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Quantum gates — in which stationary quantum bits are combined with ‘flying’ quantum bits, that is, photons — will be essential in quantum networks; such a gate, between a laser-trapped atomic quantum bit and a single photon, is now reported. Through the gate to quantum networks The development of a quantum gate between a flying optical photonic qubit (polarization) and a single trapped atomic qubit (spin) has been a long-standing goal in quantum information science. Such gates are required both for quantum computation to be scaled to a large number of qubits and for quantum communication to be scaled to long distances. Now two groups, working independently, report the successful implementation of such gates. Gerhard Rempe and colleagues demonstrate a quantum gate between a laser-trapped atomic qubit and a single photon, where the polarization of the photon is flipped depending exactly on the spin state of the atom. Mikhail Lukin and co-workers describe a similar achievement — a quantum gate effect between a single atom trapped near a photonic crystal and a single photon. The steady increase in control over individual quantum systems supports the promotion of a quantum technology that could provide functionalities beyond those of any classical device. Two particularly promising applications have been explored during the past decade: photon-based quantum communication, which guarantees unbreakable encryption 1 but which still has to be scaled to high rates over large distances, and quantum computation, which will fundamentally enhance computability 2 if it can be scaled to a large number of quantum bits (qubits). It was realized early on that a hybrid system of light qubits and matter qubits 3 could solve the scalability problem of each field—that of communication by use of quantum repeaters 4 , and that of computation by use of an optical interconnect between smaller quantum processors 5 , 6 . To this end, the development of a robust two-qubit gate that allows the linking of distant computational nodes is “a pressing challenge” 6 . Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The gate mechanism presented 7 , 8 is deterministic and robust, and is expected to be applicable to almost any matter qubit. It is based on reflection of the photonic qubit from a cavity that provides strong light–matter coupling. To demonstrate its versatility, we use the quantum gate to create atom–photon, atom–photon–photon and photon–photon entangled states from separable input states. We expect our experiment to enable various applications, including the generation of atomic 9 and photonic 10 cluster states and Schrödinger-cat states 11 , deterministic photonic Bell-state measurements 12 , scalable quantum computation 7 and quantum communication using a redundant quantum parity code 13 .

Small registers of spin qubits in silicon can exhibit hour-long coherence times and exceeded error-correction thresholds. However, their connection to larger quantum processors is an outstanding challenge. To this end, spin qubits with optical interfaces offer key advantages: they can minimize the heat load and give access to modular quantum computing architectures that eliminate cross-talk and offer a large connectivity. Here, we implement such an efficient spin-photon interface based on erbium dopants in a nanophotonic resonator. We demonstrate optical single-shot readout of a spin in silicon whose coherence exceeds the Purcell-enhanced optical lifetime, paving the way for entangling remote spins via photon interference. As erbium dopants can emit coherent photons in the minimal-loss band of optical fibers, and tens of such qubits can be spectrally multiplexed in each resonator, the demonstrated hardware platform offers unique promise for distributed quantum information processing based on scalable, integrated silicon devices. Recently, there has been significant effort in combining spin qubits in silicon with an optical interface for increased efficiency and scalability. Here the authors demonstrate the optical initialization, coherent control, and single-shot readout of a single erbium spin in a nanophotonic silicon resonator.

The stability and outstanding coherence of dopants and other atomlike defects in tailored host crystals make them a leading platform for the implementation of distributed quantum information processing and sensing in quantum networks. Albeit the required efficient light-matter coupling can be achieved via the integration into nanoscale resonators, in this approach the proximity of interfaces is detrimental to the coherence of even the least-sensitive emitters. Here, we establish an alternative: By integrating a19μmthin crystal into a cryogenic Fabry-Perot resonator with a quality factor of9×106, we achieve a two-level Purcell factor of 530(50). In our specific system, erbium-doped yttrium orthosilicate, this leads to a 59(6)-fold enhancement of the emission rate with an out-coupling efficiency of 46(8)%. At the same time, we demonstrate that the emitter properties are not degraded in our approach. We thus observe ensemble-averaged optical coherence up to 0.54(1) ms, which exceeds the 0.19(2) ms lifetime of dopants at the cavity field maximum. While our approach is also applicable to other solid-state quantum emitters, such as color centers in diamond, our system emits at the minimal-loss wavelength of optical fibers and thus enables coherent and efficient nodes for long-distance quantum networks.

