Patent ID: 12212669

DETAILED DESCRIPTION

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 toFIG.1, a quantum communications system is illustrated generally at20and may be operative as a quantum secure direct communications (QSDC) system, which permits direct communication, such as without use of a cryptographic key. The quantum communications system20includes a transmitter node26referred to as Alice, a receiver node28referred to as Bob, and a quantum communications channel30coupling the transmitter node and receiver node. The quantum communications system20may use the pulse division apparatus to place a quantum state into a superposition of time bins. Neighboring quantum states thus experience interference with each other, which scrambles the original data stream. Coupled with the he no-cloning theorem of quantum mechanics this acts as a physical layer of security that can be used to more securely transmit quantum data streams within the quantum communications channel30without the additional use of a cryptographic key in some embodiments.

Quantum states that have had their probability distribution functions broadened through the pulse divider apparatus so that these probability distribution functions interfere with neighboring states in the quantum data stream in public sections of the quantum communications channel30scramble the original data stream. The pulse recombiner at the receiver node28where Bob is located reconstructs the original data stream. The input state at the transmitter node26is divided into many temporal copies that are separated in time from each other which broadly redistributes its probability distribution function and spoofs attempts to gain information about it within the public sections of the quantum communications channel30where an eavesdropper, referred to as Eve, would be located.

Photon self-interference in the spatial domain and use of a spatial filter in the spatial domain may reveal data tampering. Because the spatial probability distribution function cannot be perfectly categorized, cloned and reintroduced to the public section of the quantum communications channel30, any attempt to monitor, misrepresent or omit temporal data may introduce spatial probability distribution changes that will not cleanly exit a spatial filter at the receiver node28, thus revealing tampering by an interferer within the public link of the quantum communications channel.

In an example, the quantum communications channel30may include a fiber optic communications channel34, which may be a single mode optical fiber or a multi-mode fiber. The single mode optical fiber may be used for transmitting temporally modulated photons, and the multi-mode optical fiber may be used for transmitting both temporally and spatially modulated photons. The quantum communications channel30may include a free-space optical (FSO) communications channel36that includes satellite or line-of-sight communications, or an underwater communications channel or bulk medium38.

As illustrated, the transmitter node26includes a pulse transmitter40as a laser pulse source and a pulse divider42downstream therefrom, such as positioned directly at the output or further downstream. The transmitter node26also includes a controller48that operates the pulse transmitter40and a transceiver50that connects to a communication system52as will be explained in further detail below and which may be part of the quantum communications channel30. The receiver node28includes a pulse recombiner54and a pulse receiver56downstream. The pulse receiver56may include opto-electric (OE) circuitry60having a spatial filter62and a beam splitter64that splits signals into a phase basis or time basis at an optical detector circuit66, which includes a phase detector76and single photon detector74. The receiver node24includes a controller78and transceiver80connected thereto. The transceiver80is coupled to the communications system52.

The pulse transmitter40may be configured to generate temporally modulated photons. The pulse receiver56includes opto-electric (OE) circuitry60that detects phase bin states using the optical detector circuit66. The pulse receiver56may include at least one single photon detector74. The pulse transmitter40may also be configured to generate spatially modulated photons and perform optical polarization encoding. The pulse transmitter40may be configured to generate a bit stream of quantum pulses in a quantum key distribution (QKD) protocol as explained in further detail below.

In an example, the quantum pulses may be time bin photons and the pulse divider42at the transmitter node25may divide the pulses across other time bins. The pulse recombiner54at the receiver node28may recombine the pulses. In an example, the pulse divider42and pulse recombiner54may operate to provide a divided pulse quantum key distribution that may be applied on top of existing QKD protocols and implemented downstream of the pulse transmitter and upstream of the pulse receiver56to improve the performance of existing QKD protocols.

In an example, the transmitter node26may be configured to generate temporally modulated photons that are communicated over the fiber optic communications channel34, which may be single mode optical fiber. The transmitter node26may also be configured to generate spatially modulated photons that are transmitted over a multi-mode optical fiber. In both cases, the temporally or spatially modulated photons may use optical polarization encoding, and each photon may have a transmitted quantum basis.

