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Journal of Modern Optics

Enhancement of discord and entanglement with detuning for two coupled quantum dots in a driven cavity

Willa Rawlinson, Reeta Vyas

Published Mar 11, 2015• 3 citations
Publisher

Abstract

We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.

DOI
10.1080/09500340.2015.1019383

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We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.
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.
Generation and dissipation of entanglement between two coupled quantum dots (QDs) in a cavity driven by a coherent field is studied. We find that it is possible to generate and sustain a large amount of entanglement between the quantum dots in the steady state, even in the presence of strong decay in both the cavity and the dots. We investigate the effect of different parameters (decay rates, coupling strengths, and detunings) on entanglement. We find that the cavity field shows bistability and study the effect of relevant parameters on the existence of this bistable behavior. We also study the correlation between the cavity field and the entanglement between the dots. The experimental viability of the proposed scheme is discussed.
The time evolution of entanglement between two quantum dots (QDs) trapped inside a cavity driven by a coherent quantized field is studied. In the presence of dissipation, entanglement shows many interesting features such as sudden death and revival, and finite steady state value after sudden death. We also investigate dependence of entanglement on dot variables and its relation to bistability. It is found that entanglement vanishes when the cavity field intensity approaches the upper branch of the bistability curve. When the cavity is driven by a modulated field in the presence of dissipation, it can periodically generate entanglement, which is much larger than the maximum value attained in the steady-state for this system but the dots are never fully entangled.