When using quantum key distribution (QKD), one of the trade-offs for security is that the generation rate of a secret key is typically very low. Recent works have shown that using a weak coherent source allows for higher secret key generation rates compared to an entangled photon source, when a channel with low loss is considered. In most cases, the system that is being studied is over a fiber-optic communication channel. Here a theoretical QKD system using the BB92 protocol and entangled photons over a free-space maritime channel with multiple spatial modes is presented. The entangled photons are generated from a spontaneous parametric down conversion (SPDC) source of type II. To employ multiple spatial modes, the transmit apparatus will contain multiple SPDC sources, all driven by the pump lasers assumed to have the same intensity. The receive apparatuses will contain avalanche photo diodes (APD), modeled based on the NuCrypt CPDS-1000 detector, and located at the focal point of the receive aperture lens. The transmitter is assumed to be located at Alice and Bob will be located 30 km away, implying no channel crosstalk will be introduced in the measurements at Alice’s side due to turbulence. To help mitigate the effects of atmospheric turbulence, adaptive optics will be considered at the transmitter and the receiver. An eavesdropper, Eve, is located 15 km from Alice and has no control over the devices at Alice or Bob. Eve is performing the intercept resend attack and listening to the communication over the public channel. Additionally, it is assumed that Eve can correct any aberrations caused by the atmospheric turbulence to determine which source the photon was transmitted from. One, four and nine spatial modes are considered with and without applying adaptive optics and compared to one another.
When using quantum key distribution (QKD), one of the trade-offs for security is that the generation rate of a secret key is typically very low. Recent works have shown that using a weak coherent source allows for higher secret key generation rates compared to an entangled photon source, when a channel with low loss is considered. In most cases, the system that is being studied is over a fiber-optic communication channel. Here a theoretical QKD system using the BB92 protocol and entangled photons over a free-space maritime channel with multiple spatial modes is presented. The entangled photons are generated from a spontaneous parametric down conversion (SPDC) source of type II. To employ multiple spatial modes, the transmit apparatus will contain multiple SPDC sources, all driven by the pump lasers assumed to have the same intensity. The receive apparatuses will contain avalanche photo diodes (APD), modeled based on the NuCrypt CPDS-1000 detector, and located at the focal point of the receive aperture lens. The transmitter is assumed to be located at Alice and Bob will be located 30 km away, implying no channel crosstalk will be introduced in the measurements at Alice’s side due to turbulence. To help mitigate the effects of atmospheric turbulence, adaptive optics will be considered at the transmitter and the receiver. An eavesdropper, Eve, is located 15 km from Alice and has no control over the devices at Alice or Bob. Eve is performing the intercept resend attack and listening to the communication over the public channel. Additionally, it is assumed that Eve can correct any aberrations caused by the atmospheric turbulence to determine which source the photon was transmitted from. One, four and nine spatial modes are considered with and without applying adaptive optics and compared to one another.
Transmitting a beam through the atmosphere over long distances can be a challenging problem. Factors from the channel, such as atmospheric turbulence causing beam wander and distortions to the wavefront, or the optical design can change the loss induced in the channel. This work simulates a quantum key distribution system implementing the BB84 protocol with entangled photons operating over a 30 km maritime channel using one, four, and nine spatial modes. For the cases of multiple spatial modes, atmospheric turbulence increases the channel cross talk, resulting in an increase in the noise for each channel. To compensate for distortions in the wavefront, adaptive optics at the transmitter or receiver are used to pre- or post-correct the wavefront.
When light is propagated through a channel that is not a waveguide, such as a fiber, it is known as a free-space optical channel. When used in communication systems, free-space optical channels provide the advantages of high communication rates, lower power, and minimal infrastructure installation. When the free-space optical channel has a random medium in it, such as the atmosphere, the distortions will be applied to the phase. The phase distortions can lead to intensity fluctuations and additional beam spreading. To compensate for the distortions adaptive optics can be used, allowing for the stability of the system to be improved. Additionally, post processing techniques on transmitted data can also be used to improve the stability of the channel. By significantly decreasing the number of photons transmitted, a classical communication channel can be considered as a quantum channel. Encoding information into the quantum states of the transmitted photons, quantum communication can be performed. When transmitting on the order of 1 photon per pulse, classical post processing techniques cannot be used to improve the stability of the channel. To stabilize the free-space optical channel, adaptive optics can be used at the transmitter or receiver. As no measurement of the quantum state is performed by the adaptive optics system, no information in the photon is lost. In this work, quantum key distribution will be implemented over the quantum channel. The focus of this work is on the design and study of how the channel degradations can be overcome, as well as the establishment of a free-space optical channel with adaptive optics in a quantum networking testbed. Quantum key distribution systems were simulated over a 30 km maritime channel as well as over a underwater channel. For the 30 km channel, the use of multiple parallel spatial modes results in the highest increases of the secure key rate at the cost of transmitting a lower average number of photons. When adaptive optics is applied, the secure key rates have a small increase, however as the strength of the turbulence increases from weak to moderate, the relative improvement is higher. For the underwater channel, operation at lower depths is optimal as fewer background photons reach the receiver and the channel attenuation is lower. In the experimental setup, it was found that the single photon detectors that are being used are the limiting the channel loss to be at most 6 dB. The free space optical link established between the Electrical and Computer Engineering building and College of Optical Sciences uses a corner cube retro-reflector to yield a distance of 1.45 km. The channel has been characterized at various times of day to determine the strength of the atmospheric turbulence, which ranges from a scintillation index of 0.07 to 2.21. A classical communication experiment was performed, resulting in error-free transmission while the input power to the EDFA before the photodetector was consistently above -25 dBm.
