Telecom-heralded entanglement between multimode solid-state quantum memories

Creation of memory-memory entanglement in a metropolitan quantum network

Towards realizing the future quantum internet1,2, a pivotal milestone entails the transition from two-node proof-of-principle experiments conducted in laboratories to comprehensive multi-node set-ups on large scales. Here we report the creation of memory-memory entanglement in a multi-node quantum network over a metropolitan area. We use three independent memory nodes, each of which is equipped with an atomic ensemble quantum memory3 that has telecom conversion, together with a photonic server where detection of a single photon heralds the success of entanglement generation. The memory nodes are maximally separated apart for 12.5 kilometres. We actively stabilize the phase variance owing to fibre links and control lasers. We demonstrate concurrent entanglement generation between any two memory nodes. The memory lifetime is longer than the round-trip communication time. Our work provides a metropolitan-scale testbed for the evaluation and exploration of multi-node quantum network protocols and starts a stage of quantum internet research.

Frequency-multiplexed on-demand storage in five modes of atomic frequency comb through simultaneous application of control pulses

In quantum communication with quantum repeaters, multiplexed quantum memory is expected to enhance communication rates. When using an atomic frequency comb (AFC) for on-demand storage, the frequency mode number is often limited by the optical power of the control pulses. Here, using a space-coupled waveguide electro-optic modulator, we increased the output power, allowing us to apply control pulses to multiple modes simultaneously. Further, through enhancement of an experimental setup that increases power density, we increased the number of modes. Consequently, we pioneered, to the best of our knowledge, on-demand storage using five modes of AFC. This technology is a significant achievement toward frequency-multiplexed on-demand quantum memory.

New classifications for quantum bioinformatics: Q-bioinformatics, QCt-bioinformatics, QCg-bioinformatics, and QCr-bioinformatics

Bioinformatics has revolutionized biology and medicine by using computational methods to analyze and interpret biological data. Quantum mechanics has recently emerged as a promising tool for the analysis of biological systems, leading to the development of quantum bioinformatics. This new field employs the principles of quantum mechanics, quantum algorithms, and quantum computing to solve complex problems in molecular biology, drug design, and protein folding. However, the intersection of bioinformatics, biology, and quantum mechanics presents unique challenges. One significant challenge is the possibility of confusion among scientists between quantum bioinformatics and quantum biology, which have similar goals and concepts. Additionally, the diverse calculations in each field make it difficult to establish boundaries and identify purely quantum effects from other factors that may affect biological processes. This review provides an overview of the concepts of quantum biology and quantum mechanics and their intersection in quantum bioinformatics. We examine the challenges and unique features of this field and propose a classification of quantum bioinformatics to promote interdisciplinary collaboration and accelerate progress. By unlocking the full potential of quantum bioinformatics, this review aims to contribute to our understanding of quantum mechanics in biological systems.

Observation of Acoustically Induced Dressed States of Rare-Earth Ions

Acoustically induced dressed states of long-lived erbium ions in a crystal are demonstrated. These states are formed by rapid modulation of two-level systems via strain induced by surface acoustic waves whose frequencies exceed the optical linewidth of the ion ensemble. Multiple sidebands and the reduction of their intensities appearing near the surface are evidence of a strong interaction between the acoustic waves and the ions. This development allows for on-chip control of long-lived ions and paves the way to highly coherent hybrid quantum systems with telecom photons, acoustic phonons, and electrons.

Heralded quantum multiplexing entanglement between stationary qubits via distribution of high-dimensional optical entanglement

Quantum entanglement between pairs of remote quantum memories (QMs) is a prerequisite for realizing many applications in quantum networks. Here, we present a heralded protocol for the parallel creation of quantum entanglement among multiple pairs of QMs placed in spatially separated nodes, where each QM, encoding a stationary qubit, couples to an optical cavity and deterministically interacts with single photons. Our protocol utilizes an entangled photon pair encoded in the high-dimensional time-bin degree of freedom to simultaneously entangle multiple QM pairs, and is efficient in terms of reducing the time consumption and photon loss during transmission. Furthermore, our approach can be extended to simultaneously support spatial-temporal multiplexing, as its success is heralded by the detection of single photons. These distinguishing features make our protocol particularly useful for long-distance quantum communication and large-scale quantum networks.

