Towards the integration of a PIC-based entangled photon-pair source and an ensemble-based quantum memory compatible with the telecommunication C-band
Gustavo Castro do Amaral, B.P.F. Dirks, Christine Satter, Breno Perlingeiro, D.L. Bakker, N.M. ten Haaf, Jacob Dalle, Ruud Schmits, D.P. van der Vuurst, J.W.M. van Uffelen, Giorgos Mazarakis, A. Sousanis, Timour Javar-Magnier, R. van Duijn, Tim van Duivenbode, C. Esteve Boncompte
Reaching a quantum internet of global scale will require a hybrid architecture of ground and satellite quantum repeater nodes [1]. The functional blocks of a generalized quantum repeater are four: the quantum channel; the Bell State measurement; the source of entanglement; and the quantum memory. Over the last 40 years, the technological achievements towards the development of these functional blocks can be categorized in epochs: the first epoch is characterized by the control of a single quantum system, such as the demonstration of entangled photon-pair emission from a non-linear material [2], or the coherent control of the spin in a colour-centre with an optical interface [3]; the second epoch involves the combined control of two quantum systems based on the same platform, such as the three-node network experiments with Rydberg atoms [4] and colour-centres [5]; the third epoch will usher demonstrations of combined control over different quantum systems. This presentation will detail the third-epoch roadmap towards the integration of an entangled photon-pair source (EPPS) and an ensemble-based quantum memory, both operating within the telecommunication C-band. The EPPS leverages the high quality factor of a micro-ring resonator built on a silicon-oninsulator (SOI) platform to achieve high photon-pair generation rates based on spontaneous four-wave mixing (SFWM). Active control of the pair emission wavelength allows preparing photons that fall within the bandwidth of an atomic frequency comb (AFC) carved into the inhomogeously-broadened absorption line of an Erbium-doped Lithium Niobate crystal. The AFC quantum memory stores input photonic quantum states for a fixed storage time, after which it emits a socalled photon echo, so that non-classical correlations between the stored quantum state and its pair can be evaluated. Techniques to increase the spectral overlap between source and memory, as well as to improve the memory storage efficiency are defined in the roadmap. The demonstration of such an integration step will pave the way towards a quantum repeater architecture that finds applications both in ground and space links.
Reaching a quantum internet of global scale will require a hybrid architecture of ground and satellite quantum repeater nodes [1]. The functional blocks of a generalized quantum repeater are four: the quantum channel; the Bell State measurement; the source of entanglement; and the quantum memory. Over the last 40 years, the technological achievements towards the development of these functional blocks can be categorized in epochs: the first epoch is characterized by the control of a single quantum system, such as the demonstration of entangled photon-pair emission from a non-linear material [2], or the coherent control of the spin in a colour-centre with an optical interface [3]; the second epoch involves the combined control of two quantum systems based on the same platform, such as the three-node network experiments with Rydberg atoms [4] and colour-centres [5]; the third epoch will usher demonstrations of combined control over different quantum systems. This presentation will detail the third-epoch roadmap towards the integration of an entangled photon-pair source (EPPS) and an ensemble-based quantum memory, both operating within the telecommunication C-band. The EPPS leverages the high quality factor of a micro-ring resonator built on a silicon-oninsulator (SOI) platform to achieve high photon-pair generation rates based on spontaneous four-wave mixing (SFWM). Active control of the pair emission wavelength allows preparing photons that fall within the bandwidth of an atomic frequency comb (AFC) carved into the inhomogeously-broadened absorption line of an Erbium-doped Lithium Niobate crystal. The AFC quantum memory stores input photonic quantum states for a fixed storage time, after which it emits a socalled photon echo, so that non-classical correlations between the stored quantum state and its pair can be evaluated. Techniques to increase the spectral overlap between source and memory, as well as to improve the memory storage efficiency are defined in the roadmap. The demonstration of such an integration step will pave the way towards a quantum repeater architecture that finds applications both in ground and space links.
