Patent Description:
In a quantum communication system, information is exchanged between a transmitter node and a receiver node using encoded single photons. Each photon carries information that is encoded on a property of the photons, such as polarization, phase, or energy in time. Quantum key distribution (QKD) allows the sharing of cryptographic keys between the transmitter node, usually referred to as "Alice," and the receiver node, usually referred to as "Bob," allowing a more secure communication between the two parties. The QKD system provides a test whether any part of the key would be known to an unauthorized third party eavesdropper, usually referred to as "Eve. " Individual bits of the bit stream are transmitted using single photons. By using complementary properties to which Heisenberg's uncertainty principle applies, information may be encoded into a photon to prevent the unauthorized third party, e.g., "Eve," from monitoring the photon since it would disturb its state. When a secret key is established between the two parties by this QKD system, the two parties may then encrypt data transmitted over any conventional communications channel.

In the QKD system, the two parties as Alice and Bob at the respective transmitter node and receiver node may use two or more non-orthogonal bases to encode bit values. The laws of quantum mechanics apply to the photons and any measurement of the photons by an eavesdropper, e.g., Eve, without prior knowledge of the encoding basis of each photon, causes an unavoidable change to the state of some of the photons. These changes to the states of the photons may cause errors in the bit values sent between the transmitter node and receiver node, and by comparing a part of the common bit steam, the two parties may determine if the eavesdropper, e.g., Eve, has gained information.

Photon polarization is often used to provide the complementary properties for encoding, and is used in the common QKD protocol, BB84, and may be applied to conjugate states, such as phase encoding. Other QKD protocols, such as E91, may be based on entanglement of photon pairs and used in a QKD system. The optical path between the transmitter node, e.g., Alice, and the receiver node, e.g., Bob, are connected by a quantum communications channel, which may be free-space or an optical fiber, for example. The transmitter node and receiver node are also each connected to each other via a conventional communications channel, which is used for key exchange or as commonly referred, key sifting.

Each bit of information, such as a "<NUM>" or "<NUM>", may be encoded onto an individual photon by selecting from a pair of orthogonal polarization states. In the BB84 protocol, for example, two pairs of states are used, and each pair of orthogonal states is referred to as a "basis. " The Heisenberg uncertainty principle of quantum indeterminacy indicates that the different states cannot in general be measured definitely without disturbing the original state. Also, the "no cloning theorem" indicates that the creation of identical copies of the non-orthogonal states is forbidden.

Two bases are commonly used and provide polarization state pairs in a rectilinear basis of vertical and horizontal polarization, e.g., <NUM>° and <NUM>°, and a diagonal basis, e.g., <NUM>° and <NUM>°. It is possible to use a third circular basis of left-handedness and right-handedness, depending on what other bases are used that are conjugate to each other. Generally, the transmitter node, e.g., Alice, will create a random bit and random basis, and transmit a single photon in the polarization state defined by the bit and basis, and record the time the photon was transmitted over the quantum communications channel. This process is repeated for a string of bits as single photons.

The receiver node, e.g., Bob, will select at random a basis for measuring each bit and record the time of receipt, the measurement basis, and measurement result for each received bit. The receiver node, e.g., Bob, may communicate the basis in which each photon was received, and the transmitter node, e.g., Alice, may communicate the basis in which each photon was transmitted. Any bits in which a different basis was used are discarded, leaving the remaining bits as the basis for a shared key. This process is often referred to as key verification or the key sifting phase. A subset of shared bits used by both parties at the respective transmitter and receiver nodes, e.g., Alice and Bob, may be used to check against eavesdropping by an unauthorized third party, e.g., Eve, which would have introduced errors. Different reconciliation and privacy amplification techniques may be used to determine a shared key.

