Synthesis Unit

Abstract

To provide a combining unit that can produce more stable polarization states.SOLUTION: A combining unit combines multiple light inputs into a single light output, and includes a photonic integrated circuit. The photonic integrated circuit includes: a plurality of low birefringence waveguides (LBWs), each LBW being coupled to a light input of the multiple light inputs; and at least one low birefringence coupler (LBC) coupled to the plurality of LBWs and coupled to the single light output, in order to combine a plurality of light signals received by the multiple light inputs into an output light signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION The embodiments described herein relate to a combining unit and a quantum communication system and method. [Background technology]

[0002] In quantum communication systems, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information, which can be encoded on a property of the photon, such as polarization.

[0003] Quantum key distribution (QKD) is a technique for sharing a cryptographic key between two parties: a transmitter, often called "Alice," and a receiver, often called "Bob." The appeal of this technique is that it provides a test for whether any part of the key could have been known to an unauthorized eavesdropper, often called "Eve." In many forms of quantum key distribution, Alice and Bob use two or more nonorthogonal bases to encode bit values. The laws of quantum mechanics require that an unknown quantum state cannot be cloned without introducing errors into the copy. Measurement of a photon by Eve without prior knowledge of each encoding base introduces errors into her estimation of the encoding symbol. When she retransmits the photon to Bob, the state is no longer the same as the one originally sent by Alice, introducing errors into the final bit value shared between Alice and Bob. Thus, by comparing portions of their common bit string, Alice and Bob can quantify how much information Eve has gained.

[0004] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a schematic diagram of a synthesis unit according to an embodiment. [Figure 2A] FIG. 2A is a schematic diagram of a synthesis unit according to an embodiment. [Figure 2B]FIG. 2B is a schematic diagram of a synthesis unit according to an embodiment. [Figure 3] FIG. 3 is a diagram illustrating a transmitter according to an embodiment. [Figure 4] FIG. 4 is a cross-sectional view of a first end and a second end of a fiber assembly according to an embodiment. [Figure 5] FIG. 5 is a schematic diagram of a light source according to an embodiment. [Figure 6] FIG. 6 is a schematic diagram of a light source according to an embodiment. [Figure 7A] FIG. 7A is a schematic diagram of a transmitter according to an embodiment. [Figure 7B] FIG. 7B is a cross-sectional view of an end of a light source according to an embodiment. [Figure 8] FIG. 8 is a schematic diagram of a quantum communication system according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0006] Various aspects and embodiments are set forth in the following claims.

[0007] In an embodiment, a combining unit for combining multiple optical inputs into a single optical output, comprising: The photonic integrated circuit includes: a plurality of low birefringence waveguides (LBWs), each LBW coupled to an optical input of the plurality of optical inputs; at least one low birefringence coupler (LBC) coupled to the plurality of LBWs for combining multiple optical signals received at the multiple optical inputs into an output optical signal, and coupled to a single optical output; A synthesis unit is provided, comprising:

[0008] Information can be encoded by modulating the polarization state of light, and in practical applications, the states can be chosen from the following mutually exclusive basis with two orthogonal basis states: These can be mutually exclusive bases including horizontal "H" and vertical "V" linear polarization states oriented at angles 0 and 90 degrees, respectively, and diagonal "D" and anti-angle "A" diagonal linear polarization states oriented at 45 and -45 degrees, respectively.

[0009] In quantum communication system embodiments, quantum information can be encoded onto single quanta, such as photons, using polarization bases. Photons are transmitted through optical components in optical devices, such as quantum encoders and decoders, and over quantum communication channels in free space, optical fiber, or on-chip. Polarized photons can also be combined together within optical devices to produce additional polarization states.

[0010] The embodiments described herein facilitate the communication of quantum information with improved stability, a small footprint, and precise alignment that ensures indistinguishability of the encoded photons.

[0011] The use of the disclosed photonic integrated circuits to optically confine the received optical pulse within a waveguide provides improved alignment precision, and the use of waveguides and couplers, particularly polarization-independent waveguides, results in more stable polarization states.

[0012] The embodiments disclosed herein provide improved alignment compared to free-space optical combiners, which require very precise mechanical alignment of the laser and optical beams to ensure indistinguishability. Additionally, the embodiments described herein provide an improvement over fiber-based optical combiners, which can introduce polarization changes due to birefringence present in the fiber and mechanical rotation of the fiber. Both free-space and fiber-based optical combiners require a large footprint (including by providing multiple bulky components in free space or by using multiple fibers and connectors).

[0013] In an embodiment, a synthesis unit as described above is provided, wherein the plurality of optical inputs comprises at least three optical inputs; wherein the at least one LBC comprises a plurality of LBCs arranged in a plurality of sequential combining stages; Here, the plurality of sequential combining stages comprises a final stage and at least one preceding stage, the final stage having an output LBC coupled to a single optical output, and the at least one preceding stage having at least one LBC configured to combine a first input and a second input into a single LBC output coupled to an input of the subsequent stage.

[0014] In an embodiment, a transmitter is provided, the transmitter comprising: A composite unit such as the one described above, a light source having a plurality of outputs, each output coupled to an optical input of a plurality of optical inputs; Equipped with Here, the light source is configured to output pulses of light at a first wavelength at each of a plurality of outputs.

[0015] In an embodiment, the light source is configured to receive at least one control signal for modulating the polarization of the polarized light output at each of the plurality of outputs.

[0016] In an embodiment, the combining unit as described above further comprises a fiber array, the fiber array comprising: a plurality of polarization-maintaining optical fibers (PMFs), each PMF coupled to an optical input of the plurality of optical inputs of the combining unit; Here, each PMF has a fast optical axis and a slow optical axis, and wherein the PMFs are rotated at a non-zero angle relative to each other.

[0017] In an embodiment, the plurality of optical inputs comprises four optical inputs and the plurality of PMFs includes four PMFs.

[0018] In an embodiment, a transmitter is described, the transmitter comprising: A composite unit such as the one described above, a light source having a plurality of outputs, each output coupled to a PMF of the fiber array, wherein the light source is configured to output a pulse of light at a first wavelength at each of the plurality of outputs; Equipped with.

[0019] The light source comprises a plurality of lasers, each laser of the plurality of lasers being coupled to an output of the plurality of outputs.

[0020] In an embodiment, each laser of the plurality of lasers is configured to receive a control signal for modulating the intensity of the laser.

