In recent years, nonlinear quantum interferometry has been widely explored for applications like spectroscopy, optical coherence tomography, and imaging [1]. Quantum infrared spectroscopy (QIRS) uses quantum interference of generation processes of frequency-entangled visible-IR photon pairs to perform infrared spectroscopy using visible light sources and silicon-based detectors [2]. This technique utilizes a nonlinear quantum interferometer configuration where the generated IR photons pass through the sample, and the correlated visible photons are detected. The change in the detected visible photon flux with and without the sample helps extract the complex transmittance of the sample in the IR region.
Quantum entanglement is one of the most intriguing features of quantum mechanics. To investigate the entanglement between two excitons in a biexciton, an experimental technique called density matrix spectroscopy (DMS) has recently been developed. DMS combines stimulated emission tomography and pump-probe techniques to obtain a time-resolved density matrix of the polarization state of a photon pair emitted from the biexciton. The reconstructed density matrix is expected to encode information about the biexciton state and its entanglement dynamics, but the precise nature of this connection has remained unclear. In this paper, we derive an analytical relationship between the density matrix obtained by DMS and the biexciton state. In addition, we perform numerical simulations to compare the entanglement dynamics obtained by DMS with the biexciton's entanglement dynamics in a two-dimensional electron-hole system using an extended ionic Hubbard model. We find that DMS can partially capture the entanglement in the biexciton, in particular, the dynamics of the difference
Sbi−Sk, where
Sbi is the entanglement entropy of the biexciton and
Sk is the entanglement in terms of the wavevectors of the excitons that constitute the biexciton. These results demonstrate the validity of DMS for obtaining information about the entanglement dynamics of the biexciton.
We present a method for quantum tomography of multiqubit states. We apply the method to spin-multiphoton states, which we generate using a quantum dot based device. Periodic excitation of a quantum dot confined spin deterministically generates strings of entangled photons in a cluster state, threefold faster than previously demonstrated. Our tomography method uses time-resolved polarization-sensitive multiphoton correlation measurements to measure both the polarization of the confined spin and that of the emitted photons. We develop an edge-sensitive gradient-descent algorithm and apply it to the acquired data to map the periodic physical process that generates the cluster state. We utilize our tomographic method to optimize our quantum dot based device and show that the enhanced generation rate increases the entanglement robustness of the generated multiqubit state.