Photonic assembly

Abstract

To provide a photonic assembly that implements encoding and decoding of a quantum state using an interferometer.SOLUTION: A photonic assembly comprises a first portion comprising a first substrate and a second portion comprising a second substrate. The photonic assembly comprises an interferometer comprising a plurality of passive photonic elements and a phase modulator. The phase modulator is provided on the second portion. The plurality of passive photonic elements is provided on the first portion.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE The embodiments described herein relate to photonic assemblies. [Background technology]

[0002] Quantum key distribution (QKD) is a technique that results in the sharing of a cryptographic key between two parties, a sender, often called "Alice", and a receiver, often called "Bob". The attraction of this technique is that it provides a test of whether any part of the key may have been learned by an unauthorized eavesdropper, often called "Eve". In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases for encoding bit values. The laws of quantum mechanics dictate that if Eve measures photons without prior knowledge of each encoding base, this will inevitably cause some photons to change state. These photon state changes will introduce errors into the bit values ​​sent between Alice and Bob. Thus, by comparing parts of their common bit strings, Alice and Bob can determine whether Eve has obtained the information.

[0003] QKD methods are implemented in quantum communication systems in which information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. The transmitters and receivers in quantum communication systems utilize interferometers to implement the encoding and decoding of quantum states.

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

[0005] [Figure 1] FIG. 1 is a schematic diagram of an encoding photonic assembly according to one embodiment. [Diagram 2] FIG. 2 is a schematic diagram of an encoding photonic assembly according to one embodiment. [Diagram 3]FIG. 3 is a schematic diagram of a composite photonic assembly according to one embodiment. [Figure 4] FIG. 4 is a schematic diagram of an encoding or decoding photonic assembly according to one embodiment. [Diagram 5] FIG. 5 is a schematic diagram of an encoding or decoding photonic assembly according to one embodiment. [Figure 6] FIG. 6 is a schematic diagram of an encoding photonic assembly according to one embodiment. [Figure 7] FIG. 7 is a schematic diagram of an encoding photonic assembly according to one embodiment. [Figure 8] FIG. 8 is a schematic diagram of a composite photonic assembly according to one embodiment. [Figure 9] FIG. 9 is a schematic diagram of a composite photonic assembly according to one embodiment. [Figure 10] FIG. 10 is a schematic diagram of an intensity modulator photonic assembly according to one embodiment. [Figure 11] FIG. 11 is a schematic diagram of an intensity modulator photonic assembly according to one embodiment. [Figure 12] FIG. 12 is a schematic diagram of a photonic assembly including a Sagnac interferometer according to one embodiment. [Figure 13] FIG. 13 is a schematic diagram of a photonic assembly including a polarizing element according to one embodiment. [Figure 14] FIG. 14 is a schematic diagram of a photonic assembly including a reflector element according to one embodiment. [Figure 15A] FIG. 15A is a schematic diagram of a different phase modulator arrangement according to embodiments described herein. [Figure 15B] FIG. 15B illustrates a Mach-Zehnder interferometer. [Figure 16A] FIG. 16A illustrates a schematic diagram of a photonic assembly having an optical input and output arrangement according to embodiments described herein. [Figure 16B]FIG. 16B illustrates a schematic diagram of a photonic assembly having an optical input and optical output arrangement according to embodiments described herein. [Figure 16C] FIG. 16C illustrates a schematic diagram of a photonic assembly having an optical input and output arrangement according to embodiments described herein. [Figure 16D] FIG. 16D illustrates a schematic diagram of a photonic assembly having an optical input and output arrangement according to embodiments described herein. [Figure 16E] FIG. 16E illustrates a schematic diagram of a photonic assembly having an optical input and optical output arrangement according to embodiments described herein. [Figure 16F] FIG. 16F illustrates a schematic diagram of a photonic assembly having an optical input and optical output arrangement according to embodiments described herein. [Figure 17] FIG. 17 is a schematic diagram of a quantum communication system according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] In one embodiment, a photonic assembly is provided, the photonic assembly comprising: a first portion comprising a first substrate; a second portion comprising a second substrate; an interferometer comprising a plurality of passive photonic elements and a phase modulator; Equipped with the phase modulator is disposed on the second portion; A plurality of passive photonic elements are disposed on the first portion.

[0007] The photonic assembly provides a photonic integrated circuit having an interferometer that minimizes losses while operating active photonic components at fast switching speeds. The photonic assembly is a hybrid photonic assembly and includes a first portion including at least one first photonic component and a second portion including at least one second photonic component. The at least one first photonic component includes passive photonic circuit elements such as waveguides, couplers, and delay lines and is formed on the first portion. The at least one second set of photonic components includes high speed phase modulators and is formed on the second portion.

[0008] The passive photonic elements and the phase modulator are manufactured through different fabrication processes, using different machining and tools. The passive photonic elements and the phase modulator may also be formed using different materials. By providing the passive elements and the high-speed phase modulator on different parts of the photonic assembly, each part can be manufactured separately. This results in a more efficient and faster manufacturing process.

[0009] Furthermore, the fabrication and structure of each part of the photonic assembly can be optimized independently. The fabrication of the first part and the passive photonic elements thereon can be optimized to minimize propagation losses through the passive photonic circuit elements. Separately, the fabrication of the second part and the phase modulator thereon can be optimized to enable the phase modulator to operate with a fast response time and to be able to switch modulation configurations at faster speeds (e.g., GHz or faster).

[0010] Thus, an interferometer on a photonic assembly assembled from independently optimized portions will operate with lower propagation losses compared to an interferometer having all interferometer elements on a single portion optimized for faster modulator switching speeds. The assembled photonic assembly can also operate at faster switching speeds than an interferometer formed having all interferometer elements on a single portion optimized for low propagation losses.

[0011] In one embodiment, the phase modulator is an electro-optic phase modulator.

[0012] In one embodiment, the plurality of passive photonic elements comprises one or more waveguides.

[0013] In one embodiment, the interferometer further comprises one or more further waveguides provided on the second portion, at least one of the one or more waveguides coupled to at least one of the one or more further waveguides.

[0014] In one embodiment, the interferometer comprises a first optical path and a second optical path; The plurality of passive photonic elements includes a first coupler, a second coupler, and a delay line, the first coupler coupling an input of the interferometer to an input of the first optical path and an input of the second optical path; a second coupler that combines an output of the first optical path with an output of the second optical path; The delay line may be provided in the first optical path or in the second optical path.

[0015] In one embodiment, a phase modulator is provided at the output of the second coupler.

[0016] In an alternative embodiment, the phase modulator is provided in the first optical path. Optionally, the interferometer further comprises a further phase modulator provided on the second portion, the further phase modulator being provided in the second optical path.