[0021] In some embodiments, the light source comprises: A laser, an output intensity modulator coupled to the laser and configured to modulate the output of the laser; a plurality of intensity modulators, each intensity modulator coupled to an output of the plurality of outputs and configured to transmit or block light output at the output in response to a control signal; Equipped with.

[0022] In some embodiments, the light source further comprises a plurality of phase modulators, each phase modulator coupled to a respective intensity modulator of the plurality of intensity modulators and configured to adjust the phase of the received light in response to a control signal.

[0023] In some embodiments, the light source generates a plurality of light pulses as a sequence of pulses.

[0024] In some embodiments, the light source generates a plurality of light pulses having an average photon number of <1.

[0025] In some embodiments, the single optical output of the combining unit is coupled to a free-space output, or the single optical output is coupled to a single-mode optical fiber, and the transmitter optionally comprises one or more birefringent elements coupled to the output of the single-mode optical fiber.

[0026] In some embodiments, a quantum encoder is described that includes a transmitter as described above.

[0027] In some embodiments, a quantum communication system is described that comprises a quantum encoder as described above and a quantum decoder.

[0028] In some embodiments, there is provided a method for combining multiple optical inputs into a single optical output, comprising the steps of: providing an optical input of a plurality of optical inputs to one of a plurality of low birefringence waveguides (LBW); providing outputs of the plurality of LBWs to at least one low birefringence coupler (LBC); and combining the input of the at least one LBC into the plurality of LBWs and the output of the at least one LBC into a single optical output. A method is described comprising:

[0029] In some embodiments, there is provided a method for generating a polarization-encoded signal, the method comprising: generating a plurality of light pulses; directing the plurality of optical pulses into a fiber array, the fiber array comprising a plurality of polarization-maintaining optical fibers (PMFs), each PMF having a fast optical axis and a slow optical axis, the plurality of PMFs being rotated at a non-zero angle relative to one another such that the fiber array outputs a plurality of polarized pulses; By synthesizing the multiple outputs of multiple PMFs according to the above method, A method is described comprising:

[0030] In some embodiments, there is provided a method for generating a polarization-encoded signal, the method comprising: generating a plurality of polarized light pulses; Combining a plurality of polarized light pulses according to the method described above; A method is described comprising:

[0031] In some embodiments, there is provided a method of quantum communication, comprising: generating a polarization-encoded signal according to claim 18 or 19; and a plurality of polarization pulses output from the PMF being randomly selected from two polarization basis states. receiving the polarization-encoded signal at a receiver; Decoding the received polarization-encoded signal based on two polarization bases; A method is described comprising:

[0032] Figure 1 shows a combining unit 100. The combining unit 100 is configured to combine multiple optical inputs 102 into a single optical output 104, thus acting as a multiplexer of input optical signals. Although four optical inputs 102a-102d are shown in Figure 1, the number is not limited to four. The number may be greater (e.g., eight) or less, and may be at least two.

[0033] The combining unit 100 comprises a photonic integrated circuit (PIC) 110. The PIC 110 comprises a plurality of waveguides 112 having low birefringence, hereinafter "low birefringence waveguides" or "LBW". Examples of low birefringence waveguides include optically isotropic waveguides, which are waveguides comprising a material that is optically isotropic (i.e., the refractive index is the same in all directions within the material). A further example includes a waveguide with uniaxial birefringence. In this example, the refractive index is equal in all directions orthogonal to the direction of the optical axis, which is aligned with the direction of light propagation within the waveguide. A further example is a waveguide with a refractive index of <5×10 in the direction orthogonal to the optical axis. -5 birefringence (5×10 -5 Each LBW is a waveguide with a low birefringence (i.e., a smaller birefringence). Each LBW with low birefringence is configured to propagate light with no or minimal axial or phase rotation. Thus, the LBW preserves the polarization of any given polarization of light pulses entering the LBW. Each LBW is formed of a low-birefringence material and has a circularly symmetric cross-section. LBWs may be fabricated by layer deposition on a substrate or by directional laser writing on a glass substrate. When forming the waveguide, the internal stress of the waveguide is minimized to maintain the low birefringence of the formed waveguide. Stress reduction methods include avoiding abrupt changes in direction, avoiding sharp edges on the waveguide sidewalls, or using controlled multimode waveguides.

[0034] Each LBW is coupled to an optical input of the plurality of optical inputs 102. While FIG. 1 shows four LBWs 112a-112d, each coupled to one of the four optical inputs 102a-102d, in other embodiments, the number may be different and be at least two. The number of LBWs 112 may correspond to the number of optical inputs 102. The PIC 110 further includes at least one coupler 114 having low birefringence, hereinafter low birefringence coupler or "LBC." Each LBC having low birefringence is configured to propagate and couple light with no or minimal axial or phase rotation. Thus, the LBC preserves the polarization of optical pulses of any given polarization entering the LBC.

[0035] At least one LBC is coupled to the multiple LBWs 112 and to the single optical output 104 to combine optical signals received at the multiple optical inputs 102 into an output optical signal. Each LBC may be a 2x1 beam combiner that combines input pulses into pulses at the output port. The LBC may be formed from the same materials used to form the LBWs described above. For example, the LBC may be formed by joining the ends of three LBWs together, with two of the LBWs forming the input branch and the third LBW forming the output branch. The symmetry of the LBWs forming the LBC is maintained during the formation process; for example, the LBWs are bent and bonded together under low stress conditions to maintain low birefringence in the components of the LBC.

[0036] LBW and LBC allow the PIC to receive an optical input of any polarization at any of multiple optical inputs 102 and transmit the optical input to a single output while maintaining the desired polarization.

[0037] The combining unit 100 in Figure 1 is configured to combine four optical inputs into one optical output, and the combining unit includes three LBCs. Such a combining unit may be applied to quantum key distribution applications in which the combining unit receives pulses encoded in {H,V} or {D,A} basis states, where each optical input is configured to receive optical pulses in a different polarization basis. For example, this combining unit may be used to receive and combine pulses encoded in the four-state BB84 protocol.

