TU Wien Four State Photon Quantum Gate

TU Wien and Chinese Scientists Achieve Breakthrough in Four-State Photon Quantum Gate

Major Milestone in Optical Quantum Computing

Researchers from TU Wien, working in partnership with Chinese research group, have achieved a key breakthrough in quantum technology. They have implemented an innovative quantum logic gate that enables calculations using pairs of photons, each capable of occupying four separate quantum states or their superpositions.

The achievement marks an important milestone in the evolution of optical quantum computers. The study appears in Nature Photonics.

Understanding the Core Principle of Quantum Computing

The fundamental principle behind quantum computing is straightforward: whereas a classical computer processes information using binary values—"0" and "1"—quantum mechanics permits combinations of these states. A quantum bit, or qubit, can exist in both states simultaneously, enabling algorithms capable of solving certain problems far more rapidly than conventional machines.

Yet quantum superposition is not limited to just two states. Depending on the physical property under consideration, a system such as a photon may possess multiple measurable outcomes rather than merely two. In such cases, it is described as a "qudit" instead of a qubit.

From Qubits to Qudits - A Major Leap Forward

While qudits offer notable computational advantages, they require a controlled method for enabling interaction between two such systems. Researchers at TU Wien developed a theoretical framework to process two photon-encoded qudits simultaneously. Their collaborators in China then successfully implemented the concept experimentally, creating a new form of quantum gate with potentially transformative applications.

Until recently, many photon-based quantum computing experiments relied on polarization—a property that offers only two measurable outcomes. In quantum terms, a photon may exist in a superposition of these alternatives, much like travelling north and east simultaneously when heading north-east.

A Fundamentally Different Approach to Photon Processing

"We take a fundamentally different approach," explained Nicolai Friis of the Institute of Atomic and Subatomic Physics at TU Wien. "Our focus is not on polarization, but on the spatial waveform of photons, which can assume infinitely many distinct states associated with different orbital angular momenta."

Friis and his colleagues devised a method enabling two such photons—each capable of occupying arbitrary superpositions of waveforms—to be processed together. Through precise manipulation, two initially independent photons can be merged into a shared, entangled state. The newly developed quantum gate can also reverse the process, disentangling the photons in a controlled manner and restoring their independence.

Why Entangling Quantum Gates Matter

Such an operation—an entangling quantum gate— is precisely what is required to construct functional quantum computers capable of processing multiple inputs simultaneously. For their initial demonstration, the researchers chose to operate with four distinct quantum states.

"It is comparable to having, in addition to the north-south and east-west directions, two entirely new axes at one's disposal," explained Friis of TU Wien. "In effect, we are navigating a four-dimensional space, allowing us to manipulate arbitrary combinations of these states."

Turning Theory into Experimental Reality

Turning theory into reality demanded more than a new conceptual protocol; it required substantial advances in experimental capability and technical precision. In this respect, the research group led by Hui-Tian Wang in China achieved notable progress.

"We succeeded in implementing a quantum logic gate that operates with two photons prepared in superpositions of four distinct states," said Friis of TU Wien. "We are able to entangle the photons in a heralded manner, meaning we can confirm when the protocol has worked. If it has not, the procedure can simply be repeated. That level of reliability is essential in practice."

Efficiency, Stability and the Future Quantum Information

The researchers believe the approach could enhance both the efficiency and stability of quantum information systems.

"We require fewer particles to encode the same quantity of quantum information," explained Marcus Huber, also from the Institute of Atomic and Subatomic Physics at TU Wien. "This offers significant benefits, particularly in improving the reliability of quantum operations." In doing so, the study opens entirely new dimensions for quantum technologies.

Source

Key Highlights of the Breakthrough

• Implementation of a four-state photon quantum logic gate
• Use of qudits instead of traditional qubits
• Successful experimental realization through international collaboration
• Heralded entanglement enabling repeatable and reliable operations
• Potential to improve efficiency and stability in optical quantum computing

Explore More from Our Network:

Science & technology news | Health & Medical research updates | Environmental science insights | Natural wellness features | Wildlife & biodiversity stories | Travel & global exploration