The transmitter node26communicates with the receiver node28over the quantum communications channel30. Both transmitter and receiver nodes26,28may communicate via the communications system52, which may include a classical communications channel and may be fiber optic, free-space, wired, or another conventional communications channel. This communications system52may be used if additional functions are desired, such as cryptographic key generation and quantum key distribution (QKD), or communication with networked devices using conventional transceivers. The quantum communications system20may use cryptographic key sifting or operate as a QSDC system. The quantum communications channel30may be part of the communication system52as indicated by the dashed lines connecting the two, indicating that both the quantum communications and non-quantum communications may be transmitted over any communications channel as part of the communications system52.

In an example, the optical pulse output from the pulse transmitter40at the transmitter node26may be an output bit stream of photons that are encoded bit values. The photon polarization may provide a complementary property used for encoding purposes, such as in the QKD protocol, BB84. Other protocols, such as the E91 protocol, may be used that includes the entanglement of photon pairs. Each bit of information such as a “0” or “1” 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 basis may provide polarization state pairs in a rectilinear basis having vertical and horizontal polarization, such as 0° and 90°, and a diagonal basis having opposite diagonal direction polarization, such as 45° and 135°.

It is also possible to use a circular basis of left-handedness and right-side handedness depending on what other bases are used that are conjugate to each other. The quantum communications system20may use an unencrypted non-quantum communications channel, such as the communications system52, for the key exchange or key sifting process, as key exchange is commonly called. It is possible to use a continuous-variable QKD (CV-QKD) protocol or a discrete-variable (DV-QKD) protocol. Single protons may be generated for the DV-QKD protocol, usually as optical pulses, and requires single photon detectors74at the receiver node28, for example, as an array of single photon optical detectors. In contrast to the DV-QKD protocol, the CV-QKD protocol may use 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 node28, the field quadratures of each light pulse may be measured using as an example, shot-noise limited, balanced homodyne or heterodyne detectors, such as phase detectors76, 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 (OL) signal may be generated at the transmitter node26and the CV-QKD protocol may involve polarization encoding and multiplexing techniques.

As noted before, the transceiver50at the transmitter node and the transceiver80at the receiver node28may communicate with the communications system52, which may be a conventional or non-quantum communications system. For example, Bob as the party at the receiver node28may communicate with Alice as the party at the transmitter node26over the conventional communications system52, and transmit data regarding the basis in which each photon was received at the receiver node28. The transmitter node26, e.g., Alice, may transmit data about the basis in which each photon was transmitted to the receiver node28, e.g., Bob, using the communications system52. 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 shifting phase. The subset of shared bits used by both parties, e.g., Alice and Bob as to the respective transmitter node26and receiver node28, 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 node26with the pulse transmitter40may 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 and circuitry that generates phase bin states, and an attenuation filter.

As noted before, the transmitter node26includes the pulse transmitter40for generating a bit stream of “quantum” optical pulses. Although a pulse transmitter40as a laser is described, it should be understood that other sources of the pulses may be used that generate the bit stream of pulses.

The pulse output from the transmitter node26is an output bit stream of photons that are encoded bit values. The photons may be temporally or spatially modulated photons and have a transmitted quantum basis that includes time and phase parameters, including optical polarization encoding. The photon polarization provides the complementary property used for encoding purposes and in the quantum communications system20as described, with optical pulses arranged in time bins in this example, photon polarization may be applied to conjugate states, such as phase encoding. The quantum communications system20may use entanglement of photon pairs. Each bit of information such as a “0” or “1” may be encoded onto an individual photon by selecting from a pair of orthogonal polarization states. In an example, two pairs of orthogonal states may be used, and each pair of orthogonal states may be referred to as a “basis.” The bases may provide polarization state pairs in a rectilinear basis having vertical and horizontal polarization, such as 0° and 90°, and a diagonal basis having opposite diagonal direction polarization, such as 45° and 135°. 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 node26includes its controller48operatively connected to the laser pulse transmitter40and other components at the transmitter node26for controlling their operation, such that the pulse transmitter is controlled for transmitting a photon in a polarization state defined by the bit and basis and in time bins, and record the time the photon was transmitted. This process is repeated for the string of bits as a stream of photons. The transmitter node26may include its transceiver50connected to the controller48and operative to communicate with conventional networked components via the communications system52. Additional functions for Quantum Key Distribution (QKD) with the receiver node28may be provided via the communications system52.

The transmitter node26may transmit the stream of pulses via the pulse divider42over the quantum communications channel30, such as the fiber optic communications channel34, and as either separate or a part of the communications system52, and either as temporally modulated photons or spatially modulated photons in an example. It is possible to use the same fiber optic communications channel34for both quantum communications and conventional communications.

