Entanglement, a cornerstone of future quantum networks and distributed computing, typically suffers from signal loss as it travels between quantum modules. James D. Teoh, Nathanael Cottet, and Patrick Winkel, all from Yale University and the Yale Quantum Institute, alongside Luke D. Burkhart, Luigi Frunzio, and Robert J. Schoelkopf, demonstrate a novel approach to generating entanglement that largely overcomes this limitation. The team successfully creates highly faithful entangled states, even in conditions where direct signal transfer fails, by treating microwave communication channels as single standing wave modes. This breakthrough allows them to herald entanglement with near-optimal probabilities and deterministically teleport quantum information between modules with remarkably high fidelity, despite using a link with extremely low direct transfer efficiency. This work fundamentally alters the design considerations for future quantum networks, proving that fast coupling rates and low-loss links are not essential for achieving high-fidelity quantum communication in the microwave frequency range.
The team employed encoding methods, including the two-legged cat code and dual-rail encoding, to represent quantum information within the cavities. The process relies on carefully controlling the interaction between the cavities and a ‘bus mode’, a lossy resonator that facilitates entanglement. A crucial step involves verifying the cavities are in their initial vacuum state using precisely timed microwave pulses and qubit readout. A fast reset sequence quickly returns the cavities to the vacuum state if an attempt fails, improving entanglement efficiency. The experimental procedure begins by preparing the cavities and qubits in a known state. Entanglement is generated by controlling the interaction between the cavities and the bus mode, followed by a vacuum check to confirm the correct state.
If the check fails, the fast reset sequence prepares the system for another attempt. Successful entanglement is characterized using techniques like Wigner tomography to confirm its quantum properties. The team achieved an entanglement fidelity of 86-89%, demonstrating the quality of the generated entangled states. The fast reset sequence significantly increased the entanglement rate to 43kHz, making the process more practical. Optimization of pulse sequences and feedback cooling further enhanced fidelity.
This research demonstrates a scalable approach to entanglement generation, relying on local interactions between cavities and a bus mode. The scheme is robust to noise and imperfections, as the dissipation process filters unwanted states. The fast reset sequence improves the entanglement rate, making it practical for quantum information processing. The versatility of the scheme allows adaptation to different qubit encodings.
Heralded Entanglement via Microwave Standing Waves
Scientists have pioneered a new approach to generating entanglement, overcoming limitations imposed by signal loss in quantum networks. Their work demonstrates a heralded entanglement scheme utilizing microwave links as single standing wave modes, enabling robust entanglement even in high-loss environments. The team coupled two quantum modules, each containing a bosonic mode representing a qubit, to a shared ‘bus mode’ using beamsplitter interactions, carefully controlling the couplings to induce specific quantum dynamics. The experiment prepared the quantum modules in either ‘bright’ or ‘dark’ states, exploiting their differing responses to the coupling Hamiltonian.
Bright states fully transfer their excitation into the bus mode, while dark states remain unaffected, forming the basis of the heralded entanglement protocol. To generate entanglement, the team prepared a superposition of bright and dark states. Following the interaction, a ‘vacuum check’ measurement determined whether the cavities remained excited, indicating a dark state and heralding the successful generation of an entangled Bell state. This state achieved a success probability approaching 50% per attempt. The method is resilient to loss in the bus mode, as dark states are unaffected by dissipation, maintaining entanglement fidelity even with significant energy loss. Researchers demonstrated that even substantial loss may aid in resetting the bus mode between attempts.
High-Fidelity Entanglement Via Standing Wave Heralding
Scientists have achieved a breakthrough in entanglement generation, overcoming limitations imposed by signal loss in quantum networks. Their work demonstrates a new method for creating entangled states between separated quantum modules, even in conditions where direct state transfer fails. The team produced Bell states with a fidelity of 92±1%, accounting for all sources of error. This achievement relies on treating the communication channel as a single standing wave, enabling them to herald entanglement with success probabilities approaching a theoretical limit of 50% per attempt. Experiments revealed that the success of this method is strongly linked to the amplitude of the encoded information, with optimal results obtained at approximately √2.
At this amplitude, the team measured the highest Bell state fidelity, demonstrating robust entanglement even in challenging environments. The research identified photon loss as the dominant source of error, providing valuable insights for future network design. Furthermore, scientists leveraged this heralded Bell state to deterministically teleport a quantum state between modules with an average fidelity. This was accomplished despite the communication link exhibiting only 2% single-photon transfer efficiency, demonstrating that high coupling rates and low loss links are no longer strict requirements for high-fidelity quantum communication in the microwave regime. Scientists successfully created high-fidelity Bell states between separated quantum modules, even when the direct transfer of single photons through the link was highly inefficient. The team achieved this by exploiting the physics of standing waves within microwave links and employing a heralding scheme, reaching success probabilities approaching the theoretical limit of 50% per attempt. Importantly, this method enabled the deterministic teleportation of quantum states between modules with a fidelity of 90%, despite the communication link exhibiting only 2% single-photon transfer efficiency. This achievement indicates that designing quantum networks no longer requires exclusively focusing on minimizing signal loss or maximizing coupling speeds, provided heralding measurements are incorporated. The researchers suggest this approach broadens the range of viable physical links for quantum communication, potentially including materials like copper or standard printed circuit boards, and could be beneficial for hybrid quantum systems where losses are inherent.