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A unitary interaction coupling two parties enables quantum communication in both the forward and backward directions. Each communication capacity can be thought of as a tradeoff between the achievable rates of specific types of forward and backward communication. Our first result shows that for any bipartite unitary gate, coherent classical communication is no more difficult than classical communication -- they have the same achievable rate regions. Previously this result was known only for the unidirectional capacities (i.e., the boundaries of the tradeoff). We then relate the tradeoff curve for two-way coherent communication to the tradeoff for two-way quantum communication and the tradeoff for coherent communiation in one direction and quantum communication in the other.
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We show how to implement several continuous-variable coherent protocols with linear optics. Noise can accumulate when implementing each coherent protocol with realistic optical devices. Our analysis bounds the level of noise accumulation. We highlight the connection between a coherent channel and a nonlocal quantum nondemolition interaction and give two new protocols that implement a coherent channel. One protocol is superior to a previous method for a nonlocal quantum nondemolition interaction because it requires fewer communication resources. We then show how continuous-variable coherent superdense coding implements two nonlocal quantum nondemolition interactions with a quantum channel and bipartite entanglement. We finally show how to implement continuous-variable coherent teleportation experimentally and provide a way to verify the correctness of its operation.
We present and experimentally demonstrate a communication protocol that employs shared entanglement to reduce errors when sending a bit over a particular noisy classical channel. Specifically, it is shown that, given a single use of this channel, one can transmit a bit with higher success probability when sender and receiver share entanglement compared to the best possible strategy when they do not. The experiment is realized using polarization-entangled photon pairs, whose quantum correlations play a critical role in both the encoding and decoding of the classical message. Experimentally, we find that a bit can be successfully transmitted with probability 0.891 pm 0.002, which is close to the theoretical maximum of (2 + 2^-1/2)/3 simeq 0.902 and is significantly above the optimal classical strategy, which yields 5/6 simeq 0.833.
We propose and experimentally demonstrate a simple and efficient scheme for photonic communication between two remote superconducting modules. Each module consists of a random access quantum information processor with eight-qubit multimode memory and a single flux tunable transmon. The two processor chips are connected through a one-meter long coaxial cable that is coupled to a dedicated communication resonator on each chip. The two communication resonators hybridize with a mode of the cable to form a dark communication mode that is highly immune to decay in the coaxial cable. We modulate the transmon frequency via a parametric drive to generate sideband interactions between the transmon and the communication mode. We demonstrate bidirectional single-photon transfer with a success probability exceeding 60 %, and generate an entangled Bell pair with a fidelity of 79.3 $pm$ 0.3 %.
We present an optimal scheme to realize the transformations between single copies of two bipartite entangled states without classical communication between the sharing parties. The scheme achieves the upper bound for the success probabilities [PRA 63, 022301 (2001), PRL 83, 1455 (1999)] of generating maximally entangled states if applied to entanglement concentration. Such strategy also dispenses with the interaction with an ancilla system in the implementation. We also show that classical communications are indispensable in realizing the deterministic transformations of a single bipartite entangled state. With a finite number of identical pairs of two entangled bosons, on the other hand, we can realize the deterministic transformation to any target entangled state of equal or less Schmidt rank through an extension of the scheme.
Quantum state verification provides an efficient approach to characterize the reliability of quantum devices for generating certain target states. The figure of merit of a specific strategy is the estimated infidelity $epsilon$ of the tested state to the target state, given a certain number of performed measurements n. Entangled measurements constitute the globally optimal strategy and achieve the scaling that epsilon is inversely proportional to n. Recent advances show that it is possible to achieve the same scaling simply with non-adaptive local measurements, however, the performance is still worse than the globally optimal bound up to a constant factor. In this work, by introducing classical communication, we experimentally implement an adaptive quantum state verification. The constant-factor is minimized from ~2.5 to 1.5 in this experiment, which means that only 60% measurements are required to achieve a certain value of epsilon compared to optimal non-adaptive local strategy. Our results indicate that classical communication significantly enhances the performance of quantum state verification, and leads to an efficiency that further approaches the globally optimal bound.
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