Quantum storage of entangled photons at telecom wavelengths in a crystal

Quantum storage and distribution of entanglement are the key ingredients for realizing a global quantum internet. Compatible with existing fiber networks, telecom-wavelength entangled photons and corresponding quantum memories are of central interest. Recently, 167Er3+ ions have been identified as a promising candidate for an efficient telecom quantum memory. However, to date, no storage of entangled photons, the crucial step of quantum memory using these promising ions, 167Er3+, has been reported. Here, we demonstrate the storage and retrieval of the entangled state of two telecom photons generated from an integrated photonic chip. Combining the natural narrow linewidth of the entangled photons and long storage time of 167Er3+ ions, we achieve storage time of 1.936 μs, more than 387 times longer than in previous works. Successful storage of entanglement in the crystal is certified using entanglement witness measurements. These results pave the way for realizing quantum networks based on solid-state devices.

Manipulating Growth and Propagation of Correlations in Dipolar Multilayers: From Pair Production to Bosonic Kitaev Models

We study the nonequilibrium dynamics of dipoles confined in multiple stacked two-dimensional layers realizing a long-range interacting quantum spin 1/2 XXX model. We demonstrate that strong in-plane interactions can protect a manifold of collective layer dynamics. This then allows us to map the many-body spin dynamics to bosonic models. In a bilayer configuration we show how to engineer the paradigmatic two-mode squeezing Hamiltonian known from quantum optics, resulting in exponential production of entangled pairs and generation of metrologically useful entanglement from initially prepared product states. In multilayer configurations we engineer a bosonic variant of the Kitaev model displaying chiral propagation along the layer direction. Our study illustrates how the control over interactions, lattice geometry, and state preparation in interacting dipolar systems uniquely afforded by AMO platforms such as Rydberg and magnetic atoms, polar molecules, or trapped ions allows for the control over the temporal and spatial propagation of correlations for applications in quantum sensing and quantum simulation.

Indistinguishable telecom band photons from a single Er ion in the solid state

Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication1. Recently, there has been significant interest in rare earth ions2-4, in particular Er3+ for its telecom band optical transition5-7 that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er3+ into CaWO4, a material that combines a non-polar site symmetry, low decoherence from nuclear spins8 and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong-Ou-Mandel interference9 between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T1,s = 3.7 s and T2,s > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er3+ ions.

Erbium emitters in commercially fabricated nanophotonic silicon waveguides

Quantum memories integrated into nanophotonic silicon devices are a promising platform for large quantum networks and scalable photonic quantum computers. In this context, erbium dopants are particularly attractive, as they combine optical transitions in the telecommunications frequency band with the potential for second-long coherence time. Here, we show that these emitters can be reliably integrated into commercially fabricated low-loss waveguides. We investigate several integration procedures and obtain ensembles of many emitters with an inhomogeneous broadening of <2 GHz and a homogeneous linewidth of <30 kHz. We further observe the splitting of the electronic spin states in a magnetic field up to 9 T that freezes paramagnetic impurities. Our findings are an important step toward long-lived quantum memories that can be fabricated on a wafer-scale using CMOS technology.

Telecom-Wavelength Quantum Repeater Node Based on a Trapped-Ion Processor

A quantum repeater node is presented based on trapped ions that act as single-photon emitters, quantum memories, and an elementary quantum processor. The node's ability to establish entanglement across two 25-km-long optical fibers independently, then to swap that entanglement efficiently to extend it over both fibers, is demonstrated. The resultant entanglement is established between telecom-wavelength photons at either end of the 50 km channel. Finally, the system improvements to allow for repeater-node chains to establish stored entanglement over 800 km at hertz rates are calculated, revealing a near-term path to distributed networks of entangled sensors, atomic clocks, and quantum processors.

Hertz-rate metropolitan quantum teleportation

Quantum teleportation can transfer an unknown quantum state between distant quantum nodes, which holds great promise in enabling large-scale quantum networks. To advance the full potential of quantum teleportation, quantum states must be faithfully transferred at a high rate over long distance. Despite recent impressive advances, a high-rate quantum teleportation system across metropolitan fiber networks is extremely desired. Here, we demonstrate a quantum teleportation system which transfers quantum states carried by independent photons at a rate of 7.1 ± 0.4 Hz over 64-km-long fiber channel. An average single-photon fidelity of ≥90.6 ± 2.6% is achieved, which exceeds the maximum fidelity of 2/3 in classical regime. Our result marks an important milestone towards quantum networks and opens the door to exploring quantum entanglement based informatic applications for the future quantum internet.

