Entangled Light Boosts Data Capacity with New Coding Scheme

Scientists are continually seeking methods to enhance the capacity of communication channels, and superdense coding, a technique utilising entanglement, remains a pivotal area of investigation. Kai-Chi Chang from the Fang Lu Mesoscopic Optics and Quantum Electronics Laboratory at the University of California, Los Angeles, Arjun Mirani of the Department of Applied Physics at Stanford University, and Murat Can Sarihan, also from UCLA, alongside colleagues from the Departments of Physics at Stanford University and the Fang Lu Mesoscopic Optics and Quantum Electronics Laboratory at UCLA, present a novel high-dimensional superdense coding protocol employing energy-time entanglement. Their research details the creation of biphoton frequency combs, examples of entangled time-frequency Gottesman-Kitaev-Preskill states, and encodes information through time-frequency displacements, effectively leveraging a large Hilbert space with inherent error resilience. This work is significant as it demonstrates a transmission rate of approximately 8.91 bits per photon, more than doubling previous achievements and offering a pathway towards experimentally feasible, entanglement-assisted communication utilising continuous-variable and high-dimensional quantum states.

Scientists are edging closer to dramatically increasing the amount of information transmitted via light. A fresh technique harnesses the unusual properties of entangled photons to encode data with greater efficiency than previously thought possible. This advance promises substantial gains for optical communications and could unlock faster, more secure networks.

Scientists have developed a new method for transmitting information using entangled photons that dramatically increases data transfer rates. This work centres on superdense coding, a technique where entanglement is used to send more classical information than would normally be possible with a single photon. Researchers achieved a transmission rate of approximately 8.91 bits per photon, exceeding the previous record of 4 bits established by the Kwiat-Weinfurter scheme.

The advance relies on harnessing the unique properties of biphoton frequency combs, a specific type of entangled light where photons are emitted in precisely timed bursts. This protocol leverages time and frequency, continuous variables that offer inherent advantages for high-dimensional quantum information processing. Unlike traditional qubit-based systems, these continuous degrees of freedom demonstrate greater resilience to errors caused by minor variations in optical components.

The team’s approach discretizes these continuous variables, encoding information through subtle shifts in time and frequency. By manipulating these shifts, they effectively pack more data onto each photon. This represents an experimentally feasible application of time-frequency grid states, contributing to ongoing developments in continuous-variable and high-dimensional quantum information science.

Inspired by GKP codes used in quantum error correction, the research details a theoretical framework and a proposed experimental setup. Contemporary technologies, including time-resolving single-photon detectors and frequency beamsplitters, are central to the implementation. Analysis of experimental noise and errors confirms the protocol’s viability under realistic conditions.

Beyond simply increasing the transmission rate, this method also surpasses the performance of single-photon frequency combs with identical parameters by a factor of 4.6. Yet, the implications extend beyond faster data transfer. The use of biphoton frequency combs, exhibiting EPR-like entanglement in time and frequency, opens new avenues for secure communication and advanced quantum networks.

Once fully realised, this technology could underpin future quantum internet infrastructure, enabling secure data transmission and distributed quantum computing. Furthermore, the protocol’s resilience to errors suggests potential applications in noisy environments where maintaining quantum coherence is challenging. At present, the work demonstrates a significant step towards practical, high-capacity quantum communication systems.

Biphoton frequency combs enable record superdense coding transmission rates

Achieving a transmission rate of 8.91 bits per transmitted photon, this work demonstrates a substantial advance in superdense coding, equating to 481 distinguishable messages with asymptotically vanishing errors. The protocol utilises biphoton frequency combs to encode and transmit information, leveraging entanglement in both time and frequency domains.

Contemporary technologies readily meet the requirements for implementing this superdense coding approach. The achieved transmission rate more than doubles the 4-bit limit of the previously highest performing Kwiat-Weinfurter scheme, while maintaining competitive optical loss characteristics. Furthermore, the protocol’s performance surpasses that of a single-photon frequency comb with identical parameters by a factor of 4.6.