All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme that does not rely on absorption. Instead, an incoming photon is reflected from an optical resonator containing a single atom prepared in a superposition of two states. The reflection toggles the superposition phase, which is then measured to trace the photon. Characterizing the device with faint laser pulses, a single-photon detection efficiency of 74% and a survival probability of 66% are achieved. The efficiency can be further increased by observing the photon repeatedly. The large single-photon nonlinearity of the experiment should enable the development of photonic quantum gates and the preparation of exotic quantum states of light.

The realization of a scalable architecture for quantum information processing is a major challenge for quantum science. A promising approach is based on emitters in nanostructures that are coupled by light. Here, we show that erbium dopants can be reproducibly integrated at well-defined lattice sites by implantation into pure silicon. We thus achieve a narrow inhomogeneous broadening, less than 1 GHz, strong optical transitions, and an outstanding optical coherence even at temperatures of 8 K, with an upper bound to the homogeneous linewidth of around 10 kHz. Our study thus introduces a promising materials platform for the implementation of on-chip quantum memories, microwave-to-optical conversion, and distributed quantum information processing.

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.

The realization of a network of quantum registers is an outstanding challenge in quantum science and technology. We experimentally investigate a network node that consists of a single nitrogen-vacancy center electronic spin hyperfine coupled to nearby nuclear spins. We demonstrate individual control and readout of five nuclear spin qubits within one node. We then characterize the storage of quantum superpositions in individual nuclear spins under repeated application of a probabilistic optical internode entangling protocol. We find that the storage fidelity is limited by dephasing during the electronic spin reset after failed attempts. By encoding quantum states into a decoherence-protected subspace of two nuclear spins, we show that quantum coherence can be maintained for over 1000 repetitions of the remote entangling protocol. These results and insights pave the way towards remote entanglement purification and the realization of a quantum repeater using nitrogen-vacancy center quantum-network nodes.

Solid-state quantum emitters operating in the telecom wavelength range are pivotal for the development of scalable quantum information processing technologies. In this review, we provide a comprehensive overview of the state-of-the-art solid-state emitters of single photons targeting quantum information processing in the discrete-variable regime and telecom wavelength range. We focus on quantum dots, color centers, and erbium ion dopants, detailing their synthesis methods and their applications. The review addresses the strategies for the integration of these quantum emitters into photonic devices alongside the associated challenges. We also discuss their applications in quantum technologies, examining current limitations, including performance constraints, decoherence, and scalability. Finally, we propose future directions for advancing photonic-based quantum technologies.

The reliable measurement and accurate control of the temperature within nanophotonic devices is a key prerequisite for their application in both classical and quantum technologies. Established approaches use sensors that are attached in proximity to the components, which only offers a limited spatial resolution and thus impedes the measurement of local heating effects. Here, we, therefore, study an alternative temperature sensing technique that is based on measuring the luminescence of erbium emitters directly integrated into nanophotonic silicon waveguides. To cover the entire temperature range from 295 K to 2 K, we investigate two different approaches: The thermal activation of nonradiative decay channels for temperatures above 200 K and the thermal depopulation of spin and crystal field levels at lower temperatures. The achieved sensitivity is 0.22(4) %/K at room temperature and increases up to 420(50) %/K at approximately 2 K. Within a few-minute measurement interval, we thus achieve a measurement precision that ranges from 0.04(1) K at the lowest studied temperature to 6(1) K at ambient conditions. In the future, the measurement time can be further reduced by optimizing the excitation pulse sequence and the fiber-to-chip coupling efficiency. Combining this with spatially selective implantation promises precise thermometry from ambient to cryogenic temperatures with a spatial resolution down to a few nanometers.

Nanophotonics offers opportunities for engineering and exploiting the quantum properties of light by integrating quantum emitters into nanostructures, and offering reliable paths to quantum technology applications such as sources of quantum light or new quantum simulators, among many others. In this Review, we discuss common nanophotonic platforms for studying light–matter interactions, explaining their strengths and experimental state-of-the-art. Each platform works at a different interaction regime: from standard cavity quantum electrodynamics (QED) setups to unique quantum nanophotonic devices, such as chiral and non-chiral waveguide QED experiments. When several quantum emitters are integrated into nanophotonic systems, collective interactions emerge, enabling miniaturized, versatile and fast-operating quantum devices. We conclude with a perspective on the near-term opportunities offered by nanophotonics in the context of quantum technologies.Quantum nanophotonics examines the interaction between emitters and light confined at the nanoscale. This Review highlights the experimental progress in the field, explains new light–matter interaction regimes and emphasizes their potential applications in quantum technologies.