Current cryptographic standards, such as the FIPS <NUM> encryption as an Advanced Encryption Standard (AES), may ensure security for many types of data. These well-known cryptographic standards, however, may become obsolete as advances occur in quantum computing that allow encryption codes to be broken more readily. As a result, newer techniques, such as QKD systems, may ensure more secure communication, especially in banking and other communications that require high security and efficient cryptographic standards. QKD techniques are attractive, but they depend on the adaptability of modern communication systems since secure cryptographic communications often use specific types of communication links, including optical fiber and free-space optical (FSO) communications, such as satellite links. These communication links are influenced by atmospheric effects, time of day and different seasons, link distances, and transmitter node and receiver node characteristics. The demanding technical requirements associated with QKD systems typically require that cryptographic keys may have to be distributed across a dynamic topography of layered networks, having both stationary and mobile nodes, and communication links that cover different domains across the ground, air and space. A QKD system may desirably provide guaranteed, secured communications across a disparate set of communication links.

Prior art can be found in <CIT> which generally relates to a system and method for exchangeable quantum key distribution and in <CIT> which generally relates to an apparatus and method for quantum key distribution with enhanced security and reduced trust requirements.

In general, a quantum communications system may include a communications system operative with a quantum key distribution (QKD) system, which includes a transmitter node, a receiver node, and a quantum communications channel coupling the transmitter node and receiver node. The transmitter node may be configured to transmit to the receiver node a bit stream of optical pulses, and switch between first and second QKD protocols based upon at least one channel condition.

The transmitter node may comprise a switch for switching between the first protocol and the second protocol. A channel monitoring device may be configured to monitor the at least one channel condition and operate the switch responsive thereto. The transmitter node may comprise a continuous-variable QKD (CV-QKD) protocol device for generating the first QKD protocol, and a discrete-variable QKD (DV-QKD) protocol device for generating the second QKD protocol.

The channel may comprise a free-space optical (FSO) communications channel. The at least one channel condition may comprise a link distance for the FSO optical communications channel. The transmitter node may switch from the first QKD protocol to the second QKD protocol when the link distance exceeds a threshold, for example.

Another aspect is directed to a method of operating a quantum communications system that may comprise a communications system and a quantum key distribution (QKD) system operable therewith. The QKD system may comprise a transmitter node, a receiver node, and a quantum communications channel coupling the transmitter node and receiver node. The method may comprise operating the transmitter node to transmit to the receiver node a bit stream of optical pulses and switching the transmitter node between first and second QKD protocols based upon at least one channel condition.

The present description is made with reference to the accompanying drawings, in which exemplary embodiments are shown. However, many different embodiments may be used, and thus, the description should not be construed as limited to the particular embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Referring initially to <FIG>, a quantum communication system is illustrated generally at <NUM> and includes a communications system <NUM> as a non-quantum communications system and a quantum key distribution (QKD) system <NUM> operable with the communication system. The QKD system <NUM> includes a transmitter node <NUM>, a receiver node <NUM>, and a quantum communications channel <NUM> coupling the transmitter node and receiver node. In an example, the quantum communications channel <NUM> may include a free-space optical (FSO) communications channel indicated at <NUM>, such as point-to-point, or using a satellite; or the quantum communications channel may be a fiber optic communications channel indicated at <NUM>, such as a single mode optical fiber 36a or multi-mode optical fiber 36b.

The transmitter node <NUM> not only communicates with the receiver node <NUM> over the quantum communications channel <NUM>, but also communicates via the communications system <NUM>, which may include a non-quantum or conventional communications channel and may be fiber optic, free-space, wired, or another communications channel. The quantum communications channel <NUM> may be part of the communication system <NUM> as indicated by the dashed lines connecting the two, indicating that both the quantum communications and non-quantum communications may be transmitted over fiber optic communications channel or over an FSO communications channel that is part of the communications system <NUM>.

When describing a quantum communication systems <NUM>, the transmitter node <NUM> is usually referred to as "Alice" and the receiver node <NUM> is usually referred to as "Bob. " Any eavesdropper on the quantum communications system <NUM> is usually referred to as "Eve. " The transmitter node <NUM> includes a laser pulse source <NUM> for generating a bit stream of optical pulses. Although a laser pulse source <NUM> is described, it should be understood that other sources may possibly be used that generate the bit stream of optical pulses. It is possible depending on system design for one or more light emitting diodes (LED's) to be used to generate the bit stream of optical pulses.

