Combining highly coherent spin control with efficient light-matter coupling offers great opportunities for quantum communication and computing. Optically active semiconductor quantum dots have unparalleled photonic properties but also modest spin coherence limited by their resident nuclei. The nuclear inhomogeneity has thus far bound all dynamical decoupling measurements to a few microseconds. Here, we eliminate this inhomogeneity using lattice-matched GaAs–AlGaAs quantum dot devices and demonstrate dynamical decoupling of the electron spin qubit beyond 0.113(3) ms. Leveraging the 99.30(5)% visibility of our optical π-pulse gates, we use up to Nπ = 81 decoupling pulses and find a coherence time scaling of $${N}_{\uppi }^{0.75(2)}$$ . This scaling manifests an ideal refocusing of strong interactions between the electron and the nuclear spin ensemble, free of extrinsic noise, which holds the promise of lifetime-limited spin coherence. Our findings demonstrate that the most punishing material science challenge for such quantum dot devices has a remedy and constitute the basis for highly coherent spin–photon interfaces. Optically active semiconductor quantum dots have so far suffered from nuclear inhomogeneity limiting all dynamical decoupling measurements to a few microseconds. Lattice-matched GaAs–AlGaAs quantum dots now enable decoupling schemes to achieve a 0.11 ms spin coherence time. 

/pdf/ideal-refocusing-of-an-optically-active-spin-qubit-under-1zo5tey0.pdf

Ideal refocusing of an optically active spin qubit under strong hyperfine interactions

Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of five dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.

/pdf/quadrupolar-spectra-of-nuclear-spins-in-strained-ingaas-dq1q8glynr.pdf

Quadrupolar spectra of nuclear spins in strained InGaAs quantum dots

In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a red-shifted photon is created. In a quantum dot, radiative Auger is predicted for charged excitons. For a singly-charged trion, a photon is created on electron-hole recombination, leaving behind a single electron. Radiative Auger determines the quantisation energies of this single electron, information which is otherwise difficult to acquire. For this reason, radiative Auger is a powerful tool. However, radiative Auger has not been observed on single quantum dots. Here, we report radiative Auger on trions in single quantum dots. There are sharp red-shifted emission lines with intensities as high as 1% of the main emission enabling the single-electron quantisation energies to be measured with high precision. Going beyond the original proposals, we show how quantum optics -- an analysis of the photon correlations -- gives access to the single-electron dynamics, notably relaxation and tunneling. All these properties of radiative Auger can be exploited on other semiconductor nanostructures and colour centres.

Radiative Auger Process in the Single Photon Limit on a Quantum Dot

Despite all of the fundamental research carried out on them, artificial atoms continue to be a source of surprise. The intersection between the electrons in a quantum dot and a nearby sea of electrons can create unusual many-body states. A spectroscopic study now makes these states observable.

/pdf/many-body-exciton-states-in-self-assembled-quantum-dots-3xtr9aymfb.pdf

Many-body exciton states in self-assembled quantum dots coupled to a Fermi sea

There is a growing interest in hybrid solid-state quantum systems where nuclear spins, interfaced to the electron spin qubit, are used as quantum memory or qubit register. These approaches require long nuclear spin coherence, which until now seemed impossible owing to the disruptive effect of the electron spin. Here we study InGaAs semiconductor quantum dots, demonstrating millisecond-long collective nuclear spin coherence even under inhomogeneous coupling to the electron central spin. We show that the underlying decoherence mechanism is spectral diffusion induced by a fluctuating electron spin. These results provide new understanding of the many-body coherence in central spin systems, required for development of electron-nuclear spin qubits. As a demonstration, we implement a conditional gate that encodes electron spin state onto collective nuclear spin coherence, and use it for a single-shot readout of the electron spin qubit with >99% fidelity. 

/pdf/harnessing-many-body-spin-environment-for-long-coherence-1k9av2k5.pdf

Harnessing many-body spin environment for long coherence storage and high-fidelity single-shot qubit readout

Combining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

/pdf/fundamental-limits-of-electron-and-nuclear-spin-qubit-ek4vxvg1r0.pdf

George Gillard

Papers

Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot