11 research outputs found
Pointer States Via Engineered Dissipation
Pointer states are long-lasting high-fidelity states in open quantum systems. We show how any pure state in a non-Markovian open quantum system can be made to behave as a pointer state by suitably engineering the coupling to the environment via open-loop periodic control. Engineered pointer states are constructed as approximate fixed points of the controlled open-system dynamics, in such a way that they are guaranteed to survive over a long time with a fidelity determined by the relative precision with which the dynamics is engineered. We provide quantitative minimum-fidelity bounds by identifying symmetry and ergodicity conditions that the decoherence-inducing perturbation must obey in the presence of control, and develop explicit pulse sequences for engineering any desired set of orthogonal states as pointer states. These general control protocols are validated through exact numerical simulations as well as semiclassical approximations in realistic single- and two-qubit dissipative systems. We also examine the role of control imperfections, and show that while pointer-state engineering protocols are highly robust in the presence of systematic pulse errors, the latter can also lead to unintended pointer-state generation in dynamical decoupling implementations, explaining the initial-state selectivity observed in recent experiments
Polytype control of spin qubits in silicon carbide
Crystal defects can confine isolated electronic spins and are promising
candidates for solid-state quantum information. Alongside research focusing on
nitrogen vacancy centers in diamond, an alternative strategy seeks to identify
new spin systems with an expanded set of technological capabilities, a
materials driven approach that could ultimately lead to "designer" spins with
tailored properties. Here, we show that the 4H, 6H and 3C polytypes of SiC all
host coherent and optically addressable defect spin states, including spins in
all three with room-temperature quantum coherence. The prevalence of this spin
coherence shows that crystal polymorphism can be a degree of freedom for
engineering spin qubits. Long spin coherence times allow us to use double
electron-electron resonance to measure magnetic dipole interactions between
spin ensembles in inequivalent lattice sites of the same crystal. Together with
the distinct optical and spin transition energies of such inequivalent spins,
these interactions provide a route to dipole-coupled networks of separately
addressable spins.Comment: 28 pages, 5 figures, and supplementary information and figure
NMR multiple quantum coherences in quasi-one-dimensional spin systems: Comparison with ideal spin-chain dynamics
The 19F spins in a crystal of fluorapatite have often been used to
experimentally approximate a one-dimensional spin system. Under suitable
multi-pulse control, the nuclear spin dynamics may be modeled to first
approximation by a double-quantum one-dimensional Hamiltonian, which is
analytically solvable for nearest-neighbor couplings. Here, we use solid-state
nuclear magnetic resonance techniques to investigate the multiple quantum
coherence dynamics of fluorapatite, with an emphasis on understanding the
region of validity for such a simplified picture. Using experimental,
numerical, and analytical methods, we explore the effects of long-range
intra-chain couplings, cross-chain couplings, as well as couplings to a spin
environment, all of which tend to damp the oscillations of the multiple quantum
coherence signal at sufficiently long times. Our analysis characterizes the
extent to which fluorapatite can faithfully simulate a one-dimensional quantum
wire.Comment: 14 pages, 11 eps color figure
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Quantum entanglement at ambient conditions in a macroscopic solid-state spin ensemble
Entanglement is a key resource for quantum computers, quantum-communication networks, and high-precision sensors. Macroscopic spin ensembles have been historically important in the development of quantum algorithms for these prospective technologies and remain strong candidates for implementing them today. This strength derives from their long-lived quantum coherence, strong signal, and ability to couple collectively to external degrees of freedom. Nonetheless, preparing ensembles of genuinely entangled spin states has required high magnetic fields and cryogenic temperatures or photochemical reactions. We demonstrate that entanglement can be realized in solid-state spin ensembles at ambient conditions. We use hybrid registers comprising of electron-nuclear spin pairs that are localized at color-center defects in a commercial SiC wafer. We optically initialize 103 identical registers in a 40-μm3 volume (with 0:95+0:05-0:07 fidelity) and deterministically prepare them into the maximally entangled Bell states (with 0.88 ± 0.07 fidelity). To verify entanglement, wedevelop a register-specific quantum-state tomography protocol. The entanglement of a macroscopic solid-state spin ensemble at ambient conditions represents an important step toward practical quantum technology
Nonlinear response and crosstalk of strongly driven silicon spin qubits
Micromagnet-based electric dipole spin resonance (EDSR) offers an attractive
path for the near-term scaling of dense arrays of silicon spin qubits in
gate-defined quantum dots while maintaining long coherence times and high
control fidelities. However, accurately controlling dense arrays of qubits
using a multiplexed drive will require an understanding of the crosstalk
mechanisms that may reduce operational fidelity. We identify a novel crosstalk
mechanism whereby the Rabi frequency of a driven qubit is drastically changed
when the drive of an adjacent qubit is turned on. These observations raise
important considerations for scaling single-qubit control.Comment: 12 pages, 9 figure
Pointer states via engineered dissipation
Pointer states are long-lasting high-fidelity states in open quantum systems.
We show how any pure state in a non-Markovian open quantum system can be made
to behave as a pointer state by suitably engineering the coupling to the
environment via open-loop periodic control. Engineered pointer states are
constructed as approximate fixed points of the controlled open-system dynamics,
in such a way that they are guaranteed to survive over a long time with a
fidelity determined by the relative precision with which the dynamics is
engineered. We provide quantitative minimum-fidelity bounds by identifying
symmetry and ergodicity conditions that the decoherence-inducing perturbation
must obey in the presence of control, and develop explicit pulse sequences for
engineering any desired set of orthogonal states as pointer states. These
general control protocols are validated through exact numerical simulations as
well as semi-classical approximations in realistic single and two qubit
dissipative systems. We also examine the role of control imperfections, and
show that while pointer-state engineering protocols are highly robust in the
presence of systematic pulse errors, the latter can also lead to unintended
pointer-state generation in dynamical decoupling implementations, explaining
the initial-state selectivity observed in recent experiments.Comment: 22 pages, 12 figure
Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet
The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ∼99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ∼400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. This work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification