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arXiv:2205.12844 · npj Quantum Information

On-chip spin-photon entanglement based on photon-scattering of a quantum dot

Ming Lai Chan, Alexey Tiranov, Martin Hayhurst Appel, Ying Wang, Leonardo Midolo, Sven Scholz, Andreas Dirk Wieck, Arne Ludwig, Anders Søndberg Sørensen, Peter Lodahl

Published May 19, 2023• 19 citations
arXivHTMLPDF

Abstract

Abstract The realization of on-chip quantum interfaces between flying photons and solid-state spins is a key building block for quantum-information processors, enabling, e.g., distributed quantum computing, where remote quantum registers are interconnected by flying photons. Self-assembled quantum dots integrated into nanostructures are one of the most promising systems for such an endeavor thanks to their near-unity photon-emitter coupling and fast spontaneous emission rate. Here we demonstrate high-fidelity on-chip entanglement between an incoming photon and a stationary quantum-dot hole spin qubit. The entanglement is induced by sequential scattering of the time-bin encoded photon interleaved with active spin control within a microsecond, two orders of magnitude faster than those achieved in other solid-state platforms. Conditioning on the detection of a reflected photon renders the entanglement fidelity immune to the spectral wandering of the emitter. These results represent a major step towards realizing a quantum node capable of interchanging information with flying photons and on-chip quantum logic, as required for quantum networks and quantum repeaters.

DOI
10.1038/s41534-023-00717-5

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Abstract The realization of on-chip quantum interfaces between flying photons and solid-state spins is a key building block for quantum-information processors, enabling, e.g., distributed quantum computing, where remote quantum registers are interconnected by flying photons. Self-assembled quantum dots integrated into nanostructures are one of the most promising systems for such an endeavor thanks to their near-unity photon-emitter coupling and fast spontaneous emission rate. Here we demonstrate high-fidelity on-chip entanglement between an incoming photon and a stationary quantum-dot hole spin qubit. The entanglement is induced by sequential scattering of the time-bin encoded photon interleaved with active spin control within a microsecond, two orders of magnitude faster than those achieved in other solid-state platforms. Conditioning on the detection of a reflected photon renders the entanglement fidelity immune to the spectral wandering of the emitter. These results represent a major step towards realizing a quantum node capable of interchanging information with flying photons and on-chip quantum logic, as required for quantum networks and quantum repeaters.
Abstract The realization of on-chip quantum gates between photons and solid-state spins is a key building block for quantum-information processors, enabling, e.g., distributed quantum computing, where remote quantum registers are interconnected by flying photons. Self-assembled quantum dots integrated in nanostructures are one of the most promising systems for such an endeavor thanks to their near-unity photon-emitter coupling and fast spontaneous emission rate. Here we demonstrate an on-chip entangling gate between an incoming photon and a stationary quantum-dot spin qubit. The gate is based on sequential scattering of a time-bin encoded photon with a waveguide-embedded quantum dot and operates on sub-microsecond timescale; two orders of magnitude faster than other platforms. Heralding on detection of a reflected photon renders the gate fidelity fully immune to spectral wandering of the emitter. These results represent a major step in realizing a quantum node capable of both photonic entanglement generation and on-chip quantum logic, as demanded in quantum networks and quantum repeaters.
Entanglement plays a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a major challenge is the efficient generation of entanglement between distant stationary (spin) qubits. Here, we report the realization of heralded quantum entanglement between two semiconductor quantum dot spins separated by more than five meters. Our results extend the previous demonstrations of distant spin entanglement in single trapped ions or neutral atoms, in atom ensembles and nitrogen vacancy centers to the domain of artificial atoms in semiconductor nanostructures that allow for onchip integration of electronic and photonic elements. Moreover the efficient spin-photon interface provided by self-assembled quantum dots[1] allows us to reach an unprecedented rate of 2300 entangled spin pairs per second, which represents an improvement of four orders of magnitude as compared to prior experiments[2]. The entanglement generation scheme relies on single photon interference of Raman scattered light from both dots[3]. A single photon detection projects the system into a maximally entangled state. We developed a delayed two-photon interference scheme that allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum repeaters and quantum networks on a chip. [1] W.-B. Gao et al., Nature 491, 426 (2012) [2] L. Slodicka et al., Phys. Rev. Lett. 110, 083603 (2013) [3] C. Cabrillo et al. Phys. Rev. A 59, 1025 (1999) -1 0 1 2 3 4 5 6 7 8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 data sine fit ra ti o pulse duration (arb. u.) F x = 29.8 ± 2.6 % 180 min 0.13 /s 0 19 37 56 74 93 112 0.0 0.1 0.2 0.3 0.4 0.5 0.6 total fidelity = 55.2 ± 3.5 % F Z = 80.6 ± 6.6 % c o in c id e n c e s / 1 0 6 .5 h measurement coincidences ideal case
Efficient generation of entanglement between a stationary matter-qubit and a propagating photonic-qubit is a scientific and technological challenge of utmost importance for the realisation of fully-fledged quantum networks. Solid-state quantum emitters in the telecom C-band are a promising platform due to the minimal absorption of photons at these wavelengths and deterministic generation of high purity single photon flying qubits. Here, we use an InAs/InP quantum dot to implement an optically active spin-qubit and demonstrate high fidelity spin initialisation and coherent spin control of the resident electron. Lastly, for the first time we demonstrate high-fidelity spin-photon entanglement in a solid-state system with direct emission into the telecom C-band and obtain a lower bound on the entanglement fidelity of 80.07 %.
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.
Solid-state spin-photon interfaces promise to scale quantum networks through on-chip photonic integration and multiplexed entanglement generation. To date, remote entanglement between integrated emitters has been realized only in cavity-enhanced systems, where fabrication yield and spectral matching remain major obstacles. Here we demonstrate heralded remote entanglement between diamond tin-vacancy spin qubits embedded in separate on-chip waveguides. Combining intrinsically efficient photon emission with a broadband waveguide architecture provides high device yield and obviates the need for spectral matching to cavity modes. We realize coherent optical and spin control and achieve high-visibility two-photon interference. By combining photon-mediated entanglement generation with real-time feedforward, we produce a consistent entangled state independent of the heralding pattern. These results establish waveguide-integrated tin-vacancy centers as a compelling platform for scalable quantum network nodes.
Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing. Remarkably, it is possible to generate entanglement even in the absence of direct interactions: provided that which path information is erased, weak spin-state dependent light scattering can be used to project two distant spins onto a maximally entangled state upon detection of a single photon. Even though this approach is necessarily probabilistic, successful generation of entanglement is heralded by the photon detection event. Here, we demonstrate heralded quantum entanglement of two quantum dot heavy-hole spins separated by 5 meters using single-photon interference. Thanks to the long coherence time of hole spins and the efficient spin-photon interface provided by self-assembled quantum dots embedded in leaky microcavity structures, we generate 2300 entangled spin pairs per second, which represents an improvement approaching three orders of magnitude as compared to prior experiments. Delayed two-photon interference scheme we developed allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum networks in semiconductor nanostructures. Combined with schemes for transferring quantum information to a long-lived memory qubit, fast entanglement generation we demonstrate could also impact quantum repeater architectures.