[0038] In other embodiments, the combining unit may include a different number of optical inputs, and LBWs and LBCs may be provided. Figures 2A and 2B show two exemplary embodiments. In Figure 2A, combining unit 200-A combines three optical inputs into one optical output. In Figure 2B, combining unit 200-B combines eight optical inputs into one optical output. Generally, the plurality of optical inputs 102 may include at least three optical inputs, and at least one LBC includes multiple LBCs arranged in a sequential combining stage 202. The combining stage includes a final stage 202-F and at least one preceding stage 202-P. The final stage includes an output LBC 204 coupled to a single optical output and to at least one preceding stage 202-P. At least one preceding stage includes at least one LBC, each configured to combine a first input and a second input into a single LBC output. The single LBC output is then coupled to the input of the next stage.

[0039] In the example of FIG. 2A , combining unit 200-A includes only one preceding stage configured to receive two optical inputs 212a and 212b via LBW and couple them to the input of LBC 204 of final stage 202-F. A third optical input 212c of combining unit 200-A is coupled to the input of LBC 204. The combining unit 200-A of FIG. 2A may be applied to quantum key distribution applications in which the combining unit receives pulses encoded in the {H,V} or {D,A} basis, where each optical input is configured to receive optical pulses in one of three states in two polarization bases. For example, this combining unit may be used to receive and combine pulses encoded in the three-state BB84 protocol.

[0040] In the example of FIG. 2B, the combining unit 200-B includes two predecessor stages 202-P1 and 202-P2. The first predecessor stage 202-P1 is configured to receive eight optical inputs 212a-212h and couple them to the inputs of four LBCs located in the first stage 202-P1. The outputs of the four LBCs are then coupled to the inputs of two LBCs located in the second stage 202-P2. The outputs of the two LBCs are then coupled to the inputs of the LBC 204 in the final stage 202-F. The combining unit 200-A of FIG. 2A can be applied to quantum key distribution applications in which the combining unit receives pulses encoded by the polarization of light. For example, a pulse may be encoded in one of eight possible states, such as states in the {H,V} basis, the {D,A} basis, and two additional orthogonal bases (e.g., orthogonal bases corresponding to the {H,V} basis and the {D,A} basis, each further rotated by an amount δ rad). For example, the combining unit may be used to receive and combine QKD protocol encodings using five or more states (e.g., six-state or eight-state BB84).

[0041] Alternatively, pulses may be encoded only in the {H,V} and {D,A} bases, but each optical input may be configured to receive pulses of a different intensity. For example, four optical inputs 212a-212d may be configured to receive pulses encoded in the {H,V} and {D,A} bases at a first intensity, and four optical inputs 212a-212d may be configured to receive pulses encoded in the {H,V} and {D,A} bases at a second intensity.

[0042] In some embodiments, each combining unit 200 may include additional LBWs. For example, there may be additional LBWs that combine the output of an earlier stage LBC with the input of a later stage LBC, and there may be an LBW that combines the final stage with the output of the combining unit.

[0043] In any of the combining units described herein, the single optical output of combining unit 100 may be coupled to a single-mode optical fiber or to a free-space output. Thus, combining unit 100 may be adapted for a wide range of applications, including transmitting quantum information over an optical fiber channel and transmitting quantum information over a free-space channel. Multiple optical inputs may be coupled to the free-space input or the fiber input. For example, the combiner input may be coupled to a light source via a fiber array (as described below), or may be coupled to the light source directly or via a free-space optical link.

[0044] Free-space combining followed by direct output into free space avoids compensating for phase drift due to single-mode output fibers, making the combining unit particularly useful in free-space communication situations such as satellite communications.

[0045] In some embodiments, all inputs and outputs of the combining unit include fiber-coupled components. Thus, the entire combining unit can be a module in a modular assembly (with other modules, including light sources, as described below, and fiber optic cables or other fiber optic outputs that form quantum communication channels). Thus, the combining unit provides a versatile, flexible, "plug-and-play" system.

[0046] The combining units described herein may be used in a method for combining multiple optical inputs into a single optical output, the method comprising providing an optical input of the multiple optical inputs to one of a multiple LBWs and providing an output of the multiple LBWs to at least one LBC, the input of the at least one LBC being combined with the multiple LBWs and the output of the at least one LBC being combined with the single optical output.

[0047] Each combining unit is provided for receiving light and may be deployed as part of a QKD encoding device. In a first set of embodiments, the combining unit comprises a polarizing element and is adapted to receive light pulses from a light source. In this first set of embodiments, the combining unit itself is adapted to polarize the light into a desired polarization basis before the light is input to the PIC. In a second set of embodiments, the combining unit is configured to receive light already polarized in a desired polarization basis from a light source that outputs light in the desired polarization basis.

[0048] In an exemplary embodiment of the first set of embodiments, FIG. 3 shows a combining unit 310 including a fiber array 320. While the fiber array 320 is shown in combination with the combining unit 110, a fiber array could be added to any of the combining units described above. The fiber array 320 comprises a plurality of polarization-maintaining optical fibers 320a-320d (hereinafter, PMFs). A first end of each PMF is coupled to an optical input of the plurality of optical inputs 120 of the combining unit 110. A second end of each PMF is combined into a respective one of the plurality of LBWs of the PIC.

[0049] Figure 4 shows the cross section of each fiber in the fiber array 320. The first end 320-1 of the fiber array represents the cross section 320a-1 through 320d-1 of the first end of each optical fiber, while the end 320-2 of the fiber array represents the cross section 320a-2 through 320d-2 of the second end of each optical fiber. Each PMF has birefringence that allows two well-defined polarization modes to propagate along each fiber at different speeds. Because light propagates along the slow axis, the polarization state does not change with atmospheric conditions (such as temperature changes). Thus, each PMF has a fast optic axis 330 and a slow optic axis 325. There are many different types of polarization-maintaining fiber. The fiber may be geometrically asymmetric or may have an asymmetric refractive index profile. Alternatively, stress may be permanently induced in the fiber to create stress birefringence. This may be achieved using rods of other materials contained within the cladding. Figure 3 shows a so-called "PANDA" type where there are two rods around a central core that create stress birefringence, however this is only an example and other types of fibers (e.g. bowtie type or elliptical core) may also be used.

[0050] The multiple PMFs are arranged in an array so that each of the first ends 320a-1 through 320d-1 of the optical fibers has the same alignment of its fast and slow axes. The second ends of the multiple PMFs are rotated at a non-zero angle relative to each other. The rotation of the fiber causes a rotation of the fiber's slow axis. Light propagates along the slow axis due to the mechanical rotation of the fiber, resulting in a rotation of the polarization state propagating within the fiber. Therefore, the different polarization states output by each fiber in the fiber array are the result of the rotation of the optical fiber end. In the example of Figure 4, the second ends of the optical fibers are rotated to output polarizations of 0°, 90°, 45°, and 135° at the second ends 320a-2, 320b-2, 320c-2, and 320d-2, respectively, resulting in H, V, D, and AD polarization states.