[0017] In one embodiment, the photonic assembly further comprises a light source; The output of the light source is coupled to the input of the interferometer; The light source is provided on the second portion, or the light source is provided on the third portion.

[0018] In an alternative embodiment, a phase modulator is provided at the input of the first coupler.

[0019] In one embodiment, the photonic assembly further comprises at least one photodetector; at least one photodetector is coupled to the output of the interferometer; At least one photodetector is provided on the second portion or at least one photodetector is provided on the third portion.

[0020] In one embodiment, the interferometer comprises a first optical path and a second optical path; the plurality of passive photonic elements comprises a first coupler and a second coupler; a first coupler couples an input of the interferometer to an input of the first optical path and to an input of the second optical path; a second coupler that combines an output of the first optical path with an output of the second optical path; The phase modulator is provided in the first optical path.

[0021] In one embodiment, the second coupler is a polarization splitter and combiner, and the interferometer further comprises a polarization rotator, the polarization rotator being provided in the first optical path or the second optical path.

[0022] In one embodiment, the plurality of passive photonic elements comprises a first optical reflector in the first optical path and a second optical reflector in the second optical path.

[0023] In one embodiment, the photonic assembly further comprises at least one further phase modulator provided on the second portion and at an output of the first phase modulator, wherein the at least one further phase modulator comprises an electro-optic phase modulator or a Mach-Zehnder modulator.

[0024] In one embodiment, the interferometer comprises an optical path, the plurality of passive photonic elements comprises a coupler, the coupler coupling an input of the interferometer to a first end of the optical path and to a second end of the optical path, and the phase modulator is provided in the optical path.

[0025] In one embodiment, the first substrate is a substrate of a passive material and the second substrate is a substrate of an active material. Optionally, the passive material is optically passive for light having a wavelength between 1300 nm and 1700 nm, and the active material is optically active for light having a wavelength between 1300 nm and 1700 nm.

[0026] In one embodiment, the first substrate is made of a silicon or nitrogen comprising material.

[0027] In one embodiment, the second substrate is made of a material comprising a III-V or II-VI semiconductor material.

[0028] In one embodiment, there is provided a quantum state encoder comprising a photonic assembly according to the above embodiments.

[0029] In one embodiment, there is provided a quantum state decoder comprising the photonic assembly described above.

[0030] In one embodiment, there is provided a quantum communication system comprising a quantum state encoder as described above and a quantum state decoder as described above.

[0031] Various embodiments of the photonic assembly are illustrated in each of FIGS. 1-14. In each embodiment, the photonic assembly 100 includes a first portion 120, the first portion 120 including a first substrate. The photonic assembly also includes a second portion 130, the second portion 130 including a second substrate. An interferometer is provided on the photonic assembly 100. The interferometer includes a plurality of passive photonic elements and a phase modulator. The passive photonic elements are components of the photonic assembly that are transparent or nearly transparent to light propagating therethrough. The number and arrangement of the passive photonic elements vary depending on the type of interferometer, but may include one or more waveguides, one or more couplers, and a delay line. The phase modulator is provided on the second portion 130, and the passive photonic elements are provided on the first portion 120.

[0032] Each of the first and second portions described herein comprises a material substrate on which photonic circuit elements are provided. Each portion may therefore be referred to as a "photonic integrated circuit (PIC)" or a "photonic chip" or an "optical chip." Thus, the first portion 120 may be considered a first photonic chip or a first PIC, and the second portion may be considered a second photonic chip or a second PIC. Thus, the photonic assemblies described herein may be considered as assemblies of photonic chips or assemblies of PICs. Thus, each photonic assembly described herein may be considered as a photonic chip assembly, or a photonic integrated circuit assembly, or an optical assembly.

[0033] In one embodiment, each part of the photonic chip assembly can be optimized by using different materials for the substrate of each part. Using different substrates for each part allows separate optimization of each part according to the photonic elements present on each part. For example, the second part can be provided with a substrate of a material (such as an active material described below) that interacts with propagating photons, thereby causing absorption, generation, or modulation of photons. The second part is therefore optimized for providing phase control elements. And the first part can be provided with a substrate of a material that has a higher level of optical transparency than the second material. For example, the first material can have a lower photon absorption coefficient than the second material. The first part is therefore optimized for providing passive photonic elements.

[0034] In one embodiment, the first substrate is a substrate of passive material and the second substrate is a substrate of active material, so the first portion is a passive portion and the second portion is an active portion.

[0035] An active material may also be referred to as an active photonic material, which is a material that exhibits an optoelectronic effect in which a photon propagating through the material interacts with an electron in the material to generate a photon, modulate a photon, or absorb a photon. For example, a propagating photon can interact with an electron in the valence band or conduction band, causing a transition across the band gap to generate, absorb, or modulate a photon.

[0036] Passive materials may also be referred to as passive photonic materials. Passive photonic materials are materials with low photon absorption that do not exhibit optoelectronic effects or that exhibit reduced electro-optic effects. Photons propagating in materials that do not exhibit electro-optic effects do not interact with electrons in the material. As a result, these photons do not create excited electronic states that can be used to generate photons, do not modulate the material refractive index, or do not absorb photons. Thus, passive materials transmit or nearly transmit photons, so that the propagating light maintains its optical properties, such as phase, amplitude, and polarization. Thus, passive materials are generally materials with low propagation losses.

[0037] The level of photon absorption, and therefore the passive or active properties of a material, may depend on the wavelength of light propagating within the material. For example, a material may have a band gap between the conduction band and the valence band, which requires that photons have energies close to the value of the band gap in order to interact with electrons in the material. Thus, a material may act as an active material for photons above the band gap energy and as a passive material for photons well below the band gap energy.

[0038] In one embodiment, the first material used for the first substrate is passive over a first wavelength range, and the second material used for the second substrate is active over the first wavelength range. The photonic assembly can then be configured to have an operating range corresponding to the first wavelength range. For example, when the photonic assembly is configured as an encoder, the photonic assembly is configured to generate photons in the first wavelength range, or when the photonic assembly is configured as a decoder, the photonic assembly is configured to receive photons in the first wavelength range. For example, the first wavelength range corresponds to a wavelength range used for telecommunications. In this example, the wavelength range is between 1300 nm and 1700 nm. Thus, the first material is passive for light having a wavelength between 1300 nm and 1700 nm, and the second material is active for light having a wavelength between 1300 nm and 1700 nm. In one example, the photons have a wavelength of 1550 nm.

