Quantum Operations on Density Operators

Zhaofang Bai, Shuanping Du

The unital quantum operation acting on infinite dimensional quantum states fixing a convex cone of density operators is completely characterized. Based on this result, we classify the commutativity of two quantum operations and determine what kind of measurement statistics are preserved by a unital quantum operation. Our work extends the results of Lee et al (2013 J. Phys. A: Math. Theor. 46 205305) to the infinite dimensional case and also corrects part (a) of theorem 6 in Lee et al (2013 J. Phys. A: Math. Theor. 46 205305).

Maurice A. de Gosson

Steven M. Moore

Gerhard G. Kramer, Serap A. Savari

The problem of compressing strings of quantum states whose density operators commute is considered. Two versions of the problem are discussed. In the visible case arbitrary operations on the strings are permitted and in the blind case only quantum operations on the density operator of the strings are permitted. Both cases have been reformulated as classical information-theoretic problems, and these problems are solved.

Rajnish Kumar

Abstract The characterization of quantum noise is fundamental to quantum information processing, quantum control and reliable operation of near-term quantum devices. Conventional descriptions of quantum noise are based on completely positive trace-preserving (CPTP) maps acting on density operators. These models describe effective quantum dynamics after the underlying physical interactions have been represented as a quantum channel. However, in practical cases, imperfections such as calibration errors, control-field fluctuations, finite-resolution waveform generation, and analog hardware noise can act upon the probability amplitudes of a quantum state earlier. The effect of such perturbation in quantum state after normalization has largely been unexplored. In this work, we develop a rigorous mathematical analysis of the normalized additive amplitude noise in a finite-dimensional quantum system. Assuming a zero-mean complex perturbation, we derive a second-order closed form expression for the ensemble-averaged density operator. The analysis leads to state-dependent evolution of the quantum state which cannot be represented by a CPTP map. We further prove that in case of isotropic noise, the ensemble-averaged density operator becomes an effective depolarizing channel. For anisotropic perturbations, we derive the orientation-dependent Bloch-vector contraction which is determined by the covariance geometry of the additive noise. Closed-form expressions are further obtained for measurement probabilities, state fidelity, and ensemble purity, showing that fidelity degradation depends only on the component of the perturbation covariance orthogonal to the input state. Monte Carlo simulations closely match the theoretical analysis, especially for small noise regime. The proposed model establishes a connection between classical additive-noise models and effective quantum dynamics generated by normalized state-space perturbations.

C Peltzer, Julius J. Brandstatter