[0051] In the above-described embodiments, because the light is polarized by the fiber's geometric orientation, the output polarization is independent of both the input wavelength and the fiber's temperature (since the output polarization is refractive index independent). This contrasts with other forms of chip-based polarization devices and methods that rely on amplitude mixing (i.e., interference). In these other forms of chip-based polarization devices, the final polarization depends on the characteristics of the optical path taken by the light that is interfered to create the final polarization state. Thus, the final polarization may depend on path length differences and refractive index variations in the chip-based components. Therefore, the above-described embodiments utilizing a mechanically rotated PMF provide encoded polarization states with greater stability. Additionally, a mechanically rotated PMF alleviates the need for active polarization control during operation. Because the PIC is also wavelength and temperature independent, the assembled combiner system provides greater polarization state stability.

[0052] Fiber-based polarization rotation provides more stable polarization states, but requires additional fiber and fiber connector lengths to create the combining unit. Therefore, utilizing fiber for both polarization state combining and combining requires combining units with large footprints and weight. Because PICs are more compact than fiber-based designs, they significantly reduce the footprint of the combining unit and, therefore, the transmitter. Because the fiber array is used only for polarization encoding, the fiber length can be kept to a minimum (e.g., a few millimeters). The entire multiplexing device can be as small as a few centimeters when combined with a chip-based light source (described later).

[0053] The embodiments described herein utilizing fiber arrays achieve a balanced advantage by implementing both optical fiber and PICs to obtain the polarization stability benefits of fiber with the size benefits of PICs. Additionally, the low birefringence of the waveguides provides thermally independent polarization states. This makes the transmitters described herein suitable for deployment on satellite nodes.

[0054] Furthermore, because the combining unit described herein does not rely on free-space optics, but rather on fibers and PICs, the weight of the device can be significantly reduced because bulky free-space combiners are not required.

[0055] A fiber array can be created by mounting a plurality of optical fibers side-by-side within a corresponding plurality of grooves 330. For each optical fiber, the second end of each optical fiber is mechanically rotated in the rotation described above, and the optical fiber is positioned within a groove of the fiber array assembly. The ends of the fiber array are then fixed in place, such as by gluing.

[0056] In some embodiments, the transmitter 300 may further comprise a single-mode optical fiber 316 coupled to the output of the combining unit 110. In further embodiments, the transmitter 300 may further comprise one or more birefringent elements 318, which are coupled to the output of the single-mode optical fiber 316. The one or more birefringent elements 318 may be used to correct for an error or shift in the polarization of the light due to propagation in the single-mode optical fiber. Although the light pulses output from the combining unit may be of different polarizations depending on the input light signal, each of these modes travels through the same optical fiber 316, and therefore, any polarization error / shift imparted by the single-mode optical fiber 316 will affect each different light pulse output in the same way. Therefore, the birefringent element may be configured to impart a polarization shift opposite to that imparted by the single-mode optical fiber 316.

[0057] 4 illustrates an embodiment in which the plurality of optical inputs comprises four optical inputs and the plurality of PMFs includes four PMFs. As discussed above, such an embodiment may be provided for use with a four-state quantum key distribution protocol such as the BB84 protocol. However, it is understood that any number of optical fibers may be provided, such that there is one optical fiber coupled to the optical input of the combining unit and the LBW of the PIC. For example, three optical fibers may be used to couple to the combining unit of FIG. 2A, and eight optical fibers may be used to couple to the combining unit of FIG. 2B.

[0058] The combining unit 310 may be included as part of the transmitter 300. In these embodiments, the transmitter 300 includes a combining unit 310 having a polarization element configured to generate pulses in a desired polarization basis (e.g., PMF). Figures 5 and 6 show an exemplary embodiment of a light source 350. The light source 350 comprises an emitter / light-emitting element 360, such as a laser or LED. Each output of the light source 350 is coupled to a respective PMF of the fiber array, and the light source is configured to output a pulse of light at a first wavelength at each of the multiple outputs. Generating pulses of the same wavelength at each of the multiple outputs improves the indistinguishability of the pulses input to the polarization optics and combining unit 310. The outputs of the light source 350 are typically polarized pulses, and each light source output is coupled to a PM fiber such that the polarization axis of the light pulse is aligned with the slow axis of the PM fiber.

[0059] As shown in FIG. 5, the light source 500 includes multiple emitters, each of which may emit pulsed radiation. For example, each emitter may be a light-emitting diode (LED) or a laser, such as a vertical-cavity surface-emitting laser (VCSEL) or a distributed feedback laser (DFB). The lasers may be chip-based lasers. Each laser corresponds to a combination of intensity level and polarization (I, P). The laser output may be modulated using an RF signal containing the information to be encoded. In this case, low-power VCSELs may be used that can be directly driven using a field-programmable gate array (FPGA). In some embodiments, the emitters may be configured to output pulsed radiation in a selected polarization basis. In these embodiments, the lasers may be polarized laser diodes or LEDs combined with linear polarization filters.

[0060] In the example of FIG. 5 , the light source 500 includes multiple emitters, e.g., multiple lasers 520 fabricated on a chip, each coupled to one of the multiple outputs of the light source 500. The light source can include drive electronics connected to each of the multiple emitters and configured to selectively excite each laser to generate an output from the emitter. The drive electronics can include an FPGA and provide high-speed signals to selectively excite each emitter. For example, the drive electronics can include a laser driver implemented such that the emitters are gain-switched, thus generating optical pulses with random phases on each clock cycle. In an embodiment, the laser driver receives a digital signal from the FPGA and generates an analog output capable of driving a laser diode or LED with a gain-switching operation, completely turning off the light source between each pulse. Driving the light source in this manner means that each pulse has a random phase due to being seeded from vacuum photons (spontaneous emission).

[0061] The outputs of the chip can have a similar pitch to the outputs of the PM fiber array, so that chips can be glued onto the chip to couple each emitter to a respective PM fiber. Each emitter can be tuned to produce pulses of the same intensity and wavelength.

