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

Abstract

To provide an optical device such as a compact quantum random number generator (QRNG) and a compact optical transmitter for quantum communication.SOLUTION: An optical device 1 comprises a semiconductor laser 3 having a first output facet 5 and a second output facet 7. The semiconductor laser 3 emits coherent light both from the first output facet 5 and from the second output facet 7. The optical device 1 further comprises a first optical channel 9 coupled to the first output facet 5 of the semiconductor laser 3, a second optical channel 11 coupled to the second output facet 7 of the semiconductor laser 3, and an optical combiner 13 for combining the first optical channel 9 and the second optical channel 11 into a combiner output channel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The embodiments described herein relate to optical devices, such as optical transmitters and optical quantum random number generators. [Background technology]

[0002] Optical devices are widely deployed in modern optical telecommunications 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. It is typically desirable to reduce the footprint and increase the efficiency of such devices. Optical devices of growing interest include optical quantum random number generators (QRNGs) and optical transmitters for quantum communications.

[0003] QRNGs generate random number sequences for use in a variety of applications, including cryptography and numerical simulation. In optical QRNGs, the source of randomness is physical and relies on the unpredictability of measurements; 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 photon's properties, such as its polarization, phase, or energy / time. Quantum communication systems can be used to implement quantum key distribution (QKD), a technique for sharing a cryptographic key between a transmitter, often called "Alice," and a receiver, often called "Bob." A notable feature of this technique is that it provides a test of whether any part of the key can be known to a malicious eavesdropper, often called "Eve."

[0005] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Brief explanation of the drawings]

[0006] [Figure 1]FIG. 1 is a schematic diagram of an optical device according to one embodiment. [Figure 2] FIG. 2 is a schematic diagram of a tunable coupler of the optical device of FIG. [Figure 3] FIG. 3 is a schematic diagram of a further optical device according to an embodiment. [Figure 4A] FIG. 4A is a schematic diagram of an optical device for a QRNG according to a comparative example. [Figure 4B] FIG. 4B is a schematic diagram of a quantum transmitter according to a comparative example. [Figure 5] FIG. 5 is a schematic diagram of an optical transmitter according to an embodiment. [Figure 6] FIG. 6 is a schematic diagram of an optical communication system according to an embodiment. [Figure 7] FIG. 7 is a schematic diagram of an optical device for a QRNG according to one embodiment. [Figure 8] FIG. 8 shows the digitization and processing of the photodetector output to obtain a random number. DETAILED DESCRIPTION OF THE INVENTION

[0007] To avoid unnecessary repetition, like reference numerals will be 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 compact devices such as compact quantum random number generators (QRNGs) and compact optical transmitters for quantum communications. Furthermore, embodiments may be more resource-efficient than conventional designs. As described below with reference to Figures 1-3 and 5-8, the proposed device achieves 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 comprises a semiconductor laser (e.g., a distributed feedback laser, a Fabry-Perot laser diode, or the like) having a first output facet (e.g., a front facet of the laser chip) and a second output facet (e.g., a rear facet of the laser chip). The semiconductor laser is configured to (simultaneously) emit coherent light from both the first output facet and the second output facet. The optical device further comprises 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 the first optical channel or the second optical channel may comprise a delay element, such as a delay line.

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

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

[0013] In one embodiment, either the first optical channel or the second optical channel may comprise an intensity control element (e.g., an electroabsorption 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 the respective optical channel.

[0014] In one embodiment, either the first optical channel or the second optical channel may comprise a phase control element configured to impart a phase shift to coherent light propagating in the respective optical channel.

[0015] In one embodiment, the optical combiner may be an adjustable optical combiner configured to combine 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 may include a Mach-Zehnder interferometer for adjusting the adjustable combining ratio. The adjustable optical combiner may include a first combiner output channel and a second combiner output channel, and may 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 the adjustable combining ratio.

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

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

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

[0019] In one embodiment, either 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 the respective optical channel, the optical combiner may comprise a first combiner output channel and a second combiner output channel (i.e., the combiner may be a 2x2 combiner), and the phase shift imparted 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 into only one of the first combiner output channel and the second combiner output channel.