The optical pulse output from the transmitter node <NUM> is an output bit stream of photons that are encoded bit values. The photon polarization provides the complementary property used for encoding purposes, such as in the QKD protocol, BB84, and may be applied to conjugate states such as phase encoding. Other protocols, such as the E91 protocol, may be used that includes the entanglement of photon pairs. Each bit of information such as a "<NUM>" or "<NUM>" may be encoded onto an individual photon by selecting from a pair of orthogonal polarization states. In the BB84 QKD protocol, two pairs of orthogonal states are used, and each pair of orthogonal states is referred to as a "basis. " The bases provide polarization state pairs in a rectilinear basis having vertical and horizontal polarization, such as <NUM>° and <NUM>°, and a diagonal basis having opposite diagonal direction polarization, such as <NUM>° and <NUM>°. It is also possible to use a circular basis of left-handedness and right-handedness depending on what other bases are used that are conjugate to each other.

The transmitter node <NUM> includes a controller <NUM> operatively connected to the laser pulse source <NUM> and other components at the transmitter node for controlling their operation, such that the laser pulse source is controlled for transmitting a photon in a polarization state defined by the bit and basis, and record the time the photon was transmitted. This process is repeated for the string of bits as a stream of photons. The transmitter node <NUM> may include a transceiver <NUM> connected to the controller <NUM> and operative to communicate with the receiver node <NUM> via the communications system <NUM> using, for example, an unencrypted non-quantum communications channel for the key exchange or key sifting process, as key exchange is commonly called.

The transmitter node <NUM> transmits the bit stream of optical pulses over the quantum communications channel <NUM>, which as noted above, could be the FSO communications channel <NUM>, via satellite or point-to-point, or the fiber optic communications channel <NUM>, and as either separate or a part of the communications system <NUM>. It is possible to use the same optical fiber for both quantum communications and conventional communications for key exchange, such as unencrypted communications used in key sifting.

The transmitter node <NUM> is configured to transmit to the receiver node <NUM> via its output <NUM>, the bit stream of optical pulses and switch between first and second QKD protocols based upon at least one channel condition. As illustrated in <FIG>, the transmitter node <NUM> includes a switch <NUM> for switching between the first and second QKD protocols, which in an embodiment are respectively a continuous-variable QKD (CV-QKD) protocol, and a discrete-variable (DV-QKD) protocol. The transmitter node <NUM> includes a continuous-variable QKD (CV-QKD) protocol device <NUM> for generating the CV-QKD protocol and a discrete-variable QKD (DV-QKD) protocol device <NUM> for generating the DV-QKD protocol. Single photons are generated for the DV-QKD protocol, usually as optical pulses, and as explained in further detail below, requiring single photon detectors at the receiver node <NUM>, for example, as an array of single photon optical detectors. In contrast to the DV-QKD protocol, the CV-QKD protocol uses conjugate, continuous degrees of freedom (field quadratures) of a light pulse prepared in a Gaussian (coherent or squeezed) state to transmit signals that constitute a shared randomness. At the receiver node <NUM>, the field quadratures of each light pulse may be measured using as an example, shot-noise limited, balanced homodyne or heterodyne detectors, which have an advantage of not requiring single photon detection and operating at high GHz speed detection rates. In the CV-QKD protocol, often a local oscillator (LO) signal may be generated at the transmitter node <NUM> and the CV-QKD protocol may involve polarization encoding and multiplexing techniques.

A channel monitoring device <NUM> is configured to monitor at least one channel condition and operate the switch <NUM> responsive to the measured channel condition. For example, the quantum communications channel <NUM> may be an FSO communications channel <NUM>, and the monitored channel condition may be its link distance. The transmitter node <NUM> may switch from transmitting optical pulses using, for example, the first CV-QKD protocol to the second DV-QKD protocol when the link distance exceed a threshold, for example, <NUM> kilometers.

