Optical device, optical transmitter, and optical quantum random number generator

Abstract

To provide a new and useful optical device for use in quantum communication systems.SOLUTION: An optical device 1 includes a semiconductor laser 3 having a first output facet and a second output facet. The laser is configured to emit coherent light from both first output facet and second output facet. The optical device further includes a first optical channel coupled to the first output facet of the laser, a second optical channel coupled to the second output facet of the laser, and an optical combiner 13 configured to combine the first and second optical channels into a combiner output channel.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The embodiments described in this specification relate to optical devices, such as optical transmitters and optical quantum random number generators.

Background Art

[0002] Optical devices are widely deployed in modern optoelectronic communication systems and computing systems. Optical devices are known to include semiconductor laser diodes as primary light sources that provide optical signals to other components of the device. Typically, it is desirable to reduce the footprint and increase the efficiency of such devices. Of particular interest are optical devices that include optical quantum random number generators (QRNGs) and optical transmitters for quantum communication.

[0003] QRNGs generate random number sequences for use in a variety of applications, including cryptography and numerical simulations. In optical QRNGs, the source of randomness is physical and relies on the unpredictability of measurements, and in particular, the unpredictability relies on the quantum mechanical properties of light.

[0004] 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 encoded according to the properties of the photon, such as its polarization, phase, or energy / time. Quantum communication systems can be used to implement quantum key distribution (QKD), which is a technique for sharing cryptographic keys between a transmitter, often called "Alice," and a receiver, often called "Bob." A notable feature of this technique is that it provides a test as to whether any part of the key can be known to an unauthorized eavesdropper, often called "Eve."

[0005] Next, embodiments of the present invention will be described by way of example only with reference to the accompanying schematic diagrams.

Brief Description of the Drawings

[0006]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5

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Figure 7

Figure 8

[0007] To avoid unnecessary repetition, like reference numerals are used to denote like features in the figures.

[0008] The present disclosure aims to provide new and useful optical devices. In particular, embodiments may enable small devices such as small quantum random number generators (QRNGs) and small optical transmitters for quantum communication. Further, embodiments may be more resource-efficient than conventional designs. As described below with reference to FIGS. 1-3 and FIGS. 5-8, the proposed devices achieve this by using coherent light emitted from two output facets of a chip-based semiconductor laser.

[0009] In one embodiment, an optical device is provided. The optical device includes a semiconductor laser (e.g., a distributed feedback laser, a Fabry-Perot laser diode, or the like) having a first output facet (e.g., the front facet of the laser chip) and a second output facet (e.g., the rear facet of the laser chip). The semiconductor laser is configured to emit coherent light (simultaneously) from both the first output facet and the second output facet. The optical device further includes a first optical channel coupled to the first output facet of the semiconductor laser, a second optical channel coupled to the second output facet of the semiconductor laser, and an optical combiner configured to combine the first optical channel and the second optical channel into a combiner output channel.

[0010] In one embodiment, either one of the first optical channel or the second optical channel may include a delay element such as a delay line.

[0011] In one embodiment, either one of the first optical channel or the second optical channel may include a polarization control element configured to control the optical polarization of the coherent light propagating in each optical channel.

[0012] In one embodiment, either one of the first optical channel or the second optical channel may include a frequency control element configured to control the optical frequency of the coherent light propagating in each optical channel.

[0013] In one embodiment, either one of the first optical channel or the second optical channel may include an intensity control element (e.g., an electro-absorption modulator, a Mach-Zehnder modulator, a semiconductor optical amplifier, or an optical switch, or the like) configured to control the intensity of the coherent light propagating in each optical channel.

[0014] In one embodiment, either one of the first optical channel or the second optical channel may include a phase control element configured to apply a phase shift to the coherent light propagating in each optical channel.

[0015] In one embodiment, the optical combiner can be an adjustable optical combiner configured to combine the coherent light emitted by the first optical channel and the second optical channel into a combiner output channel based on an adjustable combining ratio. In this case, the optical combiner can include a Mach-Zehnder interferometer for adjusting the adjustable combining ratio. The adjustable optical combiner includes a first combiner output channel and a second combiner output channel, and can combine the coherent light in the first optical channel and the second optical channel into the first combiner output channel and the second combiner output channel based on an adjustable combining ratio.

[0016] In one embodiment, the semiconductor laser can be a gain-switching laser and can be configured to emit a stream of a plurality of optical pulses such that the phase of each pulse in the stream of a plurality of optical pulses is randomized.

[0017] In one embodiment, the optical device can further include a photodetector coupled to the combiner output channel.

[0018] In one embodiment, the optical device can further include a seed laser optically coupled to the combiner output channel and configured to simultaneously seed the semiconductor laser via a first output facet and via a second output facet.

[0019] In one embodiment, either one of the first optical channel or the second optical channel includes a phase control element configured to impart a phase shift to the coherent light propagating in the respective optical channel, the optical combiner can include a first combiner output channel and a second combiner output channel (i.e., the combiner can be a 2×2 combiner), and the phase shift imparted by the phase control element is selected such that in the optical combiner, the light from the first optical channel and the second optical channel interferes and is combined into only one of the first combiner output channel and the second combiner output channel.

[0020] In one embodiment, the optical device may further include a semiconductor substrate. The semiconductor laser, the first optical channel and the second optical channel, and the optical combiner may be integrated on the semiconductor substrate to form a photonic integrated circuit.

