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Marcus Huber (left) and Nicolai Friis
Photo Credit: © Alexander Rommel / TU Wien
Scientific Frontline: Extended "At a Glance" Summary: High-Dimensional Quantum Computing
The Core Concept: A novel type of quantum logic gate that processes information using qudits—particles capable of existing in four or more quantum states simultaneously—rather than traditional binary qubits. This advancement exponentially expands computational capacity by encoding multiple dimensions of data into a single photon pair.
Key Distinction/Mechanism: Traditional optical quantum computers rely on photon polarization, which restricts the system to two potential measurement outcomes (0 and 1). In contrast, this new mechanism manipulates the spatial wave forms and orbital angular momenta of photons, allowing the system to operate in a four-dimensional state space. It achieves and reverses entanglement using a heralded process, meaning the system can actively detect and confirm whether the quantum operation was successful.
Origin/History: Published in Nature Photonics in February 2026, this breakthrough is the result of a collaboration between theoretical physicists at TU Wien (including Nicolai Friis and Marcus Huber) and an experimental research team in China led by Hui-Tian Wang.
Major Frameworks/Components:
- Qudits: Multidimensional quantum units of information that utilize more than two states, offering significantly higher data density than standard qubits.
- Orbital Angular Momentum: The specific physical property and degree of freedom manipulated within the photons' spatial wave forms to achieve multidimensional states.
- Entanglement Gate: A controlled protocol that brings two initially independent photons into a synchronized joint state, and can subsequently separate them.
- Heralded Protocol: A built-in verification mechanism that alerts researchers when the entanglement succeeds, allowing for immediate repetition if an operation fails.
Branch of Science: Quantum Physics, Quantum Information Theory, and Photonics.
Future Application: The engineering of highly efficient, stable optical quantum computers that require drastically fewer particles to process, compute, and transmit large volumes of complex quantum data.
Why It Matters: Expanding quantum hardware from two-dimensional qubits to multi-dimensional qudits reduces the physical resource overhead necessary for running advanced quantum algorithms. Furthermore, the inclusion of heralded entanglement is a critical prerequisite for quantum error correction, marking a vital milestone toward the practical scaling and reliability of next-generation quantum technologies.
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| The novel quantum gate Image Credit: © Alexander Rommel / TU Wien |
Together with a team in China a team at TU Wien extends the capabilities of quantum computers: Instead of combinations of 0s and 1s, the new technology uses four different states simultaneously.
The collaboration of TU Wien with research groups in China represents a crucial building block for a new kind of quantum computers: 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, or combinations thereof – an important milestone for optical quantum computers that opens up new opportunities. The study has now been published in the scientific journal “Nature Photonics“.
Qudits instead of qubits
The basic idea of quantum computers is simple: While a classical computer only works with the values “0“and “1“, quantum physics allows for arbitrary combinations of these states. In a certain sense, a quantum bit (“qubit“) can be in the states 0 and 1 simultaneously. This makes it possible to develop algorithms that can solve some problems much faster than a comparable classical computer.
However, such superpositions can in principle involve more than two states. Depending on what degree of freedom one considers, a quantum system such as a photon may not just have two different settings—two different outcomes of a potential measurement—but many. In this case, one refers to the system as a “qudit“ rather than a “qubit”. For quantum computations, this can bring significant advantages, but ultimately one requires a mechanism by which two such qudits can interact in a controlled way. A research team at TU Wien was able to theoretically design a scheme to jointly process two qudits encoded in two photons—and a team in China successfully realized this scheme in their laboratory, resulting in a novel type of quantum gate, with potentially revolutionary applications.
Quantum physics in four dimensions
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.
“We use photons in a fundamentally different way”, explains 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 surrounding Nicolai Friis has 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.
Such an operation—an entangling quantum gate—is needed to build quantum computers, to carry out calculations on multiple inputs. For the first experiment, the researchers decided to work with four different states. “This is as if, in addition to the North-South and East-West directions, one would have access to two additional axes”, says Friis. “In some sense one is moving in a four-dimensional space, and we can work with arbitrary combinations of such states.”
One finds out if it worked
Realizing their theoretical ideas did not just require a new protocol but also made it necessary to significantly improve the state of the art in technology and experimental precision—an area in which the team of Hui-Tian Wang in China made remarkable progress.
“We were successful in realizing a quantum logic gate that works with two photons that can be prepared in combinations of four different states”, says Nicolai Friis. “We can entangle the photons—and we can do so in a heralded fashion, meaning that we can tell, when the protocol works. And if it does not, we can repeat the procedure. This is what is needed in practice.”
The new approach is hoped to make quantum information technology more efficient and stable in different areas. “We need fewer particles to carry the same amount of quantum information”, says Marcus Huber (also from the Institute of Atomic and Subatomic Physics of TU Wien). “This has many advantages, also with a view towards the reliability of quantum operations.” The new study thus—quite literally—opens new dimensions for quantum technologies.
Published in journal: Nature Photonics
Title: Heralded high-dimensional photon–photon quantum gate
Authors: Zhi-Feng Liu, Zhi-Cheng Ren, Pei Wan, Wen-Zheng Zhu, Zi-Mo Cheng, Jing Wang, Yu-Peng Shi, Han-Bing Xi, Marcus Huber, Nicolai Friis, Xiaoqin Gao, Xi-Lin Wang, and Hui-Tian Wang
Source/Credit: Technische Universität Wien
Reference Number: qs022426_01