A single cryptographic key may be obtained by incorporating the secret bits obtained from communications in both the CV-QKD protocol and the DV-QKD protocol. Depending on the length of the cryptographic key and the properties of the quantum communications channel <NUM>, such as the FSO communications channel <NUM>, over which the cryptographic key is distributed, either the CV-QKD or the DV-QKD protocol may be selected to obtain the maximum secret key rate (SKR).

The receiver node <NUM> may include receiver optoelectronic (OE) circuitry <NUM> that receives via the input <NUM> the bit stream of optical pulses from the transmitter node <NUM> over the quantum communications channel <NUM>. An optical detector circuit <NUM> receives the bit stream of optical pulses from the OE circuitry <NUM> and detects the optical pulses, such as via at least one single photon detector <NUM>, and generates appropriate signals that may be processed via a controller <NUM> at the receiver node <NUM> demodulate depending on the type of CV-QKD or DV-QKD protocol. The OE circuitry <NUM> may include in an example a circuit that detects the specific CV-QKD or DV-QKD protocols and employ appropriate circuitry at both the OE circuitry and optical detector circuit <NUM> for processing signals depending on the protocol.

At the receiver node <NUM>, the optical detector circuit <NUM> may be formed as one or more single photon optical detectors <NUM>, for example, formed as a photon detector array. The optical detector circuit <NUM> may be connected to the controller <NUM>, which may process and demodulate the signals received from the optical detector circuitry based upon the CV-QKD protocol or DV-QKD protocol. The optical detector circuit <NUM> may also include a phase detector <NUM> and include balanced homodyne or heterodyne detectors that are configured to detect optical signals modulated using the CV-QKD protocol.

The controller <NUM> at the receiver node <NUM> may be connected to a conventional transceiver <NUM> also located at the receiver node <NUM>. This transceiver <NUM> may communicate via the conventional or non-quantum communication system <NUM> with the transceiver <NUM> located at the transmitter node <NUM>. For example, Bob as the party at the receiver node <NUM> may communicate with Alice as the party at the transmitter node <NUM> over the conventional communications system <NUM>, and transmit data regarding the basis in which each photon was received at the receiver node <NUM>. The transmitter node <NUM>, e.g., Alice, may transmit data about the basis in which each photon was transmitted to the receiver node <NUM>, e.g., Bob, using the conventional communication system <NUM>. Any bits having a different basis may be discarded, leaving the remaining bits as the basis for a shared cryptographic key in the key verification or key sifting phase. The subset of shared bits used by both parties, e.g., Alice and Bob, as to the respective transmitter and receiver nodes <NUM>, <NUM>, may be used to check against eavesdropping by the unauthorized party, e.g., Eve, which would have introduced errors into the communications stream of bits.

The transmitter node <NUM> may include other components not illustrated in detail, such as a spatial light modulator (SLM) that imposes a spatially varying modulation by modulating intensity and phase, a waveguide array that increases bit generation and phase bin states, and an attenuation filter. As noted above, the receiver node <NUM> may include the phase detector <NUM> and homodyne detection applicable for the CV-QKD protocol. The OE circuitry <NUM> may include a beam splitter and other circuitry to split any incoming optical pulse streams for time processing and phase processing.

The use of the CV-QKD protocol and DV-QKD protocol may be used with other protocols that provide a desired secret key rate (SKR) for the communications link as part of the quantum communication channel <NUM>. An example may include satellite links where the link limits the raw key rate due to signal loss and may be more vulnerable to attack, and thus, will use well-defined security proofs. In a non-limiting example, the high raw key rate that the CV-QKD protocol provides may be combined with the security that the DV-QKD protocol supports and may be used to create a hybrid protocol for high SKR key distribution across lossy and turbulent communication links. As compared to DV-QKD protocol, the CV-QKD protocol may enable higher raw bit rates due to compatibility with standard telecommunications multiplexing techniques, while the DV-QKD protocol offers well-defined security proofs, but may have lower overall raw key rates, and thus, the achievable SKR is low. The quantum communication system <NUM> as described increases the key rate while also maintaining security, encoding the quantum bit stream and switching between protocols such as an RF-assisted and modulated CV-QKD protocol and a time and phase bin DV-QKD protocol.