[0021] In one embodiment, an optical quantum random number generator is provided. The optical quantum random number generator includes an optical device and a processing circuit for generating a random number from a numerical value provided by an optical detector. In this case, the semiconductor laser is a gain-switching laser and may be configured to emit a stream of a plurality of optical pulses such that the phase of each pulse in the stream of a plurality of optical pulses is randomized. The first optical channel or the second optical channel may include a delay element that provides a delay time equal to an integer multiple of the temporal separation between a plurality of pulses in the stream of a plurality of optical pulses emitted by the gain-switching laser.

[0022] In one embodiment, the delay element provides a delay time equal to the temporal separation between a plurality of pulses in the stream of a plurality of optical pulses emitted by the gain-switching laser, whereby a plurality of consecutive optical pulses are interfered by the optical combiner to convert the phase difference between the plurality of consecutive optical pulses into an intensity modulation of an output optical signal provided to the optical detector via the combiner output channel.

[0023] In one embodiment, an optical transmitter for quantum communication is provided. The optical transmitter includes an optical device.

[0024] In one embodiment, the optical transmitter may be configured to perform quantum state encoding to generate a time-bin encoded quantum state, a frequency-encoded quantum state, a polarization-encoded quantum state, a path-encoded quantum state, or a phase-encoded quantum state from an optical pulse emitted from a first output facet of the semiconductor laser and a corresponding optical pulse emitted from a second output facet of the semiconductor laser.

[0025] FIG. 1 shows an exemplary optical device 1 according to an embodiment. The optical device 1 is provided as (and hereinafter described as) an integrated device, i.e., the components of the device 1 are integrated on a common semiconductor substrate (or on a plurality of semiconductor substrates suitably assembled / connected). However, in other embodiments, the optical device 1 may also be implemented using discrete (optically fibre pigtailed or free space) components.

[0026] The optical device 1 includes a semiconductor laser 3 and a control device 21 for controlling the operation of the semiconductor laser 3. The semiconductor laser 3 is operable to emit coherent light. For example, the semiconductor laser 3 can be operated in continuous wave mode or in pulsed mode (i.e., emit a stream of multiple optical pulses, e.g., nanosecond or picosecond long pulses). When operated in pulse mode, the semiconductor laser 3 can be driven (by the control device 21) at a fixed repetition rate to output a stream of multiple pulses having a well-defined pulse duration and a well-defined constant time interval.

[0027] The semiconductor laser 3 can be a gain-switching semiconductor laser that is driven to output a stream of phase-randomized pulses (i.e., each pulse from the stream of multiple pulses can have a random phase). Generally, a gain-switching laser generates light when the laser is switched above the lasing threshold and generates little light when the laser is switched below the lasing threshold. Thus, the control device 21 can control the modulation of the gain of the laser 3 by modulating the electrical drive current applied to the laser 3 in a time-varying manner. For example, the semiconductor laser 3 can be periodically switched above and below the lasing threshold by the application of a time-varying current. In this way, the laser generates a plurality of optical pulses. It should be understood that the control device 21 can be provided with (or connected to) a suitable drive circuit for generating and applying such a time-varying current.

[0028] The semiconductor laser 3 includes a first output facet 5 and a second output facet 7. Typically, the semiconductor laser 3 includes a laser chip having a front facet corresponding to the first output facet 5 and a rear facet corresponding to the second output facet 7 (therefore, hereinafter, the first and second output facets are referred to as the front and rear outputs 5, 7). More generally speaking, the front output 5 and the rear output 7 correspond to (or are optically coupled to) the front and rear ends of the laser resonator portion of the semiconductor laser 3. As an example, the semiconductor laser 3 can be a Fabry-Perot laser diode including a laser resonator portion having a gain element between the first and second partial reflection surfaces / elements. In this case, the front output 5 can correspond to the first partial reflection surface / element, and the rear output 7 can correspond to the second partial reflection surface / element. As another example, the semiconductor laser 3 can be a distributed feedback (DFB) laser including a periodically structured gain element as the laser resonator portion. In this case, the front facet of the gain element can correspond to the front output 5, and the rear facet of the gain element can correspond to the rear output 7.

[0029] During operation, i.e., when the semiconductor laser 3 generates light, the semiconductor laser 3 emits light from both the forward output 5 and the backward output 7. For example, in an embodiment where the semiconductor laser 3 is driven (by the control device 21) at a certain repetition rate to emit a stream of a plurality of optical pulses, the laser 3 emits optical pulses (simultaneously) from each of the forward and backward outputs 5, 7 in each cycle. Therefore, in this case, two optical pulses are generated in each cycle. FIG. 1 illustrates a pair of corresponding pulses 25, 23 that are generated in the same cycle and emerge from the forward and backward outputs 5, 7, respectively. Generally, since the corresponding pulses 23, 25 are emitted from different output ports of the semiconductor laser 3, the pulses 23, 25 are emitted in different directions. The two output ports are provided at opposite ends of a laser resonator portion that is typically a linear portion (i.e., the laser resonator portion is typically straight and not curved). Therefore, in these cases, the corresponding pulses 25, 23 are emitted in opposite directions (e.g., forward and backward with respect to the semiconductor laser 3).

