Profs. Grzegorz Karczewski and Tomasz Wojtowicz of the Institute of Physics of the Polish Academy of Sciences, beneficiaries of the Foundation for Polish Science’s MASTER/MISTRZ programme, are among the scientists responsible for a groundbreaking achievement – they have developed a new method for storing optical information using electron spins. The results of their research are presented in a paper published in the prestigious Nature Photonics journal.
The possibility of storing optically encoded information (in the form of light pulse) provides a chance for further development of ultra-fast computers and quantum telecommunication. The currently available information storage media do not guarantee such fast processing speeds as optical coding can offer. In practice, however, it is very difficult to achieve long-term storage with fast recording and reading of optical information. This is because of the quantum-mechanical physical properties of light and materials which could potentially be used as memory storage media.
Previously, one of the main problems of quantum optoelectronics had been the very short lifetime (information storage time) of quantum states in semiconductor quantum structures used as a very fast and energy-efficient information recording system. The work of the team including the Polish physicists has allowed this time to be extended by over 1000 times. The results of the research documenting this achievement are presented in a paper published in the prestigious Nature Photonics journal .
The international research team, featuring scientists from Dortmund and St Petersburg as well as the Institute of Physics of the Polish Academy of Sciences, presented a new and unique information storage method. It uses a stimulated photon echo, a physical phenomenon in which the information contained in an optical field (light pulse) is transferred to the electron spin system of the semiconductor structure – the so-called quantum well – and after a while again radiated in the form of a light pulse that is a copy (in terms of the intensity, optical coherence and phase) of the original light pulse – its “echo”. In this state, information in the form of the quantum excitation of the spin system, isolated from the optical field of the vacuum, can last considerably longer than the optical excitation itself.
The system that “stores” information using this method is an electron gas spin system in a quantum well. Such special, ultra-high quality quantum wells were grown in the Laboratory of Growth and Physics of Low-Dimensional Crystals (SL3) of the Institute of Physics in Warsaw from a cadmium telluride semiconductor using the molecular beam epitaxy method. Utilising the subtle phenomenon of photon echo with trion transitions (trions are charged three-particle complexes, in this case comprising of two electrons and one hole) in quantum wells in the magnetic field, the authors of the publication demonstrated that the time of information storage in this new type of spintronic memory can be extended by more than three orders of magnitude, from the picosecond range up to tens of nanoseconds.
This represents a significant step towards achieving efficient and long-term storage of optical information, thereby constituting a model of a new type of spintronic optical memory. Profs. Grzegorz Karczewski and Tomasz Wojtowicz also tell us that in the same international group of scientists they are currently working on further extension of the storage time of optical information using electron spins, this time not in two-dimensional objects, but in zero-dimensional objects containing electrons, CdTe and CdSe quantum dots, produced in the SL3 lab of the Institute of Physics using the molecular beam epitaxy method.
 “Access to long-term optical memories using photon echoes retrieved from semiconductor spins”, L. Langer, S. V. Poltavtsev, I. A. Yugova, M. Salewski, D. R. Yakovlev, G. Karczewski, T. Wojtowicz, I. A. Akimov, and M. Bayer, Nat. Photon. 8, 851 (2014).
prof. dr hab. Grzegorz Karczewski
Institute of Physics, Polish Academy of Sciences
tel. +48 22 1162541, +48 22 8431331
prof. dr hab. Tomasz Wojtowicz
Institute of Physics, Polish Academy of Sciences
tel. +48 22 1163123, +48 22 8431331
Department of Physics and Astronomy, Purdue University,
525 Northwestern Avenue, West Lafayette, IN 47907-2036
tel. +01 765 5883415
Fig. 1. a) Diagram of CdTe quantum well with CdMgTe barriers containing a two-dimensional electron gas immediately after resonant optical excitation creating trions inside it (indicated by circles containing two red dots – electrons and one hole – marked with a plus sign);
b) position of conduction bands (PP) and valence bands (PW) in the structure of this well with quantum levels and a single trion T marked;
c) scheme of the recording and optical information reading system in the quantum well with the direction of the magnetic field B marked as well as the directions of optical pulse beams: memorised (1), writing (2), reading (3), photon echo (EF) and stimulated photon echo (SEF);
d) temporal sequence of pulses and example of experimentally observed photon echo and short-term stimulated photon echo before switching on the magnetic field permitting to record an information in the spin system (Figure adapted from arXiv:1401.3377).