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arXiv:quant-ph/0512244 · Physica E Low-dimensional Systems and Nanostructures

The effect of local structure and non-uniformity on decoherence-free states of charge qubits

Tetsufumi Tanamoto, Shinobu Fujita

Published Apr 20, 2006• 1 citation
arXivHTMLPDF
DOI
10.1016/j.physe.2006.03.098

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Quantum computing promises significant speed-up for certain types of computational problems. However, robust implementations of semiconducting qubits must overcome the effects of charge noise that currently limit coherence during gate operations. Here we describe a scheme for protecting solid-state qubits from uniform electric field fluctuations by generalizing the concept of a decoherence-free subspace for spins, and we propose a specific physical implementation: a quadrupole charge qubit formed in a triple quantum dot. The unique design of the quadrupole qubit enables a particularly simple pulse sequence for suppressing the effects of noise during gate operations. Simulations yield gate fidelities 10-1,000 times better than traditional charge qubits, depending on the magnitude of the environmental noise. Our results suggest that any qubit scheme employing Coulomb interactions (for example, encoded spin qubits or two-qubit gates) could benefit from such a quadrupolar design.
We analyze the robustness of a decoherence-free (DF) subspace and subsystem in charge qubits, when the difference from the collective decoherence measurement condition is large $(\ensuremath{\sim}5%)$ in the long time period, which is applicable for solid-state qubits using as a quantum memory. We solve master equations of up to four charge qubits and a detector as a quantum point contact. We show that robustness of DF states is strongly affected by local nonuniformities. We also discuss the possible two-qubit logical states by exactly solving the master equations.