Reaching a quantum internet of global scale will require a hybrid architecture of ground and satellite quantum repeater nodes [1]. The functional blocks of a generalized quantum repeater are four: the quantum channel; the Bell State measurement; the source of entanglement; and the quantum memory. Over the last 40 years, the technological achievements towards the development of these functional blocks can be categorized in epochs: the first epoch is characterized by the control of a single quantum system, such as the demonstration of entangled photon-pair emission from a non-linear material [2], or the coherent control of the spin in a colour-centre with an optical interface [3]; the second epoch involves the combined control of two quantum systems based on the same platform, such as the three-node network experiments with Rydberg atoms [4] and colour-centres [5]; the third epoch will usher demonstrations of combined control over different quantum systems. This presentation will detail the third-epoch roadmap towards the integration of an entangled photon-pair source (EPPS) and an ensemble-based quantum memory, both operating within the telecommunication C-band. The EPPS leverages the high quality factor of a micro-ring resonator built on a silicon-on-insulator (SOI) platform to achieve high photon-pair generation rates based on spontaneous four-wave mixing (SFWM). Active control of the pair emission wavelength allows preparing photons that fall within the bandwidth of an atomic frequency comb (AFC) carved into the inhomogeously-broadened absorption line of an Erbium-doped Lithium Niobate crystal. The AFC quantum memory stores input photonic quantum states for a fixed storage time, after which it emits a so-called photon echo, so that non-classical correlations between the stored quantum state and its pair can be evaluated. Techniques to increase the spectral overlap between source and memory, as well as to improve the memory storage efficiency are defined in the roadmap. The demonstration of such an integration step will pave the way towards a quantum repeater architecture that finds applications both in ground and space links.
As the reach of quantum technologies extends ever further in communication and information science, a reliable way of transferring quantum information between distant locations becomes ever more crucial. While photons are widely accepted as excellent carriers due to their speed and low decoherence, losses of transmission (in free space or fibre) and the impossibility of cloning quantum information still pose a great challenge. The quantum repeater architecture was suggested as a solution to both problems [1]. In a quantum repeater the information encoded in an input state is transferred to a new one through entanglement swapping, that is then sent on along the channel. In this work we present our advances towards the realisation of a quantum repeater. Our system of choice combines a solid-state quantum memory with a source of photon pairs. The memory is based on a Rare-Earth Doped crystal, where quantum information can be stored in Pr3+ ions as a collective excitation using the Atomic Frequency Comb technique. On demand retrieval of the information is realised by transferring the excitation to a long-lived spin state. Record values of storage times and retrieval efficiencies have been demonstrated in this system [2]. Entangled pairs of single photons are generated by parametric down conversion in a periodically poled crystal placed inside a bow-tie cavity. This allows us to generate narrow band photons pairs, where the signal is spectrally matched to the memory (606nm), while the idler is in the telecom band [3]. Such a configuration allows us to benefit from the high performance of the memory, that also allows for temporal [2] and frequency [4] multimodality, while at the same time overcoming the high optical losses of 606nm photons by pair generation of a telecom photon. The first stepping stone, progress towards which is presented in this work, is the successful demonstration of energy-time entanglement between the telecom idler photon and the signal photon, stored as spin-wave excitation. The entanglement of the original pair is maintained by the memory temporal multimodality. The entanglement analysis will be made through time-bin qubit analysers made of a fibre-based Mach-Zehnder interferometer, for the former, and a solid-state equivalent based on two AFC with different storage times, for the latter [5]. In this direction we have already doubled the efficiency of the AFC storage protocols, that will be beneficial to count rates and signal-to-noise ratio. With respect to [2], we also increased the spectral-matching between the source and the memory [4]. Our experiment will provide an increase in storage time of 3 orders of magnitude with respect to previous demonstrations, as well as introducing for the first time on-demand read-out in a highly multi-mode memory. Demonstration of the successful transfer of quantum information between the signal photon and the long-lived solid-state excitation will open the way to the demonstration of long-distance entanglement between individual nodes in a quantum network.
Scalable quantum networks require the efficient generation and storage of entanglement between photonic qubits and quantum memories. Quantum repeaters based on absorptive rare-earth-ion photonic memories offer a promising route toward highly multiplexed quantum networking, but unifying spectrally matched photon sources and quantum memories within a common architecture remains a major challenge. Here we demonstrate an integrated photonic architecture for telecom photon-memory entanglement generation based on dual self-similar silicon carbide microring resonators. Connected by a fiber link, one resonator operates as an entangled photon-pair source, while the other functions as a cavity-enhanced atomic-frequency-comb quantum memory. The memory resonator reaches an ensemble cooperativity of 1.9 after hyperfine initialization and is spectrally matched to the source, enabling storage of entangled telecom photons without spectral modification. We generate and characterize photon-memory entanglement from a quantum interference visibility preserved before and after storage. Harnessing the strong source correlations and the high multimode capacity of the memory, we access high-dimensional entanglement spanning 63 temporal modes, reaching a maximum photon information efficiency of 5.1 Ebits per detected photon and a peak on-chip photon-memory entanglement rate of 5.6 kEbits per second. These results establish a route toward chip-scale quantum networking hardware operating over telecommunications infrastructure.