The receiver node28includes the pulse recombiner54that recombines the pulses and the pulse receiver56includes in this example the opto-electronic (OE) circuitry60that receives the bit stream of pulses from the transmitter node26over the quantum communications channel30. This OE circuitry60may include a spatial filter62and a beam splitter64for splitting the optical signal into an optical phase or time streams for measurement in the phase basis or time basis as explained below. The spatial filter62may be used to “clean up” the stream of optical pulses and produce a smooth intensity profile as a cleaner Gaussian signal that has unwanted multiple-order energy peaks removed such that the central maximum of a diffraction energy pattern will be passed through the OE circuitry60. The spatial filter62may include a microscopic objective lens, a pinhole aperture and a positioning mechanism having precision X-Y movement at the center of the pinhole that operates as the focal point of the objective lens in a non-limiting example. The spatial filter62may also be advantageous because it operates as a filter for the spatial probability distribution function that may not be characterized, cloned and reintroduced to the public portion of the quantum communications channel30. Thus, any spatial probability distribution disturbances that are introduced may not cleanly exit the spatial filter62, and thus, Bob at the receiver node28may use this information as a metric to reveal tampering.

The optical detector circuit66receives the bit stream of optical pulses from the OE circuitry60and detects the optical pulses and generates signals that may be processed at the controller78, which processes and demodulates the signals representative of the optical pulses depending on the communications protocol. At the receiver node28, the optical detector circuit66may be formed as a single photon detector74for measuring photons in the time basis and in respective time bins, where the optical pulses are transmitted in respective time bins for data encoding. In an example, the optical detector circuit66may include an array of single photon detectors74. The optical detector circuit66may also include a phase detector apparatus76for measuring the photons in the phase basis.

The controller78at the receiver node28may be connected to the conventional transceiver80, also located at the receiver node28. This transceiver80may communicate via the conventional or non-quantum communications system52with the other networked components or to the transceiver50located at the transmitter node26. The transmitter node26may 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. These components may be used to transmit temporally modulated photons or spatially modulated photons and perform optical polarization encoding.

As noted before, the OE circuitry60at the receiver node28may include a spatial filter62and a beam splitter64to split any incoming optical pulse streams for time processing and phase processing as explained in greater detail below. The optical detector circuit66may include the phase detector apparatus76and single photon detector74for phase basis and time basis measurements, respectively.

Generally, an eigenstate |Ψas a photon of a particular basis is prepared and transmitted from the transmitter node26as Alice over the quantum communications channel30to the receiver node28as Bob. In a conventional QKD system, if the eigenstate |Ψwas prepared in the same photon basis that Eve or Bob as the receiver node28chooses to measure the quantum state in, both will measure the same state that Alice at the transmitter node26initially prepared. If Eve or Bob at the receiver node28choose a different basis than the one Alice at the receiver node26initially prepared the quantum state in, both would collapse the eigenstate |Ψinto one of the eigenstates of the basis they were measuring in, and would have a 50% chance in a d=2 data structure, for example, corresponding to a random guess, of correctly identifying the associated bit value of the state that Alice sent.

This use of mutually unbiased bases, and the impact of preparing and measuring in inconsistent bases, is used to establish a more secure communications link between Alice as the transmitter node26and Bob as the receiver node28over the quantum communications channel30. As Eve is forced to annihilate the state Alice26has prepared as a single photon in order to gain any information about it, and as Eve must randomly choose a basis to measure the state in, on average Eve will choose the wrong basis 50% of the time, both resulting in measurements which do not provide Eve information about the original state, and revealing Eve's presence to Bob as the receiver node28downstream through a quantum bit error rate (QBER) that is higher than a certain threshold value.

It is generally assumed that the eigenstate |Ψprepared in a particular basis, does not change as it propagates. Thus, If Eve and Bob as the receiver node28choose the same basis to measure the state that Alice as the transmitter node26initially used to prepare the state in, Eve and Bob will both measure it accurately. For a 4-state transmission, however, Eve has on average a 75% chance of correctly retrieving the bit value that Alice as the transmitter node26sends, as she has a 50% chance of correctly choosing the right basis and 100% accuracy of retrieving the associated bit value in the correct basis, and a 50% chance of choosing the wrong basis, and a 50% accuracy of retrieving the associated bit value when measuring in the wrong basis. The amount of error that Bob28can tolerate before knowing that the quantum communications channel30is insecure and that Eve is present, is in part dependent on this probability, which essentially reflects the amount of information that Eve has access to.

