11 research outputs found
A many-body singlet prepared by a central spin qubit
Controllable quantum many-body systems are platforms for fundamental
investigations into the nature of entanglement and promise to deliver
computational speed-up for a broad class of algorithms and simulations. In
particular, engineering entanglement within a dense spin ensemble can turn it
into a robust quantum memory or a computational platform. Recent experimental
progress in dense central spin systems motivates the design of algorithms that
use a central-spin qubit as a convenient proxy for the ensemble. Here we
propose a protocol that uses a central spin to initialize two dense spin
ensembles into a pure anti-polarized state and from there creates a many-body
entangled state -- a singlet -- from the combined ensemble. We quantify the
protocol performance for multiple material platforms and show that it can be
implemented even in the presence of realistic levels of decoherence. Our
protocol introduces an algorithmic approach to preparation of a known many-body
state and to entanglement engineering in a dense spin ensemble, which can be
extended towards a broad class of collective quantum states.Comment: 11 pages, 6 figures, and supplementary material
Collective Quantum Memory Activated by a Driven Central Spin
Coupling a qubit coherently to an ensemble is the basis for collective quantum memories. A single driven electron in a quantum dot can deterministically excite low-energy collective modes of a nuclear spin ensemble in the presence of lattice strain. We propose to gate a quantum state transfer between this central electron and these low-energy excitations—spin waves—in the presence of a strong magnetic field, where the nuclear coherence time is long. We develop a microscopic theory capable of calculating the exact time evolution of the strained electron-nuclear system. With this, we evaluate the operation of quantum state storage and show that fidelities up to 90% can be reached with a modest nuclear polarization of only 50%. These findings demonstrate that strain-enabled nuclear spin waves are a highly suitable candidate for quantum memory.We thank E. Chekhovich for helpful discussions. This work was supported by the ERC PHOENICS grant (617985), the EPSRC Quantum Technology Hub NQIT (EP/M013243/1), and the Royal Society (RGF/EA/181068). D. A. G. acknowledges support from St. John’s College Title A Fellowship. E. V. D. and J. M. acknowledge funding from the Danish Council for Independent Research (Grant No. DFF-4181-00416). C. L. G. acknowledges support from a Royal Society Dorothy Hodgkin Fellowship
Velocity tuning of friction with two trapped atoms
Our ability to control friction remains modest, as our understanding of the underlying microscopic processes is incomplete. Atomic force experiments have provided a wealth of results on the dependence of nanofriction on structure velocity and temperature but limitations in the dynamic range, time resolution, and control at the single-atom level have hampered a description from first principles. Here, using an ion-crystal system with single-atom, single-substrate-site spatial and single-slip temporal resolution we measure the friction force over nearly five orders of magnitude in velocity, and contiguously observe four distinct regimes, while controlling temperature and dissipation. We elucidate the interplay between thermal and structural lubricity for two coupled atoms, and provide a simple explanation in terms of the Peierls–Nabarro potential. This extensive control at the atomic scale enables fundamental studies of the interaction of many-atom surfaces, possibly into the quantum regime
Kinks and nanofriction: Structural phases in few-atom chains
The frictional dynamics of interacting surfaces under forced translation are critically dependent on lattice commensurability. The highly nonlinear system of an elastic atomic chain sliding on an incommensurate periodic potential exhibits topological defects, known as kinks, that govern the frictional and translational dynamics. Performing experiments in a trapped-ion friction emulator, we observe two distinct structural and frictional phases: a commensurate high-friction phase where the ions stick-slip simultaneously over the lattice, and an incommensurate low-friction phase where the propagation of a kink breaks that simultaneity. We experimentally track the kink's propagation with atom-by-atom and sublattice site resolution and show that its velocity increases with commensurability. Our results elucidate the commensurate-incommensurate transition and the connection between the appearance of kinks and the reduction of friction in a finite system, with important consequences for controlling friction at nanocontacts
Ideal refocusing of an optically active spin qubit under strong hyperfine interactions
Combining highly coherent spin control with efficient light-matter coupling
offers great opportunities for quantum communication and networks, as well as
quantum computing. Optically active semiconductor quantum dots have
unparalleled photonic properties, but also modest spin coherence limited by
their resident nuclei. Here, we demonstrate that eliminating strain
inhomogeneity using lattice-matched GaAs-AlGaAs quantum dot devices prolongs
the electron spin coherence by nearly two orders of magnitude, beyond 0.113(3)
ms. To do this, we leverage the 99.30(5)% fidelity of our optical pi-pulse
gates to implement dynamical decoupling. We vary the number of decoupling
pulses up to N = 81 and find a coherence time scaling of N^{0.75(2)}. This
scaling manifests an ideal refocusing of strong interactions between the
electron and the nuclear-spin ensemble, holding 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
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Ideal refocusing of an optically active spin qubit under strong hyperfine interactions
Combining highly coherent spin control with efficient light-matter coupling
offers great opportunities for quantum communication and networks, as well as
quantum computing. Optically active semiconductor quantum dots have
unparalleled photonic properties, but also modest spin coherence limited by
their resident nuclei. Here, we demonstrate that eliminating strain
inhomogeneity using lattice-matched GaAs-AlGaAs quantum dot devices prolongs
the electron spin coherence by nearly two orders of magnitude, beyond 0.113(3)
ms. To do this, we leverage the 99.30(5)% fidelity of our optical pi-pulse
gates to implement dynamical decoupling. We vary the number of decoupling
pulses up to N = 81 and find a coherence time scaling of N^{0.75(2)}. This
scaling manifests an ideal refocusing of strong interactions between the
electron and the nuclear-spin ensemble, holding 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