Editing Full Quantum Process Tomography with Linear inversion (section)

==Properties==
* '''Figure of merit''': Density Matrix of the quantum process
* Multiple copies of the quantum state are required in this method.
* The process <math>\varepsilon</math> can be completely determined by <math>\chi</math> once the set of operators <math>\tilde{A_i}</math> has been fixed.
* <math>\chi</math> will contain <math>N^4 - N^2</math> parameters
* In the case of a “nonselective” quantum evolution, such as arises from uncontrolled interactions with an environment the <math>A_i</math> operators satisfy an additional completeness relation <math>\sum_i A_i^{\dagger} A_i = I</math>. This relation ensures that the trace factor <math>tr(\varepsilon(\rho))</math> is always equal to one, and thus the state change experienced by the system can be written <math>\rho \xrightarrow{} \varepsilon(\rho)</math>
* Quantum state tomography has to be performed multiple times in this experiment.
* This method is extremely resource intensive.
* The main drawback of this technique is that sometimes the computed solution of the density matrix will not be a valid density matrix. This issue occurs when not enough measurements are made. Due to this drawback, the maximum likelihood estimation technique [link here] is preferred over this linear inversion technique.
* Another drawback of this method is that an infinite number of measurement outcomes would be required to give the exact solution.
* Entanglement fidelity can be evaluated with the knowledge of <math>A_i</math> operators. This quantity can be used to measure how closely the dynamics of the quantum system under consideration approximates that of some ideal quantum system. This quantity can be determined robustly, because of its linear dependence on the experimental errors. Suppose the target quantum operation is a unitary quantum operation <math>U(\rho) = U\rho U^\dagger</math> and the actual quantum operation implemented experimentally is <math>\varepsilon</math>. Then the entanglement fidelity is <math>F_e(\rho, U, \varepsilon) = \sum_i {|tr(U^{\dagger}A_i \rho )|}^2 = \sum_{mn} \chi_{mn} tr(U^{\dagger}\tilde{A_m}\rho) tr(\rho \tilde{A_n^{\dagger}} U)</math>
* The minimum value of <math>F_e</math> over all possible states <math>\rho</math> is a single parameter which describes how well the experimental system implements the desired quantum logic gate
* Minimum fidelity of the gate operation is <math>F = min_{|\psi \rangle} \langle\psi| U^{\dagger} \varepsilon (|\psi\rangle\langle\psi|) U|\psi\rangle</math>
* Quantum channel capacity which is a measure of the amount of quantum information that can be sent reliably using a quantum communications channel which is described by a quantum operation <math>\varepsilon</math> can also be defined.
* This procedure can also be used to determine the form of the [[Lindblad operator]] used in Markovian master equations of a certain form.
* Quantum operations can also be used to describe measurements. For each measurement outcome, <math>i</math>, there is a associated quantum operation <math>\varepsilon_i</math>. The corresponding state change is given by <math>\rho \xrightarrow{} \varepsilon_i(\rho)/tr(\varepsilon_i(\rho))</math> where the probability of the measurement outcome occurring is <math>p_i = tr(\varepsilon_i(\rho))</math>. To determine the process the steps are the same, except the measurement has to be taken a large enough number of times that the probability pi can be reliably estimated. Next <math>\rho^{'}_{j}</math> is determined using tomography, thus obtaining <math>\varepsilon_i(\rho_j) = tr(\varepsilon_i(\rho_j))\rho^{'}_{j}</math> for each input <math>\rho_j</math>. The other steps remain to same to estimate <math>\varepsilon_i</math>.