[0062] In some embodiments, the light source 500 further comprises a first plurality of intensity modulators 520, each coupled to a respective emitter of the plurality of emitters. In the example of FIG. 5, four first plurality of intensity modulators 520-A through 520-D are shown coupled to emitters 510-A through 510-D, respectively. The light source 500 may further comprise a second plurality of intensity modulators 530, each coupled to a respective intensity modulator of the first plurality of intensity modulators. In the example of FIG. 5, four second plurality of intensity modulators 530-A through 530-D are shown coupled to intensity modulators 520-A through 520-D, respectively. The light source may further comprise a plurality of phase modulators 535, each coupled to a respective intensity modulator of the second plurality of intensity modulators 530. In the example of FIG. 5, four phase modulators 535-A to 535-D are shown combined into intensity modulators 530-A to 530-D, respectively.

[0063] The first plurality of intensity modulators 520 are adapted to modulate the intensity of the output of the emitter 510 in response to a control signal (eg, control signal 550-A) provided to each of the intensity modulators.

[0064] The second plurality of intensity modulators 530 are adapted as “ON-OFF” switches that allow or block the passage of light through the switching modulator in response to a control signal provided to each modulator (e.g., control signal 550-B).

[0065] The plurality of phase modulators 535 are adapted, in response to control signals 550-C, to modulate the phase of the pulses from each emitter before they are output from light source output 540. Thus, the phase modulators can be used to correct phase errors in the light pulses.

[0066] The second plurality of intensity modulators 530 may be used to encode quantum bit streams according to the QKD protocol. Each emitter output is coupled to a single input of the fiber array 320, so that the fiber array outputs light with a different polarization state relative to the light generated from each emitter. By providing control signals to the multiple switching modulators, the transmitting entity ("Alice") can select which of the multiple emitters is output to the fiber array 320, and therefore the polarization of the optical pulse output to the combining unit 110. The transmitter 300 generates a series of pulses at each of the multiple emitters 510 and receives control signals at the multiple modulators to randomly turn on different single modulators in accordance with the pulses generated by the multiple emitters. Thus, the transmitter output is a polarized pulse randomly selected from the polarization states provided by the fiber array 320.

[0067] The first plurality of intensity modulators 520 may be used to implement a QKD decoy state protocol. In some embodiments, the emitter generates multi-photon pulses to improve the reliability and efficiency of the detector and, therefore, the QKD protocol. However, such pulses are susceptible to photon number splitting attacks. Further security may be provided by additionally randomly varying the intensity of the pulses generated by the transmitter, thus introducing varying photon statistics into the channel. By publishing and monitoring the intensity levels of the pulses, Alice and Bob are able to detect photon number splitting attacks.

[0068] For example, the transmitter may be configured to employ a two-state QKD decoy protocol, where a random two-level control signal (e.g., an RF signal) may be applied to each of the intensity modulators to select either the signal state or the decoy state.

[0069] The illustrated order of the first and second plurality of intensity modulators and phase modulators is illustrative only, and alternative orderings are possible. For example, the second plurality of intensity modulators may be provided before the first plurality of intensity modulators, and the plurality of phase modulators may be provided at the output of the plurality of emitters 510 or between the first plurality of intensity modulators 520 and the second plurality of intensity modulators 530. Furthermore, the first plurality of intensity modulators and the second plurality of intensity modulators for each emitter may be implemented as a single intensity modulator for each emitter, with each single modulator performing both the switching and intensity modulation functions. The plurality of intensity modulators may use RF amplifiers, which have high power demands and are expensive to manufacture. As an alternative to the first plurality of intensity modulators, the plurality of emitters themselves may be configured / tuned to output pulses at the required intensity. Thus, a control signal 550-A may be provided directly to emitters 510-A through 510-D to directly modulate the intensity of the emitter pulses to implement a decoy state protocol. Also, as an alternative to the second plurality of intensity modulators, control signals 550-A can be provided directly to emitters 510-A through 510-D to directly and selectively switch the emitters on / off to generate a stream of output pulses of selected states of polarization basis.

[0070] According to a further embodiment, a light source 600 that may form part of the transmitter 300 is shown in FIG. 6. The light source 600 comprises a single emitter, such as a laser 610, coupled to multiple optical outputs 640, such that light generated by the emitter 610 can be output to one or more of the multiple optical outputs 640. The light source 600 may comprise drive electronics coupled to the emitter 610 and configured to selectively generate an output from the emitter. The drive electronics may comprise an FPGA and provide high-speed signals to selectively excite each emitter. For example, the drive electronics may include a laser driver implemented such that the emitter is gain-switched, thus generating optical pulses with a random phase on each clock cycle. In an embodiment, the laser driver receives a digital signal from the FPGA and generates an analog output capable of driving a laser diode or LED with a gain-switching operation, completely turning the light source off between each pulse. Driving the light source in this manner means that each pulse has a random phase due to being seeded from vacuum photons (spontaneous emission).

[0071] The light source can include a beam splitting assembly 605 that includes one or more couplers. As shown in Figure 6, the splitting assembly can include a hierarchy of 2x1 beam combining units. The beam splitting assembly is adapted to receive a single optical input and output multiple optical outputs, each corresponding to a respective one of the multiple optical outputs 640 (e.g., 640-A through 640-D).

[0072] The light source 600 may further comprise a plurality of intensity modulators 630, each coupled to an output of the plurality of outputs and configured to transmit or block light output at the output in response to a control signal 650-B. The light source may also comprise an output intensity modulator 620 coupled to an output of the emitter 610 and configured to modulate the output of the emitter in response to a control signal 650-A. Because the single emitter 610 is coupled to the multiple outputs via the beam splitting assembly, only a single light modulator 620 is needed to modulate the intensity of each of the outputs of the light assembly.

[0073] The light source may further include a plurality of phase modulators 635, each coupled to a respective one of the plurality of intensity modulators 630. The phase modulators are configured to control the phase of the light pulses in response to a control signal 650-C. In the example of FIG. 6, four intensity modulators 635-A to 635-D are shown combined into intensity modulators 630-A to 630-D, respectively. The illustrated order of the intensity modulators and phase modulators is merely exemplary, and alternative orderings are possible. For example, the plurality of intensity modulators 630 may be disposed after the plurality of phase modulators 635.

[0074] In some embodiments, light source 600 includes a second emitter (e.g., a laser) coupled to the input of intensity modulator 620 and can output light to the intensity modulator instead of emitter / laser 610. In this manner, the second emitter is provided for redundancy.