[0039] Different elements provide different bandgaps. The gap can also be tuned by varying the concentration of each element in the substrate. Thus, elements and element concentrations can be selected for use in the first or second substrate depending on whether passive or active properties are desired for a particular wavelength. For example, gallium arsenide (GaAs) has a gap of about 1.5 eV corresponding to a wavelength of 820 nm, while silicon has a bandgap of about 1.1 eV (1100 nm), indium gallium arsenide (InGaAs) has a bandgap of about 0.75 eV (1500 nm), and gallium nitride (GaN) has a bandgap of about 3.4 eV (360 nm).

[0040] In one embodiment, the first material comprises silicon or nitrogen. Silicon is an indirect transition material, and light at wavelengths used in telecommunications (between about 1300 nm and about 1700 nm, in one example, the wavelength is about 1550 nm) does not interact with electrons in the valence band or the conduction band in a single photon process. Although there may still be some small losses due to two-photon absorption effects, light at telecommunication wavelengths will propagate through the silicon material with reduced material interaction with the material (e.g., reduced absorption). Silicon nitride (Si 3 N 4 Other silicon-based materials, including SiO 2 , may also be used as passive photonic materials at telecommunication wavelengths. Silicon nitride has a wider band gap than silicon, so the two-photon effect is reduced and the loss of propagating photons is lower. Alternatively, other wide band gap materials, such as diamond, may be used as passive photonic materials. As long as the band gap is larger than the energy of the photon propagating in the material, the single photon process will not excite electrons between the bands and will therefore propagate in the material with low loss. Thus, materials with very wide band gaps can be selected to be passive materials for the wide range of photon wavelengths that the material supports.

[0041] Due to the passive optical properties of passive materials, in the embodiments described herein, the passive photonic elements of the interferometer may be formed on a substrate formed from a passive material, and thus, the propagation losses of light passing through the passive photonic elements may be minimized in these embodiments.

[0042] Active optical elements, such as phase modulators, may be formed from optoelectronically active materials. Active materials may exhibit an electro-optic effect, in which an electric field applied to the material changes the refractive index of the material. The change in refractive index may be caused by, but is not limited to, - Change in refractive index due to the linear photoelectric effect (also known as the Pockels effect), where the refractive index depends linearly on the applied electric field, - The change in refractive index due to the quantum confined Stark effect (QCSE), in which the band gap of a material changes when an electric field is applied. This can result from a variety of effects, including

[0043] Active materials can provide very fast response times upon application of an electric field, which means that phase modulators formed from active materials can switch between modulation states at high speeds, allowing fast optical switching on the picosecond time scale. Such phase modulators can be called high-speed phase modulators. Switching between modulation states can be achieved at GHz frequencies and above (i.e., frequencies used in QKD systems). Traveling-wave modulators use electrodes along an active waveguide designed to propagate radio waves at the same speed as light waves, thus enhancing the electro-optic effect along the propagating waveguide. This allows fast modulation speeds to be reached.

[0044] In one embodiment, the active material is a III-V or II-VI semiconductor material. Exemplary III-V semiconductor materials include indium-based materials (including indium phosphide or InP) and gallium-based materials (including gallium arsenide or GaAs).

[0045] An exemplary embodiment of electro-optic modulation based on the Pockels effect is LiNbO 3 Based on waveguides.

[0046] It is very difficult to provide a platform with layered active and passive photonic materials. Thus, fabricating a phase modulator made of active materials on a passive substrate requires complex, expensive and time-consuming fabrication techniques. Thus, integrated photonic circuits utilizing passive photonic substrates such as silicon are not very efficient for phase control elements made of active photonic materials. Furthermore, phase modulation elements that can be formed from passive materials on passive photonic platforms offer low switching speeds (for example, thermo-optical modulators offer modulation state switching speeds on the order of kHz). Such integrated photonic circuits may not be suitable for use in systems that require fast reconfiguration of the phase modulator, such as QKD systems.

[0047] Integrated photonic circuits utilizing active substrates may be provided with active photonic elements, such as phase modulators, to provide high switching speeds. However, when using active substrates, other photonic elements required for PICs (such as waveguides, couplers, and delay lines) are also formed from active materials. Due to the optoelectronic effects (e.g., optical gain) present in active materials, such photonic elements suffer from relatively higher losses in photon intensity than photonic elements formed from passive materials. Due to this high loss, PICs formed only from active materials are not suitable for use in, for example, QKD receivers.

[0048] Typical losses experienced on an active platform may be on the order of a few dB per centimeter, whereas typical losses experienced on a passive platform may be on the order of a few dB per meter (i.e., two orders of magnitude less). By minimizing the number of photonic elements on the active platform and maximizing the number of photonic elements on the passive platform, the overall losses to the assembled photonic circuit are minimized. Thus, the interferometer on the assembled photonic assembly operates with lower propagation losses compared to an interferometer having all interferometer elements on the portion comprising the active material. The assembled photonic assembly may also operate at faster switching speeds than an interferometer formed having all interferometer elements on the portion comprising the passive material.

[0049] In further embodiments, other forms of phase modulators may be provided on the second portion. For example, the phase modulator may be a microelectromechanical system (MEMS) modulator, which is constructed from photonic circuit elements with moving parts that can be actuated in response to an applied voltage signal at a fast switching speed. These modulators may be provided on a variety of materials, including passive materials such as silicon-based substrates. However, MEMS require more complicated fabrication processes than passive photonic components such as waveguides. It is easier, faster, more efficient to fabricate the MEMS modulator on one chip and other circuit components on separate chips, and then bond the chips together, with less chance of defects in either the MEMS modulator or the other circuit components.

[0050] Other types of phase modulators include carrier injection modulators (CIMs) or carrier depletion modulators (CDMs). In these modulators, the phase is induced by increasing or decreasing, respectively, the carriers occupying the region that overlaps with the optical mode. CIMs and CDMs are formed by doping a semiconductor region to create a PN junction in the core of the waveguide, and the number of carriers can be modulated with an external signal. Different materials may be used to realize CIMs and CDMs, including using silicon as the substrate. However, CIMs and CDMs require more complicated fabrication processes than the fabrication of other photonic elements such as waveguides. Therefore, it is easier, faster, more efficient, and less likely to have defects in either the CIM / CDM modulator(s) or the other circuit components on separate chips, and then to bond the chips together.

[0051] Other examples of modulators may include plasmonic modulators, in which very strong modulation of the propagation velocity of strongly confined photoelectron quasiparticles (known as surface plasmon polaritons) at the interface between a material and a metal is achieved by modulating the refractive index of a dielectric material using an electric field.

[0052] Other examples of modulators may include piezoelectric modulators, in which an electrical signal changes the mechanical stress, which in turn changes the refractive index of the material through the stress-optic effect.