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

[0021] In one embodiment, an optical random number generator is provided. The optical random number generator comprises an optical device and processing circuitry for generating random numbers from numerical values ​​provided by a photodetector. In this case, the semiconductor laser may be a gain-switched laser and configured to emit a stream of optical pulses such that the phase of each pulse in the stream is randomized. The first optical channel or the second optical channel may comprise a delay element providing a delay time equal to an integer multiple of the temporal separation between pulses in the stream of optical pulses emitted by the gain-switched laser.

[0022] In one embodiment, the delay element provides a delay time equal to the temporal separation between pulses in the stream of optical pulses emitted by the gain-switched laser, thereby allowing consecutive optical pulses to be interfered by the optical combiner and converting the phase difference between consecutive optical pulses into intensity modulation of the output optical signal provided to the photodetector via the combiner output channel.

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

[0024] In an 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] 1 shows an example optical device 1 according to one embodiment. Optical device 1 is provided (and will be described as such hereinafter) as an integrated device, i.e., the components of device 1 are integrated on a common semiconductor substrate (or on multiple appropriately assembled / connected semiconductor substrates). However, in other embodiments, optical device 1 may also be implemented using discrete (optical fiber pigtailed or free space) components.

[0026] The optical device 1 comprises a semiconductor laser 3 and a controller 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 may be operated in continuous wave operation or in pulsed operation (i.e., emitting a stream of light pulses, e.g., nanosecond or picosecond long pulses). When operated in pulsed mode, the semiconductor laser 3 may be driven (by the controller 21) at a fixed repetition rate to output a stream of pulses having well-defined pulse durations and well-defined, regular temporal intervals.

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

[0028] The semiconductor laser 3 comprises a first output facet 5 and a second output facet 7. Typically, the semiconductor laser 3 comprises 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 (hence, the first and second output facets are hereinafter referred to as the front and rear outputs 5, 7). More generally, the front output 5 and rear output 7 correspond to (or are optically coupled to) the front and rear ends of a laser cavity portion of the semiconductor laser 3. As an example, the semiconductor laser 3 may be a Fabry-Perot laser diode comprising a laser cavity portion comprising a gain element between first and second partially reflective surfaces / elements. In this case, the front output 5 may correspond to the first partially reflective surface / element, and the rear output 7 may correspond to the second partially reflective surface / element. As another example, the semiconductor laser 3 may be a distributed feedback (DFB) laser comprising a periodically structured gain element as the laser cavity portion. In this case, the front facet of the gain element may correspond to the front output 5 and the rear facet of the gain element may 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 front output 5 and the rear output 7. For example, in an embodiment in which the semiconductor laser 3 is driven (by the controller 21) at a repetition rate to emit a stream of light pulses, the laser 3 emits a light pulse (simultaneously) from each of the front and rear outputs 5, 7 in each cycle. Thus, in this case, two light pulses are generated in each cycle. FIG. 1 illustrates a pair of corresponding pulses 25, 23 generated in the same cycle and exiting the front and rear outputs 5, 7, respectively. Typically, the corresponding pulses 23, 25 are emitted from different output ports of the semiconductor laser 3, such that the pulses 23, 25 are emitted in different directions. The two output ports are provided at opposite ends of a laser resonator portion, which is typically a linear portion (i.e., the laser resonator portion is typically straight and not curved). Therefore, in these cases, corresponding pulses 25, 23 are emitted in opposite directions (for example forward and backward relative 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 outputs 5, 7 may 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 laser 3 is pulsed to emit a stream of multiple optical pulses, the pulses of pulse pair 23, 25 are inherently correlated due to their (common) generation process, i.e., because they are generated during the same lasing process. For example, the pulses of pulse pair 23, 25 generally have the same optical properties (e.g., the same optical frequency), and the pulses of pulse pair 23, 25 have a fixed phase relationship relative to each other. Conceptually, the pulses of pulse pair 23, 25 can be considered to be two (counter-propagating) portions of the same pulse.

