Creating Ultrafast Qubits In Zinc Selenide Crystal

By Jof Enriquez,
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Physicists at Polytechnique Montréal have successfully demonstrated how to create a qubit faster than all other existing methods by using a zinc selenide (ZnSe) crystal with tellurium impurities.

Zinc selenide is a crystal in which atoms are precisely organized, and it is considered a well-known semiconductor material, conducive to introducing tellurium impurities, which can effectively trap positively-charged "holes." Electron holes are not physical particles like negatively-charged electrons, but can be thought of as the absence of an electron in a particular place in an atom.

A zinc-selenide/tellurium impurity-host system is an ideal environment to protect a hole's spin from which a quantum bit (qubit) can be formed, and for its coherence time can be sustained. Scientists aim to achieve for the longest coherence time in qubits to store, encode and process robust quantum information.

Polytechnique Montréal scientists claim to have attained this feat by using lasers to initialize a hole and record quantum information on it.

"The initialization scheme is based on the spin-preserving tunneling of a resonantly excited donor-bound exciton to a positively charged Te IC [tellurium isoelectronic center], thus forming a positive trion. The radiative decay of the trion within less than 50 ps leaves a heavy hole in a well-defined polarization-controlled spin state. The initialization fidelity exceeds 98.5 percent for an initialization time of less than 150 ps," the researchers wrote in the abstract of their study, "High-Fidelity and Ultrafast Initialization of a Hole Spin Bound to a Te Isoelectronic Center in ZnSe," published in the journal Physical Review Letters.

In order to read the quantum information, the hole is excited anew with the laser and the emitted photons are collected.

"The result is a quantum transfer of information between the stationary qubit, encoded in the spin of the hole held captive in the crystal, and the flying qubit - the photon, which of course travels at the speed of light," according to a news release. "This new technique shows that it is possible to create a qubit faster than with all the methods that have been used until now. Indeed, a mere hundred or so picoseconds, or less than a billionth of a second, are sufficient to go from a flying qubit to a static qubit, and vice-versa."

Their research successfully demonstrates ultrafast quantum information processing, but the next phase for the Canadian researchers is to scale this technique to work on a quantum computer capable of performing complex calculations or handling secure banking transactions, according to the announcement.

The Natural Sciences and Engineering Research Council of Canada (NSERC) funded this research study led by Professor Sébastien Francoeur, Associate Professor Department of Engineering Physics, Polytechnique Montréal.