A new quantum key distribution (QKD) scheme, based on quantum state fusion and fission, is proposed. By using quantum state fusion and a spatial-temporal mode conversion circuit, two polarization modes of two photons can be converted into one temporal-polarization mode of one photon. Also, by using quantum state fission and a temporal-spatial mode conversion circuit one temporal-polarization mode of one photon can be converted into two polarization modes of two photons with the help of an ancilla photon. When the state fusion and fission processes are applied in QKD system, the sender can transmit two polarization qubits at one time and the receiver detects two single photons separately after state fission. Thus, the total count rate of photons is improved. More importantly, the quantum bit error rate decreases. The numerical analysis results show that by using this scheme the key generation rate is improved efficiently and communication distance can be increased by 6~30Km depending on different polarization misalignment errors.
A new quantum key distribution (QKD) scheme, based on quantum state fusion and fission, is proposed. By using quantum state fusion and a spatial-temporal mode conversion circuit, two polarization modes of two photons can be converted into one temporal-polarization mode of one photon. Also, by using quantum state fission and a temporal-spatial mode conversion circuit one temporal-polarization mode of one photon can be converted into two polarization modes of two photons with the help of an ancilla photon. When the state fusion and fission processes are applied in QKD system, the sender can transmit two polarization qubits at one time and the receiver detects two single photons separately after state fission. Thus, the total count rate of photons is improved. More importantly, the quantum bit error rate decreases. The numerical analysis results show that by using this scheme the key generation rate is improved efficiently and communication distance can be increased by 6~30Km depending on different polarization misalignment errors.
The hybrid modulation quantum key distribution(QKD) via single-photon two-qubit states scheme is analyzed,and an improvement scheme is proposed.The input-output model of polarization is established.And the influence of the interferometers on the polarization coding is analyzed.Quantum bit error rate(QBER) of polarization coding caused by the phasic error of fast and slow axes and the alignment error between the basis vectors of polarization and polarization axis is simulated.Furthermore,the stability conditions of polarization are derived on this basis.The QBER of polarization coding can be influenced deeply by the fiber M-Z interferometers.The theoretic analysis reveals that the jitter of polarization state is not only derived from the birefringency,but also derived from coordinate system difference between incident light and fast and slow axes in fiber interferometer.Some necessary conditoins for the QKD via single-photon two-qubit states are given.The simulation show that when ρ and θ are both below 0.2 rad,the polarization QBER is less than 0.015.When ρ and θ are both below 0.4 rad,the polarization QBER is less than 0.06.The polarization QBER of 2%-5% in two-qubit QKD scheme could be accepted as far as the safety upper limit of QBER(11%) is concerned.
Quantum Key Distribution (QKD) systems based on single photon transmission can generate unconditionally secure common key between remote users. Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. High key generation rate is realized by increasing photon detection rate and by reducing error rate. Therefore, a high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals. A solution for these issues are given by employing planar lightwave circuit interferometers, single photon detection circuits and modules, and clock synchronization based on Wavelength Division Multiplexing (WDM) technique.
Here, we present a proof-of-principle high-dimensional quantum key distribution (QKD) protocol utilizing the position and momentum entanglement of photon pairs. The protocol exploits the fact that position and momentum form mutually unbiased bases, linked via a Fourier transform. One photon of the entangled pair is measured by the sender in a randomly chosen basis-either position or momentum-selected passively via a beam splitter. This projective measurement remotely prepares the partner photon in a corresponding spatial mode, which is sent to the receiver, who similarly performs a random measurement in one of the two bases. In this implementation, we achieve a photon information efficiency of 5.07 bits per photon using 90 spatial modes, and a maximum bit rate of 0.9 Kb/s with 361 modes. To assess the scalability of this spatial-mode encoding scheme, we theoretically show that using a brighter entangled photon source along with next-generation single-photon cameras - featuring improved quantum efficiency, timing and spatial resolution - this approach could achieve 9 bits per photon at 2000 spatial modes, and a bit rate of over 700 Mb/s at 4400 modes while accounting for finite-key effects. These results quantify the opportunities and performance bounds of spatially encoded, entanglement-based QKD and provide a benchmark for future high-dimensional quantum communication systems.
It has been widely recognized that the underlying principles of quantum mechanics could be used to enable secure communications, without any additional conventional cryptographic systems, while utilizing the properties of the photons. Considerable research efforts have been invested to develop an efficient quantum key distribution (QKD) scheme over either free-space optical or fiber-optics channels. Most of these research efforts have been focused on a two-dimensional QKD, commonly realized by the use of the polarization state of photons. However, the data rates for the quantum key exchange in two-dimensional QKD are still low, while transmission distance is limited. On the other hand, it is well known that photons can carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM), associated with the polarization and azimuthal phase of the complex electric field, respectively. Accordingly, we can define the combined-OAM-SAM state of a photon as |l, σ〉, where l and σ correspond to OAM and SAM indexes, respectively. Since the OAM eigenstates are orthogonal, an arbitrary number of bits per single photon can be transmitted, which could considerably increase the total secure bit rate. To improve secure data rates, we propose two types of protocols, namely, nonentangled-based (such as weak coherent state) and entanglement-assisted protocols, both employing the photon combined-OAM-SAM state, which can be used for secure key distribution over free-space optical and few-mode fiber channels. Two types of entanglement-assisted protocols are described, namely, two-basis and (D + 1)-basis protocols ( D is dimensionality of corresponding Hilbert space). We further describe how to implement the qudit gates required for implementation of these protocols. Finally, we discuss the security issues of the proposed protocols and determine both infinite and finite secret key fraction rates.
In order to solve the low key distribution efficiency problem of the BB84 protocol, an improved BB84 quantum key distribution scheme is presented in this paper. Firstly the same polarization basis is used by the sender and the receiver with the consistent agreement. This process could make quantum key distribution rate higher. Then signature combined with the error correction method based on the odd and even parity is used to ensure the safety of the transmission. The analyses results indicate that the advantage of the improved scheme lies in that the all photons sent to Bob by Alice are useful. It can not only reduce the time extracting the original key, but also solve the authentication problem in the process of communication. The key distribution efficiency is two times of the original BB84 protocol improved scheme.
We investigate a quantum key distribution (QKD) scheme which utilizes a biased basis choice in order to increase the efficiency of the scheme. The optimal bias between the two measurement bases, a more refined error analysis, and finite key size effects are all studied in order to assure the security of the final key generated with the system. We then implement the scheme in a local entangled QKD system that uses polarization entangled photon pairs to securely distribute the key. A 50/50 non-polarizing beamsplitter with different optical attenuators is used to simulate a variable beamsplitter in order to allow us to study the operation of the system for different biases. Over 6 hours of continuous operation with a total bias of 0.9837/0.0163 (Z/X), we were able to generate 0.4567 secure key bits per raw key bit as compared to 0.2550 secure key bits per raw key bit for the unbiased case. This represents an increase in the efficiency of the key generation rate by 79%.