[0053] In the embodiments described herein, a photonic assembly is provided that includes a first portion that includes a first substrate and a second portion that includes a second substrate. The first portion and the second portion are heterogeneously integrated to form a single compact assembly. A phase modulator is provided as part of the second portion. The second portion can be constructed to be optimized for phase modulation. For example, the second portion can be constructed from active materials and therefore optimized to switch at a fast speed, and the portion can also be constructed to include quantum wells. A passive photonic element is provided as part of the first portion. The first portion can be constructed to be optimized for low-loss light propagation (e.g., constructed from passive materials and / or constructed to include waveguides with smooth sidewalls). The same optimization provided for the phase modulator portion may not be required for the passive photonic element (e.g., waveguides and couplers do not require fast switching speeds). Thus, an interferometer provided on the photonic assembly formed from the first portion and the second portion can provide fast switching of phase modulation while minimizing propagation losses through the assembly.

[0054] Photonic assemblies can be applied to QKD applications. In QKD schemes, a sender ("Alice") encodes photons in a selected basis quantum state with different states, where the selected basis and state are changed randomly. Quantum information is typically encoded into pulses with pulse generation at GHz frequencies. Thus, to randomly shift the basis between different pulses, a phase modulator needs to switch between different phase values ​​at a frequency of at least the QKD pulse frequency (GHz frequency, e.g., 1 GHz or 2 GHz). QKD schemes may also employ intensity modulation of the pulses transmitted through the quantum channel, for example, in the generation of decoy states. Intensity modulation can be implemented by an interferometer that utilizes a phase modulator. To vary the intensity between each pulse, the phase modulator needs to shift between different phase values ​​at a frequency of at least the QKD pulse frequency.

[0055] The photonic assemblies described herein provide a platform that can provide fast switching speeds while minimizing propagation losses: the phase modulator is provided on a platform that allows efficient switching at the speeds required for QKD encoding and intensity modulation, and the passive components are provided on a platform that minimizes propagation losses.

[0056] The first and second parts may be connected or attached to each other to form a heterogeneous integrated assembly. For example, the first and second parts may be attached by edge bonding (also called butt bonding), where an edge of the first substrate is attached to an edge of the second substrate by means of a bonding material such as an adhesive or by metallization. Alternatively, the first and second substrates may be attached vertically using a bonding material or by wafer bonding (materials are thoroughly cleaned and then attached by van der Waals forces).

[0057] In the embodiments described herein, the interferometer is disposed across both the first and second portions, with some photonic components of the interferometer disposed on the first portion and some photonic components disposed on the second portion, and the photonic elements on the first portion are optically coupled to the photonic elements on the second portion.

[0058] In one embodiment, the plurality of passive photonic components comprises one or more waveguides. The one or more waveguides are formed on a substrate of the first portion (and may be formed from a passive material). The waveguides are provided as part of the interferometer to couple other elements of the interferometer together to form a photonic circuit. For example, in an embodiment having a first coupler and a second coupler, the first waveguide may couple a first output of the first coupler to a first input of the second coupler, and the second waveguide may couple a second output of the first coupler to a second input of the second coupler. The waveguides may also couple any input or output port of the first portion to a photonic element of the interferometer present on the second portion.

[0059] In another embodiment, the interferometer further comprises one or more additional waveguides. The one or more additional waveguides are provided on the second portion. The one or more additional waveguides are provided as part of the interferometer to couple the phase modulator to other components of the interferometer. The one or more additional waveguides may also couple any input or output port of the second portion to the photonic element of the interferometer on the first portion. In this embodiment, at least one of the one or more waveguides is coupled to at least one of the one or more additional waveguides. Thus, the photonic circuit element provided on the first substrate is coupled to the photonic circuit element on the second substrate to form an interferometer across the portions forming the photonic assembly.

[0060] Coupling a waveguide on a first portion to a waveguide on a second portion can be achieved by aligning the waveguides end-to-end such that the optical modes in the waveguides match. For example, a first waveguide on the first portion can have a gradually tapering cross section to geometrically confine the optical mode to a predefined diameter. A second waveguide on the corresponding second portion can also be gradually tapered to geometrically confine the optical mode to the same predefined diameter. Thus, the optical modes in each waveguide have the same diameter and can therefore be efficiently coupled, allowing light to propagate across the portions with minimal photon loss. If different materials are used on the different portions, the gradual tapering of the first waveguide can be different from the gradual tapering of the second waveguide to achieve the same optical mode diameter.

[0061] Alternatively, the first waveguide of the first section and the second waveguide of the second section are evanescently coupled. In this embodiment, each of the first and second waveguides has an evanescent mode region. For example, in the evanescent mode region, the diameter of each of the first and second waveguides gradually narrows to a thin diameter. Thus, the optical mode propagating in the first waveguide in the thin diameter region is not confined to the waveguide, but extends outward from the waveguide in the evanescent field. The evanescent field couples to the second waveguide, resulting in a mode in the second waveguide, which becomes increasingly confined inside the second waveguide as the mode propagates away from the thin evanescent region into the thicker waveguide region where the mode is completely confined. In this embodiment, the first and second portions are vertically arranged such that the first and second waveguides are vertically aligned in the evanescent region of the waveguides.

[0062] In a further alternative, the first waveguide is coupled to a first 45 degree reflector and the second waveguide is coupled to a second 45 degree reflector opposite the first 45 degree reflector.

[0063] Photon reflections at the interface between the first and second waveguides can be minimized by applying a refractive index-matching adhesive to the interface and / or by using an anti-reflective coating. The anti-reflective coating can be formed to have a thickness of a quarter wavelength of the optical mode such that the mode reflected at the interface between the first waveguide and the anti-reflective coating destructively interferes with the mode reflected at the interface between the second waveguide and the anti-reflective coating. Thus, the reflections will minimally interfere with the photon modes in the first or second waveguide.

[0064] The interferometers described herein may be used as part of QKD applications, including generating encoded states, decoding received encoded states, and performing intensity modulation to generate decoy pulses. The embodiments of Figures 1-9 and 12-14 are examples of photonic assemblies configured to perform quantum state encoding and / or quantum state decoding. The embodiments of Figures 10, 11, and 12 are examples of photonic assemblies that provide intensity modulation.

[0065] In one embodiment, a photonic assembly 100 is shown in FIG. 1. The photonic assembly 100 comprises a delay line interferometer. The delay line interferometer comprises a plurality of passive elements comprising a first coupler 102, a second coupler 104, and a delay line 108. The interferometer further comprises a first optical path 106A, a second optical path 106B, a first input 112, a second input 114, a first output 116, and a second output 118. The first coupler 102 couples the first input 112 and the second input 114 of the interferometer to the input of the first optical path 106A and the input of the second optical path 106B. The second coupler 104 couples the output of the first optical path 106A and the output of the second optical path 106B. The interferometer of the photonic assembly 100 further comprises a phase modulator 110. The passive photonic element is provided on the first portion 120 and the phase modulator 110 is provided on the second portion 130 .

