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arXiv:2207.13680 · Physical Review A

Entanglement between a trapped-ion qubit and a 780-nm photon via quantum frequency conversion

John Hannegan, James Siverns, Qudsia Sara Quraishi

Published Oct 26, 2022• 18 citations
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Abstract

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.

DOI
10.1103/physreva.106.042441

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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≥0.93\ensuremath{\ge}0.93\ensuremath≥0.93(2) and \ensuremath≥0.84\ensuremath{\ge}0.84\ensuremath≥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.
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.
Trapped ions are one of the leading candidates for scalable and long-distance quantum networks because of their long qubit coherence time, high-fidelity single- and two-qubit gates, and their ability to generate photons entangled with the qubit state of the ion. One method for creating ion-photon entanglement is to exploit optical transitions from the 6 P 1/2 to 6 S 1/2 levels, which naturally emit spin-photon entangled states. But these optical transitions typically lie in the ultraviolet and visible wavelength regions of the spectrum. These wavelengths exhibit significant fiber-optic propagation loss, thereby limiting the transfer of quantum information to tens of meters. Quantum frequency conversion is essential to convert these photons to telecom wavelengths so that they can propagate over long distances in fiber-based networks, as well as for compatibility with the vast number of telecom-based optoelectronic components. Here, we generate O-band telecom photons via a low-noise quantum frequency conversion scheme from photons emitted from the 6 P 1/2 to 6 S 1/2 dipole transition of a trapped barium ion. We use a two-stage quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz between the input visible photon and the output telecom photon, achieving a conversion efficiency of 11%. We attain a signal-to-background ratio of over 100 for the converted O-band telecom photon with background noise of less than 15 counts/s. These results are an important step toward achieving trapped ion quantum networks over long distances for distributed quantum computing and quantum communication.
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.
Abstract Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. It has been demonstrated for trapped ions, trapped atoms, color centers in diamond, or quantum dots. These systems have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss, low-dispersion telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion. Here we use an efficient polarization-preserving frequency converter connecting 854 nm to the telecom O-band at 1310 nm to demonstrate entanglement between a trapped 40Ca+ ion and the polarization state of a telecom photon with a high fidelity of 98.2 ± 0.2%. The unique combination of 99.75 ± 0.18% process fidelity in the polarization-state conversion, 26.5% external frequency conversion efficiency and only 11.4 photons/s conversion-induced unconditional background makes the converter a powerful ion–telecom quantum interface.
Abstract Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. It has been demonstrated for trapped ions, trapped atoms, color centers in diamond, or quantum dots. These systems have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss, low-dispersion telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion. Here we use an efficient polarization-preserving frequency converter connecting 854 nm to the telecom O-band at 1310 nm to demonstrate entanglement between a trapped 40Ca+ ion and the polarization state of a telecom photon with a high fidelity of 98.2 ± 0.2%. The unique combination of 99.75 ± 0.18% process fidelity in the polarization-state conversion, 26.5% external frequency conversion efficiency and only 11.4 photons/s conversion-induced unconditional background makes the converter a powerful ion–telecom quantum interface.
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.