The long-awaited next breakthrough in computing is expected to come from so-called quantum computers, which will use quantum phenomena to lead to faster processing and radically improved data storage. Now, a physics graduate at Havard University has assembled a remarkable and unique system which may lead to the breakthrough quantum-computing has been waiting for.

What may be most extraordinary about the system that Harry Levine and his team, led by Mikhail Lukin, has built in his small basement lab at the University is that there are no processor chips involved. Levine’s computer is powered by 51 rubidium atoms stored in a glass cell. These atoms are lined up single file by a laser split into 51beams.

The atoms are slowed until almost motionless by further lasers, whilst another set of lasers allows the user to encourage the atoms to interact. It is from these interactions that calculations can be performed.

The experimental set-up used by Levine and his Harvard team (Levine, Keesling, Omran)

The advantage a quantum computer confers is the ability of a quantum system to be in a superposition of states. This means that whereas a normal ‘bit’ can only have two possible states-0 or 1- a qubit can encode for ones and zeroes simultaneously in a superposition of states.

If scaled up, this set-up should vastly outperform traditional computers.

Even amongst quantum computers, this set-up is quite remarkable. Most qubits that have been explored thus far have been built on silicon, superconducting wire, and semiconductor structures known as quantum dots. This work builds upon recent research using neutral atoms to form qubits.

Neutral atoms have previously been believed to be poor choices for qubits due to their lack of an electrical charge and therefore don’t interact with other atoms easily. Physicists can overcome this difficulty by using specifically timed laser bursts to excite an atom’s outermost electron and move it from the atomic nucleus, into what is called a ‘Rydberg state.’ This has the effect of massively increasing the atom’s size.

V. Altounian (Science)

In this Rydberg state, the atom behaves more like an ion, an atom with electrons stripped from it meaning that they are more likely to interact electromagnetically with neighbouring atoms. The interaction has the overall effect of preventing these neighbouring atoms from entering a Rydberg state themselves.

This confers an entangled state on the atoms, the state that is required to perform quantum calculations. A measurement on either atom collapses the superposition created by one atom being in the Rydberg state and the other not.

The advantage conferred by neutral atoms is that they are all identical, they can be packed into a much tighter space than silicon-based qubits and also do not need to be held at super-cold temperatures as superconducting qubits have to. Also, because neutral atoms are less likely to interact it means that they are less likely to interfere with each other and lose the stored quantum information.

Thus neutral atoms offer the advantage of scalability and a better overall performance.

Lukin’s work was published in the latest edition of the journal Physical Review Letters demonstrated the ability to programme a two-Rubinium logic gate with a 97% accuracy. This means the Rydberg method of qubit creation is close to the fidelity of superconducting qubits, which currently stands at 99%.

In addition to this, another piece of research published around the same time has added support to the versatility of Rydberg qubits.

A team of French researchers published a study in the September edition of Nature in which they were able to demonstrate remarkable control over a 3D array 72 neutral atoms. They were able to densely pack the atoms in a way that cannot be done with ions as they latter repeal each other due to their alike charge.

Whilst Levine is positive that the system he has helped create will benefit the telecommunications industry, others are less convinced.

“Compared with other qubits, neutral atoms tend not to stay put,” Varun Vaidya, a physicist at Xanadu, a quantum computing company, told Science. This means that systems using neutral atoms may not be suitable for performing longer tasks as their stability is lacking.

There is no question that there is still a multitude of questions remaining regarding the potential of quantum computing and how to deliver the best qubit. Rydberg systems may well deliver the required answers.

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