2010-12-01
10:15 at HCI J 574

Quantum control of superconducting networks in the presence of solid state noise

Elisabetta Paladino

Dipartimento di Fisica e Astronomia - Universita' di Catania, Italy

Intense research on solid state nanodevices during the last 10 years has established their potentiality to combine quantum coherent behavior with the existing integrated-circuit fabrication technology. High-fidelity single qubit gates based both on semi- and super-conducting technologies are nowadays available and two-qubit logic gates have been demostrated. Limitations in the performances arise from noise due to material and device dependent sources. Contrary to the quantum optics realm, solid state noise has broadband character being often strong at low-frequencies. Optimizing the tradeoff between efficient addressing and noise is a central issue for the achievement of advanced quantum control comparable to atomic systems. Effects due to non Gaussian and non Markovian noise, as 1/f noise or noise due to strongly coupled impurities are well known in single qubits. In larger logical systems they may limit seriously the performance of advanced protocols, since controlled degrees of freedom may exhibit small energy splittings and enhanced sensitivity to noise. Because of this, constrains on device design to observe phenomena as coherent population transfer via adiabatic passage emerge. Another key example is the implementation of qubit couplings, where also the presence of several noise sources, correlated to a different extent, may play an important role. Controlled generation of entangled states and preservation of quantum correlations represent critical issues towards the achievement of the high performances required to overcome classical processors. Implications are implementation of universal two-qubit gates, the possibility to store entangled states in solid-state memories and entanglement preservation during local operations in quantum algorithms. Identification of strategies to counteract physical processes detrimental to quantum coherent behavior is a fundamental step towards this goal. Here we present a general route to reduce inhomogeneous broadening in nanodevices due to 1/f noise. We apply this method to a universal two-qubit gate and demonstrate that for selected optimal couplings, a high-efficient gate can be implemented even in the presence of 1/f noise. Entanglement degradation due to interplay of 1/f and quantum noise is quantified via the concurrence. A charge-phase sqrt(i-SWAP) gate for spectra extrapolated from single qubit experiments is analyzed. Beside the interest for quantum gates engineering, on a more fundamental level our analysis is relevant for optimizing fault-tolerant architectures, showing that the influence of 1/f fluctuations in the solid state can be limited by exploiting the band structure of coupled nanodevices. In addition, motivated by the fact that disentanglement may markedly differ from the single qubit decoherence, we study entanglement degradation of two non-interacting qubits subject to independent baths with broadband spectra. We obtain the analytic form of the concurrence in the presence of adiabatic noise for classes of entangled initial states presently achievable in experiments. We find that adiabatic (low frequency) noise affects entanglement reduction analogously to pure dephasing noise. Due to quantum (high frequency) noise, entanglement is totally lost in a state-dependent finite time. The possibility to implement on-chip both local and entangling operations is briefly discussed.