Thursday, August 1, 2013

1307.8406 (Teck Seng Koh et al.)

High fidelity gates in quantum dot spin qubits    [PDF]

Teck Seng Koh, S. N. Coppersmith, Mark Friesen
Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. Differences between the schemes can make it difficult to compare them meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet (ST) and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance (LQR), and a gate-like process known as stimulated Raman adiabatic passage (STIRAP). These gates are all mediated by an exchange interaction that depends on a small number of experimental control parameters, including the interdot tunnel coupling $g$ and the detuning $\epsilon$, which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity ($f$) for fixed $g$ as a function of the other control parameters; this yields an $f^\text{opt}(g)$ that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound $f^\text{max}$ that is specific to the qubit-gate combination. We show that similar gate fidelities ($\sim 99.5$%) should be attainable for ST qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields $\Delta B$ produced by nuclear spins.
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