Robust charge-based qubit encoding

Quantum computation faces considerable hurdles, one of the most serious being engineering physical systems performing coherent operations without the deleterious effects of decoherence,1 particularly in the solid state. However by isolation and manipulation of states of quantum dot QD structures,2 it may be possible to perform many unitary operations within the dephasing time, a pre-requisite for quantum error correction QEC by means of Calderbank–Shor– Steane codes.3 Underlying logical QEC, a complementary strategy is to use Hilbert subspaces which couple least to noise processes, decoherence free subspaces DFS .4–7 Practical quantum computing will undoubtedly use elements of both. Chargebased QD quantum computing8–13 is a prime candidate for DFS encoding as electric field coupling is a major source of decoherence.14,15 Here, we present an architecture incorporating charge symmetry of the logical states to protect against electromagnetic fluctuations, analyze its resistance to charge trap noise and present single-qubit gates. Coupling to charge trap noise and decoherence is suppressed by several orders of magnitude compared to a conventional charge qubit, depending on charge trap density. Alternatives to the passive control implied by DFS encoding include active control sequences, such as Bang-Bang control.16,17 In a typical charge-based QD qubit Fig. 1 a the position of an excess electron defines the logical states. Ideally, the logical states of the system should be eigenstates of the system Hamiltonian when the system is idle, i.e., tunneling should be suppressed on practical timescales by V . Furthermore, we assume that the system can be tuned, via V0,1, such that the logical states are degenerate, hence known relative dynamical phases can be neglected.