[0075] The multiple intensity modulators 630 may be used to encode a quantum bit stream according to a QKD protocol in the manner described above in connection with the second multiple intensity modulators 530 of FIG. 5. The multiple phase modulators 635 may be used for phase error correction in the manner described above in connection with the phase modulators 535 of FIG. 5. The single intensity modulator 620 may be used to implement a QKD decoy state protocol in the manner described above in connection with the first multiple intensity modulators 520 of FIG. 5 above. A single control signal 650-A may be applied to the intensity modulator 620 to randomize the intensity of the emitter's output pulses, which may then be provided to each of the switching modulators 630. A control signal 650-B is provided to randomly activate one of the switching modulators to select an output polarization state.

[0076] The light source 600 of FIG. 6 is provided with a single emitter. Because all optical power is provided from the same emitter, the final output pulse train has improved pulse-to-pulse wavelength indistinguishability (e.g., compared to multiple emitters, where emitter-to-emitter variations can affect the wavelength of the pulses output by the light source). By using only a single emitter (or two emitters with redundancy), the light source 600 is more compact and therefore has a smaller footprint. The use of a single laser also relaxes the thermal stability requirements that may be required for multiple lasers (i.e., ensuring consistent temperatures across all lasers). The single emitter (and redundant emitters, if used) can generate pulses with polarization bases aligned with the slow axis of each fiber in the fiber array to which the light source 600 is coupled.

[0077] The transmitter described above may be used in a method for generating a polarization-encoded signal, the method comprising generating a plurality of optical pulses, directing the plurality of optical pulses into a fiber array, and combining the outputs of the plurality of PMFs according to the method described above in connection with Figures 1, 2A, and 2B, where the fiber array comprises a plurality of PMFs, each PMF having a fast optical axis and a slow optical axis, and where the PMFs are rotated at a non-zero angle relative to one another, such that the fiber array outputs a plurality of polarized pulses.

[0078] As described above, in the second set of embodiments, the combining unit is coupled to a light source that provides a polarized light output. An exemplary transmitter 700 according to the second set of embodiments is shown in FIG. 7A. The transmitter 700 includes a combining unit 110. The combining unit may be any of the combining units described above in connection with FIGS. 1, 2A, and 2B. The transmitter 700 further includes a light source 750 having multiple optical outputs, each coupled to an optical input of the combining unit 110. The light source 750 is configured to output pulses of polarized light at a first wavelength at each of the multiple outputs. Generating pulses of the same wavelength at each of the multiple outputs improves the indistinguishability of the pulses input to the combining unit 810. In these embodiments, the light source may be edge-coupled to the PIC such that each emitter output is coupled to an input of the PIC. The light source may be coupled by a coupling interface 712, which may include an epoxy or other transparent adhesive, or may include a microlens array, a grating coupler, or a free-space coupler.

[0079] In some embodiments, light source 750 is configured to output light having a different polarization at each light output of the light source. For example, in a four light output embodiment, the outputs output light in each of the H, V, A, and D polarization basis states. The light source may comprise multiple laser light sources, each configured to generate polarized light. Alternatively, each laser light source may be provided with a polarizing element attached to its output.

[0080] Light source 750 can include multiple light-emitting elements 760, e.g., multiple lasers. Each light-emitting element can be combined into a respective one of light source 750's multiple optical outputs, outputting light of a different polarization at each output. For example, in some embodiments, light source 750 can include elements of light source 500 described above in connection with FIG. 5. In these embodiments, laser 510 is a laser configured to generate polarized light. Alternatively, light source 750 can include the same intensity modulation and switching functionality described above in connection with light source 500. Light source 750 can thus be adapted for a QKD encoding protocol and, optionally, a QKD decoy state protocol.

[0081] In an exemplary embodiment, the multiple light sources are multiple VCSELs. VCSELs provide high efficiency and high-speed operation, and are therefore well-suited for generating optically encoded states. The multiple VCSELs may be provided at the edge of the light source 750 to output light from the edge. FIG. 7B illustrates an example showing the edge 720 of the light source 750, where the emitting surfaces of multiple VCSELs 725a-725d are located, serving as light outputs 740a-740d of the light source 750. To achieve different polarization states, the VCSELs are physically oriented at 0°, 90°, −45°, and 45°. The embodiment of FIG. 7B is merely exemplary, and it will be understood that one VCSEL is provided for each light output of the light source 750, and that more than four or fewer than four VCSELs may be provided. The physical orientation of each VCSEL may further vary depending on which polarization basis is desired for the light of each light output.

[0082] The edge 720 may be edge-coupled to an edge of the combining unit (including edge-coupled to an edge of the PIC). The edge coupling may be formed by an interface 712 (e.g., a microlens array, a grating coupler, or another edge coupler). Alternatively, the interface may be a free-space imaging system that allows transmission of light from the light source through free space. In such an embodiment, the combining unit 110 coupled to the light source 750 may include a free-space edge coupler that couples the free-space input to the optical input of the combining unit.

[0083] The transmitter embodiment described above in connection with Figures 7A and 7B can be made more compact because no polarizing element is required in the combining unit.

[0084] In some embodiments, the transmitter 700 may further comprise a single-mode optical fiber 716 coupled to the output of the combining unit 110. In further embodiments, the transmitter 700 may further comprise one or more birefringent elements 718 coupled to the output of the single-mode optical fiber 716. The one or more birefringent elements 718 are configured to compensate for a shift in polarization from the single-mode optical fiber 716 in the same manner as described above in connection with the one or more birefringent elements 318 of FIG.

[0085] In some examples of light sources 500, 600, 750, multiple switching transistors can be operated simultaneously, resulting in multiple optical outputs being simultaneously input to the input of the combining unit. In this manner, the combining unit is configured to combine multiple polarized pulses into a single optical pulse. Thus, the combining unit can be used to perform amplitude multiplexing (i.e., interference) of different optical pulses. In examples using multiple emitters 510, each emitter can be individually driven by drive electronics to transmit pulses of the desired phase and / or amplitude (and / or polarization, in the case of light source 750) for each laser that are combined by the combining unit. The amplitude, phase, and / or polarization are selected to generate the desired final output state.

[0086] In some embodiments, light source 750 is configured to receive at least one control signal 630 to modulate the polarization of the polarized light output at each of the multiple outputs. For example, a control signal may be provided for each laser of the multiple lasers that generate the polarized light output from light source 750. In this manner, the light source may be configured to vary the polarization basis of the encoded pulses.

