Quantum Tomography–Measurability of the Spin Density Matrix Revisited

Roger G. Newton, Bing-Lin Young

J. A. Aguilar-Saavedra, Pier Paolo Giardino

We revisit quantum tomography of $H \to ZZ$ and $H \to WW$ in the presence of higher-order corrections. We verify that neither the use of an effective spin analysing power (only for $ZZ$) or a photon veto are sufficient to render the naively-constructed spin density operators physical. A subtraction of higher-order corrections is thus necessary to perform consistent quantum tomography. Such corrections are small when compared to expected experimental uncertainties with current data. As a by-product, we point out the striking possibility to observe parity-violating effects in $H \to WW$.

Iaakov Exman, Ariel Zvulunov

Quantum Tomography partially measures and then recovers the remaining density matrix quantum state, in order to verify that a certain device -processor or detector -indeed outputs the intended quantum state.However, single matrix element measurements rapidly increase in number with the density matrix dimension and are error-prone.This work proposes a novel quantum software viewpoint on quantum tomography.Instead of individual matrix elements, measurements and density matrix recovery are performed on higher-abstraction modules, i.e. semantically meaningful groups of matrix elements.Quantum Modules Tomography potentially reduce the necessary number of explicit measurements, while still allowing recovery of the whole density matrix.A Recovery Theorem is formulated and proved: density matrix diagonal measurements suffice to recover the whole system density matrix, in terms of modules.The recovery procedure is illustrated by a few case studies.

Roger G. Newton, Bing-Lin Young

William J. Munro, Daniel F. V. James, Andrew G. White +1 more

Quantum computation depends on quantum entanglement, a correlation between subsystems that cannot occur classically. A variety of theoretical measures exist for quantifying the degree entanglement in such schemes, all of which are functions of the system density matrix. How can the entanglement be measured experimentally? Using quantum tomography techniques developed for two photon entangled states, the density matrix can be reconstructed from the appropriate experimental data. In this case the state tomography gives the complete characterization of the physical system (for the relevant degree of freedom, such as spin or polarization). It gives information on both the degree of nonclassical correlation, that is entanglement, as well as the amount of decoherence in the system. In this proceedings we discuss the general state tomography procedure required to characterize a few qubit quantum computer, for any architecture.