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In a recent article [X. Lai et al., Phys. Rev. Lett. 133, 033601 (2024)], the coherence time of degenerate entangled photon pairs (biphotons) generated via backward spontaneous four-wave mixing in a cold atomic ensemble was shown to be immune to optical loss and dephasing. This finding is crucial for practical applications in quantum information processing, quantum communication, and networking, where loss is inevitable. However, in studying the underlying mechanism for this loss- and dephasing-insensitive biphoton coherence time, the previous article did not take quantum noise into account. In this work, we employ the Heisenberg-Langevin approach to study this effect and provide a rigorous theoretical proof of the symmetry-protected biphoton coherence by taking quantum noise into consideration, as compared to the perturbation theory in the interaction picture. 

We propose a distributed quantum computing (DQC) architecture in which individual small-sized quantum computers are connected to a shared quantum gate processing unit (S-QGPU). The S-QGPU comprises a collection of hybrid two-qubit gate modules for remote gate operations. In contrast to conventional DQC systems, where each quantum computer is equipped with dedicated communication qubits, S-QGPU effectively pools the resources (e.g., the communication qubits) together for remote gate operations, and, thus, significantly reduces the cost of not only the local quantum computers but also the overall distributed system. Our preliminary analysis and simulation show that S-QGPU's shared resources for remote gate operations enable efficient resource utilization. When not all computing qubits (also called data qubits) in the system require simultaneous remote gate operations, S-QGPU-based DQC architecture demands fewer communication qubits, further decreasing the overall cost. Alternatively, with the same number of communication qubits, it can support a larger number of simultaneous remote gate operations more efficiently, especially when these operations occur in a burst mode.