Increasing the maximum threshold of the quantum bit error rate (QBER) that Bob28can tolerate before concluding that the quantum communications channel30is insecure may increase secure link lengths, increase secure bit rates, and enable more efficient and cost effective implementations of the quantum communications system20in existing communication links and better enable secure communications for QKD systems, and transmission of quantum information in general, for instance for distributed quantum computing or sensing applications.

The quantum communications system20increases the maximum QBER threshold where an initial state |Ψhas its temporal probability distribution is broadened so that it interferes with other neighboring bits in the bit stream, and scrambles the state and bit stream in the public link of the quantum communications channel30that Eve has access to. This results in any measurements made at a location other than where Alice as the transmitting node26and Bob as the receiving node28are located will reduce the information available to Eve, even if Eve chooses to measure |Ψin the same basis that the state was initially prepared in. The quantum communications system20may reduce Eve's information about the eigenstate |Ψin the public segment of the quantum communications channel30even for measurements she conducts in the correct basis. The QBER threshold required for unconditional security may be increased even when Eve chooses the right basis. The probability that Eve will measure the state Alice at the transmitter node26initially sent is reduced. As a result, using the quantum communications system20ofFIG.1as a QSDC system, Alice26and Bob28may tolerate higher system losses, increase communication link distances, relax optical detector requirements, and more easily adapt the system into existing telecom networks.

Further details of pulse detection occurring at the receiver node28are explained relative toFIG.2, where a schematic block diagram is illustrated, and showing the optical detector circuit66having a phase basis section as part of the phase detector76, which includes detectors D1 and D2 for phase basis measurements, and in this example, a single photon detector74for time basis measurements and including detector D3. The quantum communications system20results in the correct determination of the state for the quantum basis it is intended to be measured in, and inconclusive results of the state when measured in an unintended basis. As shown inFIG.2, the phase detector76includes detectors D1 and D2, and the single photon detector74includes detector D3 and operating to measure the time bin photons. Time basis measurements may be performed with direct detection to resolve the arrival times of pulses associated with the various bit values that Alice26sends. It is also possible to use the time to frequency conversion as disclosed in commonly assigned U.S. patent application Ser. No. 16/583,346 filed Sep. 26, 2019, under the title, “Quantum Communication System Having Time to Frequency Conversion and Associated Methods,” the disclosure which is hereby incorporated by reference in its entirety.

An incoming photon may be randomly directed by the beam splitter64to either a time basis measurement at the single photon detector74(D3) or a phase basis measurement at the phase detector apparatus76(D1 and D2). For time basis measurements, detector D3 as the single photon detector74detects the arrival time of the photon, which correlates with a particular time bin and associated bit value.

Phase basis measurements may be performed by passing the single photon state through a Mach-Zender interferometer84, which has a delay set by the time bin width of a protocol for the quantum communications system20or a half width of the waveguide for the quantum communications system. Single photon interference occurs in a central time window, which the two outputs of the Mach-Zender interferometer84resolve constructively or destructively depending on the eigenstate of the phase basis that was sent. For example, if phase state1was sent with an associated bit value 0, the phase detector76would yield a detection event for P1 on Detector 1, and no detection event on P1 of Detector 2. There is a non-zero probability amplitude of a detection event in the other arrival time bins (P2 and P3) for both of detectors D1 and D2 in the phase basis. However, detection events in these time bins do not help discriminate between the two states, and so they are not used to make state determinations, e.g., only the central time bins “P1” are used.

A flowchart illustrating a method of operating the quantum communication system is illustrated inFIG.3at100. The process starts (Block102), and the method includes operating the transmitter node26to generate quantum pulses at a pulse transmitter40(Block104). The method further includes dividing the quantum pulses at the pulse divider42(Block106), operating the receiver node28to recombine the quantum pulses at the pulse recombiner54(Block108), and receiving the recombined pulses at the pulse receiver56(Block110). The process ends (Block112).

Referring now toFIG.4, there is illustrated at110a graph showing the improvements resulting from the quantum communications system20to resist any tampering such as by Eve, when the pulse divider42and pulse recombiner54are employed. As illustrated as point A1 on the graph, Eve's measurements are reduced to random and Eve's ability to recreate the state is impossible without a matched receiver. As shown at point B1 on the graph, Alice and Bob may receive the same bits after sifting.

