In this work we experimentally and theoretically characterize the converted signal and the noise level of a quantum frequency conversion (QFC) device implemented for frequency down conversion from the visible to telecommunication wavelengths. This frequency converter is a useful device able to interface quantum memories operating at different wavelengths. We observe that the main noise source is directly induced by the pump via spontaneous parametric down-conversion (SPDC), enhanced by imperfections in the periodic poling structure of the waveguide. We also measure conversion with high signal to noise ratio for microsecond long weak coherent states at the single photon level. Finally, we show theoretically that the converted Stokes/Anti-Stokes non-classical correlations of photon pairs emitted by quantum memories could be largely preserved for pulse durations lower than 4µs.
In this work we experimentally and theoretically characterize the converted signal and the noise level of a quantum frequency conversion (QFC) device implemented for frequency down conversion from the visible to telecommunication wavelengths. This frequency converter is a useful device able to interface quantum memories operating at different wavelengths. We observe that the main noise source is directly induced by the pump via spontaneous parametric down-conversion (SPDC), enhanced by imperfections in the periodic poling structure of the waveguide. We also measure conversion with high signal to noise ratio for microsecond long weak coherent states at the single photon level. Finally, we show theoretically that the converted Stokes/Anti-Stokes non-classical correlations of photon pairs emitted by quantum memories could be largely preserved for pulse durations lower than 4µs.
We report on quantum frequency conversion of memory-compatible narrow-bandwidth photons at 606 nm to the telecom C-band at 1552nm. The 200ns long photons, compatible with Praseodymium-based solid-state quantum memories are frequency converted using a single-step difference frequency generation process in a periodically poled Lithium Niobate waveguide. We characterize the noise processes involved in the conversion and by applying strong spectral filtering of the noise, we demonstrate high signal-to-noise ratio conversion at the single photon level (SNR$\,>\,100forameaninputphotonnumberperpulseof1).Wefinallyobservethatamemorycompatibleheraldedsinglephotonwithabandwidthof1.8\,$MHz, obtained from a spontaneous parametric down conversion pair source, still shows a strong non-classical behavior after conversion. We first demonstrate that correlations between heralding and converted heralded photons stay in the non-classical regime. Moreover, we measured the heralded autocorrelation function of the heralded photon using the converter device as a frequency-domain beam splitter, yielding a value of 0.19±0.07. The presented work represents a step towards the connection of several quantum memory systems emitting narrow-band visible photons to the telecommunication wavelengths.
We report an experiment demonstrating quantum frequency conversion of weak light pulses compatible with atomic quantum memories to telecommunication wavelengths. We use a PPLN nonlinear waveguide to convert weak coherent states at the single photon level with a duration of 30 ns from a wavelength of 780 nm to 1552 nm. We measure a maximal waveguide internal (external) conversion efficiency η(int) = 0.41 (η(ext) = 0.25), and we show that the signal to noise ratio (SNR) is good enough to reduce the input photon number below 1. In addition, we show that the noise generated by the pump beam in the crystal is proportional to the spectral bandwidth of the device, suggesting that narrower filtering could significantly increase the SNR. Finally, we demonstrate that the quantum frequency converter can operate in the quantum regime by converting a time-bin qubit and measuring the qubit fidelity after conversion.
Quantum frequency conversion is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here, we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single-photon level compatible with Pr3+:Y2SiO5 quantum memories, with a high signal-to-noise ratio (SNR). We use a single-step difference-frequency-generation process with a continuous-wave pump at 994 nm in a periodically poled lithium niobate (MgO:ppLN) waveguide and ultranarrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 \textmu{}s with a device efficiency of about 25 % and an SNR >460 for 10-$\text{\ensuremath{\mu}}\mathrm{s}$-long pulses containing one photon on average. This SNR is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on Pr3+:Y2SiO5 and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.
In this paper, we present a novel single stage quantum frequency conversion (QFC) scheme based on an enhancement cavity, which allows for high conversion efficiencies with record low noise levels, 100 times smaller than in previous systems and close to the thermal background. Our approach represents a significant improvement over traditional QFC methods that rely on periodically poled waveguide crystals, which typically produce high levels of noise with input wavelengths in the visible spectrum. One of the key challenges in QFC is the generation of noise photons - photons that are not part of the desired output state. Those noise photons are one of the major limitations of today’s quantum network experiments. It has been widely believed that single stage QFC of qubit photons in the visible range would result in too many noise photons in the telecom band, because of SPDC induced by the short wavelength driver laser. However, using the enhancement cavity design, we are able to overcome this challenge and demonstrate high-fidelity QFC from the visible to telecom band. Our results have important implications for the development of practical QFC devices for a range of applications, including quantum communication and quantum networks, where low noise levels are critical.
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.