Nonlinear Optical Quantum Communication with a Two-Dimensional Perovskite Light Source

Abstract

Two-dimensional organic-inorganic hybrid perovskite (2D-OIHP) quantum wells are emerging as promising light sources for quantum communication technologies, owing to their ability to generate polarization-encoded optical signals. In this work, we explore how nonlinear optical phenomena can be exploited for quantum information applications, demonstrating the versatility that arises from resonant coupling among excited states. By tracking changes in the ellipticities of signal photons on femtosecond timescales in four-wave-mixing experiments, we first establish a method for information encoding based on exciton spin dynamics and biexciton correlations. Using single-photon detection, we then implement the BB84 quantum key distribution protocol by mapping these polarization states onto binary sequences. While the polarizations of weak coherent pulses are typically manipulated with optical elements in traditional quantum key distribution approaches, the intrinsic electronic structure and spin relaxation processes within the 2D-OIHP system determine the characteristics of the signal photons in our method. As a demonstration, an ASCII message consisting of 56 bits is transmitted through the polarization states of photons emitted by 2D-OIHP quantum wells. These results show that the information transmission efficiency depends strongly on contributions from biexciton states, highlighting the potential of spin-dependent nonlinear optical processes for quantum communication.

Figures (10)

• Figure 1: (a) Spin relaxation induces an elliptical-to-linear transformation in the polarization of the transient grating signal field. The relative amplitudes of the wave components that contribute to this effect are governed by the populations of the exciton states. (b) Circularly polarized light generates a macroscopic spin alignment within the ensemble of quantum wells. The radiated signal field is elliptically polarized due to the nonequilibrium distribution of populations within the exciton fine structure. (c) Single- and biexciton resonances radiate fields with distinct polarization ellipses, reflecting differences in spin dynamics and optical selection rules.
• Figure 2: Transient-grating experiments were conducted with a diffractive-optic--based interferometer and square beam geometry. The initial pulse pair was circularly polarized, whereas the third pulse was vertically polarized. Conventional dispersed signal detection was accomplished with a spectrograph and array detector. Alternatively, the signal beam was attenuated to a mean fluence of $\sim$1 photon per pulse and detected with a pair of silicon photomultiplier modules (SiPM). The transient imbalance between horizontal and vertical signal polarizations was determined by passing the beam through the quarter-wave plate and polarizer after the sample. The quarter-wave plate in the photon-resolved detection path was mounted on a motorized rotation stage.
• Figure 3: The (PEA)$_2$PbI$_4$ system exhibits a fine structure consisting of three bright single-exciton states with angular momentum quantum numbers $M_J = -1, 0, +1$. The basis set also includes three biexciton states with energies determined by whether the angular momentum vectors of the individual excitons have parallel or antiparallel orientations. The transition dipole moments, $\boldsymbol{\mu}_{g,+}$ and $\boldsymbol{\mu}_{g,-}$, are circularly polarized within the $x$–$y$ plane of the quantum well, whereas $\boldsymbol{\mu}_{g,0}$ is linearly polarized along the out-of-plane $z$-axis.
• Figure 4: Four-wave mixing signal fields are calculated at a pump--probe delay of $T = 0$ using the parameters in Table \ref{['table1']}. In this model, the single-exciton resonance energy is located at $E_{\mathrm{det}}=0$, whereas the biexcitons are spectrally shifted from the origin. Real (absorptive) and imaginary (dispersive) signal fields are shown for the (a) RRRR and (b) RRLL conditions, where R and L denote right- and left-handed circularly polarized fields, respectively. (c) The RRRR condition yields the largest signal intensity, while the peak of the RRLL spectrum is slightly red-shifted. Real and imaginary signal field components for the (d) RRVV and (e) RRVH conditions are also shown, where V and H denote vertical and horizontal linear polarizations, respectively. (f) The red shift of the RRVH signal intensity relative to RRVV reflects the linear combination of the RRRR and RRLL signals defined in Equation \ref{['eq:signal_combination_VH']}.
• Figure 5: Transient grating signal intensities measured at the indicated quarter-wave plate angles and detection wavelengths. The angle between the vertically polarized probe and the fast axis of the quarter-wave plate used for signal analysis is set to either (a)--(c) $\theta_{\mathrm{qwp}} = 0^{\circ}$ or (d)--(f) $\theta_{\mathrm{qwp}} = 45^{\circ}$. The relative intensities of the horizontally and vertically polarized signals vary weakly across the 500--525 nm range but show significant dispersion in the spectral region corresponding to the lower-energy biexciton. In each panel, the intensities are normalized to the peak value of the signal with the largest magnitude.