We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well-suited to achieving the strong coupling regime. Our approach combines the favorable coherence properties of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

%B Physical Review B %V 94 %P 205421 %8 2016/11/16 %G eng %U https://doi.org/10.1103/PhysRevB.94.205421 %N 20 %R 10.1103/PhysRevB.94.205421 %0 Journal Article %J Physical Review B %D 2015 %T Capacitively coupled singlet-triplet qubits in the double charge resonant regime %A V. Srinivasa %A J. M. Taylor %X We investigate a method for entangling two singlet-triplet qubits in adjacent double quantum dots via capacitive interactions. In contrast to prior work, here we focus on a regime with strong interactions between the qubits. The interplay of the interaction energy and simultaneous large detunings for both double dots gives rise to the double charge resonant regime, in which the unpolarized (1111) and fully polarized (0202) four-electron states in the absence of interqubit tunneling are near degeneracy, while being energetically well-separated from the partially polarized (0211 and 1102) states. A controlled-phase gate may be realized by combining time evolution in this regime in the presence of intraqubit tunneling and the interqubit Coulomb interaction with refocusing {\pi} pulses that swap the singly occupied singlet and triplet states of the two qubits via, e.g., magnetic gradients. We calculate the fidelity of this entangling gate, incorporating models for two types of noise - classical, Gaussian-distributed charge fluctuations in the single-qubit detunings and charge relaxation within the low-energy subspace via electron-phonon interaction - and identify parameter regimes that optimize the fidelity. The rates of phonon-induced decay for pairs of GaAs or Si double quantum dots vary with the sizes of the dipolar and quadrupolar contributions and are several orders of magnitude smaller for Si, leading to high theoretical gate fidelities for coupled singlet-triplet qubits in Si dots. We also consider the dependence of the capacitive coupling on the relative orientation of the double dots and find that a linear geometry provides the fastest potential gate. %B Physical Review B %V 92 %P 235301 %8 2015/12/01 %G eng %U http://arxiv.org/abs/1408.4740v2 %N 23 %0 Journal Article %J Physical Review Letters %D 2015 %T Tunable Spin Qubit Coupling Mediated by a Multi-Electron Quantum Dot %A V. Srinivasa %A H. Xu %A J. M. Taylor %X We present an approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantum dots via a multi-electron, two-level quantum dot. The effective exchange interaction mediated by the dot can be understood as the simplest manifestation of Ruderman-Kittel-Kasuya-Yosida exchange, and can be manipulated through gate voltage control of level splittings and tunneling amplitudes within the system. This provides both a high degree of tuneability and a means for realizing high-fidelity two-qubit gates between spatially separated spins, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system. %B Physical Review Letters %V 114 %P 226803 %8 2015/06/04 %G eng %U http://arxiv.org/abs/1312.1711v3 %N 22 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.114.226803 %0 Journal Article %J Physical Review Letters %D 2013 %T Electrically-protected resonant exchange qubits in triple quantum dots %A J. M. Taylor %A V. Srinivasa %A J. Medford %X We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates. %B Physical Review Letters %V 111 %8 2013/7/31 %G eng %U http://arxiv.org/abs/1304.3407v2 %N 5 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.111.050502