[0030] Broadly speaking, the coherent light emitted from these outputs 5, 7 has the same optical frequency and a well-defined phase relationship (e.g., the coherent light emitted from the outputs 5, 7 can have the same phase or a well-defined relative phase offset). This is because the coherent light emitted from these outputs 5, 7 is generated in the same lasing process. More specifically, when the laser 3 is pulsed to emit a stream of a plurality of optical pulses, the pulses of the pulse pair 23, 25 are essentially correlated because they are generated by a (common) generation process, i.e., they are generated during the same lasing process. For example, the pulses of the pulse pair 23, 25 generally have the same optical characteristics (e.g., the same optical frequency), and the pulses of the pulse pair 23, 25 have a fixed phase relationship with respect to each other. Conceptually, the pulses of the pulse pair 23, 25 can be regarded as two (counter-propagating) portions of the same pulse.

[0031] The semiconductor laser 3 can be configured to emit light in which the forward output 5 and the backward output 7 have substantially the same light intensity (for example, by appropriately designing the laser resonator portion of this laser) (for example, the average photon numbers of the corresponding pulses 25, 23 can be made substantially the same). In an embodiment, the ratio of the intensities of the light emitted from the forward output 5 and the backward output 7 respectively is in the range of 0.5 to 2, and preferably can be in the range of 0.8 to 1.2.

[0032] The optical device 1 further includes a first optical channel 9 coupled to the forward output 5 of the laser 3 so as to receive the light emitted from the forward output 5. Similarly, the optical device 1 further includes a second optical channel 11 coupled to the backward output 7 of the laser 3 so as to receive the light emitted from the backward output 7. The first and second optical channels 9, 11 can be integrated optical waveguides.

[0033] The optical device 1 further includes an integrated beam splitter (or optical “2×2 coupler”) 13 having two input ports respectively coupled to the first and second optical channels 9, 11 and two output ports. The optical coupler 13 is configured to combine the optical signals in the first and second optical channels 9, 11 and output the combined signal to the output channels 17, 19 (that is, the light emitted from the forward and backward outputs 5, 7 can interfere in the optical coupler 13). The coupler 13 can combine the optical signals in the first and second optical channels 9, 11 based on a combining ratio (for example, 50:50). As will be described below, in some embodiments, the coupler 13 has an adjustable combining ratio. Further, in some embodiments, the coupler 13 can be provided as a “2×1 coupler”, that is, having two input ports and one output port.

[0034] The coupler 13 can be implemented in any known and suitable manner. As an example, in one embodiment, the coupler 13 may use evanescent coupling to couple light from one waveguide to one or several other waveguides. In this embodiment, the optical coupler 13 comprises optical waveguides coupled to the first and second optical channels 9, 11 respectively at the input ports of the coupler 13. The coupler 13 may comprise a contact region where these waveguides are disposed in close contact. In the contact region, light can be evanescently coupled so as to oscillate from one waveguide to the other, i.e., the length of the coupling region determines how much light is coupled from one waveguide to the other. In other words, the length of the coupling region can determine the splitting ratio. The length of the coupling region can be, for example, such that 50% of the light is coupled from one waveguide to the other. In other embodiments, the coupler 13 can be implemented using different methods for splitting the signal (i.e., the coupler 13 does not necessarily use evanescent coupling).

[0035] As another example, in one embodiment, the coupler 13 may comprise a multimode interference (MMI) coupler. The MMI coupler has single-mode input / outputs, and a multimode section is coupled between these inputs and outputs. Light is inserted from a single-mode input waveguide into the multimode waveguide region, and interference between several modes excited in the multimode waveguide region results in the generation of self-images of the input light distribution for a certain propagation distance in the multimode region. The output single-mode waveguides are positioned at a suitable distance from the input waveguide so as to couple the light from the input to the output waveguides with a certain intensity distribution. For example, a 2×2 coupler can be designed such that the length of the multimode waveguide region generates two lobes (e.g., each having 50% of each input optical pulse). The output single-mode waveguides are disposed at the positions where these lobes are generated.

[0036] The optical device further includes a phase control element 14 in the optical channel 9. The phase control element 14 is configured to impart a phase shift to the coherent light propagating in the optical channel 9. Accordingly, the phase control element 14 enables control of the phase difference between the light emitted from the forward output 5 (i.e., the light propagating in the first optical channel 9) and the light emitted from the rearward output 7 (i.e., the light propagating in the second optical channel 11). By controlling this phase difference, the interference in the coupler 13 can be controlled (i.e., the interference by the optical coupler 13 converts the phase difference between the light emitted from the forward output 5 and the light emitted from the rearward output 7 into intensity modulation of the output optical signals provided to the combiner output channels 17, 19). For example, the phase control element 14 can impart a phase shift such that the light emitted by the forward output 5 and the light emitted by the rearward output 7 interfere in the coupler, whereby substantially all of the light is output to the output channel 17 (and substantially no light is output to the output channel 19). The phase control element 14 can be used to dynamically (i.e., during use) vary the phase shift imparted in order to adjust the output intensities in the output channels 17 and 19 according to a desired target intensity output distribution. The control device 21 can be further configured to control or adjust the phase shift imparted by the phase control element.

[0037] Accordingly, the optical device 1 enables the light emitted from both laser outputs 5, 7 to be used for application (i.e., the light emitted from both laser outputs is available for the optical components downstream of the output channels 17, 19). This is in contrast to known designs where only the light emitted from the front facet is used for application. In those designs, the light emitted from the rear facet of the laser chip is either simply lost (i.e., absorbed) or detected by a photodetector to track the optical output power of the laser and thus is not available for application. Although it is known to provide a highly reflective rear facet to direct the entire laser output towards the front facet, such a design is complex to manufacture (and thus more expensive). The optical device 1 provides a simple and elegant solution to the problem of "wasted" output power from the rear facet of the laser chip.

