Discord and Global Discord in Systems of Coupled Quantum Dots in Driven Cavities with Dissipation, and a Method for the Calculation of Global Discord

In the field of quantum information, which is subdivided into quantum computing and quantum cryptography, quantum correlations are essential for a performance or security boost not achievable with classical means. Various quantum correlation measures exist for evaluating a state’s potential to be a qubit (quantum bit). Entanglement, or nonseparability of quantum states, is the older, better known class of measures. However, for a mixed state, quantum entanglement is an incomplete measure of quantumness. Quantum discord, and its multibody extension global discord, encompass all quantum correlations. We study systems of coupled quantum dots using these measures. We study the discord of two quantum dots in the steady state with dissipation and detuning. The entanglement of the system was previously studied by Mitra and Vyas. We compare quantum discord to entanglement, finding high discord in an unentangled region, namely the upper branch of the bistability curve, where the driving field is high. By adjusting the detuning between the dots and the driving field, we can optimize the quantum discord and entanglement. We present an efficient numerical method for calculating global discord and analyze its speed and scaling. We verify that the method works for two, three, and four qubits and run speed tests. We present further simplifications that greatly enhance the scaling of the method with system size provided that the bodies are identical, but which also improve the speed otherwise. We compare our system of two quantum dots to an otherwise identical system with three quantum dots. We observe tristability in the cavity field as a function of driving field. The high field limit of global discord is a larger portion of the maximum than the discord in the same limit for the system of two quantum dots. The improvement in correlations due to detuning is still present but diminished compared to that in our two quantum dot system. The decreased effectiveness of detuning is, however, offset by the increased effect of high dot coupling, whereby the peak found in said limit with no detuning may exceed the asymptotic value without the aid of detuning