[0066] In the illustrated embodiment of Figure 1, the first coupler 102 and the second coupler 104 are 2x2 couplers. Thus, an interferometer having two inputs and two outputs is shown. However, it will be understood that the first coupler 102 and / or the second coupler 104 may be 1x2 couplers such that the interferometer is provided with a single input and / or a single output.

[0067] The delay line 108 is provided in the first optical path 106A or in the second optical path 106B. As a result, the first optical path 106A and the second optical path 106B have different overall path lengths. A pulse of light traversing an optical path with a delay line will arrive at the second coupler 104 at a later time than light traversing an optical path without a delay line. When a single optical pulse is provided to the interferometer at the input 112, the interferometer will output two time-separated pulses, where the time separation is determined by the overall difference in path length between the first optical path 106A and the second optical path 106B. The passive photonic element may comprise at least one waveguide, where the at least one waveguide couples the interferometer elements together. For example, the first optical path 106A and the second optical path 106B may be formed by a waveguide provided on the first portion.

[0068] In the embodiment of Fig. 1, the phase modulator 110 is provided at the output of the second coupler 104. The output of the second coupler 104 may be coupled to the input of the phase modulator using the coupled waveguides described above. The output of the phase modulator 110 may be coupled to the interferometer output 116 or to further photonic elements of the photonic assembly 100 using the coupled waveguides described above.

[0069] Thus, the phase modulator 110 receives the time-separated pulses output by the delay line interferometer of FIG. 1. The phase modulator 110 is configured to have its modulation state switched by an external signal. For example, the phase modulator 110 is a multi-level electro-optic phase modulator that is responsive to an applied voltage. Light propagating through the phase modulator 110 will be phase shifted by an amount that depends on the applied voltage. For example, the phase modulator may shift the phase of the received light by an amount of 0, π / 2, π, 3π / 2, or −π / 2, or any phase.

[0070] The phase modulator 110 is configured to switch modulation states in time to selectively modulate either one of the time-separated pulses. Thus, the time-separated pulse pair is phase modulated such that a first pulse of the time-separated pulse pair is phase shifted by a different amount than a second pulse of the time-separated pulse pair. Thus, quantum information can be encoded by the time-separated pulses output by the interferometer. For example, the early pulse may represent a |0> state and the late pulse may represent a |1> state. By varying the phase shift φ provided by the phase modulator, the relative phase shift between the output |0> and |1> states can be varied by |ψ enc >=|0>+e iφ The time-bin coding may be shifted to encode any selected final state |1>. Encoding the states in this manner may be called "time-bin coding".

[0071] Thus, the photonic assembly 100 may form part of a quantum encoder in a quantum communication system, where the phase modulation states are selected based on a desired measurement basis.

[0072] In the illustrated embodiment of FIG. 1, the photonic assembly 100 comprises an optical input 113 coupled to the first input 112 of the interferometer. The optical input 113 is configured to receive a signal from outside the photonic assembly 100. For example, the optical input 113 may receive light pulses from a light source such as a laser. The photonic assembly 100 further comprises an optical output 117 for providing an output signal to the outside of the photonic assembly 100. For example, the output signal is an encoded state output from the interferometer, and the optical output 117 is coupled to an optical fiber forming a quantum communication channel. The second interferometer output 118 is not coupled to any component, and light output from this interferometer output is lost. These illustrated connections are merely examples. For example, the photonic assembly 100 may be provided with a second optical input coupled to the second input 114 and a second optical output coupled to the second output 118. The optical input and output may not be directly coupled to the input and output of the interferometer, but may instead be coupled via other photonic elements on the photonic assembly 100. No optical input may be provided to the photonic assembly at all, and the input to the interferometer may be coupled to another component in the second portion (discussed below in connection with Figures 6 and 7).

[0073] In another embodiment, the photonic assembly 200 shown in FIG. 2 is illustrated with an additional phase modulator. The photonic assembly 200 comprises a delay line interferometer. The delay line interferometer comprises a plurality of passive elements comprising a first coupler 202, a second coupler 204, and a delay line 208. The interferometer further comprises a first optical path 206A and a second optical path 206B. The delay line interferometer further comprises a first input 212, a second input 214, a first output 216, and a second output 218. The first coupler 202 couples the first input 212 and the second input 214 of the interferometer to the input of the first optical path 206A and the input of the second optical path 206B. The second coupler 204 couples the output of the first optical path 206A and the output of the second optical path 206B. The interferometer of the photonic assembly 200 further comprises a first phase modulator 210A and a second phase modulator 210B. The passive photonic element is provided on the first portion 220, and the first phase modulator 210A and the second phase modulator 210B are provided on the second portion 230. The delay line 208 is provided in the first optical path 206A or in the second optical path 206B.

[0074] 2, the first coupler 202 is a 2x2 coupler. Thus, an interferometer having two inputs is shown. However, it will be understood that the first coupler 202 may be a 1x2 coupler such that a single input is provided to the interferometer.

[0075] The passive photonic elements may include at least one waveguide, where the at least one waveguide couples the interferometer elements together. For example, the first optical path 206A and the second optical path 206B may be formed by waveguides disposed on the first portion.

[0076] In the embodiment of FIG. 2, the first phase modulator 210A is provided at a first output of the second coupler 204, and the second phase modulator 210B is provided at a second output of the second coupler 204. The output of the second coupler 204 may be coupled to the input of the phase modulator using the above-mentioned coupled waveguides. Using the above-mentioned coupled waveguides, the first phase modulator 210A is coupled to the first interferometer output 216, and the second phase modulator 210B is coupled to the second interferometer output 218. Alternatively, the outputs of the phase modulators 210A and 210B may be coupled to further photonic elements provided on the photonic assembly 200 using the above-mentioned coupled waveguides.

[0077] In the illustrated embodiment of FIG. 2, the photonic assembly 200 comprises an optical input 213 coupled to the first input 212 of the interferometer. The optical input 213 is configured to receive a signal from outside the photonic assembly 200. For example, the optical input 213 may receive optical pulses from a light source such as a laser. The photonic assembly 200 further comprises a first optical output 217 and a second optical output 219 for providing an output signal to the outside of the photonic assembly 200. For example, the output signal is an encoded state output from the interferometer, and each of the optical outputs 217 and 219 is coupled to an optical fiber forming a quantum communication channel. These illustrated connections are merely examples. The optical input and the optical output may not be directly coupled to the input and output of the interferometer, but may instead be coupled via other photonic elements on the photonic assembly 200. The photonic assembly 200 may not be provided with an optical input at all, and the input to the interferometer may be coupled to another component of the second part (discussed below in connection with FIGS. 6 and 7).

