Quantum key distribution systems represent a proven method to obtain fundamentally secure communication channels. However, the loss of entangled photons which create the encryption key limit the distance over which the systems are useful. Quantum repeaters have been used to increase the distance over which entangled photons can be transmitted. The most successful quantum repeaters entangle information between trapped ions and single photons and through the use of entanglement swapping protocols, relay the encryption keys over large distances. A requirement of these quantum repeaters is the ability to work in the telecom wavelengths (1310 nm or 1550 nm), where standard silica fibers exhibit low loss. With the majority of ions emitting in the ultraviolet and visible spectrum, and ytterbium in particular emitting at 369.5 nm, a conversion process is then needed that can translate the photons from 369.5 nm to the telecom band while preserving the entangled state. The experiment presented here details the first stage of a nonlinear frequency conversion setup that uses strong pump light to enable the conversion of 369.5 nm photons to 708 nm. The efficiency of the conversion process is analyzed to determine the optimum system parameters, with an initial single pass conversion efficiency of 0.20% achieved. Noise introduced during the nonlinear conversion process is examined to determine the effect it presents on the communication channel.
Quantum key distribution systems represent a proven method to obtain fundamentally secure communication channels. However, the loss of entangled photons which create the encryption key limit the distance over which the systems are useful. Quantum repeaters have been used to increase the distance over which entangled photons can be transmitted. The most successful quantum repeaters entangle information between trapped ions and single photons and through the use of entanglement swapping protocols, relay the encryption keys over large distances. A requirement of these quantum repeaters is the ability to work in the telecom wavelengths (1310 nm or 1550 nm), where standard silica fibers exhibit low loss. With the majority of ions emitting in the ultraviolet and visible spectrum, and ytterbium in particular emitting at 369.5 nm, a conversion process is then needed that can translate the photons from 369.5 nm to the telecom band while preserving the entangled state. The experiment presented here details the first stage of a nonlinear frequency conversion setup that uses strong pump light to enable the conversion of 369.5 nm photons to 708 nm. The efficiency of the conversion process is analyzed to determine the optimum system parameters, with an initial single pass conversion efficiency of 0.20% achieved. Noise introduced during the nonlinear conversion process is examined to determine the effect it presents on the communication channel.
Generating entangled states of quantum memories over a long distance is an essential component for scalable quantum communication and its application such as quantum key distribution. Generation of entangled ion pairs mediated by photonic qubits has been demonstrated with trapped ytterbium (Yb) ions [1] and is a promising approach. However the wavelength of a single photon emitted by most trapped ions is in the ultraviolet (UV) or short-wavelength visible range where the maximum distribution distance of the photons is severely limited due to high absorption coefficient in a single mode optical fiber. To overcome this limitation, we are currently developing a frequency down-conversion scheme [2,3] which can coherently transfer a qubit stored in a 370 nm photon emitted by a Yb ion to another photon at 1310 nm which coincides with the low loss window of a telecom fiber.
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.
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.
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.