[0031] The semiconductor laser 3 may be configured (e.g., by appropriately designing the laser cavity portion of the laser) so that the forward output 5 and the rear output 7 emit light having substantially the same optical intensity (e.g., the average number of photons in corresponding pulses 25, 23 may be substantially the same). In an embodiment, the ratio of the intensities of the light emitted from the front output 5 and the rear output 7, respectively, may be in the range of 0.5 to 2, and preferably in the range of 0.8 to 1.2.

[0032] The optical device 1 further comprises a first optical channel 9 coupled to the front output 5 of the laser 3 to receive light emitted from the front output 5. Similarly, the optical device 1 further comprises a second optical channel 11 coupled to the rear output 7 of the laser 3 to receive light emitted from the rear output 7. The first and second optical channels 9, 11 may be integrated optical waveguides.

[0033] The optical device 1 further comprises an integrated beam splitter (or optical “2×2 coupler”) 13 comprising two input ports coupled to the first and second optical channels 9, 11, respectively, 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 output channels 17, 19 (i.e., the light emitted from the forward and backward outputs 5, 7 may interfere in the optical coupler 13). The coupler 13 may combine the optical signals in the first and second optical channels 9, 11 based on a combining ratio (e.g., 50:50). As described below, in some embodiments, the coupler 13 has an adjustable combining ratio. Furthermore, in some embodiments, the coupler 13 may be provided as a “2×1 coupler,” i.e., comprising two input ports and one output port.

[0034] The coupler 13 may 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 an input port of the coupler 13. The coupler 13 may comprise a contact region where these waveguides are arranged in close contact. At the contact region, light may evanescently couple from one waveguide to the other in a vibrating manner; 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 may determine the splitting ratio. The length of the coupling region may be such that, for example, 50% of the light is coupled from one waveguide to the other. In other embodiments, coupler 13 may be implemented using a different method for splitting the signal (ie, coupler 13 does not necessarily use evanescent coupling).

[0035] As another example, in one embodiment, coupler 13 may comprise a multimode interference (MMI) coupler. An MMI coupler has a single-mode input / output, with a multimode section coupled between the input and output. Light is injected from the single-mode input waveguide into the multimode waveguide region, and interference between several modes excited in the multimode waveguide region results in the creation of self-images of the input light distribution for a specific propagation distance in the multimode region. The output single-mode waveguide is positioned a suitable distance from the input waveguide to couple the light from the input into the output waveguide with a specific intensity distribution. For example, a 2x2 coupler may be designed so that the length of the multimode waveguide region creates two lobes (e.g., each with 50% of the input light pulse). The output single-mode waveguide is positioned at the location where these lobes are created.

[0036] The optical device further comprises 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. Thus, the phase control element 14 makes it possible to control 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 rear 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 rear output 7 into intensity modulation of the output optical signal 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 rear output 7 interfere in the coupler, thereby outputting substantially all of the light to output channel 17 (and substantially no light to output channel 19). Phase control element 14 may be used to dynamically (i.e., during use) vary the phase shift imparted to adjust the output intensity at output channels 17 and 19 according to a desired target intensity output distribution. Controller 21 may be further configured to control or adjust the phase shift imparted by the phase control element.

[0037] Thus, optical device 1 allows light emitted from both laser outputs 5, 7 to be used for applications (i.e., light emitted from both laser outputs is available to downstream optical components in output channels 17, 19). This contrasts with known designs in which only light emitted from the front facet is used for applications. In those designs, light emitted from the rear facet of the laser chip is not available for applications because it is either simply lost (i.e., absorbed) or detected by a photodetector to track the laser's optical output power. While it is known to provide a highly reflective rear facet to direct the entire laser output toward the front facet, such designs are complex to manufacture (and therefore more expensive). 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 optical device 1 described above, it should be noted that embodiments may comprise additional elements. For example, the optical loss accumulated in the first optical channel 9 may be different 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 forward output 5 and the rear output 7 (e.g., to ensure that pulses 23 and 25 enter coupler 13 with substantially the same intensity). To this end, one or both optical channels 9, 11 may comprise an intensity-control element (not shown in FIG. 1 ) configured to control the intensity of the multiple optical pulses propagating in the respective optical channel. The intensity-control element may thus be operated to compensate for unequal losses. 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.

