Strangely stable Entanglement

New Hope for Quantum Computers

28. Mai 2021, 8:25 Uhr | Heinz Arnold
Künstlerische Darstellung des Science-Experiments.
Artist's rendering of the Science experiment.
© Enrique Sahagún, Scixel

A system of two entangled atoms has proved remarkably stable even under electron bombardment. This gives hope to quantum computing enthusiasts.

The experiment, described by scientists from TU Delft, RWTH Aachen University and Forschungszentrum Jülich in the prestigious journal Science, could provide an indication that quantum states in a quantum computer can in certain cases be realized more easily than previously thought.

This is because decoherence - the decay of an entangled system - is the major limiting factor for quantum computers. Indeed, only in the entangled state can the qubits be manipulated and read out. For qubits based on superconducting loops (Josephson junctions), the time period over which entanglement can be maintained is in the range of milliseconds; for qubits based on ion traps, it is as short as minutes. Only this period is available to perform calculations. As soon as the system interacts with the environment, the quantum state decays - the information contained in the qubits is then irrevocably lost.

»Systems subject to quantum physics, unlike classical objects, are not sharply defined in all their properties. Instead, they can occupy multiple states simultaneously. This is called superposition«, explains Markus Ternes, a quantum physicist at Forschungszentrum Jülich and RWTH Aachen University. »A famous example is Schrödinger's thought experiment with the cat, which is temporarily dead and alive at the same time. However, the superposition breaks down as soon as the system is disturbed or measured. What then remains is only a single state, the measured value.«
Against this background, the experiment that researchers at Delft University of Technology have now carried out is all the more astonishing. Using a new method, they succeeded for the first time in observing in real time how two atoms coupled to each other switch freely back and forth between different excited states in a flip-flop fashion.
»Each atom carries a small magnetic moment called a spin. These spins influence each other, just like compass needles do when you bring them close together. If you give one of them a nudge, they start moving in a very specific way«, explains Sander Otte, leader of the Delft research team.
Quantum effects emerge from this type of information exchange between atoms, on which various forms of quantum technologies are based. A well-known example is superconductivity: the effect in which some materials completely lose their electrical resistance below a critical temperature.
Otte and his team chose a very direct way to observe the interaction between atoms. Using a scanning tunneling microscope, they placed two titanium atoms next to each other at a distance of just over a nanometer - a millionth of a millimeter. At this distance, the atoms are just able to sense each other's spin. If one of the two spins were now to be twisted, the exchange between the atoms would begin by itself.
Normally, this flipping is done by means of precise radio signals that are sent to the atoms. This so-called spin resonance technique is reminiscent of the MRI scanners in a hospital and is already being used successfully in research. Among other things, quantum bits in certain quantum computers are programmed this way. However, the technique has a drawback. »It's just too slow«, says Delft doctoral student Lukas Veldman, lead author of the Science paper. »No sooner do you start spinning one spin than the other one starts spinning with it. That way, you can never study what happens when you put the two spins in opposite directions.«
So the researchers resorted to an unconventional approach: they abruptly reversed the spin of one of the two atoms with a sudden jolt of electricity. To the researchers' surprise, this drastic approach was followed by a textbook quantum interaction. That's because during the pulse, electrons collide with the atom and cause its spin to rotate. Otte: »We always assumed that the sensitive quantum information - known as coherence - is lost during this process. This is because the electrons you send out are incoherent: each electron had a slightly different history before the collision, and this chaos transfers to the spin of the atom and destroys any coherence.«The fact that this now does not seem to be the case caused some discussion. Apparently, every electron can produce superposition states, as they form the basis for almost every form of quantum technology. The fact that these electrons are still connected to their environment via their history is obviously irrelevant. What is at stake here, then, is the violation of a principle of quantum physics, according to which any measurement irretrievably destroys the superposition of quantum states.

Prof. Markus Ternes: »Die Krux ist, dass es auf Perspektive ankommt.«
Prof. Markus Ternes: »The crux is that perspective matters.«
© Forschungszentrum Jülich / Ralf-Uwe Limbach

»The crux is that perspective matters«, argues Markus Ternes, a co-author on the Science paper. »The electron reverses the spin of an atom so that it points to the left, for example. You could think of this as a measurement that erases all quantum memory. But from the point of view of the combined system of the two atoms, the resulting situation is not so trivial. For the two atoms together, the new state represents a perfect superposition that allows information to be exchanged between them. Crucial to this is that both spins become entangled.«
The discovery could prove momentous for the development and exploration of quantum computers, whose function relies on the entanglement and superposition of quantum states. If the findings are followed, the creation of these quantum states may need to be somewhat less careful than previously thought. For Otte and his team at TU Delft, however, the result is above all the starting point for further exciting experiments. Veldman: »Here we used two atoms, but what happens if you use three? Or ten, or a thousand? No one can predict that, because there is not enough computing power to simulate such numbers.«

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