[0087] In some embodiments, the outputs of the light sources 750 may be combined directly into the combining unit 110 by directly mounting the light sources to the combining unit to couple the light source outputs to the combining unit inputs, where the light source outputs and the combining unit inputs are provided with the same pitch to ensure alignment. In alternative embodiments, the light sources and the combining unit may be coupled by a fiber array comprising multiple single-mode optical fibers that maintain polarization between the light sources and the combining unit. In other alternative embodiments, the light sources and the combining unit may be coupled by a free-space optical link.

[0088] The transmitter described above may be used in a method for generating a polarization-encoded signal, the method comprising generating a plurality of polarized optical pulses and combining the polarized optical pulses according to the method described above in relation to Figures 1, 2A, and 2B.

[0089] For each of the above embodiments of a transmitter including a light source, the light source can be configured to generate the light pulses as a sequence of pulses. If the light source includes multiple lasers, each laser can be configured to generate the sequence of pulses. If the light source includes a single laser, the laser can be configured to generate the sequence of pulses. Each laser can be driven by a high-speed clock pulse. Each pulse can be encoded in a polarization basis using the systems and methods described herein and provided to a combining unit. In this way, the combining unit output is a sequence of polarization-encoded pulses and can be used as a channel for quantum information.

[0090] In some embodiments, the light source generates light pulses with an average photon count of less than 1 (<1). Generating pulses with a low photon count reduces the system's vulnerability to photon-splitting attacks when operating as a QKD encoder.

[0091] The light sources can be configured to generate pulses at a range of wavelengths within the wavelength range for communications applications (e.g., 1300 nm to 1700 nm), as well as near-infrared, infrared, and visible wavelength bands. In one embodiment, each laser generates pulses at 850 nm, which exhibits low absorption in free space and allows for higher transmitter efficiency when utilizing free-space optical links.

[0092] The above embodiments include a combining unit having three or more optical inputs. In some embodiments, the combining unit can have two optical inputs and a single optical output to form a 2×1 combining unit. In these embodiments, the 2×1 combining unit includes two LBWs that combine the optical inputs of the combining units into the inputs of a single 2×1 LBC, the output of which is coupled (optionally via another LBW) to the output of the PIC. Such 2×1 coupler embodiments may be useful for combining only two polarization states in applications other than QKD. However, this system may still be useful in QKD encoding schemes.

[0093] For example, a 2×1 combining unit may be configured such that each optical input receives a different orthogonal polarization state (e.g., an “H” and a “V” polarization state) to form two states in a first basis. If pulses are received at each input at different times, the 2×1 combining unit is configured to output pulses in different states of the first basis at different times. If pulses are received simultaneously at the two optical inputs, the pulses combine to form a state in a third basis. For example, an input pulse with H polarization and an input pulse with V polarization can be combined to form a “D” polarization state.

[0094] The 2x1 combining unit can thus be employed as part of a transmitter configured as a quantum encoder. Such a transmitter includes a light source according to any of the above-described embodiments, and a control signal provided to a switching modulator controls the switching modulator to transmit light from either optical output of the light source, or from both optical outputs simultaneously, to encode a quantum state. The intensity modulator also receives a control signal to modulate the intensity of the output pulses so that pulses output at each polarization state have the same intensity when they exit the combining unit.

[0095] While the 2x1 combining unit is more compact than the other combining units described herein, generating a set of states for QKD encoding requires amplitude mixing (i.e., interference) of the input wavelengths, which depends on the wavelength of the input light source. In contrast, having individual inputs of the combining unit dedicated to individual polarization states means that amplitude mixing is not required to generate all the required polarization states of a communication protocol, and the polarization of the encoding states is more stable because it does not depend on the wavelength of the light source (e.g., at least three optical inputs can be used for stable polarization states for QKD encoding applications).

[0096] The above-described components of the transmitter may be bonded together using a filler or adhesive, such as an index-matching epoxy, that is transparent to the light propagating within the transmitter.

[0097] In some embodiments, the single optical output of the combining unit is combined into an optical fiber, so that non-free space optical combining is not used, and therefore significant alignment instabilities can be significantly reduced.

[0098] An embodiment of a quantum encoder system utilizing a transmitter according to the described embodiment is described below in connection with FIG.

[0099] Figure 8 shows a quantum communication system 800 including a quantum encoder 805 and a quantum decoder 900. The quantum encoder 805 may be any of the transmitter embodiments described above. For example, Figure 8 shows that the quantum encoder 805 may include the transmitter described above in connection with Figure 3, including a combining unit 810, a fiber array 820, and a light source 820 (which may be light source 500 or 600). However, other transmitters described herein may form part of the quantum encoder 805.

[0100] The quantum encoder 805 is configured to generate optical pulses (e.g., a series of pulses encoded in the {H,V} or {D,A} basis) containing quantum-encoded information. The quantum encoder 805 is connected to the quantum decoder 900 via a communication channel 850. This communication channel may be, for example, a free-space optical channel or an optical fiber channel. The quantum decoder 900 is configured to decode the received light. The quantum decoder comprises a polarizing beam splitter PBS 904, a polarization controller 902, and detectors 904-1 and 904-2. In an embodiment, the PBS 904 is configured to split the light into two orthogonal polarization states (e.g., H and V). The polarization controller 902 is configured to be switchable between two states: in a first state, the controller provides no polarization rotation; and in a second state, the controller provides a 45° rotation to switch the incident light from a first polarization basis (e.g., {H,V}) to a second orthogonal polarization basis (e.g., {A,D}).

[0101] By switching the state of the polarization controller, the receiver ("Bob") can change the basis in which the quantum decoder 900 is configured to measure the incident light.

[0102] The above system may be used in a quantum communication method, the method comprising generating a polarization-encoded signal according to the method described above, receiving the polarization-encoded signal at a receiver, wherein a plurality of polarization pulses output from the PMF are randomly selected from two polarization basis states, and decoding the received polarization-encoded signal based on the two polarization bases.

[0103] The combining units and transmitters described herein may have further applications. For example, each transmitter may be used as a transmitter for high-speed optical communications or as a source for ultrafast Mueller polarimetry, in which a Mueller matrix is ​​constructed from multiple generated polarization states. In some embodiments, a timestamp may be generated by the transmitter for each generated polarization pulse used to create the Mueller matrix.

