(12)((2)) = 53 +/- 8; entanglement preserving storage with input-output fidelity of F-IO approximate to 93 +/- 2%; and nonlocality featuring a violation of the Clauser-Horne-Shimony-Holt Bell inequality with S = 2.6 +/- 0.2. Our proof-of-principle experiment shows that entanglement persists while propagating through different solid-state quantum memories operating at different wavelengths.

DOI
10.3929/ethz-b-000467900

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Entangling quantum systems with different characteristics through the exchange of photons is a prerequisite for building future quantum networks. Proving the presence of entanglement between quantum memories for light working at different wavelengths furthers this goal. Here, we report on a series of experiments with a thulium-doped crystal, serving as a quantum memory for 794-nm photons, an erbium-doped fiber, serving as a quantum memory for telecommunication-wavelength photons at 1535 nm, and a source of photon pairs created via spontaneous parametric down-conversion. Characterizing the photons after re-emission from the two memories, we find nonclassical correlations with a cross-correlation coefficient of g(12)((2)) = 53 +/- 8; entanglement preserving storage with input-output fidelity of F-IO approximate to 93 +/- 2%; and nonlocality featuring a violation of the Clauser-Horne-Shimony-Holt Bell inequality with S = 2.6 +/- 0.2. Our proof-of-principle experiment shows that entanglement persists while propagating through different solid-state quantum memories operating at different wavelengths.
Entanglement between photons at telecommunication wavelengths and long-lived quantum memories is one of the fundamental requirements of long-distance quantum communication. Quantum memories featuring on-demand readout and multimode operation are additional precious assets that will benefit the communication rate. In this Letter, we report the first demonstration of entanglement between a telecom photon and a collective spin excitation in a multimode solid-state quantum memory. Photon pairs are generated through widely nondegenerate parametric down-conversion, featuring energy-time entanglement between the telecom-wavelength idler and a visible signal photon. The latter is stored in a Pr^{3+}:Y_{2}SiO_{5} crystal as a spin wave using the full atomic frequency comb scheme. We then recall the stored signal photon and analyze the entanglement using the Franson scheme. We measure conditional fidelities of 92(2)% for excited-state storage, enough to violate a Clauser-Horne-Shimony-Holt inequality, and 77(2)% for spin-wave storage. Taking advantage of the on-demand readout from the spin state, we extend the entanglement storage in the quantum memory for up to 47.7 μs, which could allow for the distribution of entanglement between quantum nodes separated by distances of up to 10 km.
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.
We report on the transmission of telecom photons entangled with a multimode solid-state quantum memory over a deployed optical fiber in a metropolitan area. Photon pairs were generated through spontaneous parametric downconversion, with one photon stored in a rare-earth-based quantum memory, and the other, at telecommunication wavelengths, traveling through increasing distances of optical fiber, first in the laboratory and then outside in a deployed fiber loop. We measured highly non-classical correlations between the stored and the telecom photons for storage times up to 25 µs and for a fiber separation up to 50 km. We also report light–matter entanglement with a two-qubit fidelity up to 88%, which remains constant within error bars for all fiber lengths, showing that the telecom qubit does not suffer decoherence during the transmission. Finally, we moved the detection stage of the telecom photons to a different location placed 16 km away, and confirmed the non-classical correlations between the two photons. Our system was adapted to provide the transmission of precise detection times and synchronization signals over long quantum communication channels, providing the first steps for a future quantum network involving quantum memories and non-classical states.
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Two photons can simultaneously share entanglement between several degrees of freedom such as polarization, energy-time, spatial mode and orbital angular momentum. This resource is known as hyperentanglement, and it has been shown to be an important tool for optical quantum information processing. Here we demonstrate the quantum storage and retrieval of photonic hyperentanglement in a solid-state quantum memory. A pair of photons entangled in polarization and energy-time is generated such that one photon is stored in the quantum memory, while the other photon has a telecommunication wavelength suitable for transmission in optical fibre. We measured violations of a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality for each degree of freedom, independently of the other one, which proves the successful storage and retrieval of the two bits of entanglement shared by the photons. Our scheme is compatible with long-distance quantum communication in optical fibre, and is in particular suitable for linear-optical entanglement purification for quantum repeaters.
Abstract We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser–Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring high-visibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.