[0038] Before describing specific use cases for the above-described optical device 1, it should be noted that the embodiments may include additional elements. For example, the optical losses accumulated in the first optical channel 9 may differ compared to the second optical channel 11. For some applications, it may be desirable to control the relative intensities of the light emitted by the front output 5 and the light emitted by the rear output 7 (e.g., to ensure that pulses 23 and 25 enter the coupler 13 with substantially the same intensity). To this end, one or both of the optical channels 9, 11 may include intensity control elements (not shown in FIG. 1) configured to control the intensity of the plurality of optical pulses propagating in the respective optical channels. Accordingly, the intensity control elements may be operated to compensate for unequal losses. The intensity control elements may be implemented in any known and suitable manner, for example, as an electro-absorption modulator, a Mach-Zehnder modulator, a semiconductor optical amplifier, or an optical switch.

[0039] Additionally or alternatively, the coupler 13 may have an adjustable synthesis ratio (i.e., the synthesis ratio can typically be changed during use not by manually adjusting the physical components of the coupler, but by using an electro-optic effect or a thermo-optic effect). In one embodiment, the coupler 13 may include a Mach-Zehnder interferometer for adjusting the synthesis ratio, as illustrated in FIG. 2. The Mach-Zehnder interferometer may include an input 2×2 coupler 131 whose output is optically coupled to the output 2×2 coupler 132. A further phase control element 133 is provided in one arm of the Mach-Zehnder interferometer to control / regulate the synthesis ratio of the coupler 13. In this case, the control device 21 may be further configured to control or adjust the synthesis ratio of the coupler 13.

[0040] In the embodiment of FIG. 1, the coupler 13 includes two output ports respectively coupled to the first and second optical output channels 17, 19. The output channels 17, 19 can be configured in several ways depending on the specific requirements of the application for which the device is used. As an example, one or both of the output channels 17, 19 can be coupled to respective out-couplers for providing the optical output of the device 1 (e.g., for downstream use). For example, the out-coupler(s) can be diffraction grating couplers for coupling the received signal to free space or an external optical fiber. As another example, one or both of the output channels 17, 19 can be coupled to respective photodetectors.

[0041] In an embodiment, a polarization control element may be provided in one or both of the optical channels 9, 11 (not shown in FIG. 1). The polarization control element(s) can be configured to control the optical polarization of the optical pulses propagating in the respective channels. For example, the polarization control element(s) can include a polarization element and / or a polarization rotation element.

[0042] In an embodiment, the frequency control element can be provided in one or both of the optical channels 9, 11 (not shown in FIG. 1). The frequency control element can be configured to control the optical frequency of the coherent light propagating in each channel. For example, the frequency control element can be an electro-optic modulator (or an acousto-optic modulator) configured to modulate the optical frequency of the coherent light propagating in each channel. The frequency control element can be used to encode information onto the optical signals propagating in each channel.

[0043] In an embodiment, the second optical channel 11 includes a delay element (e.g., a delay line) configured to delay an optical pulse (e.g., pulse 23) propagating in the second channel 11 by a delay time Δ delay with respect to an optical pulse (e.g., pulse 25) propagating in the first optical channel 9. In other embodiments, the delay element can be provided in the first channel 9 instead of the second channel 11. Generally, and as described in more detail below, the delay time Δ delay of the delay element and the repetition rate of laser 3 are typically selected together depending on the application in which device 1 is to be used.

[0044] Referring to FIG. 3, a further optical device 31 is described. Similar to the optical device 1 of FIG. 1, the optical device 31 of FIG. 3 comprises a semiconductor laser 3 having front and rear outputs 5, 7 coupled to optical channels 9 and 11 respectively. Further similar to the optical device 1 of FIG. 1, the optical device 31 of FIG. 3 comprises a phase control element 14, a 2×2 coupler 13 having two output channels 17, 19, and a control device 21 for controlling the operation of the semiconductor laser 3 and the phase control element 14. The phase control element is controlled such that substantially all of the light emitted from the laser 3 is provided to the first output channel 17 of the combiner 13 (in FIG. 3, the dotted arrow 34 indicates the light emitted by the laser 3). In other words, the applied phase shift controls the interference between the light emitted from the front output 5 and the light emitted from the rear output 7, whereby all of the light is coupled to the first output channel 17 and substantially no light emitted by the laser 3 exits the combiner 13 via the second output channel 19. To (actively) stabilize the phase difference between the optical channels 9, 11, the intensity of the light exiting the combiner 13 via the first output channel 17 can be detected. For this purpose, the optical device 31 further comprises a 1×2 coupler, which couples the output channel 17 to i) a photodetector 16 and ii) an output port (for use in downstream components or applications). The control device 21 can control the phase shift applied by the phase control element 14 based on the intensity detected by the photodetector 16.

[0045] The optical device 31 further includes a seed laser 20 optically coupled to the second output channel 19 of the combiner 13 (in FIG. 3, the dashed arrow 32 indicates the light emitted by the seed laser 20). The seed laser 20 is a semiconductor laser configured to coherently seed the laser 3 by simultaneously injecting light into the laser 3 from both the front and rear facets 5, 7. As a result of being coherently seeded by the seed laser 20, the laser 3 emits coherent light having a fixed frequency and phase relationship with respect to the light emitted by the seed laser 20. The operation of the seed laser 20 can be controlled by a control device 21. Advantageously, the optical device 31 prevents the propagation of the light emitted by the laser 3 to the seed laser 20 (which would interfere with the operation of the seed laser 20) without the need for a conventional optical isolator device. In an embodiment, the optical device 31 further includes an optical attenuator (e.g., having a fixed value) in the second output channel 19 of the combiner 13 to reduce the effect of the propagation of the light emitted by the seed laser 20 so as to return to the seed laser 20.