[0039] Additionally or alternatively, coupler 13 may have an adjustable combining ratio (i.e., the combining ratio can be changed during use by using electro-optic or thermo-optic effects, rather than, as is typical, by manually adjusting the physical components of the coupler). In one embodiment, coupler 13 may comprise a Mach-Zehnder interferometer for adjusting the combining ratio, as illustrated in FIG. 2 . The Mach-Zehnder interferometer may comprise an input 2×2 coupler 131 whose output is optically coupled to an output 2×2 coupler 132. An additional phase control element 133 is provided in one arm of the Mach-Zehnder interferometer to control / adjust the combining ratio of coupler 13. In this case, controller 21 may be further configured to control or adjust the combining ratio of coupler 13.

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

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

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

[0043] In an embodiment, the second optical channel 11 delays an optical pulse (e.g., pulse 23) propagating in the second channel 11 relative to an optical pulse (e.g., pulse 25) propagating in the first optical channel 9 by a delay time Δ delay In other embodiments, the delay element may be provided in the first channel 9 instead of the second channel 11. In general, and as will be explained in more detail below, the delay time Δ delay and the repetition rate of the laser 3 are typically selected jointly depending on the application in which the 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 forward and backward outputs 5, 7 coupled to optical channels 9 and 11, respectively. Also 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 controller 21 for controlling the operation of the semiconductor laser 3 and the phase control element 14. The phase control element is controlled so 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 provided phase shift controls the interference between the light emitted from the forward output 5 and the light emitted from the backward output 7, so that all of the light is coupled to the first output channel 17 and substantially none of the light emitted by the laser 3 exits the combiner 13 via the second output channel 19. In order to (actively) stabilize the phase difference between optical channels 9, 11, the intensity of the light exiting combiner 13 via first output channel 17 can be detected. To this end, optical device 31 further comprises a 1×2 coupler, which couples output channel 17 to i) photodetector 16 and ii) an output port (for use with downstream components or applications). Controller 21 can control the phase shift imparted by phase control element 14 based on the intensity detected at photodetector 16.

[0045] The optical device 31 further comprises a seed laser 20 optically coupled to the second output channel 19 of the combiner 13 (in FIG. 3 , the dashed arrow 32 indicates 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 to the light emitted by the seed laser 20. The operation of the seed laser 20 can be controlled by the controller 21. Advantageously, the optical device 31 prevents propagation of light emitted by the laser 3 into 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 comprises an optical attenuator (e.g., of a fixed value) in the second output channel 19 of the combiner 13 to reduce the effect of light emitted by the seed laser 20 propagating back to the seed laser 20.

[0046] Before describing proposed embodiments related to a quantum random number generator (QRNG) and an optical transmitter, conventional optical QRNG and quantum transmitter designs will now be described with reference to FIGS. 4A and 4B. Both of these conventional designs use a laser that emits multiple optical pulses into the 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 pulsed laser P1 driven at a fixed repetition rate to output a stream of pulses. When the repetition rate is sufficiently low, each pulse from the stream of pulses can have a random phase. These pulses are coupled into a time-delay interferometer P2 via an input coupler P3. 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 relative to the pulses traveling in the short arm. In a QRNG device, delay element P6 is configured so that the introduced delay D is such that each delayed pulse overlaps in time with the previous reference pulse in the reference arm. The delayed and reference pulses interfere in 2x2 coupler P4 (or beam splitter) of time delay interferometer P2, and the interfered pulses are sent to photodetector P5, where the intensity of the interfered pulses is converted into an electrical signal. This signal, which corresponds to the intensity of the interfered pulses, has a random value because the phases of the reference and delayed pulses are random. Random numbers can be generated from the random intensities of the interfered pulses.

