Single-photon entanglement (SPE) is an important source in quantum communication. In this paper, we put forward a single-photon-assisted noiseless linear amplification protocol to protect the SPE of an arbitrary polarization–time-bin qudit from the photon transmission loss caused by the practical channel noise. After the amplification, the fidelity of the SPE can be effectively increased. Meanwhile, the encoded polarization–time-bin features of the qudit can be well preserved. The protocol can be realized under the current experimental conditions. Moreover, the amplification protocol can be extended to resist complete photon loss and partial photon loss during the photon transmission. After the amplification, we can not only increase the fidelity of the target state, but also solve the decoherence problem simultaneously. Based on the above features, our amplification protocol may be useful in future quantum communication.
Single-photon entanglement (SPE) is an important source in quantum communication. In this paper, we put forward a single-photon-assisted noiseless linear amplification protocol to protect the SPE of an arbitrary polarization–time-bin qudit from the photon transmission loss caused by the practical channel noise. After the amplification, the fidelity of the SPE can be effectively increased. Meanwhile, the encoded polarization–time-bin features of the qudit can be well preserved. The protocol can be realized under the current experimental conditions. Moreover, the amplification protocol can be extended to resist complete photon loss and partial photon loss during the photon transmission. After the amplification, we can not only increase the fidelity of the target state, but also solve the decoherence problem simultaneously. Based on the above features, our amplification protocol may be useful in future quantum communication.
Heralded noiseless amplification is beneficial in overcoming transmission photon loss in a noisy quantum channel. We propose a single-photon-assisted heralded noiseless amplification protocol of the single-photon entanglement (SPE), where the single-photon qubit has an arbitrary unknown polarization feature. We focus on both the complete and partial photon loss during the transmission process. After the amplification, the parties can recover the pure less-entangled SPE into a maximally entangled SPE and increase its fidelity. Moreover, the polarization feature of the single-photon qubit will be well preserved and not be leaked. Our protocol can be realized under our current experimental condition. Based on the features above, our protocol may be useful in the quantum secure communication schemes that encode information in the polarization degree of freedom of photons.
Abstract A single-photon qudit encoded in more than one degree of freedom (DOF) can effectively enlarge a single photon’s capacity and has great potential for applications in future high-capacity quantum communication fields. During the photon transmission in practical noisy channels, photon qudits may suffer from photon transmission loss, which largely limits their application. In this paper, we propose a practical noiseless linear amplification (NLA) protocol for protecting the single-photon qudit encoded in polarization and double-longitudinal momentum DOFs. This NLA adopts the imperfect entangled state generated from a practical spontaneous parametric down-conversion (SPDC) source as the auxiliary state and can be realized under current experimental conditions. After performing the NLA protocol, the fidelity of the target single-photon qudit can be increased and the encoded information in all DOFs can be well preserved. We show the adoption of the imperfect entangled state generated from SPDC source is superior to that of the imperfect single-photon state from a practical heralded single-photon source. Our NLA protocol has application potential in current and future quantum communications.
Photon loss is one of the main obstacles in current long-distance quantum communications. The approach of noiseless linear amplification (NLA) is one of the powerful way to distill the single-photon state (SPS) from a mixed state, which comprises both the SPS and vacuum state. However, existing NLA protocol can only perform the amplification for one time. That is the fidelity of the SPS cannot be increased anymore. In this paper, We put forward an efficient cascaded NLA protocol for both the SPS and single-photon entanglement (SPE), respectively, with the help of some auxiliary single photons. By repeating this protocol for sever times, the fidelity of the SPS and SPE can reach near 100\%, which may make this protocol is extremely useful to close the detection loophole in quantum key distribution. Moreover, this protocol is based on the linear optics, which makes it feasible in current technology.
Quantum communication is based on the basic principles of quantum mechanics such as the Heisenberg uncertainty and the no-cloning theorem. Quantum communication can detect the eavesdropping, so that it has unconditional security in theory. Coherent state is an important continuous-variable resource in the quantum communication field. The hyper-encoded coherent states mean encoding in two or more degrees of freedom (DOFs) of each photon in the coherent state, which can increase the capacity of single photon and has potential applications in future high-capacity quantum communication field. However, during practical applications, the channel noise may cause photon transmission loss, which will largely reduce quantum communication’s efficiency, even threaten its security. As a result, the photon transmission loss largely limits the applications of the hyper-encoded coherent states. The noiseless linear amplification (NLA) is a promising method to solve the photon transmission loss problem. However, no existing NLA schemes can be used for the hyper-encoded coherent states. In the paper, we propose an NLA protocol for arbitrary polarization-time-bin hyper-encoded coherent states assisted by auxiliary single photons. This NLA protocol adopts the quantum scissor (QS) devices in the parallel structure. The protocol first splits the input coherent state into N paths with the help of the first beam splitter array, which makes the coherent state in each path be a weak coherent state. Then, it applies the QS in each path. Only when the QS in each path obtains the successful photon-detector response, the whole NLA protocol will be successful. In this case, the output states in all paths will be finally converged into one output path with the help of the second beam splitter array. By adjusting the transmittance of the variable beam splitters in each QS, the NLA protocol can increase the average photon number of the output coherent state, and thus realize the amplification of the coherent state. Meanwhile, the hyper-encoded features in the polarization and time-bin DOFs can be perfectly preserved. Our NLA protocol only requires the common optical elements, so that it can be realized under current experimental conditions. Then, we analyze the performance of our NLA protocol with practical experimental conditions. First, the NLA protocol requires the photon-number-resolving detectors in theory, which are unavailable under practical experimental conditions. With practical imperfect photon detectors, which cannot distinguish the input photon number and have relatively low detection efficiency, the total success probability of the NLA protocol will decrease. Meanwhile, some unwanted disturb items may be introduced into the output state with a small probability, so that the fidelity of the target coherent state in the output state will decrease. Second, the NLA protocol requires an infinite N in theory. In practical experiments, we can only choose a finite N. In this way, besides the amplification, the NLA protocol will perform the N-photon truncation of the coherent state. With the growth of N, the fidelity of the output state will increase, but the total success probability of this NLA protocol will decrease. In conclusion, our NLA protocol can realize the amplification of the polarization-time-bin hyper-encoded coherent state and preserve the hyper-encoded features of the photons. Meanwhile, it can be realized under current experimental conditions. Our protocol has application potential in the future quantum communication field.
Abstract We put forward a highly efficient recyclable single-photon assisted amplification protocol, which can protect single-photon entanglement (SPE) from photon loss and decoherence. Making use of quantum nondemolition detection gates constructed with the help of cross-Kerr nonlinearity, our protocol has some attractive advantages. First, the parties can recover less-entangled SPE to be maximally entangled SPE, and reduce photon loss simultaneously. Second, if the protocol fails, the parties can repeat the protocol to reuse some discarded items, which can increase the success probability. Third, when the protocol is successful, they can similarly repeat the protocol to further increase the fidelity of the SPE. Thereby, our protocol provides a possible way to obtain high entanglement, high fidelity and high success probability simultaneously. In particular, our protocol shows higher success probability in the practical high photon loss channel. Based on the above features, our amplification protocol has potential for future application in long-distance quantum communication.
We put forward an effective amplification protocol for protecting the single-photon entangled state of the time-bin qubit. The protocol only requires one pair of the single-photon entangled state and some auxiliary single photons. With the help of the 50:50 beam splitters, variable beam splitters with the transmission of t and the polarizing beam splitters, we can increase the fidelity of the single-photon entangled state under $t