[0046] Before describing the proposed embodiments related to a quantum random number generator (QRNG) and an optical transmitter, the designs of a conventional optical QRNG and a conventional quantum transmitter are next described with reference to FIGS. 4A and 4B. Both of these conventional designs use a laser that emits a plurality of optical pulses into an input port of a time delay interferometer (also called an asymmetric Mach-Zehnder interferometer (AMZI)). More specifically, FIG. 4A shows a portion of a conventional optical QRNG that includes a single pulse laser P1 that is driven at a fixed repetition rate and outputs a stream of multiple pulses. When the repetition rate is sufficiently low, each pulse from the stream of multiple pulses can have a random phase. These pulses are coupled via an input coupler P3 to a time delay interferometer P2. The time delay interferometer P2 includes a short arm and a long arm. The long arm of the time delay interferometer P2 includes a delay element P6, which delays the pulse by a time D with respect to the pulse traveling through the short arm. In the QRNG device, the delay element P6 is configured such that the introduced delay D is such that each delayed pulse temporally overlaps with a previous reference pulse in the reference arm. The delayed pulse and the reference pulse interfere at a 2×2 coupler P4 (or beam splitter) of the time delay interferometer P2, and the interfered pulse is sent to a photodetector P5 where the intensity of the interfered pulse is converted into an electrical signal. This signal corresponding to the intensity of the interfered pulse has a random value because the phases of the reference pulse and the delayed pulse are random. A random number can be generated from the random intensity of the interfered pulse.

[0047] FIG. 4B shows a conventional quantum key distribution (QKD) transmitter device. This device is suitable for emitting time-bin encoded quantum signals to a corresponding quantum receiver. The time-bin encoded quantum signal is a pair of a plurality of optical pulses having a clearly defined temporal separation and a well-controlled phase relationship. Similar to the above, the pulse laser P1 emits a plurality of optical pulses to the time delay interferometer P2. In FIG. 4B, the repetition rate of the laser is lower than the delay time D, whereby the plurality of subsequently emitted pulses do not arrive simultaneously at the beam splitter P4 (therefore, there is no interference). Instead, each laser pulse is converted by the time delay interferometer P2 into a pair of pulses having a fixed temporal separation. The device further comprises a phase control element P7 for controlling the phase difference between the created pulse pairs (i.e., for "encoding" the phase information into the pulse pairs). Then, the time-bin quantum signal encoded in this way is sent to the quantum receiver.

[0048] The "footprint" of the optical devices of FIGS. 4A and 4B can be reduced by using a laser with two output ports, as will be described below with reference to FIGS. 5-8 (reducing the footprint of the optical QRNG and the optical quantum transmitter is advantageous as it supports large-scale deployment). The term "footprint" can generally refer to the size of the device, e.g., the area or volume occupied by the device.

[0049] Referring to FIG. 5, an optical quantum transmitter 41 for quantum communication is described (for example, the optical quantum transmitter 41 can be used to implement a quantum key distribution protocol). In a quantum communication system, information is sent between a transmitter and a receiver by encoded single quanta such as single photons. Each photon carries 1-bit of information encoded according to the characteristics of the photon, such as its polarization, phase, or energy / time. The quantum communication system can be used to implement quantum key distribution (QKD), which is a technique for sharing cryptographic keys between a transmitter, often called "Alice," and a receiver, often called "Bob." A notable aspect of QKD is that it provides a test as to whether any part of the key can be known to an unauthorized eavesdropper, often called "Eve." For example, a quantum communication system with the multi-beam OPA described above can implement the BB84 protocol [C. H. Bennett and G. Brassard, Proc. of IEEE Int. Conf. on Comp. Sys. Sign. Process. (IEEE, New York, 1984), pp. 175-179] or a distributed phase reference protocol (such as a coherent one-way protocol or a differential phase shift protocol).

[0050] The QKD protocol typically involves two steps. First, Alice sends quantum information to Bob through a quantum channel, and then, during a post-processing stage, Alice and Bob communicate with each other through a classical channel to establish a shared key. The quantum channel (sent from the transmitter to the receiver) can be encoded in several ways, for example, by phase (i.e., time-bin encoding) or by polarization. Time-bin encoding means that the quantum transmitter provides pairs of (short) optical pulses with well-defined temporal separation and well-controlled phase relationships.

[0051] The optical transmitter 41 of FIG. 5 is configured to emit a time-bin encoded quantum signal at the output port 43 into a quantum channel (e.g., free space channel or optical fiber). It should be understood that such a quantum signal typically has an average photon number of less than 1 photon per quantum signal.

