  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.