[0078] The first coupler 202, the second coupler 204, and the delay line 208 of the photonic assembly 200 function as described above for the photonic assembly 100 illustrated in FIG. 1, such that when a single pulse of light enters the input 212, the second coupler 204 outputs a pair of time-separated pulses at a first output of the second coupler 204 and a second pair of time-separated pulses at a second output of the second coupler 204. Each of the first and second phase modulators is configured to have its modulation state switched by an external signal. Each phase modulator can be switched to a different modulation state in response to a received pre-pulse and / or post-pulse to encode two different quantum information streams. The phase modulators can be configured to encode in different bases of the QKD scheme. Thus, the photonic assembly can be configured to output two streams of encoded data, i.e., the first stream at the first output 216 and the second stream at the second output 218. The two streams may be used to provide twice the amount of data transmitted, or may be provided for redundancy purposes.

[0079] In a further embodiment, a photonic assembly 300 shown in FIG. 3 is illustrated. The photonic assembly 300 comprises a delay line interferometer. The delay line interferometer comprises a plurality of passive elements comprising a first coupler 302, a second coupler 304, and a delay line 308. The interferometer further comprises a first optical path 306A and a second optical path 306B. The interferometer further comprises a first input 312, a second input 314, a first output 316, and a second output 318. The first coupler 302 couples the input 312 of the photonic assembly 300 to the input of the first optical path 306A and the input of the second optical path 306B. The second coupler 304 couples the output of the first optical path 306A and the output of the second optical path 306B. The interferometer of the photonic assembly 300 further comprises a phase modulator 310. The passive photonic element is provided on the first portion 320 and the phase modulator 310 is provided on the second portion 330 .

[0080] In the illustrated embodiment of Figure 3, the first coupler 302 and the second coupler 304 are 2x2 couplers. Thus, an interferometer having two inputs and two outputs is shown. However, it will be understood that the first coupler 302 and / or the second coupler 304 may be 1x2 couplers such that the interferometer is provided with a single input and / or a single output.

[0081] The passive photonic elements may include at least one waveguide, where the at least one waveguide couples the interferometric elements together. For example, the first optical path 306A and the second optical path 306B may be formed by waveguides disposed on the first portion.

[0082] 3, the phase modulator 310 is provided at the input of the first coupler 302. The output of the phase modulator 310 may be coupled to the input of the first coupler 302 by a coupled waveguide as described above. The input of the phase modulator 310 is coupled to an optical input 313 of the optical device assembly 300 (which may be coupled by a coupled waveguide as described above).

[0083] The phase modulator 310 is configured to have its modulation state switched by an external signal. For example, the phase modulator 310 is a multi-level electro-optic phase modulator that is responsive to an applied voltage. Light propagating through the phase modulator 310 will be phase shifted by an amount that depends on the applied voltage. For example, the phase modulator may shift the phase of the received light by an amount of 0, π / 2, π, 3π / 2, or -π / 2, or any phase.

[0084] In one embodiment, photonic assembly 300 is configured to receive a pair of time-separated pulses (e.g., time-separated pulses output from photonic assemblies 100 or 200) at optical input 313. Phase modulator 310 is configured to switch modulation states for the time-separated pulse pair such that a first pulse of the time-separated pulse pair is phase shifted a different amount than a second pulse of the time-separated pulse pair. Thus, the relative phase between each pulse of the pulse pair is shifted based on the selected modulation state.

[0085] The delay line 308 is provided in the first optical path 306A or in the second optical path 306B. As a result, the first optical path 306A and the second optical path 306B have different overall path lengths. A pulse of light traversing an optical path with a delay line will arrive at the second coupler 304 at a later time than light traversing an optical path without a delay line. After the time-separated pulse pair passes through the phase modulator 310, both pulses will enter the first coupler 302 and be passed to each of the first optical path 306A and the second optical path 306B. The pulse entering the optical path with the delay line (first optical path 306A in FIG. 3) will be delayed compared to the pulse entering the optical path without the delay line. In this exemplary embodiment, the pulse pair entering the photonic assembly 300 has a time separation that corresponds to the time difference between the pulse entering the first optical path 306A and the pulse entering the second optical path 306B. Thus, the front pulse passing through the first optical path 306A will arrive at the second coupler 304 at the same time as the rear pulse entering the second optical path 306B. These pulses will then interfere and an interference output will be output from the second coupler at the first output 316 and the second output 318. Depending on the modulation state of the phase modulator 310, the pulses will interfere constructively or destructively.

[0086] Thus, the photonic assembly 300 may form part of a quantum decoder in a quantum key distribution scheme, where the phase modulation states are selected based on a desired measurement basis.

[0087] In the illustrated embodiment of FIG. 3, the photonic assembly 300 comprises an optical input 313 coupled to the first input 312 of the interferometer. The optical input 313 is configured to receive a signal from outside the photonic assembly 300. For example, the optical input 313 may be coupled to an optical fiber and receive the encoded states in the form of time-separated optical pulses as described above. The photonic assembly 300 further comprises a first optical output 317 and a second optical output 319 for providing an output signal to the outside of the photonic assembly 300. For example, the output signal is an interference signal of the optical pulses traversing the first optical path 306A and the second optical path 306B. Each of the first optical output 317 and the second optical output 319 may be coupled to a photodetector that measures the photon count and indicates the measurement in a selected basis. These illustrated connections are merely examples. For example, the photonic assembly 300 may be provided with a second optical input coupled to the second input 314 or only one optical output coupled to the interferometer output. The optical input and output may not be directly coupled to the input and output of the interferometer, but may instead be coupled via other photonic elements on the photonic assembly 300. No optical output may be provided to the photonic assembly at all, and the output of the interferometer may be coupled to another component in the second portion (discussed below in connection with Figures 8 and 9).

[0088] In a further embodiment, a photonic assembly 400 shown in FIG. 4 is illustrated. The photonic assembly 400 comprises a delay line interferometer. The delay line interferometer comprises a plurality of passive elements comprising a first coupler 402, a second coupler 404, and a delay line 408. The interferometer further comprises a first optical path 406A, a second optical path 406B, a first input 412, a second input 414, a first output 416, and a second output 418. The first coupler 402 couples the first input 412 and the second input 414 to the input of the first optical path 406A and the input of the second optical path 406B. The second coupler 404 couples the output of the first optical path 406A and the output of the second optical path 406B. The interferometer of the photonic assembly 400 further comprises a phase modulator 410. The passive photonic element is provided on the first portion 420 and the phase modulator 410 is provided on the second portion 430 .

