Capacitively coupled singlet-triplet qubits in the double charge resonant regime

Abstract

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 rapid 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 π 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 -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.

Universal Quantum Transducers Based on Surface Acoustic Waves

“…(v) Finally, there is a large body of previous theoretical work on electron-phonon coupling in gate-defined QDs (see, for example, Refs. [25,87,88,98]) where any effects due to the structure defining the QDs have been neglected as well. As a matter of fact, our description for the electron-phonon coupling emerges directly from these previous treatments in the limit where the continuum of phonon modes is replaced by a single relevant SAW cavity mode (similar to cavity QED, other bulk modes are still present and contribute to the decoherence of the qubit on a time scale T 2 ).…”

Simulation of the coupling strength of capacitively coupled singlet-triplet qubits

“…The |SS state can be modeled by taking two copies of the single-qubit Hamiltonian H S and adding a constant dipole-dipole interaction energy D to the |S 02 S 02 state, as done in Ref. 29, yielding the following Hamiltonian:…”

Two-qubit logical operations in three quantum dots system

“…Mehl et al [16] proposed the high-fidelity entangling quantum gate in two S-T qubits mediated by one quantum state from a quantum dot between them. The S-T qubits can be also coupled via an exchange [18] and a capacitive interaction [17].One of the most promising concept is an exchange-only qubit encoded in a doublet subspace of three spins [19]. The advantage of this proposal is easy control of the qubit states by purely electrical manipulations of the exchange interactions between the spins.…”