Japanese scientists have achieved a breakthrough in quantum computing research by developing a technology able to control and sustain the lifetime of qubits that is 60 times more successful than any previous developments in the field.

Researchers from the University of Tokyo have found a way to solve the problem of qubits losing their coherence and ceasing to be quantum bits by developing a precise, continuous control technology out of laser light to create entangled states that last for as long as needed.

Although currently quantum computers are merely a concept, numerous computer science researchers around the world and billions of dollars has been invested to create them, and it is believed that these new super-powerful computers will be available within the next 50 years.

### Quantum entanglement

Computers today are coded using binary bits, which have a value of 0 or 1. When put together, the bits create codes such as 00, 01, 10 or 11. That's two bits with four possible combinations, of which only one can be used.

Quantum computing bits (qubits) also make use of 0 and 1 values, but can exist in a superposition of probability - being both 0 and 1 until the qubit is measured, at which point it collapses into one of the definite states, 0 or 1. This means that two qubits in superposition can be in four possible configurations at once.

The number of values stored in parallel increases exponetially as more qubits are used. A quantum computer can essentially calculate all possible probable outcomes at once, however sorting through the observed outcomes to produce the correct result is a challenge.

By using a single photon as a qubit, another useful property of particles can be used to narrow down quantum computing calculations - entanglement. When qubits are entangled in pairs, you can directly correlate the opposite state of one qubit by observing the state of its paired qubit. These qubits can remain entangled no matter the distance between them, as long as they are isolated.

One mathematical concept for a quantum computer system requires the measurement of clusters of entangled pairs of light photons. Entangling pairs of light photons to make qubits is one challenge, while another is how to get the entangled photons into a robust cluster state so that there is no way for them to be separated and they can be used to factor large numbers.

While working to solve these problems, the University of Tokyo researchers entangled more than one million different physical systems together. Their achievement is a world record in itself, but the scientists say that it could have been even more impressive, if they hadn't run out of data storage space.

By being able to entangle an infinite number of qubits together, information processing can last much longer, as previously, researchers have only been able to link tens of thousands of qubits together at the most.

### Making entangled photons that last

"There is a problem of the lifetime of qubits for quantum information processing. We have solved the problem, and we can continue to do quantum information processing for any time period we want," said Professor Akira Furusawa, of the Department of Applied Physics, School of Engineering at the University of Tokyo, who led the research.

"The most difficult aspect of this achievement was continuous phase locking between squeezed light beams, but we have solved the problem."

Now that the researchers are able to extend the lifetimes of qubits, their next step is to create 2D and 3D lattices of clusters of entangled photons, in order to add structure and make topological quantum computing possible.

The open access paper, entitled "Generation of one-million-mode continuous-variable cluster state by unlimited time-domain multiplexing" is published in the journal APL Photonics.

This is not the only successful attempt at entangling photons – scientists from the Technion-Israel Institute of Technology have built a device that can deterministically produce large clusters of entangled photons on demand and guarantee being able to repeatedly produce the same result indefinitely.