[0089] The passive photonic elements may include at least one waveguide, where the at least one waveguide couples the interferometer elements together. For example, the first optical path 406A and the second optical path 406B may be formed by waveguides disposed on the first portion.

[0090] The output of the phase modulator 410 may be coupled to the input of the second coupler 404 by the coupled waveguide described above. The input of the phase modulator 410 is coupled to the output of the first coupler 402 by the coupled waveguide described above.

[0091] In the embodiment of FIG. 4, a phase modulator 410 is provided in the first optical path 406A. As a result, pulses entering the first optical path will be phase shifted by an amount determined by the modulation state of the phase modulator 410. The phase modulator 410 is configured to have its modulation state switched by an external signal. For example, the phase modulator 410 is a multi-level electro-optic phase modulator responsive to an applied voltage. Light propagating through the phase modulator 410 will be phase shifted by an amount that depends on the applied voltage. For example, the phase modulator may shift the phase of the received light by an amount of 0, π / 2, π, 3π / 2, or −π / 2, or any phase.

[0092] The delay line 408 is provided in either the first optical path 406A or the second optical path 406B. As a result, the first optical path 406A and the second optical path 406B have different overall path lengths. A pulse of light traversing an optical path with a delay line will arrive at the second coupler 404 at a later time than light traversing an optical path without a delay line.

[0093] In a further embodiment, a photonic assembly 500 shown in FIG. 5 is illustrated. The photonic assembly 500 comprises a delay line interferometer. The delay line interferometer comprises a plurality of passive elements comprising a first coupler 502, a second coupler 504, and a delay line 508. The interferometer further comprises a first optical path 506A, a second optical path 506B, a first input 512, a second input 514, a first output 516, and a second output 518. The first coupler 502 couples the first input 512 and the second input 514 to the input of the first optical path 506A and the input of the second optical path 506B. The second coupler 504 couples the output of the first optical path 506A and the output of the second optical path 506B. In the embodiment of FIG. 5, the photonic assembly 500 comprises a first phase modulator 510A and a second phase modulator 510B. The plurality of passive elements are provided on the first portion 520 , and the first phase modulator 510 A and the second phase modulator 510 B are provided on the second portion 530 .

[0094] A first phase modulator 510A is provided in the first optical path 506A. A second phase modulator 510B is provided in the second optical path 506B. A pulse entering the first optical path 506A will be phase shifted by an amount determined by the modulation state of the phase modulator 510A. A pulse entering the second optical path 506B will be phase shifted by an amount determined by the modulation state of the phase modulator 510B. Each of the first phase modulator 510A and the second phase modulator 510B is configured to have its modulation state switched by an external signal in the same manner as described above for the phase modulator 410. The relative phase between the pulse entering the first optical path 506A and the pulse entering the second optical path 506B will be a combination of the phase shifts provided by each of the first phase modulator 510A and the second phase modulator 510B.

[0095] The passive photonic elements may include at least one waveguide, where the at least one waveguide couples the interferometer elements together. For example, the first optical path 506A and the second optical path 506B may be formed by waveguides disposed on the first portion.

[0096] The output of the phase modulator is coupled to the input of the second coupler 504 by the coupled waveguide described above. The input of the phase modulator is coupled to the output of the first coupler 502 by the coupled waveguide described above.

[0097] In the illustrated embodiments of Figures 4 and 5, each of the first and second couplers is a 2x2 coupler. Thus, each interferometer is shown as having two inputs and two outputs. However, it will be understood that the first and / or second couplers may be 1x2 couplers such that a single input and / or a single output is provided to either interferometer.

[0098] Each of the interferometers of FIG. 4 and FIG. 5 can be operated to encode information. In each embodiment, when a single optical pulse is provided to the interferometer at the optical input of the photonic assembly, the interferometer will output two time-separated pulses, where the time separation is determined by the overall difference in path length between the first optical path and the second optical path. Due to the presence of a phase modulator in the first optical path (FIG. 4) or the presence of phase modulators in the two optical paths (FIG. 5), the time-separated pulses will be phase shifted relative to each other. Thus, the photonic assembly can be implemented as an encoder, where the encoding basis is selected based on the selected phase modulation state of the phase modulator.

[0099] Each of the interferometers of FIG. 4 and FIG. 5 can also operate to decode information. In each embodiment, when the interferometer is provided with encoded time-separated pulse pairs at the optical input of the photonic assembly, the delay line will delay the pulses such that the front pulse propagating through the second optical path arrives at the same time as the rear pulse propagating through the first optical path. These pulses will then interfere at the second coupler, and an interference output will be output from the second coupler. Depending on the modulation state of one or more phase modulators, the pulses will constructively or destructively interfere. Thus, each photonic assembly can be implemented as a decoder, where the decoding basis is selected based on the selected phase modulation state of the phase modulator.

[0100] The above-mentioned embodiments relate to photonic assemblies in which a phase modulator is provided on the second part. The phase modulator is an example of an active photonic component. In further embodiments, further active components, such as a light source or a photodetector, may be provided on the second part. The provision of further active components is described below in relation to Figures 6 to 9.

[0101] In some embodiments, the photonic assembly further comprises a light source. The light source is an active optical component formed from an active photonic material. The light source may be provided on a photonic assembly configured to receive an input light pulse, such as the photonic assembly 100 of FIG. 1, the photonic assembly 200 of FIG. 2, the photonic assemblies 400 and 500 of FIG. 4 and FIG. 5. In these embodiments, the output of the light source is coupled to the input of the interferometer. The light source may be provided on the second part, or the light source may be provided on the third part. The third part may be attached to the first part or the second part. The third part may be independently optimized in the same way as the second part described above (e.g., the third part may also include a substrate of active material. The active material provides optical gain, which is advantageous as a platform for the light source).

[0102] FIG. 6 illustrates a photonic assembly 600 according to an embodiment. In this embodiment, the photonic assembly 600 comprises a first portion 620 and a second portion 630 attached to each other. The photonic assembly 600 further comprises a light source 610, a plurality of passive photonic elements 640, a first phase modulator 610A, and a second phase modulator 610B. The passive photonic elements 640 are provided on the first portion 620, and the light source 610 and the first phase modulator 610A and the second phase modulator 610B are provided on the second portion 630. The passive photonic elements 640 and the phase modulators 610A and 610B form an interferometer as described above in connection with FIG. 2. In FIG. 6, the interferometer outputs 616 and 618 are coupled to the optical output of the photonic assembly 600. The interferometer input 612 is coupled to the light source 610.

