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A scheme is presented for engineering momentum-space entanglement of fragmented magnon condensates. We consider easy-plane frustrated antiferromagnets in which the magnon dispersion has degenerate minima that represent "umbrella" chiral spin textures. We tune the Hamiltonian near a quantum critical point that is signaled by a singularity in the entanglement entropy. The ground state develops momentum-space entanglement of the chiral spin textures. The size of the entangled superposition is accessible experimentally through the magnetic structure factor. Our model is motivated by equilibrium magnon condensates in frustrated antiferromagnets such as CsCuCl3, and it can also be simulated in spin-orbit coupled Mott insulators in atomic optical lattices and circuit quantum electrodynamics. [ABSTRACT FROM AUTHOR]

We propose an optimized set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the S-T- subspace. Qubit rotations are driven by the applied magnetic field and a field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of the magnetic fields, taking advantage of "sweet spots" where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states. Our results show that, for silicon quantum dots, gate fidelities greater than 99% should be realizable, for rotations about two nearly orthogonal axes. [ABSTRACT FROM AUTHOR]