Controlled Timing of Light Echoes

Mountaineers who shout into a canyon have no way of controlling the time it takes for the acoustic echo of their yell to return to them. Researchers at two German universities have devised a laser-pulse method for precise control of optical echoes from semiconductor quantum dots (Commun. Phys., doi: 10.1038/s42005-020-00491-2).

Studying photon echoes

Quantum dots and their excitons can be modeled as simple two-level quantum systems, which makes them useful for investigating nanoscale phenomena. Photon echoes are the optical-frequency analog to spin echoes, in which a pulse of radio-frequency radiation resets the spin of a simple system such as a hydrogen atom—the technology behind magnetic resonance imaging in medicine.

“The main idea is to stop and freeze the dephasing in an inhomogeneous ensemble of oscillators—an intrinsic process that usually happens automatically and cannot be avoided if one does not act actively against it,” says Ilja A. Akimov, a physicist at the Technical University of Dortmund, Germany.

Control on picosecond timescales

First, the group, led by Akimov and Torsten Meier of Paderborn University, Germany, mathematically modeled the effect light pulses would have on the phases of the exciton ensemble of a quantum dot. Next, the team set up an experiment involving a single layer of indium/gallium arsenide quantum dots sandwiched between layers of aluminum gallium arsenide. The substrate layers, ranging in thickness from 68 nm to 82 nm, formed a Bragg mirror with a resonator mode in the spectral range of 910 nm to 923 nm.

With the sandwich cooled to 2 K, the scientists hit it with 2.5-ps excitation and control pulses from a mode-locked Ti:sapphire laser, with mechanical translation stages varying the delay times between the two types of pulses. These pulses create the photon echo. A third resonant pulse—coming from the same laser—either slows down or speeds up the emission time of the photon echo, depending on its arrival time. This delay or advance was up to 5 ps.

“Previous studies of photon echoes in atomic ensembles and rare-earth crystals used optical pulses with durations of 100 ns and longer,” Akimov says. The short duration of the photon echo pulses in these semiconductor quantum dots enabled the researchers to extend the timing control into the picosecond range.

Quantum memory applications and beyond

This type of control over photon echoes could be important in future plans to manipulate light emission from quantum dots and other nanoscale photonic systems. Akimov and the Paderborn University researchers intend to apply this work to high-bandwidth optical quantum memories based on semiconductor quantum dots.

“In general, an accurate control of timing of short optical signals is required in optoelectronic and nanophotonic circuits,” Akimov says. “In particular, our results could be used for timing corrections in quantum optical memory protocols where non-classical optical signals are stored in the QD [quantum dot] ensemble and can be retrieved in the form of photon echoes at any desired time.”

“At the same time, our studies open plenty of other possibilities such as quantum interferometry of electronic excitations in solid-state systems,” Akimov adds. “In this case the control pulse will be used to split the photon echo in two pulses in a similar way as it is done in conventional Michelson or Mach-Zehnder interferometers.”

Meier and his colleagues led the theoretical portion of the study, while Akimov and collaborators implemented the experiment work. Researchers from two other German universities (University of Würzburg and University of Oldenburg) and two Russian institutions (St. Petersburg State University and the Ioffe Institute of the Russian Academy of Sciences) also participated in the study.