Light’s Frequency Spectrum Holds 38 Dimensions for Quantum Data Transfer

Researchers Prasad Koviri and colleagues at Graduate School of Informatics and Engineering, The University of Electro-Communications, Tokyo, Japan, in collaboration with Advanced ICT Research Institute, National Institute of Information and Communications Technology, Hyogo, Japan and Japan 2Advanced ICT Research Institute, have engineered precisely controlled, high-dimensional quantum states utilising the frequency of light. They generate entangled frequency states with spacings from 12.5GHz to 750GHz. This achievement establishes lower bounds on Hilbert-space dimensionality of at least 289. This provides a key advance in the development of strong and scalable quantum optical systems, enabling applications in wavelength-multiplexed quantum networks, high-dimensional information processing, and quantum remote sensing

Engineered high-dimensional entanglement exceeds limits of photon polarisation for quantum networks

A Hilbert-space dimensionality of at least 289 has been demonstrated, representing a substantial increase over previous limitations of 17. This leap enables the creation of more complex and robust quantum systems, crucial for tackling computationally intensive tasks beyond the reach of classical computers. Conventional polarisation-entangled photons, a common method for encoding quantum information, are restricted by lower dimensional Hilbert spaces, typically limited to two dimensions. This inherent limitation restricts the amount of information that can be reliably encoded and processed. Scientists utilised time-domain Fourier optical synthesis, a sophisticated technique involving the manipulation of light pulses in the time domain and subsequent Fourier transformation to the frequency domain, to engineer these high-dimensional entangled states of light. This process effectively converts continuous broadband entanglement, where entanglement exists across a wide range of frequencies, into discrete frequency bins, making it compatible with standard telecommunications wavelengths, typically around 1550nm, used in existing fibre optic infrastructure.

The resulting entangled ‘qudits’, quantum bits capable of representing multiple values, showed spectral anticorrelations across 38 frequency bins. This signifies that the frequencies of the entangled photons are correlated in a specific way, confirming quantum nonlocality over a campus-scale fibre network and paving the way for advanced quantum communication protocols. Quantum nonlocality, a fundamental principle of quantum mechanics, ensures that the entangled particles remain correlated even when separated by large distances. Schmidt decomposition, a mathematical process for analysing quantum states and determining their inherent complexity, was employed to rigorously validate this high dimensionality. This decomposition allows researchers to quantify the entanglement present in the system and confirm that it genuinely occupies a high-dimensional Hilbert space. Transmission of these entangled states over a 1.3 kilometre campus-scale fibre network, verified by coincidence detection, a technique that identifies simultaneous arrival of photons at detectors, preserved spectral correlations and confirms the durability of the system for real-world quantum communication. Each qudit possessed a dimension of 17, meaning each qudit could exist in 17 distinct states, forming the basis for the overall 289-dimensional Hilbert space (17x 17 = 289). This high dimensionality allows for significantly increased information capacity and enhanced security against eavesdropping attempts.

Entangled states utilising frequency demonstrate potential for scalable quantum communication

Generating high-dimensional quantum states offers a pathway to more secure and efficient communication networks, exceeding the limitations of traditional systems reliant on single photons. Traditional quantum key distribution (QKD) protocols often rely on encoding information in the polarisation of single photons, which are vulnerable to eavesdropping and signal degradation. High-dimensional entanglement, by contrast, offers increased security and robustness. The increased dimensionality allows for more complex encoding schemes, making it more difficult for an eavesdropper to intercept the information without being detected. However, the current demonstration does not quantify the inevitable effects of signal loss and environmental disturbances over longer distances; this remains a key hurdle for practical implementation. Fibre optic cables inherently cause signal attenuation, and environmental factors such as temperature fluctuations and vibrations can disrupt the delicate quantum states. Addressing these challenges is vital, as even minor degradation could compromise the integrity of entangled states and undermine the benefits of increased dimensionality. Further research will need to focus on developing techniques for quantum error correction and entanglement purification to mitigate these effects.

Despite the lack of quantification of signal loss over extended distances, this demonstration establishes a strong foundation for future quantum networks, potentially enabling applications in quantum remote sensing and advanced information processing. Quantum remote sensing could utilise entangled photons to achieve higher precision measurements than classical sensors, with applications in areas such as environmental monitoring and medical imaging. Discrete frequency bins will likely expand the capacity of quantum networks and begin a new era in secure data transmission, offering more durable and secure communication. Manipulating light’s colours, specifically its frequency components, has allowed for the creation of high-dimensional entangled states, known as qudits, which provide greater information density than traditional quantum bits. A Hilbert-space dimensionality of at least 289 sharply expands the capacity for encoding and transmitting quantum data, building upon the principles established in the initial demonstration. This increased capacity is analogous to increasing the number of lanes on a highway, allowing for more data to be transmitted simultaneously. The ability to generate and manipulate these high-dimensional entangled states represents a significant step towards realising the full potential of quantum communication and computation, paving the way for a future where quantum technologies play a central role in our daily lives. The use of frequency as a degree of freedom for entanglement is particularly promising due to its compatibility with existing telecommunications infrastructure, potentially lowering the barriers to widespread adoption.

Researchers successfully engineered entangled states of light using its frequency, achieving a Hilbert-space dimensionality of at least 289 with two qudits. This represents a substantial increase in the amount of quantum information that can be encoded and transmitted compared to standard quantum bits. The team demonstrated this entanglement across a fibre network, and are now focused on developing techniques for quantum error correction and entanglement purification to improve the stability of these states. This work provides a crucial step towards building more complex and robust quantum communication systems and high-dimensional information processing.