Spatial-spectral mapping to prepare frequency entangled qudits

Entangled qudits, the high-dimensional entangled states, play an important role in the study of quantum information. How to prepare entangled qudits in an efficient and easy-to-operate manner is still a challenge in quantum technology. Here, we demonstrate a method to engineer frequency entangled qudits in a spontaneous parametric downconversion process. The proposal employs an angle-dependent phase-matching condition in a nonlinear crystal, which forms a classical-quantum mapping between the spatial (pump) and spectral (biphotons) degrees of freedom. In particular, the pump profile is separated into several bins in the spatial domain, and thus shapes the down-converted biphotons into discrete frequency modes in the joint spectral space. Our approach provides a feasible and efficient method to prepare a high-dimensional frequency entangled state. As an experimental demonstration, we generate a three-dimensional entangled state by using a homemade variable slit mask.

Long distance multiplexed quantum teleportation from a telecom photon to a solid-state qubit

Quantum teleportation is an essential capability for quantum networks, allowing the transmission of quantum bits (qubits) without a direct exchange of quantum information. Its implementation between distant parties requires teleportation of the quantum information to matter qubits that store it for long enough to allow users to perform further processing. Here we demonstrate long distance quantum teleportation from a photonic qubit at telecom wavelength to a matter qubit, stored as a collective excitation in a solid-state quantum memory. Our system encompasses an active feed-forward scheme, implementing a conditional phase shift on the qubit retrieved from the memory, as required by the protocol. Moreover, our approach is time-multiplexed, allowing for an increase in the teleportation rate, and is directly compatible with the deployed telecommunication networks, two key features for its scalability and practical implementation, that will play a pivotal role in the development of long-distance quantum communication.

Sagnac interferometer-type nondegenerate polarization entangled two-photon source with a Fresnel rhomb

Telecommunication wavelength-entangled photon sources (EPS) are indispensable systems for a fiber-based quantum network. We developed a Sagnac-type spontaneous parametric down conversion system adopting a Fresnel rhomb as a wideband and reasonable retarder. This novelty, to the best of our knowledge, enables the generation of a highly nondegenerate two-photon entanglement comprising the telecommunication wavelength (1550 nm) and quantum memory wavelength (606 nm for Pr:YSO) with only one nonlinear crystal. Quantum state tomography was performed to evaluate the degree of entanglement, and the fidelity with a Bell state |Φ+⟩ with a maximum of 94.4% was obtained. Therefore, this paper shows the potential of nondegenerate EPSs that are compatible with both telecommunication wavelength and quantum-memory wavelength to be installed in quantum repeater architecture.

Microwave-to-optical transduction with erbium ions coupled to planar photonic and superconducting resonators

Optical quantum networks can connect distant quantum processors to enable secure quantum communication and distributed quantum computing. Superconducting qubits are a leading technology for quantum information processing but cannot couple to long-distance optical networks without an efficient, coherent, and low noise interface between microwave and optical photons. Here, we demonstrate a microwave-to-optical transducer using an ensemble of erbium ions that is simultaneously coupled to a superconducting microwave resonator and a nanophotonic optical resonator. The coherent atomic transitions of the ions mediate the frequency conversion from microwave photons to optical photons and using photon counting we observed device conversion efficiency approaching 10^-7. With pulsed operation at a low duty cycle, the device maintained a spin temperature below 100 mK and microwave resonator heating of less than 0.15 quanta.

On-Demand Storage of Photonic Qubits at Telecom Wavelengths

Quantum memories at telecom wavelengths are crucial for the construction of large-scale quantum networks based on existing fiber networks. On-demand storage of telecom photonic qubits is an essential request for such networking applications but yet to be demonstrated. Here we demonstrate the storage and on-demand retrieval of telecom photonic qubits using a laser-written waveguide fabricated in an ^{167}Er^{3+}:Y_{2}SiO_{5} crystal. Both ends of the waveguide memory are directly connected with fiber arrays with a fiber-to-fiber efficiency of 51%. Storage fidelity of 98.3(1)% can be obtained for time-bin qubits encoded with single-photon-level coherent pulses, which is far beyond the maximal fidelity that can be achieved with a classical measure and prepared strategy. This device features high reliability and easy scalability, and it can be directly integrated into fiber networks, which could play an essential role in fiber-based quantum networks.

