Quantum Computers and Decoherence: Exorcising the Demon from the Machine

Decoherence is the main obstacle to the realization of quantum computers. Until recently it was thought that quantum error correcting codes are the only complete solution to the decoherence problem. Here we present an alternative that is based on a combination of a decoherence-free subspace encoding and the application of strong and fast pulses: ``encoded recoupling and decoupling'' (ERD). This alternative has the advantage of lower encoding overhead (as few as two physical qubits per logical qubit suffice), and direct application to a number of promising proposals for the experimental realization of quantum computers.Comment: 15 pages, no figures. Invited contribution to the proceedings of the SPIE Conference on Fluctuations and Noise. Section 8 contains a new result: how to eliminate off-resonant transitions induced by generic "bang-bang" pulses, by using a special type of "bang-bang" pulse

Self-protected quantum simulation and quantum phase estimation in the presence of classical noise

The decoherence phenomenon inevitably exists in quantum computing processes. Consequently, dynamic suppression of decoherence for instance via dynamical decoupling, quantum error correction codes (QECC) etc. is crucial in accurately executing known or to-be-developed quantum algorithms. While this dynamic zero noise strategy well fits into our expectations for the future of quantum computing, given the status quo, we have launched self-protected quantum algorithms for over 15 years based on the opposite living-with-noise strategy. Here we propose self-protected quantum simulations immune to a large class of classical noise. Accordingly, for readout we generalize the conventional quantum phase estimation to its upgraded version in the presence of classical noise.Comment: corrected typo