[0052] Similar to the optical device 1 of FIG. 1, the optical device 41 of FIG. 5 includes a semiconductor laser 3 having front and rear outputs 5 and 7 respectively coupled to optical channels 9 and 11. Further similar to the optical device 1 of FIG. 1, the optical device 41 of FIG. 5 includes a phase control element 14, a 2×2 coupler 13 having two output channels 17 and 19, and a control device 21 for controlling the operation of the semiconductor laser 3 and the phase control element 14. The optical device 41 has a delay element 15 (e.g., a delay line) configured to delay an optical pulse (e.g., pulse 23) propagating in the second channel 11 by a delay time Δ with respect to an optical pulse (e.g., pulse 25) propagating in the first optical channel 9 in the second optical channel. The components of the optical transmitter 41 are integrated on a common semiconductor substrate 37 (or on a plurality of appropriately assembled / connected semiconductor substrates). delay The components of the optical transmitter 41 are integrated on a common semiconductor substrate 37 (or on a plurality of appropriately assembled / connected semiconductor substrates).

[0053] To implement time-bin encoding, the laser 3 is operated to emit a stream of short laser pulses having a controlled time delay between a plurality of optical pulses in each pair. More specifically, the laser 3 is gain-switched to output a stream of a plurality of pulses that are phase-randomized (i.e., each pulse from the stream of a plurality of pulses can have a random phase). The laser 3 can be pulsed at a fixed repetition rate, i.e., a plurality of subsequently emitted pulses can have a corresponding time interval indicated by Δ. The repetition rate of the laser 3 is selected such that the time interval Δ is longer than the delay time Δ. laser The repetition rate of the laser 3 is selected such that the time interval Δ is longer than the delay time Δ. laser The repetition rate of the laser 3 is selected such that the time interval Δ is longer than the delay time Δ. delay The repetition rate of the laser 3 is selected such that the time interval Δ is longer than the delay time Δ.

[0054] During operation of the optical transmitter 41, the laser 3 simultaneously emits the pulse 25 from the forward output 5 and the pulse 23 from the backward output 7. The pulse 23 emitted from the backward output 7 is delayed by Δ delay only, and thus, at the output channel 17 of the coupler 13, a pair of pulses having a fixed time interval is provided. As described above, the pulses 23, 25 are emitted in a common lasing process and have a fixed phase relationship with respect to each other. Therefore, the phase control element 14 can be used to adjust the phase between the pulses 23, 25 in order to generate a quantum state encoded with a specific time bin. As illustrated in FIG. 5, the resulting time-bin encoded quantum state 35 provided at the output port 43 comprises a pair of temporally spaced pulses 23, 25 having a well-defined phase difference φ0.

[0055] In particular, in order to implement the aforementioned BB84 protocol, the quantum transmitter 41 can be configured to randomly select the phase between the pulses in the same pair from a set of phase differences (in each cycle of the repetition rate). In one embodiment, the set of phase differences can comprise 0, π / 2, π, and 3π / 4. As described above, the laser 3 can be operated such that there is a random phase difference between the pulses from different pairs (it is known that it is desirable to randomize the phase between consecutive pulse pairs in order to enhance the security of the protocol).

[0056] Advantageously, the optical transmitter 41 can be implemented in a more compact way than the known design of FIG. 4B (for example, by way of an example, the beam splitter P3 is not required in the embodiment of FIG. 5).

[0057] FIG. 6 shows a quantum communication system in which an optical transmitter 41 is used to emit a time-bin encoded quantum state via a quantum channel 45 (e.g., a free-space link or an optical fiber) to a quantum receiver 51 (e.g., for implementing a QKD protocol). The quantum receiver 51 is of generally known design. The quantum receiver comprises an asymmetric Mach-Zehnder interferometer (AMZI) (also called a decoding Mach-Zehnder interferometer) having a long arm and a short arm with an adjustable phase difference between the two arms. The time delay of the AMZI matches the time delay Δ of the delay element 15 of the optical transmitter 41. The output of the AMZI is coupled to photodetectors D1, D2. The receiver 51 also comprises a control device that selects a measurement basis by controlling the phase difference of the AMZI and records the detection events of the photodetectors D1, D2. As described above, the optical transmitter 41 and the receiver 51 are also in communication via a classical channel (not shown in FIG. 6). delay matches that of

[0058] In one embodiment, the optical transmitter 41 may further comprise an intensity control element (not shown in FIG. 5) coupled between a combiner 13 and an output port 43 and configured to control the intensity of the generated pulse pair 35. The intensity control element may be implemented in any known and suitable manner, for example, as an electro-absorption modulator, a Mach-Zehnder modulator, a semiconductor optical amplifier, or an optical switch. Additionally or alternatively, the optical transmitter 41 may further comprise a further phase control element (not shown in FIG. 5) coupled between a combiner 13 and an output port 43 and configured to control (i.e., further randomize) the global phase of the generated pulse pair 35.

[0059] While the optical transmitter 41 has been described as emitting time-bin encoded quantum signals, various modifications will be apparent to those skilled in the art. For example, the optical transmitter 41 may implement different encodings (i.e., quantum state encoding techniques other than time-bin encoding). As a first example, a variant of the optical transmitter 41 (not shown in the figures) may emit polarization-encoded quantum states. In this variant, the time delay element may be absent, and the optical transmitter may further comprise a polarization control element (e.g., coupled between the combiner 13 and the output port 43 and configured to control the polarization of a plurality of optical pulses emitted by the laser 3). As a second example, a variant of the optical transmitter 41 (not shown in the figures) may emit frequency-encoded quantum states. In this variant, the time delay element may be absent, and the optical transmitter may further comprise a frequency control element for controlling the frequency shift between the light propagating in the first and second optical channels 9, 11 (in the first or second optical channel 9, 11). As a third example, a variant of the optical transmitter 41 (not shown in the figures) may emit phase-encoded quantum states. In this variant, the time delay element may be absent, and the optical transmitter may further comprise a phase control element (e.g., coupled between the combiner 13 and the output port 43 and configured to control the phase of a plurality of optical pulses emitted by the laser 3). Further, in this variant, the laser 3 may not be configured to emit a stream of pulses with phase-randomized pulses. Instead, the laser 3 may be configured to emit a stream of pulses having a well-defined phase relationship (e.g., the laser 3 may be injection-locked to a reference laser that provides a stable phase and frequency reference to the laser 3). As a fourth example, a variant of the optical transmitter 41 (not shown in the figures) may emit path-encoded quantum states. In this variant, the time delay element may be absent, and the optical transmitter may further comprise a second output port coupled to the second output channel 19 of the combiner 13.In this modification, the phase control element 14 can be used to encode path information, i.e., by controlling the phase difference between a plurality of corresponding pulses propagating in channels 9 and 11, the phase control element 14 controls the interference of the plurality of pulses in the combiner 13, and as a result, controls the output port through which the interfering light propagates.

