Quantum network consisting of separate nodes processing information and linked by quantum channels is a promising and widely investigated general architecture for scalable and distributed quantum computing [1]. The most feasible proposals for realisation of such a network involve matter-based nodes (e.g. trapped atoms or ions or solid-state systems) communicating by photons [2]. This approach combines advantages of long-living and precisely manipulated systems for nodes with fast and convenient for long distance information transfer ones. Implementation of such a scheme opens up a lot of research challenges where establishing a reliable channel for quantum state transfer between two nodes by photons is essential, once the nodes are operating. It would by particularly convenient to have a standard channel that can different systems be talking to and the telecom band photons are good candidates for that because of well-established optical fiber infrastructure. In our work we implement a single photon frequency conversion to telecom band so that quantum systems with different emitting frequencies could be used in such a network. This approach also permits to minimize the losses and increase distances to 10s-kilometre range by exploiting the lowest attenuation band in fibers. The work is aimed in particular on developing device capable of converting photons produced by P3/2-D5/2transition in40Ca+ ion that corresponds to 854.2 nm wavelength as a promising candidate for the network nod.
Quantum network consisting of separate nodes processing information and linked by quantum channels is a promising and widely investigated general architecture for scalable and distributed quantum computing [1]. The most feasible proposals for realisation of such a network involve matter-based nodes (e.g. trapped atoms or ions or solid-state systems) communicating by photons [2]. This approach combines advantages of long-living and precisely manipulated systems for nodes with fast and convenient for long distance information transfer ones. Implementation of such a scheme opens up a lot of research challenges where establishing a reliable channel for quantum state transfer between two nodes by photons is essential, once the nodes are operating. It would by particularly convenient to have a standard channel that can different systems be talking to and the telecom band photons are good candidates for that because of well-established optical fiber infrastructure. In our work we implement a single photon frequency conversion to telecom band so that quantum systems with different emitting frequencies could be used in such a network. This approach also permits to minimize the losses and increase distances to 10s-kilometre range by exploiting the lowest attenuation band in fibers. The work is aimed in particular on developing device capable of converting photons produced by P3/2-D5/2transition in40Ca+ ion that corresponds to 854.2 nm wavelength as a promising candidate for the network nod.
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.
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.
Entanglement distribution is the overarching purpose of quantum networks. While communication over long distances can use deployed fiber infrastructure, it requires photons in the telecom band. However, advanced quantum network nodes do not operate at such wavelengths. Here we overcome this limitation with two tailor-made low-noise quantum-frequency converters to distribute entanglement between a single atom and a resonant photon over 14km line-of-sight via 24km of a deployed commercial fiber. The photon at wavelength 780nm is first entangled with the atom, then converted to the telecom S-band, and finally back-converted after propagation through the fiber. This link enables a photon transfer efficiency of 1.7% while affecting the atom-photon entanglement fidelity by less than 1%. This brings integration of atomic quantum nodes with existing long-distance fiber networks into reach, enabling novel applications 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.
A quantum internet connects remote quantum processors that need to interact and exchange quantum signals over a long distance through photonic channels. However, these quantum nodes operate at the wavelength ranges unsuitable for long-distance transmission. Therefore, quantum wavelength conversion to telecom bands is crucial for long-distance quantum networks based on optical fiber. Here, we propose wavelength conversion devices for single-photon polarization qubits using continuous-variable quantum teleportation that can efficiently convert qubits between near-infrared (780--795 nm suitable for interacting with atomic quantum nodes) and telecom wavelength (1300--1500 nm suitable for long-distance transmission). The teleportation uses entangled photon fields (i.e., nondegenerate two-mode squeezed state) that can be generated by four-wave mixing in a rubidium atomic gas using a diamond configuration of atomic transitions. The entangled fields can be emitted in two orthogonal polarizations with locked relative phase, making them especially suitable for interfacing with single-photon polarization qubits. Our work may pave the way for the realization of long-distance quantum networks.
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.