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The role of state preparation in quantum process tomography

Kavan Modi, University of Texas

(Session 13 : Sunday from 12:00-12:30)

Abstract. The immense computational power of a quantum computer comes with a cost - the fragility of entangled quantum states from coherence loss. Although decoherence is present in all physical systems, the effect of these logic errors can be eliminated by using error correcting codes provided gate errors fall below a fault tolerance threshold. This threshold depends on system architecture and specific forms of decoherence, but is likely to be in the 10^-4 range. The measurement of gate fidelity is thus a critical step for implementing fault tolerant quantum computing.

Most experiments determine coherence through T1 and T2 measurements, which gives only a simple description of error process in qubits. A more full and precise measurement is based on density matrix measurements of qubit states, which leads to a description of coherence in terms of state and process tomography. We study the effects of preparation of input states in quantum process tomography experiments. We study two preparation procedures, stochastic preparations and preparations by measurements. We show that for stochastic preparation procedure, linear process maps adequately describe the process. But when linear process maps are obtained from systems initially prepared using von Neumann measurements, they cannot describe the process adequately. We introduce a quadratic process map that can describe the processes initialized by preparation by measurements. I will discuss the consequences of the quadratic map and its properties.