[0104] While specific embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel devices and methods described herein may be embodied in a variety of other forms, and various omissions, substitutions, and changes in the forms of the devices, methods, and products described herein may be made without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover such forms or modifications as would be within the scope and spirit of the invention.

Claims

1. a combining unit for combining multiple optical inputs into a single optical output, a photonic integrated circuit, the photonic integrated circuit comprising: a plurality of low birefringence waveguides (LBWs), each LBW coupled to an optical input of the plurality of optical inputs; at least one low birefringence coupler (LBC) coupled to the plurality of LBWs and coupled to the single optical output for combining multiple optical signals received at the multiple optical inputs into an output optical signal; Equipped with A combining unit, wherein the LBW is configured to propagate and output an optical input while maintaining a polarization state without axial or phase rotation, and the LBC is configured to propagate and combine and output an optical input while maintaining a polarization state without axial or phase rotation.

2. the plurality of optical inputs comprises at least three optical inputs; wherein the at least one LBC comprises a plurality of LBCs arranged in a plurality of sequentially combined stages; 2. The combining unit of claim 1, wherein the plurality of sequential combining stages comprises a final stage and at least one preceding stage, the final stage having an output LBC coupled to the single optical output, and the at least one preceding stage having at least one LBC configured to combine a first input and a second input into a single LBC output coupled to an input of a subsequent stage.

3. The optical fiber further includes a fiber array, the fiber array comprising: a plurality of polarization-maintaining optical fibers (PMFs), each PMF coupled to an optical input of the plurality of optical inputs of the combining unit; 2. The combining unit of claim 1, wherein each PMF has a fast optical axis and a slow optical axis, and two PMFs selected from the plurality of PMFs, a first PMF and a second PMF, are configured such that the fast optical axis of the first PMF is rotated by a non-zero angle relative to the fast optical axis of the second PMF and the slow optical axis of the first PMF is rotated by the non-zero angle relative to the slow optical axis of the second PMF, generating polarization states in which a basis of a first optical output of the first PMF and a basis of a second optical output of the second PMF are rotated by the non-zero angle, and at least the angle is set so that the basis of the first optical output and the basis of the second optical output are not biased from each other.

4. The combining unit of claim 3 , wherein the plurality of optical inputs comprises four optical inputs and the plurality of PMFs includes four PMFs.

5. A transmitter comprising: A synthesis unit according to claim 3; a light source having a plurality of outputs, each output coupled to a PMF of the fiber array, wherein the light source is configured to output a pulse of light at a first wavelength at each of the plurality of outputs; A transmitter comprising:

6. The transmitter of claim 5 , wherein the light source comprises a plurality of lasers, each laser of the plurality of lasers coupled to an output of the plurality of outputs.

7. The transmitter of claim 6 , wherein each laser of the plurality of lasers is configured to receive a control signal for modulating an intensity of the laser.

8. The light source is A laser, an output intensity modulator coupled to the laser and configured to modulate an output of the laser; a plurality of intensity modulators, each intensity modulator coupled to an output of the plurality of outputs and configured to transmit or block light output at the output in response to a control signal; The transmitter of claim 5 , comprising:

9. 9. The transmitter of claim 8, wherein the light source further comprises a plurality of phase modulators, each phase modulator coupled to a respective intensity modulator of the plurality of intensity modulators and configured to adjust a phase of received light in response to the control signal.

10. A transmitter comprising: A synthesis unit according to claim 1; a light source having a plurality of outputs, each output coupled to an optical input of the plurality of optical inputs; Equipped with 10. The transmitter of claim 9, wherein the light source is configured to output pulses of light at a first wavelength at each of the plurality of outputs.

11. The transmitter of claim 10 , wherein the light source is configured to receive at least one control signal for modulating the polarization of the polarized light output at each of the plurality of outputs.

12. The transmitter of claim 5 , wherein the light source generates a plurality of light pulses as a sequence of pulses.

13. The transmitter of claim 5 , wherein the light source generates a plurality of light pulses having an average photon number of less than one (<1).

14. the single optical output of the combining unit is combined into a free-space output; or 6. The transmitter of claim 5, wherein the single optical output is coupled to a single mode optical fiber, and the transmitter optionally comprises one or more birefringent elements coupled to an output of the single mode optical fiber.

15. A quantum encoder comprising the transmitter of claim 5.

16. A quantum communication system comprising a quantum encoder according to claim 15 and a quantum decoder.

17. 1. A method for combining multiple optical inputs into a single optical output, comprising: providing an optical input of the plurality of optical inputs to one of a plurality of low birefringence waveguides (LBW); providing outputs of the plurality of LBWs to at least one low birefringence coupler (LBC); and coupling inputs of the at least one LBC to the plurality of LBWs and coupling outputs of the at least one LBC to the single optical output. Equipped with The LBW is configured to propagate and output an optical input while maintaining its polarization state without axial or phase rotation, and the LBC is configured to propagate and combine and output an optical input while maintaining its polarization state without axial or phase rotation.

18. 1. A method for generating a polarization-encoded signal, comprising: generating a plurality of light pulses; directing the plurality of optical pulses into a fiber array; the fiber array comprising a plurality of polarization-maintaining optical fibers (PMFs), each PMF having a fast optical axis and a slow optical axis, two selected from the plurality of PMFs, a first PMF and a second PMF, configured such that the fast optical axis of the first PMF is rotated by a non-zero angle with respect to the fast optical axis of the second PMF, and the slow optical axis of the first PMF is rotated by the non-zero angle with respect to the slow optical axis of the second PMF, generating polarization states in which a basis of a first optical output of the first PMF and a basis of a second optical output of the second PMF are rotated by the non-zero angle, and at least the angle is set such that the basis of the first optical output and the basis of the second optical output are not biased relative to each other; Combining the outputs of the PMFs according to the method of claim 17; A method comprising:

19. 1. A method for generating a polarization-encoded signal, comprising: generating a plurality of polarized light pulses; Combining a plurality of polarized light pulses according to the method of claim 17; A method comprising:

20. 1. A method of quantum communication, comprising: generating a polarization-encoded signal according to claim 18, wherein the plurality of polarization pulses output from the plurality of PMFs are randomly selected from a plurality of states in two polarization bases. receiving the polarization-encoded signal at a receiver; decoding the received plurality of polarization-encoded signals based on the two polarization bases; A method comprising:

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