On the relationship between noise squeezing and Rabi oscillations in active quantum dot ensembles

TL;DR

The paper investigates how Rabi oscillations in room-temperature quantum-dot SOAs interact with amplified spontaneous emission to shape the noise of ultrafast optical pulses. By driving the QD SOA with ~100 fs resonant pulses and performing homodyne tomography, the authors map the output into Wigner functions that reveal cyclical noise modification with pulse-area changes of $2\\pi$, including squeezed-thermal states with noise below the vacuum and a non-Gaussian two-lobed state under certain conditions. The results extend the understanding of active media as a source of engineered quantum noise and point toward on-chip noise tailoring for quantum communication and metrology. The observed phenomena persist over two orders of magnitude in input energy and arise from the gain-absorption cycling in the QD SOA during Rabi dynamics, with potential improvements via cavity feedback and pulse shaping.

Abstract

Squeezed light is usually generated using passive nonlinear materials. Semiconductor lasers and optical amplifiers (SOAs) also offer nonlinearities but they differ in that they add amplified spontaneous emission (ASE). Squeezing to below the vacuum level has been demonstrated in a semiconductor laser, and gain saturation in SOAs can likewise reduce photon-number fluctuations to, and in some cases below, the vacuum limit. Here, we demonstrate that Rabi oscillations in room-temperature quantum-dot SOAs, induced by short resonant pulses, cause cyclical noise modification that repeat with every change of 2pi in pulse area, corresponding to a fourfold increase in excitation pulse energy. Homodyne measurements reveal in those cases elliptical Wigner functions corresponding to squeezed thermal states and in certain regimes, the state is squeezed to below the vacuum level. At other pulse areas, the Wigner functions are circular representing thermal coherent states. This periodic behavior persists over two orders of magnitude in input pulse energy, spanning several 2pi cycles. Under specific bias and excitation conditions, we further observe a non-Gaussian Wigner function featuring two bright lobes. Although its precise nature remains unresolved, this structure may be consistent with a Schrodinger cat - like state whose accompanying negativity is suppressed due to an approximately 10 dB optical output loss. Notably, the emergence of this non-Gaussian state is itself periodic in excitation pulse energy.

Figures (6)

• Figure 1: Experimental setup for homodyne detection of femtosecond pulses following the interaction with a QD ensemble. The two small insets show Wigner functions of the spontaneous emission noise under linear conditions (no input signal) and of a squeezed state.
• Figure 2: Measurements of the homodyne system for three different excitation pulse energies. The left column represents the measured temporal correlation between the LO and the signal. The middle column represents the histogram of the normalized output current of the balanced photodetector as a function of the LO phase measured at 0 ps, i.e., the trailing peak, which is indicated by the dashed line in the correlation graph. The right column describes the corresponding Wigner functions. X and P are the quadratures for which $X^2+P^2$ equals the square of the photon number.
• Figure 3: Excitation energy dependent Wigner functions. The left panel shows the Wigner distributions of the vacuum (red) and the ASE (green). At 1.56 pJ, 6.28 pJ and 9.95 pJ, the Wigner functions are round, representing coherent thermal states. Each is compared to the standard deviation of the ASE (green circle) and are found to be equal or slightly wider than the ASE. The Wigner functions at 3.96 pJ and 15.8 pJ are elliptical. $\Delta X$ for the 3.96 pJ case is equal to the width of the ASE. However, at an excitation energy four times large namely at 15.8 pJ, $\Delta X$ is slightly smaller than the standard deviation of the vacuum shown in a red circle and hence represents a squeezed state.
• Figure 4: Wigner functions for large excitation energies At 9.4 pJ, 35.3 pJ and 141 pJ, the Wigner functions are elliptical while all other energies, yield round Wigner functions representing thermal coherent states. These are equal or slightly larger than the uncertainty of the ASE, shown as green circles. The elliptical Wigner functions at 9.4 pJ 35.3 pJ and 141 pJ are squeezed thermal states with their $\Delta X$ being similar to the ASE shown in green circles.
• Figure 5: Squeezed state at an excitation of 25 pJ (a) Squeezed Wigner function distribution. The red circle represents the vacuum. (b) contour plots of the Wigner and vacuum distributions highlighting the squeezed state. The full width at half maximum is reduced from 1.43 for the vacuum state to 1.17 for the squeezed state.