As shown in the chart ofFIG.5, the improvements for increased sensitivity to tampering such as by Eve indicates that the use of the pulse divider42and pulse recombiner54may potentially tolerate up to four times more bit errors than other protocols, from sources such as channel degradation, and still be operable. The state-of-the-art for a conventional quantum system without use of pulse dividers42and pulse recombiners54is shown in the first column and the use of the pulse combiner and pulse recombiner as a divided pulse QKD in an example with four stages, and assuming Eve does not have a matched receiver, is shown in the second column. The QBER without Eve is shown in the first row, and the QBER with Eve is shown in the second row, and the relative throughput indicated.

Referring now toFIG.6, there is illustrated a schematic diagram showing inaccuracy of a measured image as in the public link of the quantum communications channel30using the quantum communication system20ofFIG.1. For example, the original image is shown at the transmitter node26(Alice) and is shown relative to the measured image120in the public link when the pulse divider42and pulse recombiner54are employed. The image is shown after recombination at the receiver node28and corresponding to the image at the transmitter node26. The measured image120in the public link cannot be discerned even if Eve somehow correctly chose the frequency of the data. The optical communication system20as described provides high security, tamper evident communications, and low probability of interception/detection that are compatible with optical fiber, free space and underwater links and achieved with, for example, pulse division techniques using the pulse divider42and pulse recombiner54.

Referring now toFIG.7, there is illustrated at200a communications system that is not a quantum communication system that includes a transmitter node226, receiver node228, and an optical communications channel230coupling the transmitter node and the receiver node. The transmitter node226includes a pulse transmitter240and a pulse divider242downstream therefrom and the receiver node238includes the pulse recombiner254and pulse receiver256downstream therefrom. The optical communications channel230may be a free-space optical communications channel236, an underwater communications channel238, or fiber optic communications channel234, such as a single mode fiber or a multi-mode fiber.

The pulse transmitter240may include pulse generation circuitry241that is configured to generate an optical signal carrying communications data and amplified simultaneous emission (ASE) noise, which may be in the spectral and temporal domains. the pulse divider242may include birefringent elements243, that divide the pulses received from the pulse transmitter240into a first group of pulses having a first polarization and a second group of pulses having a second orthogonal polarization. These first and seconds of pulses may be interleaved with each other. The pulse receiver256may include a photoelectric detector260and a signal processor264coupled to the photoelectric detector and configured to separate the amplified spontaneous emission (ASE) noise from the communications data. The transmitter node226may include a controller248connected to the pulse transmitter240and controlling operation of the pulse transmitter. The receiver node228may include a controller278connected to the pulse receiver256and controlling operation of the pulse receiver, including the signal processor264.

The data signal from the pulse transmitter240is passed through the pulse divider242, which may create copies of each bit and distribute them into neighboring time bins, foiling attempts to extract information about the data. As the power in the data signal is reduced and the pulse copies are increased, security of the data increases. The broad probability distribution function of the data created by the pulse divider242may be compatible with frequency hopping techniques and classical encryption techniques and may provide an integrated and added layer of physical security.

Different pulse dividers242and pulse recombiners254may be used for both the quantum communications system20ofFIG.1and the communications system200shown as a non-quantum optical communication system inFIG.7. The description relative to components described inFIG.7may also apply to the pulse divider42and pulse recombiner54inFIG.1.

The pulse divider242may receive an input pulse of a first energy level and divide the pulse into a sequence of temporally spaced lower-energy pulses. The pulse recombiner254may combine the temporally spaced pulses for input into the pulse receiver256. The pulse divider242may be formed from a sequence of M birefringent elements243, which divide an initial pulse into a sequence of 2Mpulses. This sequence of pulses may include a first group of pulses that have a first polarization, and a second group of pulses that have a second orthogonal polarization. It is possible that the pulses in the first and second groups are interleaved with one another, so that the sequence of pulses have alternating linear polarizations.

The birefringent elements243may be formed from a sequence of birefringent crystals 1, 2, . . . , N. Crystals at odd-numbered positions in the sequence may have their optic axes oriented at a 45-degree angle to a direction of linear polarization of the pulse, while crystals at the even-numbered positions may be oriented in the same direction as the linear polarization of the pulse, so that at each crystal, a pulse is split into two equal-intensity pulses, one as an ordinary (o) wave pulse and a second as an extraordinary (e) wave pulse. The o and e pulses are separated in time by Δt=|1/υe−1/υo|L, where υoand υeare the group velocities of the o- and e-waves and L is the crystal length. The length of the shortest crystal in the sequence of crystals may be chosen so that Δt exceeds the pulse duration. The length of the mthcrystal in the sequence may be Lm=2m-1L1to produce equally spaced pulses.