[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. A time-bin-encoded quantum signal is a pair of optical pulses with a well-defined time separation and a well-controlled phase relationship. As before, a pulsed laser P1 emits optical pulses into a time-delay interferometer P2. In FIG. 4B, the repetition rate of the laser is lower than the delay time D, so that subsequently emitted pulses do not arrive simultaneously at the beam splitter P4 (and therefore do not interfere). Instead, each laser pulse is converted by the time-delay interferometer P2 into a pulse pair with a fixed time separation. The device further includes a phase control element P7 for controlling the phase difference between the generated pulse pairs (i.e., for "encoding" phase information into the pulse pair). The encoded time-bin quantum signal is then sent to the quantum receiver.

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

[0049] Referring to FIG. 5 , an optical quantum transmitter 41 for quantum communication is described (e.g., 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 a single photon. Each photon carries one bit of information encoded according to the photon's properties, 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 a cryptographic key between a transmitter, often referred to as “Alice,” and a receiver, often referred to as “Bob.” A notable feature of QKD is that it provides a test of whether any part of the key can be known to a malicious eavesdropper, often referred to as “Eve.” For example, a quantum communication system equipped with the above-mentioned multi-beam OPA may 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 may implement a distributed phase reference protocol (such as a coherent one-way protocol or a differential phase shift protocol).

[0050] QKD protocols typically involve two steps. First, Alice sends quantum information to Bob through a quantum channel; 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 polarization. Time-bin encoding means that the quantum transmitter provides pairs of (short) optical pulses with a well-defined temporal separation and a well-controlled phase relationship.

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

[0052] 5 includes a semiconductor laser 3 having forward and backward outputs 5, 7 coupled to optical channels 9 and 11, respectively. Also like optical device 1 of FIG. 1, optical device 41 of FIG. 5 includes a phase control element 14, a 2×2 coupler 13 having two output channels 17, 19, and a controller 21 for controlling the operation of semiconductor laser 3 and phase control element 14. Optical device 41 adjusts, in the second optical channel, an optical pulse (e.g., pulse 23) propagating in second channel 11 relative to an optical pulse (e.g., pulse 25) propagating in first optical channel 9 by a delay time Δ delay The optical transmitter 41 comprises a delay element 15 (e.g., a delay line) configured to delay the optical signal by a given amount. The components of the optical transmitter 41 are integrated on a common semiconductor substrate 37 (or on multiple appropriately assembled / connected semiconductor substrates).

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

[0054] During operation of the optical transmitter 41, the laser 3 simultaneously emits a pulse 25 from the forward output 5 and a pulse 23 from the rear output 7. The pulse 23 emitted from the rear output 7 has a period Δ delay 5, the pulses 23, 25 are delayed by a fixed time difference φ, thus providing a pulse pair with a fixed time interval at the output channel 17 of the coupler 13. 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 to generate a specific time-bin-encoded quantum state. As illustrated in FIG. 5, the resulting time-bin-encoded quantum state 35 provided at the output port 43 comprises a time-spaced pulse pair 23, 25 with a well-defined phase difference φ.

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

[0056] Advantageously, the optical transmitter 41 may be implemented in a more compact manner than the known design of FIG. 4B (for example, the beam splitter P3 is not required in the embodiment of FIG. 5, to give one example).

[0057] 6 shows a quantum communication system in which an optical transmitter 41 is used to emit time-bin encoded quantum states over 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 generally of known design. The quantum receiver comprises an asymmetric Mach-Zehnder interferometer (AMZI) (also called a composite 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 is determined by the time delay Δ delay The output of the AMZI is coupled to photodetectors D1 and D2. The receiver 51 also includes a controller that selects the measurement basis by controlling the phase difference of the AMZI and records the detection events of the photodetectors D1 and D2. As mentioned above, the optical transmitter 41 and the receiver 51 are also in communication via a classical channel (not shown in FIG. 6).

[0058] In one embodiment, the optical transmitter 41 may further comprise an intensity control element (not shown in FIG. 5 ) coupled between the combiner 13 and the output port 43 configured to control the intensity of the created pulse pairs 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 the combiner 13 and the output port 43 configured to control (i.e., further randomize) the global phase of the created pulse pairs 35.

