Schrödinger's Cat With 20 Qubits

Researchers report in " Science" of a new record with folded quantum bits

Dead or alive, left-handed or right-handed - in the quantum world, particles like the famous analogy of Schrödinger's cat can all be at the same time. Together with experts from Forschungszentrum Jülich, an international team, including researchers from several top American universities, has succeeded in putting 20 entangled quantum bits in such a state of superposition. The generation of such "cat states" is considered an important step in the development of quantum computers, which could far exceed classical computers in solving certain tasks. The results were published last Friday in the journal "Science".

The physicist Erwin Schrödinger had applied the thought experiment with the quantum cat in 1935, in which the cat is enclosed in a box together with a radioactive preparation, a detector and a deadly amount of poison. Should the radioactive decay, the detector will sound an alarm and the poison will be released. The special thing about it: According to the rules of quantum mechanics, unlike in everyday life, it is not clear whether the cat is dead or alive. It would be both at the same time, for so long, until an experimenter looks up. Only then did a clear result arise.

Since the beginning of the 1980s, researchers have been able to experimentally realize this superposition of quantum states in the laboratory using various approaches. "These superposition states are extremely sensitive, however, and even the smallest thermal interactions with the environment cause them to collapse," explains Tommaso Calarco from Forschungszentrum Jülich. Among other things, he plays a leading role in Europe's big quantum initiative, the EU's quantum flagship program. "For this reason, you can realize far fewer quantum bits in the state of Schrödinger's cat than those that exist independently."

From the latter, researchers can now control more than 50 in laboratory experiments. But these quantum bits, or qubits for short, do not show the special features of Schrödinger's cat; in contrast, the 20 qubits that the research team has now generated using what is known as a programmable quantum simulator: a record that holds true even with other physical approaches involving optical photons, ion traps, or superconducting circuits.

For the development of the experiment, researchers from several of the most renowned institutions in the world had joined forces. In addition to the Jülich researchers, scientists from numerous leading American universities - Harvard, Berkeley, MIT and Caltech - as well as the Italian University of Padua were involved.

"Catfish qubits are considered the highest commodity for the development of quantum technologies," says Jian Cui. "Because the overlay is the secret of the tremendous performance, which one promises of future quantum computers," says the physicist of the Jülich Peter Grünberg Institute (PGI-8).

Classic bits in a conventional computer always have only a certain value, which is composed for example of 0 and 1. Therefore, they can only be processed bit by bit sequentially. On the other hand, qubits, which adopt several states at the same time due to the overlay principle, can store and process several values ​​in parallel in one step. The decisive factor here is the number of qubits. You can not get far with a handful.

But with 20 qubits, the number of overlapping states is already over one million. And 300 qubits can store more numbers at the same time than there are particles in the universe.

The new record of 20 qubits comes a bit closer to this value, after the old record of 14 qubits has remained unchanged since 2011. For their experiment, the researchers used a programmable quantum simulator with atoms that are in the Rydberg state. In this process, single atoms, in this case rubidium atoms, are captured by laser beams and held side by side in a row in their place. The technique is known as "optical tweezers". Another additional laser excites the atoms until about half of them reach the so-called Rydberg state, where the electrons are far beyond the nucleus.

This process is quite complicated and classically takes so much time that the sensitive cat's condition is decaying again in the meantime. The researchers at the Jülich Peter Grünberg Institute (PGI-8) were able to minimize this preparation time by changing the way the second laser was switched on and off - thus enabling the new record.

"We practically bulge the atoms until their atomic sheaths merge with neighboring atoms and simultaneously take on two opposite configurations," explains Jian Cui. "It goes so far as to superimpose the wave functions as in Schrodinger's cat and we could prove a condition that is also referred to as Greenberger-Horne-Zeilinger condition."

The success for quantum research was complemented by another work by a Chinese research group, which also appeared in the current issue of "Science". The researchers have succeeded in realizing 18 qubits in the Greenberger-Horne-Zeilinger state using superconducting circuits, which also sets a new record for this experimental approach.