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arXiv:1801.01193 · Physical Review Applied

Neutral-Atom Wavelength-Compatible 780 nm Single Photons from a Trapped Ion via Quantum Frequency Conversion

James Siverns, John Hannegan, Qudsia Sara Quraishi

Published Jan 23, 2019• 27 citations
arXivHTMLPDF

Abstract

Photonic interactions between different types of quantum memories will be crucial in the implementation of long-range hybrid quantum networks. Unfortunately, such interactions are typically impossible, due to the differing resonance frequencies of different systems. To overcome this hurdle, the authors collect photons at 493 nm from trapped ${}^{138}$Ba${}^{+}$ and convert them to 780 nm, a wavelength resonant with ${}^{87}$Rb, while preserving the quantum statistics of the photons after frequency conversion. This result increases the networking range of ions and enables hybrid networking experiments between trapped ions and neutral atoms in quantum information processing.

DOI
10.1103/physrevapplied.11.014044

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Photonic interactions between different types of quantum memories will be crucial in the implementation of long-range hybrid quantum networks. Unfortunately, such interactions are typically impossible, due to the differing resonance frequencies of different systems. To overcome this hurdle, the authors collect photons at 493 nm from trapped 138{}^{138}138Ba+{}^{+}+ and convert them to 780 nm, a wavelength resonant with 87{}^{87}87Rb, while preserving the quantum statistics of the photons after frequency conversion. This result increases the networking range of ions and enables hybrid networking experiments between trapped ions and neutral atoms in quantum information processing.
Future quantum networks will require the ability to produce matter-photon entanglement at photon frequencies not naturally emitted from the matter qubit. This allows for a hybrid network architecture, where these photons can couple to other tools and quantum technologies useful for tasks such as multiplexing, routing, and storage, but which operate at wavelengths different from that of the matter qubit source, while also reducing network losses. Here, we demonstrate entanglement between a trapped ion and a 780-nm photon, a wavelength that can interact with neutral-Rb-based quantum networking devices. A single barium ion is used to produce 493-nm photons, entangled with the ion, which are then frequency converted to 780 nm while preserving the entanglement. We generate ion-photon entanglement with fidelities $\ensuremath{\ge}0.93$(2) and $\ensuremath{\ge}0.84$(2) for 493-nm and 780-nm photons respectively with the fidelity drop arising predominantly from a reduction in the signal-noise of our detectors at 780 nm compared with at 493 nm.
Trapped ions are excellent candidates for quantum nodes, as they possess many desirable features of a network node including long lifetimes, on-site processing capability, and production of photonic flying qubits. However, unlike classical networks in which data may be transmitted in optical fibers and where the range of communication is readily extended with amplifiers, quantum systems often emit photons that have a limited propagation range in optical fibers and, by virtue of the nature of a quantum state, cannot be noiselessly amplified. Here, we first describe a method to extract flying qubits from a Ba+ trapped ion via shelving to a long-lived, low-lying D-state with higher entanglement probabilities compared with current strong and weak excitation methods. We show a projected fidelity of ≈89% of the ion–photon entanglement. We compare several methods of ion–photon entanglement generation, and we show how the fidelity and entanglement probability varies as a function of the photon collection optic’s numerical aperture. We then outline an approach for quantum frequency conversion of the photons emitted by the Ba+ ion to the telecommunication range for long-distance networking and to 780 nm for potential entanglement with rubidium-based quantum memories. Our approach is significant for extending the range of quantum networks and for the development of hybrid quantum networks compromised of different types of quantum memories.
The ability to control the optical frequency of quantum state carriers (i.e. photons) is an important functionality for future quantum networks. It allows all matter quantum systems - nodes of the network - to be compatible with the telecommunication C-band, therefore enabling long distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, thus resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents. Quantum memories are one of the building blocks of a quantum network, enabling the storage of quantum states of light and the entanglement distribution over long distances. In our group, two different types of memories are investigated: a cold atomic ensemble and an ion-doped crystal. In this thesis I investigate the quantum frequency conversion of narrow-band photons, emitted or absorbed by optical quantum memories, with two different objectives: the first one is to connect quantum memories emitting or absorbing visible single photons with the telecommunication wavelengths, where fiber transmission loss is minimum. The second and main goal is to study the compatibility between disparate quantum nodes, emitting or absorbing photons at different wavelengths. More precisely the objective is to achieve a quantum connection between the two optical memories studied using quantum frequency conversion techniques. The main core of this work is the quantum frequency conversion interface that bridges the gap between the cold ensemble of Rubidium