If a traditional qubit is like a coin with only two sides, then a four-dimensional quantum unit is like a four-sided die, offering more combined states, greater information capacity, and larger processing space.
Recently, China Mobile Research Institute "entangled" three "four-sided dice" on a silicon photonics chip measuring 16 mm in length and 1.5 mm in width, preparing the "3-ququart entangled state" GHZ entangled state and W entangled state with fidelities reaching 93.2% and 90.1%, respectively, setting a new scale record for high-dimensional multi-particle entangled states.
"Researchers have just solved a problem that has been pending for many years... and truly realized it on a photonic chip with a fidelity exceeding 90%! We are living in an interesting era," commented Alan Woodward, Chair Professor at the University of Surrey in the UK.
01. From "Coins" to "Dice": Why Quantum Technology is Advancing to Higher Dimensions
Why has the innovative endeavor of China Mobile Research Institute received high praise from international experts? Answering this question requires first returning to the fundamentals of quantum entangled states and high-dimensional quantum mechanics itself.
Quantum entangled states are the core resource of quantum technology, providing critical momentum for surpassing the limits of classical physics. For a long time, industry research has primarily focused on two-dimensional quantum systems (i.e., "qubits"), whereas many quantum computers and quantum information processing systems in the real world often involve higher dimensions.
Chi Yulin, a researcher at the Future Research Institute of China Mobile Research Institute, used a vivid metaphor to explain the difference between two-dimensional and high-dimensional systems: "A traditional qubit is like a coin with only two basic states, 0 and 1; whereas a high-dimensional quantum unit possesses multiple states. For example, a four-dimensional quantum unit has four basic states: 0, 1, 2, and 3. The information capacity of a single unit is twice that of a qubit."
Due to the increased information capacity, high-dimensional quantum entangled states exhibit three unique advantages compared to two-dimensional quantum entangled states: first, higher channel capacity to carry more information; second, stronger noise resistance, which can effectively improve the stability and accuracy of quantum information transmission; and third, simplified complexity of quantum computing circuits and enhanced quantum computing efficiency.
Based on this, China Mobile Research Institute proposed the universal definition of high-dimensional W entangled states for the first time, further improving the theoretical foundation of high-dimensional quantum entangled states.
02. Bridging with Preparation Schemes: Closing the Loop from Theory to Implementation
Currently, research on high-dimensional quantum entangled states not only needs improvement in theory but also faces bottlenecks in large-scale preparation. This is precisely the starting point of China Mobile Research Institute's recent breakthrough.
"Although we know the color, aroma, and taste of a dish, this is entirely different from getting a practical recipe and actually cooking it." Chi Yulin once again used a vivid metaphor to illustrate the importance of the preparation scheme, "A preparation scheme is an actionable 'engineering implementation path'—from theoretically knowing what a quantum state 'looks like' to actually building it step by step on a physical platform, with a complete scheme design in between."
Facing the bottleneck of large-scale preparation, China Mobile Research Institute innovatively designed a scalable preparation scheme for high-dimensional quantum entangled states. By utilizing the high-dimensional characteristics of single photons and the theory of quantum delayed measurement, the scheme breaks through the scale limitations of traditional preparation methods. Based on this scheme, researchers successfully prepared the "3-ququart entangled state" GHZ entangled state and W entangled state on a programmable silicon photonics quantum chip.
The "3-ququart entangled state" here refers to entangling three aforementioned four-dimensional units together, enabling the entire system to have 64 combined states, with information capacity and processing space significantly increased compared to a single four-dimensional unit.
In actual tests, the fidelity of the GHZ state reached 93.2%, and the fidelity of the W state reached 90.1%, setting a new scale record for high-dimensional multi-particle entangled states, which can also be efficiently scaled up to larger sizes in the future.
03. Three Key Challenges: Theoretical Definition, Circuit Design, and Physical Implementation
The transition from theoretical innovation to practical implementation is often not a smooth journey. Chi Yulin noted that China Mobile Research Institute also faced three major hurdles when researching the preparation scheme for high-dimensional quantum entangled states.
The first is the hurdle of theoretical definition. Entangled states include Bell states, GHZ states, Cluster states, and W states. The high-dimensional generalization of the first three is quite direct for the industry, but the high-dimensional generalization of the W state is "not obvious." The W state is a "total quantity conservation" type of entanglement; even if any one particle is lost, the remaining two can still maintain entanglement. Its universal definition should possess the characteristic that the higher the dimension, the stronger the loss resistance, making its importance self-evident. China Mobile Research Institute proposed for the first time to use the high-dimensional "total quantity conservation" to universally define the high-dimensional W state, utilizing all 4 energy levels, and proved that it brings stronger robustness against loss.
The second is the hurdle of circuit design. In traditional schemes, resources grow exponentially with the number of particles. This scheme achieves "linear" growth of hardware resources with scale, rather than requiring resources to multiply several times for each additional particle. Furthermore, the chip in this scheme is programmable—it is not a fixed device capable of generating only one state, but can be switched on demand to produce different entangled states such as GHZ, W, and Cluster states.
The last is the hurdle of physical implementation. Traditional linear optical methods have a ceiling—without the aid of auxiliary photons, three-photon GHZ states above three dimensions, as well as Bell measurements above three dimensions, cannot be realized. China Mobile Research Institute took a different approach, using one photon to simultaneously carry two four-dimensional quantum units (equivalent to one actor playing two roles) and another photon to carry the third unit, jointly forming a "3-ququart entangled state" quantum system.
Having successfully overcome these three hurdles, the preparation scheme of China Mobile Research Institute has connected the complete chain of "universal definition—scalable quantum circuits—physical scheme breaking scale limitations—chip verification," achieving substantial innovation at every stage.
04. Beyond Academic Breakthroughs: Industrial Impact
Quantum information is a cutting-edge field with intensive technological innovation. China Mobile Research Institute has successfully tackled the challenges of high-dimensional quantum technology, providing new ideas and technical pathways for high-dimensional quantum theoretical research and industrial development.
In terms of theoretical research, a relevant paper from China Mobile Research Institute was published in a Nature partner journal, the international top-tier optics journal Light: Science & Applications (Impact Factor: 24.0). This is the first paper in the quantum field published by China Mobile Research Institute in a Nature partner journal, forming a complete autonomous method from theory to chip. Professor Alan Woodward evaluated it as having "solved a problem that has been pending for many years."
The honors received in the academic field are inseparable from the three highlights of this breakthrough—filling the gap of the missing W entangled state in high-dimensional theory and providing a unified language and theoretical basis for the W state; achieving linear growth with scale, which means there is a feasible engineering path to "scale up"; and the programmable chip that can be switched on demand to generate different entangled states, which is the first of its kind in the high-dimensional field.
More important than academic value, this achievement lays the foundation for expanding the application of high-dimensional quantum technology in fields such as quantum communication, quantum computing, and quantum precision measurement. In the field of quantum communication, high-dimensional entangled single photons carry more information and have stronger noise resistance. Moreover, operating in the 1550nm optical communication band and manufactured using CMOS processes, they are naturally compatible with the existing optical communication industry chain and semiconductor manufacturing system. In the field of quantum computing, high-dimensional encoding can simplify circuits, improve efficiency, and support more hardware-friendly error correction schemes. It also holds application potential in the field of quantum precision measurement.
As Professor Alan Woodward said, we are living in an interesting era. From theoretical innovation to preparation schemes and then to application promotion, high-dimensional quantum technology will move from academic innovation to industrial practice, ushering in a new era of quantum technology full of imagination.
Editor/Layout: Sun Tian
Reviewed by: Mei Yaxin
Supervised by: Liu Qicheng