Researchers Successfully Place 20 Entangled Qubits in Superposition State

In the quantum realm, particles like the well-known analogy of Schrödinger’s cat can be left-spinning or right-spinning, dead or alive, all at the same time.

An international team, with scientists from various pioneering American universities, in collaboration with experts from Forschungszentrum Jülich, have now successfully placed 20 entangled quantum bits, or qubits, into such a superposition state.

It is considered that producing such atomic Schrödinger cat states is a vital step in the advancement of quantum computers that could surpass the performance of classical computers in solving specific tasks. The study outcomes were reported in Science on August 9^th, 2019.

In 1935, Erwin Schrödinger, a physicist, proposed the thought experiment with the quantum cat, where the cat is confined in a box along with a radioactive sample, a lethal amount of poison, and a detector. In case the radioactive material decays, an alarm is triggered by the detector and the poison gets released.

The unique aspect is that based on the quantum mechanical rules, in contrast to the day-to-day experience, it is obscure whether the cat is alive or dead. Until an experimenter verifies, it would be both dead and alive. A single state would only be ascertained commencing from the time of the observation.

From the early 1980s, scientists have been in a position to experimentally achieve this superposition of quantum states in the lab through a number of strategies.

However, these cat states are extremely sensitive. Even the smallest thermal interactions with the environment cause them to collapse.

Tommaso Calarco, Forschungszentrum Jülich

Apart from other things, he has played a pioneering role in Europe’s major quantum initiative, the EU’s Quantum Flagship program. “For this reason, it is only possible to realize significantly fewer quantum bits in Schrödinger cat states than those that exist independently of each other.”

Among the latter states, researchers can now regulate over 50 in-lab experiments. Yet, these qubits do not exhibit the unique properties of Schrödinger’s cat as opposed to the 20 qubits that the team has now developed with the help of a programmable quantum simulator, thus setting a new record that would remain valid even when other physical strategies with trapped ions, superconducting quantum circuits, or optical photons are considered.

Experts from a number of renowned institutions across the world collaborated to design the experiment. Besides Jülich researchers, researchers from several top American universities—such as Berkeley, Harvard, Caltech, and MIT—as well as the Italian University of Padua were involved.

Qubits in the cat state are considered extremely important for the development of quantum technologies. The secret of the enormous efficiency and performance expected of future quantum computers is to be found in this superposition of states.

Jian Cui, Physicist, Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich

In a traditional computer, classical bits always have only one specific value, 0 or 1, for instance. Hence, it is possible to process these values only bit by bit one after the other. By contrast, qubits have a number of states at the same time owing to the superposition principle; thus, they can store and process a number of values in parallel in a single step.

What is crucial here is the number of qubits. One can never achieve much with only a handful of qubits. However, the number of superimposed states that can be achieved with 20 qubits already reaches beyond one million. Moreover, 300 qubits can store even more numbers at the same time than the number of particles in the universe.

Currently, the new outcome of 20 qubits gets somewhat closer to this value, after the old record of 14 qubits stayed unaltered from 2011. For the experiment, the scientists used a programmable quantum simulator that works on the basis of Rydberg atom arrays.

In this technique, laser beams are used to capture individual atoms (here rubidium atoms), which are held in place next to each other in a row. The process is also called optical tweezers. The atoms are excited by an additional laser until they attain the Rydberg state, where the electrons are positioned far beyond the nucleus.

This method is highly complex and often requires too much time that the delicate cat state gets ruined even before it is actually measured. The team from Jülich offered their expertise in Quantum Optimal Control to overcome this problem. They skillfully turned the lasers on and off at the apt rate and realized acceleration in the preparation process, which rendered this new record feasible.

We practically inflated some atoms to such an extent that their atomic shells merge with the adjacent atoms to simultaneously form two opposite configurations, namely excitations occupying all even or odd sites. This goes so far that the wave functions overlap as in the analogy of Schrödinger’s cat and we were able to create the superposition of the opposite configurations which is also known as the Greenberger-Horne-Zeilinger state.

Jian Cui, Physicist, Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich

Their progress in quantum studies was augmented by the efforts of a Chinese research team, which was also reported in the latest issue of Science. The researchers used superconducting quantum circuits and successfully created 18 qubits in the Greenberger-Horne-Zeilinger state—another new record for this experimental technique.