In a non-limiting example of operation, at the transmitter node <NUM>, e.g., Alice, the controller <NUM> may be operative to select a discrete-modulation level from the constellation for the CV-QKD protocol, e.g., M-ary QAM or PSK. This may be followed by a modulation state being randomly slotted into one of "N" discrete DV-QKD time and phase-bins that are similar to pulse position modulation (PPM). After encoding these symbols, which represent hybrids of continuous and discrete quantum variables, onto the laser pulses, for example, with a standard I/Q modulator, the resulting bit stream may be attenuated before being transmitted over the quantum communications channel <NUM>, and in an example, the FSO communications channel <NUM>, characterized by its quantum bit error rate (QBER), transmissivity, and excess noise (ε).

At the other end of the quantum communications channel <NUM>, in an example embodiment, the receiver node <NUM> may randomly choose the state that the receiver node measures, such as the DV-QKD protocol or CV-QKD protocol, by switching between circuits as part of the OE circuitry <NUM> and optical detector circuit <NUM> based on an optimal probability of selection. After a sufficient number of symbols have been exchanged, the receiver node <NUM> may publicly announce whether the receiver node employed circuitry for detecting optical pulses employing the CV-QKD protocol or DV-QKD protocol, as well as a subset of the bit stream. With this information, the transmitter node <NUM> may calculate the QBER of the communications and compare this value to a prescribed threshold, below which the protocol may be considered secure. Once the QBER has been verified, the receiver node <NUM> may initiate a reverse reconciliation so that the bit stream is jointly derived from both the CV-QKD and DV-QKD protocols. Privacy amplification may be used to distill a secret key. In an example, this process as described may be referred to as a serial interleaved Quantum Key Distribution (iQKD) using a synchronous iQKD protocol.

Referring to <FIG>, there is illustrated at <NUM> a graph showing one example of the SKR for communications in a particular link using the DV-QKD protocol and CV-QKD protocol, such as by employing a balanced homodyne detector (BHD). The horizontal axis shows a link distance in kilometers versus the SKR in bits per second on the vertical axis. The maximum SKR may be achieved with either a CV-QKD protocol or a DV-QKD protocol depending on the link distance as illustrated, where at above a link distance of above about <NUM> in this one example, the SKR drops quality to almost zero for communications employing CV-QKD, while communications employing DV-QKD maintain a SKR up to about <NUM>. The use of both the CV-QKD and DV-QKD protocols enables high speed quantum communications at above <NUM> Mbps.

The channel monitoring device <NUM> monitors at least one channel condition, such as the link distance, and operates the switch <NUM> responsive to the monitored channel conditions and switches between a plurality of protocols, and in an example, CV-QKD and DV-QKD protocols. Besides just two protocols - one CV-QKD and one DV-QKD, the system may use a plurality of protocols, for example, a bank of protocols could be drawn from based on channel condition. For one channel condition, the system <NUM> may use all DV-QKD protocols, e.g., four protocols to complete a key transmission or mission objective. For another series of channel conditions, the system may use all CV-QKD and draw from the bank three protocols to meet that mission objective, or for yet another type of fluctuating channel condition, the system could draw, for example, six protocols from the bank that would represent a mix between CV-QKD and DV-QKD resources to satisfy mission objectives in those atmospheric conditions. In an example, the switch <NUM> may include a dynamically configurable modem to help toggle between the different protocols, and in this example, the CV-QKD protocol device <NUM> and DV-QKD protocol device <NUM> to implement the selected CV-QKD protocol or DV-QKD protocol as an example. An advantage in having the transmitter node <NUM> transmit in different protocols, such as either the CV-QKD protocol or DV-QKD protocol is the SKR may be guaranteed to lie within a well-defined range of values to equalize the SKR and facilitate communications planning and ensure that the cryptographic keys can be reliably distributed within a narrow operating window. Also, the channel monitoring device <NUM> may monitor varying weather and atmospheric conditions and make any switch to an appropriate QKD protocol. For example, one QKD protocol may be suited for a certain channel condition and time period and the QKD protocol switched, depending on the time of day, different seasons, and other channel conditions that may be monitored.

