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Maximal entanglement generation in spectrally distinct solid state qubits

Elena Callus, Pieter Kok

Published Feb 24, 2021• 0 citations
OpenAlex

Abstract

We demonstrate how to create maximal entanglement between two qubits that are encoded in two spectrally distinct solid-state quantum emitters embedded in a waveguide interferometer. The optical probe is provided by readily accessible squeezed light, generated by parametric down-conversion. By continuously probing the emitters, the photon scattering builds up entanglement with a concurrence that reaches its maximum after O(10^1) photo-detection events. Our method does not require perfectly identical emitters, and accommodates spectral variations due to the fabrication process. It is also robust enough to create entanglement with a concurrence above 99% for 10% scattering photon loss, and can form the basis for practical entangled networks.

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We demonstrate how to create maximal entanglement between two qubits that are encoded in two spectrally distinct solid-state quantum emitters embedded in a waveguide interferometer. The optical probe is provided by readily accessible squeezed light, generated by parametric down-conversion. By continuously probing the emitters, the photon scattering builds up entanglement with a concurrence that reaches its maximum after O(10^1) photo-detection events. Our method does not require perfectly identical emitters, and accommodates spectral variations due to the fabrication process. It is also robust enough to create entanglement with a concurrence above 99% for 10% scattering photon loss, and can form the basis for practical entangled networks.
We demonstrate how to create maximal entanglement between two qubits that are encoded in two spectrally distinct solid-state quantum emitters embedded in a waveguide interferometer. The optical probe is provided by readily accessible squeezed light, generated by parametric down-conversion. By continuously illuminating the emitters, the photon scattering and incremental path-erasure builds up entanglement. Our method does not require perfectly identical emitters, and accommodates spectral variations due to the fabrication process. Furthermore, for some linewidth and energy ratios, the entanglement buildup can be significantly faster than for more similar emitters. It is also robust enough to create entanglement with a concurrence above 99% in the event of scattering losses and detector inefficiencies and can form the basis for practical entangled networks.
Solid state quantum emitters with electronic spin properties can be entangled using optical interferometric which-path erasure techniques that rely on identical frequencies and linewidths of the emitters. This promises a powerful, fast and scalable platform for quantum technologies. Using current protocols, a pair of emitters embedded in a waveguide interferometer can be deterministically prepared in a maximally entangled state. However, the amount of entanglement diminishes rapidly with increasing separation of the emission lines and variations in the linewidths of the emitters. Solid state emitters such as quantum dots are subject to intrinsic variations in the fabrication process. Therefore, manufacturing the required number of identical emitters on a chip is not scalable. In this Letter, we present a method that uses readily available pairs of entangled photons to entangle solid state emitters with large transition frequency separations and linewidth variations, circumventing the need to fabricate optically identical emitters. We demonstrate a concurrence of C = 0.95 for typical quantum dots using narrowband photon inputs.
We show how to create maximal entanglement between spectrally distinct solid-state emitters embedded in a waveguide interferometer. By revealing the rich underlying structure of multiphoton scattering in emitters, we show that a two-photon input state can generate deterministic maximal entanglement even for emitters with significantly different transition energies and linewidths. The optimal frequency of the input is determined by two competing processes: which-path erasure and interaction strength. We find that smaller spectral overlap can be overcome with higher photon numbers, and quasimonochromatic photons are optimal for entanglement generation. Our work provides a new methodology for solid-state entanglement generation, where the requirement for perfectly matched emitters can be relaxed in favor of optical state optimization.
Quantum entanglement is an important and often necessary resource for quantum information processing e.g. the measurement-based quantum computer [1]. Current protocols for generating entanglement between emitters assume near identical spectral properties [2-3], which is a major obstacle for using integrated solid-state emitters in photonic structures as a scalable platform. The solid-state emitters can vary significantly from one another in terms of both lifetimes and central energies due to imperfections during fabrication and therefore need to be tuned into resonance e.g. electrically [4]. The requirement for local tuning of emitters provides a barrier to scalability.
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.
The ability to create large highly entangled "cluster" states is crucial for measurement-based quantum computing. We show that deterministic multiphoton entanglement can be created from coupled solid state quantum emitters without the need for any two-qubit gates and regardless of whether the emitters are identical. In particular, we present a general method for controlled entanglement creation by making direct use of the always-on exchange interaction, in combination with single-qubit operations. This is used to provide a recipe for the generation of two-dimensional, cluster-state entangled photons that can be carried out with existing experimental capabilities in quantum dots.