Efficiently scaling quantum networks to long ranges requires local processing nodes to perform basic computation and communication tasks. Trapped ions have demonstrated all the properties required for the construction of such a node, storing quantum information for up to 12 min, implementing deterministic high fidelity logic operations on one and two qubits, and ion–photon coupling. While most ions suitable for quantum computing emit photons in visible to near ultraviolet (UV) frequency ranges poorly suited to long-distance fibre optical based networking, recent experiments in frequency conversion provide a technological solution by shifting the photons to frequencies in the telecom band with lower attenuation for fused silica fibres. Encoding qubits in frequency rather than polarization makes them more robust against decoherence from thermal or mechanical noise due to the conservation of energy. To date, ion–photonic frequency qubit entanglement has not been directly shown. Here we demonstrate a frequency encoding ion–photon entanglement protocol in 171 Yb + with correlations equivalent to 92.4(8)% fidelity using a purpose-built UV hyperfine spectrometer. The same robustness against decoherence precludes our passive optical setup from rotating photonic qubits to unconditionally demonstrate entanglement, however it is sufficient to allow us to benchmark the quality of ion–UV photon correlations prior to frequency conversion to the telecom band.
Efficiently scaling quantum networks to long ranges requires local processing nodes to perform basic computation and communication tasks. Trapped ions have demonstrated all the properties required for the construction of such a node, storing quantum information for up to 12 min, implementing deterministic high fidelity logic operations on one and two qubits, and ion–photon coupling. While most ions suitable for quantum computing emit photons in visible to near ultraviolet (UV) frequency ranges poorly suited to long-distance fibre optical based networking, recent experiments in frequency conversion provide a technological solution by shifting the photons to frequencies in the telecom band with lower attenuation for fused silica fibres. Encoding qubits in frequency rather than polarization makes them more robust against decoherence from thermal or mechanical noise due to the conservation of energy. To date, ion–photonic frequency qubit entanglement has not been directly shown. Here we demonstrate a frequency encoding ion–photon entanglement protocol in 171 Yb + with correlations equivalent to 92.4(8)% fidelity using a purpose-built UV hyperfine spectrometer. The same robustness against decoherence precludes our passive optical setup from rotating photonic qubits to unconditionally demonstrate entanglement, however it is sufficient to allow us to benchmark the quality of ion–UV photon correlations prior to frequency conversion to the telecom band.
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 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.
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.
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.
Trapped ion quantum networking aims at distributing quantum information between distant parties. Entanglement links between distant ions are mediated with photonic qubits, necessitating the need for efficient ion-photon interfaces. This thesis reports on progress towards improving the main limitations of creating a practical quantum network by using diffractive optics for high numerical aperture imaging of trapped ions. The diffractive optics are near-planar, patterned structures which are fabricated to collimate a large fraction of ion uorescence, with low aberration, for high resolution imaging. Patterning the central grounding electrode on a chip trap allows the ion to be within less than 100 microns of the surface. Large scale quantum networks will require a large number of ions and ion-photon interfaces. To scale the ion trap system, microfabricated surface traps can be used allowing for the opportunity to etch diffractive mirrors directly onto the trap surface. Collecting ion uorescence with the diffractive mirror, we show the highest ion-single mode optical fibre coupling efficiency, which enhances the entanglement rate of a quantum network. We then build a high resolution UV spectrometer to distinguish between frequency entangled photonic qubits. Frequency qubits offer more robust encoding when compared with polarisation qubits, as a result of energy conservation. Using the spectrometer we demonstrate correlations between the ion's internal state and the frequency state of an entangled photonic qubit. Quantum networks will require local quantum information processing using ion-ion gates to distribute entanglement over the entire network. Typically, the speed of these gates is presently limited to ~10 µs by the secular trap frequency. We demonstrate the first steps towards a fast ion-ion gate, not limited by the secular frequency, based on strong optical forces generated by the coherent, resonant absorption and stimulated emission of light from an ultrafast, pulsed laser. A full-scale quantum network will require many ion trap installations each with their own laser systems. For this challenge we demonstrate simple, robust, and cost-effective methods to lock the 399 nm photoionisation laser, based on polarisation spectroscopy through a hollow cathode lamp, and an opto-mechanical switch which allows locking of multiple lasers to a wavelength meter. Another scalable diffractive optic used is a phase Fresnel lens integrated into a 3D quadrupole ion trap system. The limits of imaging with this lens are tested by demonstrating force sensing at a sub-attoNewton level with super-resolution imaging techniques to find the ion location.