In the graph of <FIG> at <NUM>, mathematical modeling results for the SKR performance of the synchronous iQKD protocol as described above is illustrated (marked as Hybrid DV-CV-QKD) as a function of the channel attenuation in decibels, and compared to both Gaussian-modulated (GM) and <NUM>-PSK discrete-modulated (DM) CV-QKD, assuming typical values for the reconciliation efficiency β, and also compared with a Decoy-state BB84 DV-QKD. It is evident from the graph <NUM> that the quantum communications system <NUM> using a hybrid DV-CV-QKD has the highest SKR compared to the other protocols. Simulation parameters are given in Table I below using a balanced coherent detector and single photon detector <NUM> for the CV-QKD and DV-QKD protocols, respectively. Using the CV-QKD protocol, the raw transmission rate was set to <NUM> Gbps. The synchronous iQKD protocol explained above as the Hybrid DV-CV-QKD outperformed the straight CV-QKD and DV-QKD protocols, with a SKR above <NUM> Mbps and theoretically achievable for up to <NUM> dB of channel loss. Thus, the quantum communications system <NUM> may achieve high SKR key distribution across long range communication links, e.g., ship-to-satellite.

A flowchart illustrating the method of operating the quantum communications system is illustrated in <FIG> at <NUM>. The process starts (Block <NUM>) and the laser pulse source <NUM> generates a bit stream of optical pulses (Block <NUM>). The channel monitoring device <NUM> monitors at least one channel condition (Block <NUM>), and based on the measured channel condition, will select a specific CV-QKD protocol or DV-QKD protocol (Block <NUM>) for transmission. The switch <NUM> is operated to select the CV-QKD protocol device <NUM> or DV-QKD protocol device <NUM> (Block <NUM>) and the bit stream of optical pulses is modulated and transmitted to the receiver node (Block <NUM>). The channel monitoring device <NUM> maintains its monitoring status, and when a channel condition changes, such as link conditions, the switch changes the protocol (Block <NUM>). The process ends (Block <NUM>).

It is possible the quantum communications system <NUM> may be incorporated within the physical and data link layers within a quantum-based mobile ad-hoc network (MANET) that includes FSO links and nodes on a range of platforms, such as unmanned aerial vehicles (UAV). It is also possible to maintain point-to-point communication links in a network that includes techniques for FSO Pointing, Acquisition and Tracking (PAT) and MANET linked protocols, including neighbor discovery and distribution of quantum resources, such as single-proton, and entangled states. It is also possible to change the type of protocol such as CV-QKD protocol and DV-QKD protocol based on different levels of security and communication rates.

Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed.

Claim 1:
A quantum communications system (<NUM>) comprising:
a communications system (<NUM>); and
a quantum key distribution, QKD, system (<NUM>) operable with the communications system (<NUM>) and comprising a transmitter node (<NUM>), a receiver node (<NUM>), and a quantum communications channel (<NUM>) coupling the transmitter node (<NUM>) and receiver node (<NUM>), wherein the transmitter node (<NUM>) comprises a continuous-variable QKD, CV-QKD, protocol device (<NUM>) for generating the first QKD protocol, and a discrete-variable QKD, DV-QKD, protocol device (<NUM>) for generating the second QKD protocol;
the transmitter node (<NUM>) configured to transmit to the receiver node (<NUM>) a bit stream of optical pulses and switching between first and second QKD protocols based upon at least one channel condition.