One way to extend the functionality of quantum dots as a platform for quantum applications is through control of the dark exciton states in quantum dots. The photon generation from quantum dots relies on the recombination of bright excitons. However, quantum dots also host excitons of parallelly-oriented spins, called dark excitons. While the bright excitons in quantum dots emit light, the dark excitons are optically inactive. Due to the suppression of emission, the dark exciton states exhibit a substantially reduced decay rate. As a result, the lifetimes of these excitons can be orders of magnitude longer than their optically bright counterparts. This attribute makes dark exciton states beneficial for storing and controlling quantum information over time. Quantum dots on a semiconductor material (black) can be precisely controlled through a new method that uses chirped laser pulses and a magnetic field. Courtesy of the University of Innsbruck. To harness this potential in dark exciton states, a research team from the University of Innsbruck devised a way to optically access and coherently manipulate dark excitons in quantum dots using a laser and an external in-plane magnetic field. “Using chirped laser pulses and a magnetic field, we can manipulate the spin state of these excitons in a controlled manner and transform bright excitons into dark ones. We can also reverse this process and turn dark excitons back into bright ones,” researchers Florian Kappe and René Schwarz said. “In this way, the state can be kept in the dark for a prolonged period of time and reactivated later.” In experiments, the team demonstrated how an exciton could be stored in a dark state and converted back into a bright state through an additional laser pulse. The researchers used state-of-the-art simulations based on product tensor methods to investigate the dark state in a numerically exact way. The experimental results agreed with the theoretical predictions that were calculated using product tensor techniques. The researchers used a laser pulse sequence consisting of a transform-limited pulse and a pair of chirped pulses for their approach. They energetically tuned the pulses using 4f pulse shapers and introduced picosecond chirps via reflection off a chirped volume Bragg grating (CVBG). They hosted the semiconductor quantum dots in a closed-cycle cryostat with a base temperature of 1.5 °K. The optical activity of the dark state was controlled by a magnetic field of up to 4 teslas. The new approach to quantum dots dark state control could open as-yet-unexplored pathways to controlling quantum information with quantum dots. Control of the dark excitons in quantum dots could improve protocols for generating time-bin entanglement states and photonic cluster states, for example. Researchers assemble a large cryostat in an experimental physics laboratory, preparing for ultra-low temperature experiments with quantum dots on a semiconductor chip. Courtesy of the University of Innsbruck. The ability of quantum dots to generate high-quality states of quantum light, such as single photons, entangled photon pairs, and correlated multiphoton states, could offer immense flexibility in establishing the key functional elements of a quantum network. Single photons and entangled photon pairs are essential resources for optical quantum computing, secure communication via quantum key distribution, and the transmission of quantum information. The ability to leverage the versatile quantum dot state manifold, including the optically dark exciton states, could increase the impact of quantum dots on quantum technologies. “This opens up new opportunities for the control of quantum memories and the generation of entangled photon pairs in quantum dots,” professor Gregor Weihs, who led the research, said. The research was published in Science Advances (www.doi.org/10.1126/sciadv.adu4261).