[0060] The optical quantum random number generator (QRNG) 61 will now be described with reference to FIGS. 7 and 8. Generally, the QRNG 61 is configured to generate (and output) random numbers from the random intensities of a plurality of interfering optical pulses. The QRNG is provided as an integrated device, i.e., the components of the QRNG 61 are integrated on a common semiconductor substrate 69 (or on a plurality of appropriately assembled / connected semiconductor substrates).

[0061] Similar to the optical device 1 of FIG. 1, the QRNG 61 of FIG. 7 includes a semiconductor laser 3 having front and rear outputs 5 and 7 respectively coupled to optical channels 9 and 11. Further similar to the optical device 1 of FIG. 1, the QRNG 61 includes a 2×2 coupler 13 having two output channels 17, 19, and in the second optical channel, a delay element 15 (e.g., a delay line) configured to delay an optical pulse (e.g., pulse 25) propagating in the first optical channel 9 by a delay time Δ delay with respect to an optical pulse (e.g., pulse 23) propagating in the second channel 11.

[0062] The laser 3 is operated to emit a stream of short laser pulses having a controlled time delay between a plurality of optical pulses in each pair. More specifically, the laser 3 is gain-switched to output a stream of pulses consisting of a plurality of phase-randomized pulses (i.e., each pulse from the stream of pulses can have a random phase). The laser 3 can be pulsed at a fixed repetition rate, i.e., the plurality of pulses subsequently emitted can have a corresponding time interval indicated by Δ laser The repetition rate of the laser 3 is such that the delay time Δ delay is the time interval Δ laserselected to be an integer multiple of (e.g., time interval Δ laser is equal to delay time Δ delay or the delay time Δ delay is twice the time interval Δ laser etc.). In this way, the pulse emitted from the rear output 7 is delayed by the delay element and interferes with the non-delayed pulse emitted from the front output 5 in the combiner 13. The phases of the delayed pulse and the non-delayed pulse are random (since these pulses are not generated in the same lasing process), and thus the intensity of the interfering pulses (e.g., at the first output channel 17 of the combiner 13) has a random value. A random number can be generated from the random intensity of the interfering pulses.

[0063] It should be understood that in other embodiments, the laser 3 of the QRNG 61 can be operated in continuous wave mode. In this case, the delay time Δ delay is selected to be longer than the coherence time of the laser 3, whereby the intensity of the interfering light is random.

[0064] Referring back to FIG. 7, the QRNG 61 further includes an intensity control element 53 configured to control the intensity of a plurality of optical pulses propagating in each optical channel in the first optical channel 9. Accordingly, the intensity control element can be operated to compensate for unequal losses in the first and second optical channels 9, 11 (e.g., to compensate for additional losses in the second channel 7 caused by the delay element 15). The intensity control element 53 can be implemented in any known and suitable manner, such as an electro-absorption modulator, a Mach-Zehnder modulator, a semiconductor optical amplifier, or an optical switch.

[0065] The QRNG 61 further includes a control device 21 for controlling the operation of the semiconductor laser 3 and the intensity control element 53.

[0066] The QRNG 61 further includes a photodetector 55 optically coupled to the output channel 17 of the combiner 13 and configured to convert the random intensity of the interfered optical pulses into an electrical signal (e.g., the photodetector 55 can be an InP-based on-chip photodiode). The photodetector 55 is coupled to a processing unit 57 configured to extract a random number from the electrical signal generated by the photodetector 55. Thus, the processing unit 57 can process the numerical values provided by the photodetector 55 using a randomness extraction algorithm. More specifically, as shown in FIG. 8, the processing unit 57 may include an analog-to-digital converter (ADC) 71 further coupled to a post-processor 73. The ADC 71 can convert the analog electrical signal received from the photodetector 55 into a digital signal provided to the post-processor 73. The output of the post-processor 73 is a random number sequence having a uniform probability distribution 75.

[0067] Advantageously, the QRNG 61 can be implemented in a more compact manner than the known design of FIG. 4A (e.g., for example, a beam splitter P3 is not required in the embodiment of FIG. 7).

[0068] Although certain embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. In fact, the novel devices and methods described herein can be embodied in various other forms, and furthermore, various omissions, substitutions, and changes in the forms of the devices, methods, and products described herein can 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 being within the scope and spirit of the invention.

Claims

1. A semiconductor laser having a first output facet and a second output facet, the semiconductor laser being configured to emit coherent light from both the first output facet and the second output facet, a first optical channel coupled to the first output facet of the semiconductor laser, a second optical channel coupled to the second output facet of the semiconductor laser, an optical combiner configured to combine the first optical channel and the second optical channel into a combiner output channel, An optical device comprising.

