Limitations of Quantum Entangled Particles

In a recent study, researchers have uncovered new evidence indicating that heat can destroy quantum entanglement, a phenomenon that has puzzled scientists since it was first highlighted by physicist Erwin Schrödinger nearly a century ago. Entanglement occurs when quantum particles interact in such a way that their individual identities merge into a collective state, leading to behavior that defies classical physics. While entanglement is well-understood in simple, idealized systems with a few particles, the situation becomes much more complex in the real world, where larger systems of atoms are influenced by both quantum mechanics and thermodynamics. The study emerged somewhat unexpectedly as researchers were developing a new quantum algorithm. During their work, they identified a hard limit to the persistence of entanglement when exposed to increasing temperatures. At very low temperatures, entanglement can extend over large distances and involve many atoms, leading to phenomena like superconductivity. However, as the temperature rises, the increased thermal activity causes atoms to move more chaotically, which disrupts the delicate connections between entangled particles. This discovery marks a significant step in understanding how entanglement interacts with thermodynamic forces in more complex systems. It provides crucial insights into the limitations of quantum entanglement, especially in practical applications where maintaining low temperatures might be challenging. The findings also help to clarify the conditions under which quantum entanglement can exist, offering a clearer picture of how heat fundamentally impacts this "spooky" phenomenon that has intrigued physicists for decades.

Delays, in various forms, can limit the entanglement distribution performance of quantum communication networks, particularly when the latter rely on quantum switches with limited resources (like single photon sources with Nitrogen Vacancy, NV, centers) and quantum memories, sensitive to noise and losses. In our recent work, that will appear in IEEE JSAC, we rigorously analyze the quantum memory decoherence noise and resulting end-to-end fidelity after distillation. Then, we leverage this analysis to jointly optimize the average entanglement distribution delay and entanglement distillation operations to improve end-to-end fidelity while taking into account the practical physics underlying NV centers. The results show considerable improvements in fidelity and delay, for this physics-informed approach: https://lnkd.in/e8X3q8pT Mahdi Chehimi

PHOTONIC QUANTUM ENTANGLEMENT VIA ANTI-PARITY-TIME SYMMETRY Quantum entanglement, a fundamental aspect of interconnected quantum states, enables instantaneous correlations across any distance. However, it is highly susceptible to quantum decoherence, which occurs when these states interact with their environment. Decoherence disrupts the delicate quantum states, causing them to lose coherence and behave more like classical systems or degraded into a mixed state. This challenge limits the practical implementation of reliable entanglement-based technologies. In order to mitigate decoherence and recover an entangled state that has degraded into a mixed state, a targeted method must be employed to selectively remove its classical components. This approach is analogous to classical optical filters, which isolate specific degrees of freedom of light, wavelength or polarization. In quantum optics, various strategies for entanglement filtering have been investigated, including techniques involving photon ancillas or leveraging the nonlinear behavior of Rydberg atoms. Since filters are inherently non-Hermitian systems, a compelling question emerges: can dissipation be strategically engineered within certain non-conservative configurations to effectively restore entanglement from a mixed input state? Non-Hermitian systems reveals a range of surprising phenomena, such as phase transitions, topological chirality, unidirectional invisibility, laser mode control, loss-induced transparency, and enhanced sensitivity. By harnessing the unique characteristics of photonic non-Hermitian anti-parity-time (APT) symmetric configurations, research team at USC developed a set of structures capable of achieving quantum-level functionalities. Their approach isolates a desired entangled state within a bosonic subspace, thereby providing a highly versatile linear mechanism for state selection through photon-photon interference. Importantly, this configuration functions as a decoherence-free subspace, preserving quantum states against dephasing while enhancing the robustness of quantum information processing. Researchers demonstrated efficient extraction of entanglement from any input state. This filter was implemented on a lossless waveguide network using Lanczos transformations, consistent with Wigner-Weisskopf theory. This scheme achieved nearunity fidelity under single- and two-photon excitation and is scalable to higher photon levels while remaining robust against decoherence during propagation. Overall, implementing APT systems within a completely Hermitian environment presents a promising path forward in non-Hermitian quantum mechanics, eliminating the need for absorbing or amplifying materials. By facilitating the on-demand generation of entangled photons and nondestructive entanglement purification directly on-chip, this research paves the way for development of quantum technologies on integrated and compact platforms. # https://lnkd.in/ebQVadcq

⚛️ No quantum advantage without classical communication: fundamental limitations of quantum networks 📜 Quantum networks connect systems at separate locations via quantum links, enabling a wide range of quantum information tasks between distant parties. Large-scale networks have the potential to enable global secure communication, distributed quantum computation, enhanced clock synchronization, and high-precision multiparameter metrology. For the optimal development of these technologies, however, it is essential to identify the necessary resources and sub-routines that will lead to the quantum advantage, but this is demanding even for the simplest protocols in quantum information processing. Here we show that quantum networks relying on the long-distance distribution of bipartite entanglement, combined with local operations and shared randomness, cannot achieve a relevant quantum advantage. Specifically, we prove that these networks do not help in preparing resourceful quantum states such as Greenberger-Horne-Zeilinger states or cluster states, despite the free availability of long-distance entanglement. At an abstract level, our work points towards a fundamental difference between bipartite and multipartite entanglement. From a practical perspective, our results highlight the need for classical communication combined with quantum memories to fully harness the power of quantum networks ℹ️ Neumann et al - 2025