[0059] While the optical transmitter 41 has been described as emitting a time-bin-encoded quantum signal, various modifications will be apparent to those skilled in the art. For example, the optical transmitter 41 could implement a different encoding (i.e., a quantum state encoding technique other than time-bin encoding). As a first example, a variation of the optical transmitter 41 (not shown in the figures) could emit a polarization-encoded quantum state. In this variation, the time delay element may not be present, 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 the multiple optical pulses emitted by the laser 3). As a second example, a variation of the optical transmitter 41 (not shown in the figures) could emit a frequency-encoded quantum state. In this variation, the time delay element may not be present, and the optical transmitter may further comprise a frequency control element (in the first or second optical channel 9, 11) to control the frequency shift between the light propagating in the first and second optical channels 9, 11. As a third example, a variation of the optical transmitter 41 (not shown in the figure) may emit a phase-encoded quantum state. In this variation, the time delay element may not be present, and the optical transmitter may further include a phase control element (e.g., coupled between the combiner 13 and the output port 43 and configured to control the phase of the optical pulses emitted by the laser 3). Furthermore, in this variation, the laser 3 may not be configured to emit a stream of 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 the laser 3 with a stable phase and frequency reference). As a fourth example, a variation of the optical transmitter 41 (not shown in the figure) may emit a path-encoded quantum state. In this variation, the time delay element may not be present, and the optical transmitter may further include a second output port coupled to the second output channel 19 of the combiner 13.In this variant, the phase control element 14 can be used to encode path information, i.e., by controlling the phase difference between corresponding pulses propagating in channels 9, 11, the phase control element 14 controls the interference of the pulses in the combiner 13 and, consequently, the output port through which the interfered light propagates.

[0060] An optical quantum random number generator (QRNG) 61 will now be described with reference to Figures 7 and 8. Generally, the QRNG 61 is configured to generate (and output) random numbers from the random intensities of multiple interfered 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 multiple appropriately assembled / connected semiconductor substrates).

[0061] 7 includes a semiconductor laser 3 having forward and backward outputs 5, 7 coupled to optical channels 9 and 11, respectively. Also similar to optical device 1 of FIG. 1, QRNG 61 includes a 2×2 coupler 13 having two output channels 17, 19, and a delay time Δ delay and a delay element 15 (eg, a delay line) configured to delay the signal by a delay of .times. ...

[0062] The laser 3 is operated to emit a stream of short laser pulses emitted with a controlled time delay between the optical pulses in each pair. More specifically, the laser 3 is gain-switched to output a phase-randomized stream of pulses (i.e., each pulse from the stream of pulses may have a random phase). The laser 3 may be pulsed at a fixed repetition rate, i.e., subsequently emitted pulses are spaced apart by a period of Δ laser The repetition rate of the laser 3 may have a corresponding time interval denoted by the delay time Δ delay is the time interval Δ laser(e.g., the time interval Δ laser is the delay time Δ delay or the delay time Δ delay is the time interval Δ laser , and so on). 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 and non-delayed pulses are random (because these pulses are not generated by the same lasing process), and therefore the intensity of the interfered pulse (e.g., in 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 interfered pulses.

[0063] It should be understood that in other embodiments, the laser 3 of the QRNG 61 may be operated in a continuous wave mode, in which case the delay time Δ delay is chosen to be longer than the coherence time of the laser 3, so that the intensity of the interfered light is random.

[0064] 7 , the QRNG 61 further comprises, in the first optical channel 9, an intensity-control element 53 configured to control the intensity of the plurality of optical pulses propagating in the respective optical channel. The intensity-control element may thus 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 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.

[0065] The QRNG 61 further comprises a controller 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 random numbers from the electrical signal generated by the photodetector 55. To this end, the processing unit 57 can process the numerical values ​​provided by the photodetector 55 using a randomness extractor algorithm. More specifically, as shown in FIG. 8 , the processing unit 57 can 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 that is 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, QRNG 61 may be implemented in a more compact manner than the known design of FIG. 4A (for example, beam splitter P3 is not required in the embodiment of FIG. 7, to give one example).

[0068] While certain 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 may be made in the forms of the devices, methods, and articles of manufacture described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover such forms or modifications as fall within the scope and spirit of the invention.

Claims

[Claim 1] a semiconductor laser having a first output facet and a second output facet, the semiconductor laser 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:

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