The pulse recombiner254may be formed from a second sequence of birefringent crystals, which may be formed from Yttrium vanadate. Any alternating pulses with orthogonal polarizations may be separated with a polarizing beam splitter, and counter-propagate through a gain medium that requires a specific direction of linear polarization. A wave plate may exchange the direction of polarization of the counter-propagating beams, ensuring the correct polarizations for the beam entering the gain medium, and reverse the pulse replicas before the replicas are recombined into a final output pulse.

A mirror may be employed at the pulse recombiner254to rotate the polarization of the divided pulses by 90 degrees before they are fully recombined so that all pulses experience the same total delay and recombine into the output pulse. The pulse divider242and the pulse recombiner254may be implemented by a single stack of birefringent crystals243. For pulse division, a pulse may be passed in a first direction through a stack of crystals and for pulse recombination, a sequence of pulses may be passed in a second, opposite direction through a stack of birefringent crystals.

Examples and descriptions of different pulse dividers42and pulse recombiners54that may be used with the quantum communications system20ofFIG.1, and pulse dividers242and pulse recombiners253that may be used with optical communications system20ofFIG.7are disclosed in U.S. Pat. Nos. 8,456,736; 10,109,976; and 10,374,376; and in the articles: Zhou et al., “Divided-Pulse Amplification of Ultrashort Pulses,” Optics Letters, 32(7), 2007, pp. 871-873; Zhang et al., “Divided Pulse Soliton Self-Frequency Shift: A Multi-Color, Dual-Polarization, Power-Scalable, Broadly Tunable Optical Source,” Optics Letters, 42(3), 2017, pp. 502-505; and Lamb et al., “Divided-Pulse Lasers,” Optics Letters, 39(9), 2014, pp. 2775-2777, all of the disclosures which are hereby incorporated by reference in their entirety.

Referring now toFIG.8, there is illustrated a graph at290showing a simulation of the results in pulse division at the transmitter node226using the conventional optical communication system20ofFIG.7. The pulse division is shown in the graph ofFIG.8, and the recombining is shown in the graph ofFIG.9at292. The different stages in both graphs290,292are illustrated as letters A-F.

Referring now to the graphs ofFIGS.10and11, shown respectively at294and296, there are illustrated the probabilities of currently detecting an original data stream at different locations in the optical communications channel230using the communications system200ofFIG.7. Bob and the Public link graph lines are shown as “A” and “B” respectively.

The communications system200having the optical communications channel230and employment of the pulse dividers242and pulse recombiners254provides a low probability of detection where weak pulses are hid in tailored noise and makes the probability of detection low. There is a low probability of intercept because each bit is divided into many copies and distributing each copy into bins provides a system200where no useful information about the original message is gained. The system200is tamper evident because attempts to measure the data mid-link may be detected by the intended recipient and it is compatible with existing methods of data encryption with added potential for protecting against the attacks. Simulations have been based on “OOK” at 10 Gb/s to send an image from the transmitter node226to the receiver node228with a five-stage version of the divided pulse communications link. Similar performance may be expected as in a demonstrated free-space optical communications channel236that is greater than 25 km and greater than 1 Gb/s for communications between ship-shore and underwater up to 100 Mb/S and 100 meter range. A single system may enable communications over multiple link types with different wavelengths and distances. Use of the pulse divider242and pulse recombiner254are compatible with existing methods of data encryption and compatible with active link monitoring techniques and phase front shaping for increasing link performance and scattering media.

Referring now toFIG.12, there is illustrated a flowchart generally at300that shows the method of operating the communications system200shown inFIG.7. The process starts (Block302). The transmitter node226is operated to generate optical pulses at the pulse transmitter240(Block304). The optical pulses are divided at the pulse divider242(Block306). The receiver node228is operated to recombine the optical pulses at the pulse recombiner254(Block308). The recombined pulses are received at the pulse receiver256(Block310). The process ends (Block312).

This application is related to patent application entitled, “COMMUNICATIONS SYSTEM USING PULSE DIVIDER AND ASSOCIATED METHODS,” which is filed on the same date and by the same assignee and inventors, the disclosure which is hereby incorporated by reference.

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, and that modifications and embodiments are intended to be included within the scope of the appended claims.