(English) Future networks for quantum communication will require a new form of technology in order to operate over long distances. The no-cloning theorem precludes the use of amplification, so quantum repeater schemes have been proposed to achieve long-distance quantum communication through the distribution of entanglement. A key device for this setup is a quantum memory, which is required in order to store entanglement and synchronise its distribution. Among the many physical systems than can be used as a quantum memory, rare-earth ion-doped crystals stand out due to their long coherence times and potential for multiplexing and integration. This thesis reports the use of one such system, Pr3+:Y2SiO5, as a quantum memory using the atomic frequency comb (AFC) protocol. Building on previous work, we improved the performance of the memory to be able to demonstrate light-matter entanglement with a telecommunications wavelength photon and the quantum memory, for both a bulk memory and an integrated version. To measure light-matter entanglement, we use photon pairs generated through cavity-enhanced spontaneous parametric down conversion, where one photon (the signal) is stored in the memory and the other (the idler) is at a telecommunications wavelength. These photon pairs exhibit energy-time entanglement, which we analyse in the time basis using fibre- and AFC-based Mach-Zehnder interferometers. In the first experiment, we demonstrate light-matter entanglement where the bulk quantum memory is used with on-demand retrieval via storage in a spin-wave. We measure interference fringes with average visibilities of 89(2)% for AFC storage, and 70(3)% for spin-wave storage, sufficient to demonstrate entanglement. This is the first such demonstration using an on-demand multimode solid-state memory and telecom photon. We show that entanglement is maintained for up to 47.7 µs of storage. These results were made possible through improvements in the AFC efficiency and the noise filtering. Furthermore, we perform on-demand storage and retrieval of up to 30 temporal modes. We take the next step towards implementing our system in a real world scenario by measuring entanglement over a distance. To do so, we send the idler photons through both optical fibre spools and deployed fibre in the metropolitan area of Barcelona, Spain, while the photon detection still occurs in the same lab. The resulting visibilities (80% or higher) demonstrate that the photonic qubit does not decohere during transmission through the optical fibre. We then decouple the photon creation and detection by measuring non-classical correlations between signal photons detected in the lab, and idler photons detected 16 km away (44 km in fibre) in a different location. The final experiment uses a fibre-integrated memory for light-matter entanglement. We use a Pr3+:Y2SiO5 crystal containing a Type I laser-written waveguide, interfaced directly with optical fibre. The fibre-integration allows for improved transmission and AFC efficiency compared to a free-space coupled waveguide memory. To analyse the performance of the memory, we demonstrate non-classical correlations for storage times up to 28 µs. We perform qubit tomography in the time basis for storage times of 3 and 10 µs, measuring two-qubit fidelities of 86(2)% and 86(4)%, respectively, demonstrating storage of entanglement. These experiments demonstrate the potential of our system for use as part of a quantum repeater, with many opportunities for further improvement to the storage time, efficiency, and multiplexing capability.
Photonic links between disparate quantum technologies−such as photon sources, memories, processors, clocks, and sensors−are key to scaling quantum networks and realizing a versatile quantum internet for secure quantum communication, distributed quantum computing, and entanglement-enhanced metrology. In practice, each technology is most suitably implemented on a different quantum platform; the substantial spectral mismatch between them, along with scarce native telecom interfaces, thus poses a major bottleneck to achieving efficient interconnections over long distances. Here we demonstrate the first deployed two-node hybrid network that operates entirely in the telecom C-band. Our approach uses no quantum frequency conversion or external filtering; instead, we develop a neutral atom single photon source and a solid-state rare-earth quantum memory that both operate in previously unexplored telecom regimes with state-of-the-art performance. The source achieves a high single-photon purity at 46 kcps, and the memory a storage efficiency of 10.6% with high multimode capacity. We leverage the intrinsic tunability of both systems to optimize their spectral overlap and demonstrate microsecond-level storage and retrieval with a large time-bandwidth product. Moreover, we showcase real-world networking competencies such as support for multiplexing across 37 temporal modes and preservation of non-classicality over fibers of 10.6 km (metropolitan) and 49.2 km (laboratory). Our work establishes a backbone for telecom-native quantum repeater links and unlocks a path towards high-bandwidth, large-scale quantum networking.
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.