Due to the absorption of water, communication between two parties submersed below the water is normally performed with acoustic waves. However, with the need for higher data rates, the use of RF or optical frequencies is needed. Currently optical wavelengths have been demonstrated for classical communication over short distances. For these short distances, if a large amounts of data needs to be transmitted securely, it is not feasible for both parties to return to the surface to exchange, and it can be assumed that a third party Eve, is located in the channel trying to gather information. The solution is to use quantum key distribution (QKD) to generate the secure key, allowing the parties to continuously encrypt and transmit the data. It is assumed the BB92 protocol using pairs of entangled photons generated from a spontaneous parametric down conversion (SPDC) source of Type-II. By using entangled photons, Eve is not able to gain information without being detected. In this work, the beam propagation through a horizontal oceanic channel is studied for various distances ranging from 10m to 100m at visible wavelengths. The secure key rates are then calculated assuming that a low-density parity check (LDPC) error correction code is used for information reconciliation, as well as the maximum distance that a QKD protocol can be implemented for a submarine environment.
Due to the absorption of water, communication between two parties submersed below the water is normally performed with acoustic waves. However, with the need for higher data rates, the use of RF or optical frequencies is needed. Currently, optical wavelengths have been demonstrated for classical communication over short distances. For these short distances, if a large amount of data needs to be transmitted securely, it is not feasible for both parties to return to the surface to communicate. Additionally, it can be assumed that a third party (Eve) is located in the channel trying to gather information. The solution is to use quantum key distribution (QKD) to generate the secure key, allowing the parties to continuously encrypt and transmit the data. It is assumed the BB84 protocol using pairs of polarization entangled photons generated from a spontaneous parametric down conversion (SPDC) source of Type-II. By using entangled photons, Eve is not able to gain information without being detected. In this work, horizontal oceanic channel is studied for various distances ranging from 10 m to 100 m, depth ranging from 100 m to 200 m, and surface chlorophyll-a concentrations at a wavelength of 532 nm. The secure key rates are calculated, assuming that a low-density parity check (LDPC) error correction code is used for information reconciliation. The maximum secure key rate and optimal number of average entangled photons transmitted are then studied for the various channels.
The information that is leaked to an eavesdropper during the error reconciliation phase of quantum key distribution (QKD) protocols limits the maximum bit error rate (BER) of a system. In a standard QKD protocol, parity bits are transmitted over an authenticated noiseless channel, to which Eve has access. This Letter presents the concept of using a covert classical communication channel to transmit the parity bits between Alice and Bob without Eve gaining information of the transmitted parity bits. This allows for higher secure key generation rates and operation of a QKD system in which the BER exceeds the limit of the standard protocol. This concept is then applied to a practical free-space optical system that contains multiple parallel channels, where channel loss, crosstalk, atmospheric turbulence effects, and noise are considered.
Multiphoton Quantum Key Distribution (QKD) has recently been proposed to exchange the secret keys using the rotational of polarization over a multi-stage protocol. It has the ability to outperform the weaknesses of a single photon QKD by improving the generation of key rate and distance range. This paper investigates the theoretical aspects of multiphoton QKD protocol’s performance over free space optic (FSO) networks. The most common setup for quantum communication is the single-beam approach. However, the single-beam setup has limitations in terms of high geometrical loss. In this paper, the symmetry multiple-beam for quantum communication which is called as Multiphoton Quantum Communication-Multiple Beam (MQC-MB) is proposed to transmit the multiphoton from the sender to the receiver in order to minimize the impact of geometrical loss that is faced by the single-beam setup. The analysis was carried out through mathematical analysis by establishing the FSO quantum model with the effects of atmospheric and geometrical loss as well as considering atmospheric turbulence modeled by log-normal distribution. The design criteria of FSO, such as the transmitter, receiver, beam divergence, and diameter of apertures, are analytically investigated. The numerical results demonstrate that the MQC-MB outperforms the single-beam in terms of reducing channel loss by about 8 dB and works well under strong turbulence channel. Furthermore, the MQC-MB reduces the quantum bit error rate (QBER) and improves the secret key rate (SKR) as compared to the single-beam system even though the distance between the sender and receiver increases.