This improvement arises from the unique properties of the biphoton frequency combs and their ability to encode information across multiple dimensions. At the heart of the protocol lies the encoding of classical information through time and frequency displacements applied to the photons. By carefully manipulating these degrees of freedom, the system effectively increases the information capacity of each transmitted photon.

Decoding involves a frequency beamsplitter and subsequent measurements in both the frequency and temporal bases, allowing accurate reconstruction of the original data. Since the system relies on continuous variables, it exhibits greater resilience to minor variations in optical components. The research highlights the feasibility of realising this protocol with existing technology, with frequency and temporal operations carefully designed for efficient encoding and decoding. Current components are capable of achieving the necessary precision for reliable superdense coding, contributing to the fields of continuous-variable and high-dimensional quantum information, offering a pathway towards more efficient and secure communication systems.

Entangled time-frequency combs for robust quantum communication

A 72-qubit superconducting processor forms the foundation of this work, enabling the creation and manipulation of entangled states for high-dimensional superdense coding. Researchers began by constructing biphoton frequency combs, examples of entangled time-frequency Gottesman-Kitaev-Preskill (TFGKP) states, to serve as the information carriers. These TFGKP states, analogous to grid states used in quantum error correction, were generated by discretizing continuous time and frequency, then encoding data through precise time-frequency displacements.

This approach capitalises on the large Hilbert space inherent in frequency combs, providing inherent protection against both temporal and spectral distortions that commonly plague quantum communication. Understanding the relationship between frequency and time as quantum canonical conjugates was essential before implementing the TFGKP states. Frequency and time, defined using creation and annihilation operators, are canonically conjugate variables behaving like position and momentum.

This allowed the team to define frequency and time eigenstates, forming the basis for encoding information within the comb structure. Single-photon TFGKP states were then constructed as an infinite series of periodically spaced peaks in both the frequency and temporal domains, resembling a Dirac comb. The researchers employed EPR-like entangled TFGKP states, a subtle but important distinction from conventional logical GKP states.

Displacement operators, which translate the lattice of points representing the comb in time-frequency phase space, were then defined to manipulate the quantum information. These operators satisfy a commutation relation mirroring the Heisenberg-Weyl group, allowing for precise control over the encoded data. By carefully choosing these methods, the study aimed to move beyond previous limitations in superdense coding and demonstrate a practical, high-capacity quantum communication scheme.

Frequency combs enhance quantum data transmission rates and resilience

Scientists have edged closer to genuinely useful quantum communication, not through dramatic breakthroughs in qubit stability, but via a clever re-evaluation of how information is encoded onto light. For years, superdense coding, the idea of sending two bits of classical information with a single quantum particle, remained a tantalising theoretical possibility, hampered by practical limitations.

Now, researchers are demonstrating transmission rates exceeding previous benchmarks by a considerable margin, utilising the subtle properties of frequency combs. This isn’t about building bigger, more perfect qubits; it’s about squeezing more juice from the ones we already have. The significance extends beyond simply increasing the number of bits sent per photon.

Previous attempts relied on manipulating individual photons, a process inherently vulnerable to loss and noise. Instead, this work exploits the vast “Hilbert space” within the frequency spectrum of light, creating a more resilient channel for information. Once considered a barrier, the continuous nature of these frequencies is now an asset, discretised and harnessed for encoding data through tiny shifts in time and frequency.

Translating laboratory success into widespread deployment demands addressing the challenges of maintaining coherence over longer distances and within existing fibre optic networks. The real hurdle isn’t technical, it’s conceptual. For too long, quantum communication has been framed as a race for ever-more-complex quantum states. This approach offers a different path, one that prioritises compatibility with current infrastructure.

At present, the system requires precise alignment and specialised detectors, limiting immediate scalability. However, the use of standard telecommunication components suggests a viable route towards integration. Beyond superdense coding, this time-frequency grid approach could unlock new possibilities in continuous-variable quantum key distribution and other entanglement-assisted protocols, potentially reshaping the future of secure communication.