Stephan Kucera

Telecom-heralded single-photon absorption by a single atom

We present, characterize, and apply the architecture of a photonic quantum interface between the near infrared and telecom spectral regions. A singly resonant optical parametric oscillator (OPO) operated below threshold, in combination with external filters, generates high-rate (

High-fidelity entanglement between a trapped ion and a telecom photon via quantum frequency conversion

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Doubly heralded single-photon absorption by a single atom

) narrowband photon pairs (

Single telecom photon heralding by wavelength multiplexing in an optical fiber

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Polarization-entangled photon pairs from a cavity-enhanced down-conversion source in Sagnac configuration

MHz bandwidth); the signal photons are tuned to resonance with an atomic transition in

Quantum Frequency Down-Conversion of Ca+-resonant Polarization-Entangled Photons to the Telecom O-Band

{\mathrm{Ca}}^{+}

Frequency conversion of narrowband single photons from a SPDC pair source

, while the idler photons are at telecom wavelength. Interface operation is demonstrated through high-rate absorption of single photons by a single trapped ion (

Quantum State Teleportation from a Single Ion to a Single Photon by Heralded Absorption

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Quantum frequency down-conversion of 40 Ca + -resonant polarization-entangled photons to the telecom O-band

), heralded by coincident telecom photons.

Photon-photon to atom-photon entanglement transfer

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.Entanglement between photons and stationary quantum nodes is a fundamental resource for quantum communication, but typical transition wavelengths are far from the telecom band. Here, the authors deal with the problem using polarisation-independent, entanglement-preserving frequency conversion.