Towards a Quantum Internet. Product Measurements Enhance Efficiency, Robustness

Quantum entanglement, where particles become interconnected and can instantly affect each other regardless of distance, is a key aspect of quantum communication. However, harnessing this phenomenon presents challenges, particularly in the entanglement-assisted prepare-and-measure (EAPM) scenario. We are moving towards an ever-increasing push towards quantum connectivity and an eventual quantum internet.

Recent research has shown that product measurements, simplifying experiments by allowing particles to be measured separately, can enhance quantum communication. Despite potential limitations, such as noise robustness, the research proposes solutions and demonstrates the power of product measurements in generating quantum correlations. This could lead to more efficient and robust quantum communication systems, with implications for secure communication and quantum computing.

What is the Significance of Entanglement in Quantum Communication?

Quantum entanglement, a phenomenon where particles become interconnected and the state of one can instantly affect the other, regardless of the distance between them, is a fundamental aspect of quantum communication. This shared entanglement between a sender and a receiver connected over a quantum channel is considered the most powerful communication resource in quantum theory. This is famously demonstrated in the dense-coding protocol, where entanglement doubles the classical capacity of a noise-free qubit channel.

In the context of quantum communication, this entanglement-assisted prepare-and-measure (EAPM) scenario can be viewed as a setting for efficient quantum communication and as a platform for semi-device-independent quantum information protocols. This is because the state of the sender and the receiver are uncharacterized devices and only knowledge of the dimension of the channel is required to deduce the quantum nature of the correlations. In this sense, the EAPM scenario offers an appealing path to certify the advantages of entanglement in experiments with limited characterization.

However, a central obstacle for harnessing entanglement advantages in the EAPM scenario is that protocols commonly need the receiver to measure both the particles, namely the one coming from the entanglement source and the one arriving over the channel, in an entangled basis. In optical systems, such measurements are well known to be impossible without extra photons or non-linear effects, which can limit experiments to using only single-photon carriers of multiple qubits.

How Can Product Measurements Enhance Quantum Communication?

In the EAPM scenario, for the simplest case of qubit systems, a series of dense-coding experiments have over time implemented increasingly sophisticated Bell basis measurements and thereby approached the theoretical limit of the entanglement advantage. For systems of higher dimension than qubit, the situation is extra challenging. Even resolving one element of a high-dimensional entangled basis is impossible with ancilla-free linear optics. The most high-dimensional optical Bell basis measurement hitherto realized is limited to three-level systems and uses ancillary photons.

In the EAPM scenario, this has led experiments based on high-dimensional entanglement and quantum communication to instead focus on simpler, suboptimal measurements compatible with standard linear optics. The challenges associated with entangled measurements are broadly relevant in the different correlation tests accommodated by the EAPM scenario.

However, while entangled measurements are provably necessary for the specific task of dense coding, this is not true in general. Interestingly, it has recently been shown that there exist well-known communication tasks that in the EAPM scenario admit their best implementation in protocols that rely only on the simplest joint measurements. These are mere product measurements of the source and message particles; they therefore constitute the classical post-processing of two completely separate single-particle measurements. In principle, this can greatly simplify the experiments as the particles do not need to interfere with each other in the measurement device and can even be measured at separate times.

What are the Limitations and Potential Solutions in Quantum Communication?

Despite the potential advantages of product measurements in quantum communication, there are still limitations that need to be addressed. One of the main concerns is the noise robustness of protocols based on product measurements. While entangled measurements are well known to reveal the correlation advantages of shared entanglement even from very noisy states, no counterpart is known for product measurements. That is, even though product measurements can sometimes be optimal under ideal conditions, their performance might deteriorate in the presence of significant amounts of noise in the entanglement distribution, rendering them unable to certify entanglement that is well within the reach of schemes that use entangled measurements.

Indeed, certifying noisy forms of entanglement is a central matter for correlation experiments. To address this, the researchers propose a natural extension of the standard scenario for these experiments and show that it circumvents this limitation. This leads them to prove entanglement advantages from every entangled two-qubit Werner state, evidence its generalization to high-dimensional systems, and establish a connection to quantum teleportation.

How Does This Research Contribute to the Field of Quantum Communication?

The findings of this research reveal the power of product measurements for generating quantum correlations in entanglement-assisted communication. They pave the way for practical semi-device-independent entanglement certification well beyond the constraints of Einstein-Podolsky-Rosen steering. This research contributes significantly to the field of quantum communication by providing a deeper understanding of the role of entanglement and product measurements in quantum communication protocols.

Moreover, the proposed extension of the standard scenario for these experiments and the proof of entanglement advantages from every entangled two-qubit Werner state not only address the limitations of current quantum communication protocols but also open up new possibilities for the development of more efficient and robust quantum communication systems. This research, therefore, represents a significant step forward in the ongoing efforts to harness the full potential of quantum entanglement for quantum communication.

What are the Future Implications of This Research?

The implications of this research are far-reaching. By demonstrating the potential of product measurements in quantum communication and proposing a solution to the limitations of current protocols, this research opens up new avenues for the development of more efficient and robust quantum communication systems. This could have significant implications for a wide range of applications, from secure communication to quantum computing.

Furthermore, by establishing a connection to quantum teleportation, this research also opens up new possibilities for the exploration of other quantum phenomena and their potential applications in quantum communication. This could lead to the development of new quantum communication protocols and technologies that could revolutionize the way we communicate and process information.

In conclusion, this research represents a significant contribution to the field of quantum communication and sets the stage for exciting developments in the future.