Implementation of Fault-Tolerant Encoding Circuit Based on Stabilizer Implementation and "Flag" Bits in Steane Code.

Quantum error correction (QEC) is an effective way to overcome quantum noise and de-coherence, meanwhile the fault tolerance of the encoding circuit, syndrome measurement circuit, and logical gate realization circuit must be ensured so as to achieve reliable quantum computing. Steane code is one of the most famous codes, proposed in 1996, however, the classical encoding circuit based on stabilizer implementation is not fault-tolerant. In this paper, we propose a method to design a fault-tolerant encoding circuit for Calderbank-Shor-Steane (CSS) code based on stabilizer implementation and "flag" bits. We use the Steane code as an example to depict in detail the fault-tolerant encoding circuit design process including the logical operation implementation, the stabilizer implementation, and the "flag" qubits design. The simulation results show that assuming only one quantum gate will be wrong with a certain probability p, the classical encoding circuit will have logic errors proportional to p; our proposed circuit is fault-tolerant as with the help of the "flag" bits, all types of errors in the encoding process can be accurately and uniquely determined, the errors can be fixed. If all the gates will be wrong with a certain probability p, which is the actual situation, the proposed encoding circuit will also be wrong with a certain probability, but its error rate has been reduced greatly from p to p2 compared with the original circuit. This encoding circuit design process can be extended to other CSS codes to improve the correctness of the encoding circuit.

Composable security for inter-satellite continuous-variable quantum key distribution in the terahertz band.

Continuous-variable quantum key distribution (CVQKD) can be effectively compatible with off-the-shelf communication systems and has been proven to be the security against collective attacks in the finite-size regime and composability. In this paper, we classify three different trust levels for the loss and noise experienced by the sender and receiver. Based on these trust levels, we derive the composable finite-size security bounds of inter-satellite CVQKD in the terahertz (THz) band. We also show how these trust levels can nontrivially increase the composable secret key rates of THz-CVQKD and tolerate higher loss. Furthermore, the numerical simulations strongly support the feasibility of inter-satellite THz-CVQKD even in the worst trust level. This work provides an efficient path for building an inter-satellite quantum communication network.

FL-QKD based on optical-THz biphotons generated by spontaneous parametric downconversion in inter-satellite wireless communication.

Floodlight quantum key distribution (FL-QKD) is a new QKD protocol that can achieve a 2 Gbps secret key rate (SKR) in a 50 km fiber link without multiplexing technology [Q. Zhuang et al., Phys. Rev. A94, 012322 (2016)PLRAAN1050-294710.1103/PhysRevA.94.012322]. In this paper, we propose a wireless FL-QKD at terahertz bands (THz-FL-QKD) in inter-satellite links. THz-FL-QKD is the two-way protocol that sends quantum signals in the forward channel, modulates and amplifies the received signals at the receiver, and then returns to the transmitter through the backward channel for homodyne detection and decoding. We analyze the security of THz-FL-QKD against individual attacks and optimum collective attacks. Numerical simulations show that THz-FL-QKD is capable of a 50 Mbps SKR at 10 THz frequency in a 200 km inter-satellite wireless link. We expect this work will provide an efficient path to build a high-speed global quantum communication network.

FL-QKD based on optical-THz biphotons generated by spontaneous parametric downconversion in inter-satellite wireless communication: publisher's note.

This publisher's note serves to identify a correction in Appl. Opt.60, 7362 (2021)APOPAI0003-693510.1364/AO.430898.

Continuous-Variable Quantum Secret Sharing Based on Thermal Terahertz Sources in Inter-Satellite Wireless Links.

We propose a continuous-variable quantum secret sharing (CVQSS) scheme based on thermal terahertz (THz) sources in inter-satellite wireless links (THz-CVQSS). In this scheme, firstly, each player locally preforms Gaussian modulation to prepare a thermal THz state, and then couples it into a circulating spatiotemporal mode using a highly asymmetric beam splitter. At the end, the dealer measures the quadrature components of the received spatiotemporal mode through performing the heterodyne detection to share secure keys with all the players of a group. This design enables that the key can be recovered only by the whole group players' knowledge in cooperation and neither a single player nor any subset of the players in the group can recover the key correctly. We analyze both the security and the performance of THz-CVQSS in inter-satellite links. Results show that a long-distance inter-satellite THz-CVQSS scheme with multiple players is feasible. This work will provide an effective way for building an inter-satellite quantum communication network.

Noise tailoring for quantum circuits via unitary 2t-design.

Because of environmental variations and imperfect operations, real-world quantum computers produce different coherent errors that are difficult to estimate. Here, we propose a method whereby the twirled noise over a unitary 2t-design (a set of unitary matrices that approximate the entire unitary group) for quantum circuits can be tailored into stochastic noise. Then, we prove that local random circuits for twirling separable noisy channel over the Clifford group can be used to construct a unitary 2t-design, which is easy to implement in experiments. Moreover, we prove that our method is robust to gate-dependent and gate-independent noise. The stochastic noise can be both estimated by average fidelity and directly obtained by randomized benchmarking via unitary 2t-designs. Obtaining such tailored noise is an important guarantee for achieving fault-tolerant quantum computation.

[Neural Cell Activity Detection Method Based on Near Infrared Spectroscopy Analysis of Spinal Cord Injury].

Traffic accidents, high-altitude drops, mechanical shock and other accidents may lead to human spinal cord injury, interrupting the transmission channel of human nerve signals, resulting in loss of some limb function. Since there is no method of spinal cord injury site of nerve cell activity, while the conventional testing instruments such as X ray, CT and other traditional instruments cannot provide any nerve cell activity, doctors couldn't have a clearer understanding of the patient's condition, which may delay the best timing of treatment, thus causing lifelong paralysis. Spectroscopy method can be used in the detection of cell organization change while near infrared spectrum technology is a kind of fast measurement and simple operation of nondestructive measuring. Therefore, aiming at the shortcomings of the existing medical testing equipment, on the basis of animal experiment, this paper uses near infrared spectrum technology, based on the absorption characteristics of the material in near infrared wave band, and combined with near infrared spectrum analysis technique of qualitative determination and quantitative analysis of the characteristics. Using clustering algorithm for spinal cord injury site of neuron-specific nuclear protein and neurotransmitters classify and simulation, partial least squares (PLS) algorithm is used to calculate the content to achieve a spinal cord accurate detection of the site of injury nerve cell activity. This method provides theoretical basis for the detection of spinal cord injury, bringing hopes to patients with limb function reconstruction and rehabilitation, for clinical provides a method to detect the activity of nerve cells in the noninvasive.

W-state Analyzer and Multi-party Measurement-device-independent Quantum Key Distribution.

W-state is an important resource for many quantum information processing tasks. In this paper, we for the first time propose a multi-party measurement-device-independent quantum key distribution (MDI-QKD) protocol based on W-state. With linear optics, we design a W-state analyzer in order to distinguish the four-qubit W-state. This analyzer constructs the measurement device for four-party MDI-QKD. Moreover, we derived a complete security proof of the four-party MDI-QKD, and performed a numerical simulation to study its performance. The results show that four-party MDI-QKD is feasible over 150 km standard telecom fiber with off-the-shelf single photon detectors. This work takes an important step towards multi-party quantum communication and a quantum network.