Time-bin encoding of photons offers a robust kind of long-distance quantum communication over lossy channels. The diversity of material nodes in quantum networks operating at natural frequencies requires coherent frequency and wave-form conversion of information-carrying photons to provide an efficient quantum interface with optical fibers and various quantum memories, which has been extensively studied. However, the quantum frequency conversion with transfer of time-bin encoding between single photons of different wavelengths has not yet been explored. In this paper, we present a method for efficiently exchanging time-bin encoding in the conversion process between two photons propagating in cold tripod atoms driven by a strong laser field. The latter is designed to slow down photons and suppress their absorption due to electromagnetically induced transparency. Photons interact parametrically through modified atomic coherence, which is utilized also to achieve equal group velocities of photons. We demonstrate the ability of our model to generate entanglement between distant atoms that equally share the original quantum information stored in the ground-state polarization qubits of both atoms. The proposed method for frequency conversion based on modified atomic coherence is promising because it can simplify the implementation of reliable and high-speed quantum communication protocols based on single-photon entanglement.
Time-bin encoding of photons offers a robust kind of long-distance quantum communication over lossy channels. The diversity of material nodes in quantum networks operating at natural frequencies requires coherent frequency and wave-form conversion of information-carrying photons to provide an efficient quantum interface with optical fibers and various quantum memories, which has been extensively studied. However, the quantum frequency conversion with transfer of time-bin encoding between single photons of different wavelengths has not yet been explored. In this paper, we present a method for efficiently exchanging time-bin encoding in the conversion process between two photons propagating in cold tripod atoms driven by a strong laser field. The latter is designed to slow down photons and suppress their absorption due to electromagnetically induced transparency. Photons interact parametrically through modified atomic coherence, which is utilized also to achieve equal group velocities of photons. We demonstrate the ability of our model to generate entanglement between distant atoms that equally share the original quantum information stored in the ground-state polarization qubits of both atoms. The proposed method for frequency conversion based on modified atomic coherence is promising because it can simplify the implementation of reliable and high-speed quantum communication protocols based on single-photon entanglement.
A new method of preparing a single photon in temporally delocalized entangled modes is proposed and analyzed. We show that in a medium of four-level atoms with tripod-level configuration a strong parametric interaction between two one-photon pulses emerges, if the medium was initially prepared in a coherent superposition of the two ground states, while a strong classical field drives the neighbor transition from the third ground state. The analytical solution of Maxwell equations reveals that under the electromagnetically induced transparency (EIT), the quantum fields propagate with significantly small absorption and with different group velocities that leads to their temporal split into well-separated pulses, the amplitudes of which are well controlled by the driving field. The proposed scheme can serve as a robust source of narrow-band one-photon qubits with an entanglement between two temporal modes, not sensitive to losses in atmosphere and in telecommunication waveguides. We show also that in our scheme a controllable degree of temporal entanglement can be achieved.
A key enabling feature of future quantum networks is interoperability between platforms that operate at different wavelengths and with different qubit encodings. We demonstrate an interface that converts atom-photon entanglement from polarization encoding at an atomic wavelength to time-bin encoding in the telecom C-band. Atom-entangled photons at 854 nm are generated from a single 40Ca+ ion. After quantum frequency conversion to 1550 nm, the photonic polarization qubit is converted into a time-bin qubit using a fiber-based Mach--Zehnder-like encoder. Full quantum tomography of the final state verifies that the process preserves entanglement with 96.3(4.2)% fidelity. Together with the independent work of Ferrari et al. [arXiv:2607.07805 (2026)], this is the first demonstration of polarization-to-time-bin conversion of photons entangled with a single atomic quantum memory. The telecom-compatible interface enables robust qubit transmission over optical fibers and provides a key building block for heterogeneous quantum networking architectures.
We propose a method that enables efficient frequency conversion of quantum information based on recently demonstrated strong parametric coupling between two single-photon pulses propagating in a slow-light atomic medium at different group velocities. We show that an incoming single-photon state is efficiently converted into another optical mode in a lossless and shape-conserving manner. The persistence of initial quantum coherence and entanglement within frequency conversion is also demonstrated. We first illustrate this result for the case of small frequency difference of converted photons, and then discuss the modified scheme for conversion of photon wavelengths in different spectral ranges. Finally we analyze the generation of a narrow-band single-photon frequency-entangled state.
The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\ifmmode\pm\else\textpm\fi{}0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.
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.
Hong-Ou-Mandel (HOM) interference with two independent photons of different wavelengths is a primary tool for controlled transfer of quantum information across a hybrid quantum network such as the quantum internet, in which the diverse material nodes operate at different resonant frequencies. The key challenge of realizing two-photon interference in the frequency domain is the need for linear frequency conversion between spectrally distinct photons, which, in contrast to frequency mixing in nonlinear optical processes, is free from multiphoton noise arising in strong laser fields. Here, we present a linear HOM scheme based on a lossless three-wave mixing between two photons of different frequencies in the laser-controlled tripod-type atoms, where the parametric interaction between photons is triggered by the induced coherence between two metastable states of the atoms [S. Petrosyan et al., Phys. Rev. A 105, 052606 (2022)]. We give the general derivation of the HOM effect and examine it for two single-pulse photons and in the important case of time-bin encoded single photons, which are well protected from decoherence in optical fibers. We show that two-photon coincidence measurements display the HOM-type ``dip and peak'' fringes according to the number of time bins in the incident photon state, allowing full recovery of the original quantum information. An essential feature of our scheme is the neutrality of the HOM effect implementation to the polarization indistinguishability of photons, which makes our model flexible for use in hybrid quantum networks.