Non-classical correlations over 1250 modes between telecom photons and 979-nm photons stored in 171Yb3+:Y2SiO5

Quantum repeaters based on heralded entanglement require quantum nodes that are able to generate multimode quantum correlations between memories and telecommunication photons. The communication rate scales linearly with the number of modes, yet highly multimode quantum storage remains challenging. In this work, we demonstrate an atomic frequency comb quantum memory with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz. The memory is based on a Y₂SiO₅ crystal doped with ¹⁷¹Yb³⁺ ions, with a memory wavelength of 979 nm. The memory is interfaced with a source of non-degenerate photon pairs at 979 and 1550 nm, bandwidth-matched to the quantum memory. We obtain strong non-classical second-order cross correlations over all modes, for storage times of up to 25 μs. The telecommunication photons propagated through 5 km of fiber before the release of the memory photons, a key capability for quantum repeaters based on heralded entanglement and feed-forward operations. Building on this experiment should allow distribution of entanglement between remote quantum nodes, with enhanced rates owing to the high multimode capacity.

Multimode operation would greatly improve the performances of quantum repeaters. Here, the authors demonstrate a fixed-delay atomic frequency comb quantum memory, based on a Y2SiO5 crystal doped with Ytterbium ions, with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz.

Photonic Indistinguishability of the Tin-Vacancy Center in Nanostructured Diamond

Tin-vacancy centers in diamond are promising spin-photon interfaces owing to their high quantum efficiency, large Debye-Waller factor, and compatibility with photonic nanostructuring. Benchmarking their single-photon indistinguishability is a key challenge for future applications. Here, we report the generation of single photons with 99.7_{-2.5}^{+0.3}% purity and 63(9)% indistinguishability from a resonantly excited tin-vacancy center in a single-mode waveguide. We obtain quantum control of the optical transition with 1.71(1)-ns-long π pulses of 77.1(8)% fidelity and show it is spectrally stable over 100 ms. A modest Purcell enhancement factor of 12 would enhance the indistinguishability to 95%.

Purcell Enhancement of Erbium Ions in TiO2 on Silicon Nanocavities

Isolated solid-state atomic defects with telecom optical transitions are ideal quantum photon emitters and spin qubits for applications in long-distance quantum communication networks. Prototypical telecom defects, such as erbium, suffer from poor photon emission rates, requiring photonic enhancement using resonant optical cavities. Moreover, many of the traditional hosts for erbium ions are not amenable to direct incorporation with existing integrated photonics platforms, limiting scalable fabrication of qubit-based devices. Here, we present a scalable approach toward CMOS-compatible telecom qubits by using erbium-doped titanium dioxide thin films grown atop silicon-on-insulator substrates. From this heterostructure, we have fabricated one-dimensional photonic crystal cavities demonstrating quality factors in excess of 5 × 10⁴ and corresponding Purcell-enhanced optical emission rates of the erbium ensembles in excess of 200. This easily fabricated materials platform represents an important step toward realizing telecom quantum memories in a scalable qubit architecture compatible with mature silicon technologies.

Perfect Photon Indistinguishability from a Set of Dissipative Quantum Emitters

Single photon sources (SPS) based on semiconductor quantum dot (QD) platforms are restricted to low temperature (T) operation due to the presence of strong dephasing processes. Although the integration of QD in optical cavities provides an enhancement of its emission properties, the technical requirements for maintaining high indistinguishability (I) at high T are still beyond the state of the art. Recently, new theoretical approaches have shown promising results by implementing two-dipole-coupled-emitter systems. Here, we propose a platform based on an optimized five-dipole-coupled-emitter system coupled to a cavity which enables perfect I at high T. Within our scheme the realization of perfect I single photon emission with dissipative QDs is possible using well established photonic platforms. For the optimization procedure we have developed a novel machine-learning approach which provides a significant computational-time reduction for high demanding optimization algorithms. Our strategy opens up interesting possibilities for the optimization of different photonic structures for quantum information applications, such as the reduction of quantum decoherence in clusters of coupled two-level quantum systems.