2. The optical device according to claim 1, wherein the first output facet is a front facet of the semiconductor laser, and the second output facet is a rear facet of the semiconductor laser.

3. The optical device according to claim 1, wherein either one of the first optical channel or the second optical channel comprises a delay element.

4. The optical device according to claim 3, wherein the delay element comprises a delay line.

5. The optical device according to claim 1, wherein either one of the first optical channel or the second optical channel comprises a polarization control element configured to control the optical polarization of the coherent light propagating in each optical channel.

6. The optical device according to claim 1, wherein either one of the first optical channel or the second optical channel comprises a frequency control element configured to control the optical frequency of the coherent light propagating in each optical channel.

7. The optical device according to claim 1, wherein either one of the first optical channel or the second optical channel comprises an intensity control element configured to control the intensity of the coherent light propagating in each optical channel.

8. The optical device according to claim 7, wherein the intensity control element comprises at least one of an electro-absorption modulator, a Mach-Zehnder modulator, a semiconductor optical amplifier, or an optical switch.

9. The optical device according to claim 1, wherein either one of the first optical channel or the second optical channel comprises a phase control element configured to impart a phase shift to the coherent light propagating in each optical channel.

10. The optical combiner according to claim 1 is an adjustable optical combiner configured to combine the coherent light emitted by the first optical channel and the second optical channel into the combiner output channel based on an adjustable combining ratio.

11. The adjustable optical combiner according to claim 10 includes a first combiner output channel and a second combiner output channel, and the adjustable optical combiner is configured to combine the coherent light emitted by the first optical channel and the second optical channel into the first combiner output channel and the second combiner output channel based on the adjustable combining ratio.

12. The optical device according to claim 10, wherein the optical combiner includes a Mach-Zehnder interferometer for adjusting the adjustable combining ratio.

13. The optical device according to claim 1, wherein the semiconductor laser is a distributed feedback laser or a Fabry-Perot laser diode.

14. The semiconductor laser according to claim 1 is a gain-switched laser, and is configured to emit the stream composed of a plurality of optical pulses such that the phase of each pulse in the stream composed of the plurality of optical pulses is randomized.

15. The optical device according to claim 1 further includes a photodetector coupled to the combiner output channel.

16. The optical device according to claim 1 further includes a seed laser optically coupled to the combiner output channel for simultaneously seeding the semiconductor laser via the first output facet and via the second output facet.

17. One of the first optical channel or the second optical channel includes a phase control element configured to apply a phase shift to the coherent light propagating in each optical channel. The optical combiner includes a first combiner output channel and a second combiner output channel. The phase shift applied by the phase control element is selected such that, in the optical combiner, light from the first optical channel and the second optical channel interferes and is combined only into one of the first combiner output channel and the second combiner output channel. The optical device according to claim 1.

18. Further comprising a semiconductor substrate, wherein the semiconductor laser, the first optical channel and the second optical channel, and the optical combiner are integrated on the semiconductor substrate to form a photonic integrated circuit. The optical device according to claim 1.

19. The optical device according to claim 15, A processing circuit for generating a plurality of random numbers from a plurality of numerical values given by the photodetector, An optical quantum random number generator comprising.

20. The semiconductor laser is a gain-switched laser and is configured to emit a stream consisting of a plurality of optical pulses such that the phase of each pulse in the stream consisting of the plurality of optical pulses is randomized. The first optical channel or the second optical channel includes a delay element configured to provide a delay time equal to an integer multiple of a time separation between a plurality of pulses in the stream consisting of the plurality of optical pulses emitted by the gain-switched laser. The optical quantum random number generator according to claim 19.

21. The delay element provides a delay time equal to the time separation between a plurality of pulses in the stream consisting of the plurality of optical pulses emitted by the gain-switched laser, whereby a plurality of consecutive optical pulses are interfered by the optical combiner to convert a phase difference between the plurality of consecutive optical pulses into an intensity modulation of an output optical signal given to the photodetector via the combiner output channel. The optical quantum random number generator according to claim 20.

22. An optical transmitter for quantum communication, comprising the optical device according to claim 1.

23. configured to generate a plurality of time-bin encoded quantum states, whereby an optical pulse emitted from the first output facet of the semiconductor laser and a corresponding optical pulse emitted from the second output facet of the semiconductor laser form a time-bin encoded quantum state, the optical transmitter according to claim 22.

24. configured to generate a plurality of frequency-encoded quantum states, whereby an optical pulse emitted from the first output facet of the semiconductor laser and a corresponding optical pulse emitted from the second output facet of the semiconductor laser form a frequency-encoded quantum state, the optical transmitter according to claim 22.

25. The optical transmitter according to claim 22, configured to generate a phase-encoded quantum state from an optical pulse emitted from the first output facet of the semiconductor laser and a corresponding optical pulse emitted from the second output facet of the semiconductor laser.

26. The optical transmitter according to claim 22, configured to generate a polarization-encoded quantum state from an optical pulse emitted from the first output facet of the semiconductor laser and a corresponding optical pulse emitted from the second output facet of the semiconductor laser.

27. The optical transmitter according to claim 22, configured to generate a path-encoded quantum state from an optical pulse emitted from the first output facet of the semiconductor laser and a corresponding optical pulse emitted from the second output facet of the semiconductor laser.

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