atoms, emitting photons at 780nm, and the Praseodymium ion doped crystal, absorbing photons at 606nm. This interface is composed of two different frequency conversion devices, where a cascaded conversions takes place: the first one converts 780nm photons to the telecommunication C-band, and the second one converts them back to visible, at 606nm. This comes with several challenges such as conversion efficiency, phase stability and parasitic noise reduction, which are important considerations to show the conservation of quantum behaviors through the conversion process. This work can be divided in three parts. In a first one, we built a quantum frequency conversion interface between 606nm and the C-band wavelength, capable of both up and down-conversion of single photon level light. We also characterized the noise processes involved in this specific conversion. In the down-conversion case we showed that memory compatible heralded single photons emitted from a photon pair source preserve their non-classical properties through the conversion process. In the up-conversion case, we showed the storage of converted telecom photons in the praseodymium doped crystal, and their retrieval with high signal to noise ratio. The second part of the work was devoted to the conversion of photons from an emissive Rubidium atomic quantum memory to the telecom C band. In this work we converted the heralding photons from the atomic ensemble and measured non-classical correlations between a stored excitation and a C-band photon, necessary for quantum repeater applications. In the last part of the thesis, we setup the full frequency conversion interface and showed that heralded photons emitted by the atomic ensemble are converted, stored in the solid state memory and retrieved with high signal to noise ratio. We demonstrated that a single collective excitation stored in the atomic ensemble is transfered to the crystal by mean of a single photon at telecom wavelength. We also showed time-bin qubit transfer between the two quantum memories. This work represents the first proof of principle of a photonic quantum connection between disparate quantum memory nodes. The results presented in this thesis pave the way towards the realization of modular and hybrid quantum networks.
Long-range quantum information processing will require the integration of different technologies to form hybrid architectures combining the strengths of multiple quantum systems. In this work, we propose a hybrid networking architecture designed to improve entanglement rates in quantum networks based on trapped ions. Trapped ions are excellent candidates as network nodes, but photon losses make long-distance networking difficult. To overcome some losses and extend the range of trapped-ion-based networks, we propose including neutral-atom-based nondestructive single-photon detection and single-photon storage in between networking nodes, forming a hybrid network. This work builds on recently demonstrated optical frequency conversion of single photons emitted by trapped ions. We derive the average two-node entanglement rate for this proposed network architecture as a function of distance. Using reasonable experimental parameters, we show that this proposed quantum network can generate remote entanglement rates up to a factor of 100 larger than that of an equivalent homogeneous network at both near-IR and C-band wavelengths for distances up to 50 km.
Trapped atomic ions are ideal single photon emitters with long-lived internal states which can be entangled with emitted photons. Coupling the ion to an optical cavity enables the efficient emission of single photons into a single spatial mode and grants control over their temporal shape. These features are key for quantum information processing and quantum communication. However, the photons emitted by these systems are unsuitable for long-distance transmission due to their wavelengths. Here we report the transmission of single photons from a single ^{40}Ca^{+} ion coupled to an optical cavity over a 10 km optical fiber via frequency conversion from 866 nm to the telecom C band at 1530 nm. We observe nonclassical photon statistics of the direct cavity emission, the converted photons, and the 10 km transmitted photons, as well as the preservation of the photons' temporal shape throughout. This telecommunication-ready system can be a key component for long-distance quantum communication as well as future cloud quantum computation.
Ion trap system is a leading candidate for quantum information science benefitting from its long coherence time, high-fidelity gate operations. In addition, the ion photon entanglement provides a versatile tool to realize quantum networks by generating an ideal pair of a stationary memory qubit and a flying communication qubit. Rapid developments in nonlinear quantum frequency conversion techniques have enhanced the potential for constructing a trapped ion quantum network via optical fiber connections. The generation of long-distance entanglement has been demonstrated with ions such as Ca+ and Ba+, which emit photons in visible or near-infrared range naturally. On the other hand, as the qubit-native photons reside in ultra-violet (UV) spectrum, the Yb+ ion has not been considered as a strong competitor for telecommunication qubits despite extensive research on it. Here, we demonstrate an efficient difference-frequency conversion of UV photons, emitted from a trapped Yb+ ion, into a visible range. We provide experimental evidence that confirms the converted photons are radiated from the Yb+ ion. Our results provide a crucial step toward realizing a long-distance trapped ion quantum network based on Yb+ ions through quantum frequency conversion.