Postselected Entanglement between Two Atomic Ensembles Separated by 12.5 km

Quantum internet gives the promise of getting all quantum resources connected, and it will enable applications far beyond a localized scenario. A prototype is a network of quantum memories that are entangled and well separated. In this Letter, we report the establishment of postselected entanglement between two atomic quantum memories physically separated by 12.5 km directly. We create atom-photon entanglement in one node and send the photon to a second node for storage via electromagnetically induced transparency. We harness low-loss transmission through a field-deployed fiber of 20.5 km by making use of frequency down-conversion and up-conversion. The final memory-memory entanglement is verified to have a fidelity of 90% via retrieving to photons. Our experiment makes a significant step forward toward the realization of a practical metropolitan-scale quantum network.

Storage and analysis of light-matter entanglement in a fiber-integrated system

The deployment of a full-fledged quantum internet poses the challenge of finding adequate building blocks for entanglement distribution between remote quantum nodes. A practical system would combine propagation in optical fibers with quantum memories for light, leveraging on the existing communication network while featuring the scalability required to extend to network sizes. Here, we demonstrate a fiber-integrated quantum memory entangled with a photon at telecommunication wavelength. The storage device is based on a fiber-pigtailed laser-written waveguide in a rare earth-doped solid and allows an all-fiber stable addressing of the memory. The analysis of the entanglement is performed using fiber-based interferometers. Our results feature orders-of-magnitude advances in terms of storage time and efficiency for integrated storage of light-matter entanglement and constitute a substantial step forward toward quantum networks using integrated devices.

Entangling single atoms over 33 km telecom fibre

Quantum networks promise to provide the infrastructure for many disruptive applications, such as efficient long-distance quantum communication and distributed quantum computing1,2. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation3,4 and loophole-free tests of Bell's inequality5,6, recently, entanglement distribution has also been achieved over telecom fibres and analysed retrospectively7,8. Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fibre links with a length up to 33 km. For this, we generate atom-photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fibres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion9. The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms10. Our results show the feasibility of entanglement distribution over telecom fibre links useful, for example, for device-independent quantum key distribution11-13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links.

Generation of entanglement between a highly wave-packet-tunable photon and a spin-wave memory in cold atoms

Controls of waveforms (pulse durations) of single photons are important tasks for effectively interconnecting disparate atomic memories in hybrid quantum networks. So far, the waveform control of a single photon that is entangled with an atomic memory remains unexplored. Here, we demonstrated control of waveform length of the photon that is entangled with an atomic spin-wave memory by varying light-atom interaction time in cold atoms. The Bell parameter S as a function of the duration of photon pulse is measured, which shows that violations of Bell inequality can be achieved for the photon pulse in the duration range from 40 ns to 50 µs, where, S = 2.64 ± 0.02 and S = 2.26 ± 0.05 for the 40-ns and 50-µs durations, respectively. The measured results show that S parameter decreases with the increase in the pulse duration. We confirm that the increase in photon noise probability per pulse with the pulse-duration is responsible for the S decrease.

Entanglement between a Telecom Photon and an On-Demand Multimode Solid-State Quantum Memory

Entanglement between photons at telecommunication wavelengths and long-lived quantum memories is one of the fundamental requirements of long-distance quantum communication. Quantum memories featuring on-demand readout and multimode operation are additional precious assets that will benefit the communication rate. In this Letter, we report the first demonstration of entanglement between a telecom photon and a collective spin excitation in a multimode solid-state quantum memory. Photon pairs are generated through widely nondegenerate parametric down-conversion, featuring energy-time entanglement between the telecom-wavelength idler and a visible signal photon. The latter is stored in a Pr^{3+}:Y_{2}SiO_{5} crystal as a spin wave using the full atomic frequency comb scheme. We then recall the stored signal photon and analyze the entanglement using the Franson scheme. We measure conditional fidelities of 92(2)% for excited-state storage, enough to violate a Clauser-Horne-Shimony-Holt inequality, and 77(2)% for spin-wave storage. Taking advantage of the on-demand readout from the spin state, we extend the entanglement storage in the quantum memory for up to 47.7  μs, which could allow for the distribution of entanglement between quantum nodes separated by distances of up to 10 km.