Abstract The realization of quantum networks and quantum computers relies on the scalable generation of entanglement, for which spin-photon interfaces are strong candidates. Current proposals to produce entangled-photon states with such platforms place stringent requirements on the physical properties of the photon emitters, limiting the range and performance of suitable physical systems. We propose a scalable protocol, which significantly reduces the constraints on the emitter. We use only a single optical transition and an asymmetric polarizing interferometer. This device converts the entanglement from the experimentally robust time basis via a path degree of freedom into a polarization basis, where quantum logic operations can be performed. The fundamental unit of the proposed protocol is realized experimentally in this work, using a nitrogen-vacancy center in diamond. This classically assisted protocol greatly widens the set of physical systems suited for scalable entangled-photon generation and enables performance enhancement of existing platforms.
Abstract The realization of quantum networks and quantum computers relies on the scalable generation of entanglement, for which spin-photon interfaces are strong candidates. Current proposals to produce entangled-photon states with such platforms place stringent requirements on the physical properties of the photon emitters, limiting the range and performance of suitable physical systems. We propose a scalable protocol, which significantly reduces the constraints on the emitter. We use only a single optical transition and an asymmetric polarizing interferometer. This device converts the entanglement from the experimentally robust time basis via a path degree of freedom into a polarization basis, where quantum logic operations can be performed. The fundamental unit of the proposed protocol is realized experimentally in this work, using a nitrogen-vacancy center in diamond. This classically assisted protocol greatly widens the set of physical systems suited for scalable entangled-photon generation and enables performance enhancement of existing platforms.
Quantum networks and distributed quantum computing rely on entanglement generation between photons and long-lived stationary qubits. For large-scale architectures, one of the most crucial parameters is the efficiency at which entanglement can be created and detected. Here, we maximize the efficiency for the detection of hybrid entanglement between a nuclear spin qubit in diamond with a photonic time-bin qubit. Our approach relies on the optimal implementation of a photonic qubit analyzer, for which we use a high-speed electro-optical deflector to direct photons deterministically along the optimal interferometer paths. This way, we eliminate all cases in which photons are randomly lost due to propagation in the wrong interferometer path. We use nitrogen-vacancy centers in diamond, for which we demonstrate nuclear spin--photon entanglement with a fidelity of 0.6 reducing the experimental acquisition time by a factor of 2. An extension to other spin--photon entanglement systems and higher-dimensional entanglement schemes is outlined.
Solid-state quantum emitters are promising candidates for the realization of quantum networks, owing to their long-lived spin memories, high-fidelity local operations, and optical connectivity for long-range entanglement. However, due to differences in local environment, solid-state emitters typically feature a range of distinct transition frequencies, which makes it challenging to create optically mediated entanglement between arbitrary emitter pairs. We propose and demonstrate an efficient method for entangling emitters with optical transitions separated by many linewidths. In our approach, electro-optic modulators enable a single photon to herald a parity measurement on a pair of spin qubits. We experimentally demonstrate the protocol using two silicon-vacancy centers in a diamond nanophotonic cavity, with optical transitions separated by 7.4 GHz. Working with distinguishable emitters allows for individual qubit addressing and readout, enabling parallel control and entanglement of both colocated and spatially separated emitters, a key step toward scaling up quantum information processing systems.
Solid-state quantum light sources offer a scalable pathway for interfacing stationary spin qubits with flying photonic qubits, forming the backbone of future quantum networks. Telecom-band spin-photonic qubits, operating in the 1260-1675 nm wavelength range, are particularly well-suited for long-distance quantum communication due to minimal loss in standard optical fibers. Achieving scalability, however, hinges on fulfilling several stringent criteria: coherent spin-state control, deterministic and indistinguishable single-photon emission, and integration with nanophotonic structures that enhance radiative properties, such as lifetime, coherence, and photon indistinguishability. This study explores the state-of-the-art spin-photonic qubits across solid-state platforms, including diamond color centers, silicon carbide defect centers, quantum dots, and two-dimensional materials. Special attention is given to silicon-based emitters, particularly G, T, C- and Ci-centers, which promise monolithic integration with complementary metal-oxide-semiconductor (CMOS) technology and telecom-band operation. We classify these systems based on spin-photon interface availability, CMOS process compatibility, and emitter scalability. We also discuss recent advances in cavity quantum electrodynamics (cQED), including Purcell enhancement and quality factor engineering in integrated photonic (circuits) environments. The work highlights emerging demonstrations of quantum networking over metropolitan scales and outlines the trajectory toward chip-scale quantum photonic integrated circuits (QPICs). It combines deterministic emitter creation, coherent spin manipulation, and quantum information processing. These developments pave the way for global quantum networks, enabling secure communication, distributed quantum computing, and quantum-enhanced sensing.
The realization of scalable systems for quantum information processing and networking is of utmost importance to the quantum information community. However, building such systems is difficult because of challenges in achieving all the necessary functionalities on a unified platform while maintaining stringent performance requirements of the individual elements. A promising approach that addresses this challenge is based on the consolidation of experimental and theoretical capabilities in quantum physics and integrated photonics. Integrated quantum photonic devices allow efficient control and read-out of quantum information while being scalable and cost effective. Here, the authors review recent developments in solid-state single photon emitters coupled with various integrated photonic structures, which form a critical component of future scalable quantum devices. Their work contributes to the further development and realization of quantum networking protocols and quantum logic on a scalable and fabrication-friendly platform.
We propose a curious protocol for swapping the intraphoton entanglement between path and polarization degrees of freedom of a single photon to interphoton entanglement between two distance photons that have never interacted. This is accomplished by using an experimental setup consisting of three suitable Mach-Zehnder interferometers along with number of beam splitters, polarization rotators, and detectors. Using the same setup, we have also demonstrated an interesting quantum state transfer protocol, symmetric between Alice and Bob. Importantly, the Bell-basis discrimination is not required in both the swapping and state transfer protocols. Our proposal can be implemented using linear optical devices.
Current photon entangling schemes require resources that grow with the photon number. We present a new approach that generates quantum entanglement between many photons, using only a single source of entangled photon pairs. The different spatial modes, one for each photon as required by other schemes, are replaced by different time slots of only two spatial modes. States of any number of photons are generated with the same setup, solving the scalability problem caused by the previous need for extra resources. Consequently, entangled photon states of larger numbers than before are practically realizable.