Researchers have taken a significant step to make quantum computers multi-dimensional. Classical computer only works with the values  “0“ and “1“, but researchers are exploring a new technology that can use four states simultaneously for quantum computers.

A joint team from TU Wien and China have achieved the new type of quantum computing.

The team revealed that the realization of a novel type of quantum logic gate makes it possible to carry out quantum computations on pairs of photons that are each in four different quantum states. The step is claimed to be an important milestone for optical quantum computers that opens up new opportunities.

Quantum-computing experiments with photon

“We use photons in a fundamentally different way,” said Nicolai Friis from the Institute of Atomic and Subatomic Physics of TU Wien.

“We aren’t interested in the polarization, but in the spatial wave form of the photons, which can be in infinitely many different states, corresponding to different orbital angular momenta.”

The team developed a procedure that works with two such photons: Both can be in arbitrary superpositions of different wave forms. Through sophisticated manipulation, two initially independent photons can be brought into a joint state—a so-called “entangled” state. Likewise, the new quantum gate can also be used to separate two entangled photons in a controlled way to make the states of the photons independent of each other again, according to a press release. 

Researchers also revealed that until now, quantum-computing experiments with photons have often been carried out by relying on the polarization of photons—a property with two different possible measurement outcomes. From the point of view of quantum physics, the photon can be in a superposition of these two options, like moving simultaneously North and East when walking Northeast.

The research team also pointed out that a major obstacle for realizing quantum gates between two individual photons is the restriction of direct interaction between photons in linear media.

Researchers addressed this challenge by presenting a protocol for realizing an entangling gate—the controlled phase-flip gate—for two photonic qudits in an arbitrary dimension.

Photonic qudits

Published in the journal Nature Photonics, the research experimentally demonstrates this protocol by realizing a four-dimensional qudit–qudit controlled phase-flip gate, whose decomposition would require at least 13 two-qubit entangling gates.

“Our photonic qudits are encoded in orbital angular momentum, and we have developed a new active high-precision phase-locking technology to construct a high-dimensional orbital angular momentum beamsplitter that increases the stability of the controlled phase-flip gate, resulting in a process fidelity within a range of [0.71 ± 0.01, 0.85 ± 0.01],” said researchers in the study.

The work represents an important advance for high-dimensional optical quantum information processing and has the potential for wider applications beyond optical system, according to researchers.