[0103] In an alternative embodiment, the photonic assembly 600 includes only a single phase modulator 610 A. In this embodiment, the arrangement of the passive photonic elements 640 and the phase modulator 610 A is similar to that described above in connection with FIG.

[0104] FIG. 7 illustrates a photonic assembly 700 according to an embodiment. In this embodiment, the photonic assembly 700 comprises a first portion 720 and a second portion 730 attached to each other. The photonic assembly 700 further comprises a light source 710, a plurality of passive photonic elements 740, a first phase modulator 710A, and a second phase modulator 710B. The passive photonic elements 740 are provided on the first portion 720, and the light source 710 and the first phase modulator 710A and the second phase modulator 710B are provided on the second portion 730. The passive photonic elements 740 and the phase modulators 710A and 710B form an interferometer as described above in connection with FIG. 5. In FIG. 7, the interferometer outputs 716 and 718 are coupled to the optical output of the photonic assembly 700. The interferometer input 712 is coupled to the light source 710.

[0105] In an alternative embodiment, the photonic assembly 700 includes only a single phase modulator 710 A. In this embodiment, the arrangement of the passive photonic elements 740 and the phase modulator 710 A is similar to that described above in connection with FIG.

[0106] The above embodiments described in relation to Figures 6 and 7 relate to photonic assemblies having a light source and a phase modulator(s) on the same part. In alternative embodiments, the photonic assembly comprises a third part. The third part may be independently optimized in the same manner as the second part described above (e.g., the third part may also include a substrate of active material). In these alternative embodiments, the light source is on a different part than the phase modulator(s). Providing all active components on a single part reduces manufacturing costs and also provides a more compact photonic assembly.

[0107] In the above-described embodiments, the light source is configured to generate light pulses to be received by the interferometer. In one embodiment, the light source includes a laser. Such a laser may be a gain-switched laser configured to generate a series of time-separated pulses having a randomized phase between the pulses.

[0108] In an alternative embodiment, the light source comprises a second laser and an optical filter. The second laser acts as a "master laser" (using conventional terminology) and the first laser is a "slave laser" (using conventional terminology). The second laser provides a seed for the first laser, which means that the laser pulses output by the first laser have a fixed phase relationship.

[0109] In one embodiment, the second laser is modulated by a phase control element. The seeds provided by the second laser are modulated by a multi-level signal to introduce different phase relationships between each of the master laser seeds, which in turn produces different phases between the output pulses of the first laser. This phase modulation can be fixed or random.

[0110] As another alternative, the laser may be a continuous wave (CW) laser used in conjunction with an intensity modulator, where the output of the CW laser is selectively modulated to produce a pulsed laser output.

[0111] In one embodiment, the light source is configured to generate light pulses having an average photon number less than 1. Alternatively, the optical device is provided with an attenuator coupled to the output of the combiner for attenuating the light pulses at the output of the combiner to have an average photon number less than 1. In these embodiments where the average photon number is less than 1, the system is adapted for quantum communications.

[0112] In an alternative embodiment, the light pulses are generated to have an average photon number greater than one, in which case the system may be adapted for classical communication.

[0113] In some embodiments, the photonic assembly further comprises at least one photodetector. The at least one photodetector is an active optical component formed from an active photonic material. The at least one photodetector may be provided on a photonic assembly configured to receive the encoded state for measurement, such as the photonic assembly 300 of FIG. 3 or the photonic assemblies 400 and 500 of FIG. 4 and FIG. 5. In these embodiments, the at least one photodetector is coupled to the output of the interferometer. The at least one photodetector may be provided on the second part, or the at least one photodetector may be provided on the third part. The third part may be attached to the first part or the second part. The third part may be independently optimized in the same manner as the second part described above (e.g., the third part may also include a substrate of an active material, which provides optical gain and absorption, which is advantageous for use in photodetectors).

[0114] FIG. 8 illustrates a photonic assembly 800 according to an embodiment. In this embodiment, the photonic assembly 800 comprises a first portion 820 and a second portion 830 attached to each other. The photonic assembly 800 further comprises a first photodetector 850A, a second photodetector 850B, a plurality of passive photonic elements 840, and a phase modulator 810. The passive photonic elements 840 are provided on the first portion 820, and the first and second photodetectors 850A and 850B and the phase modulator 810 are provided on the second portion 830. The passive photonic elements 840 and the phase modulator 810 form an interferometer as described above in connection with FIG. 3. In FIG. 8, the interferometer outputs 816 and 818 are coupled to the inputs of the first and second photodetectors 850A and 850B, respectively. The interferometer input 812 is coupled to the optical input of the photonic assembly 800.

[0115] FIG. 9 illustrates a photonic assembly 900 according to an embodiment. In this embodiment, the photonic assembly 900 comprises a first part 920 and a second part 930 attached to each other. The photonic assembly 900 further comprises a first photodetector 950A, a second photodetector 950B, a plurality of passive photonic elements 940, a first phase modulator 910A, and a second phase modulator 910B. The passive photonic elements 940 are provided on the first part 920, and the first and second photodetectors 950A and 950B and the first and second phase modulators 910A and 910B are provided on the second part 930. The passive photonic elements 940 and the phase modulators 910A and 910B form an interferometer as described above in connection with FIG. 5. 9, the interferometer outputs 916 and 918 are coupled to inputs of a first photodetector 950A and a second photodetector 950B, respectively. The interferometer inputs 912 and 914 are coupled to optical inputs of the photonic assembly 900.

[0116] In an alternative embodiment, the photonic assembly 900 includes only a single phase modulator 910 A. In this embodiment, the arrangement of the passive photonic elements 940 and the phase modulator 910 A is similar to that described above in connection with FIG.

[0117] The above embodiments described in relation to Figures 8 and 9 relate to photonic assemblies having at least one photodetector and phase modulator(s) provided on the same part. In alternative embodiments, the photonic assembly comprises a third part. In these alternative embodiments, the at least one photodetector is provided on a different part than the phase modulator(s). The third part may be independently optimized in the same manner as the second part described above (e.g., the third part may also include a substrate of active material). Providing all active components on a single part reduces fabrication costs and also provides a more compact photonic assembly.

[0118] In one embodiment, a photonic assembly 1000 is shown in FIG. 10. The photonic assembly 1000 comprises an interferometer. The interferometer comprises a plurality of passive elements comprising a first coupler 1002 and a second coupler 1004. The interferometer further comprises a first optical path 1006A, a second optical path 1006B, a first input 1012, a second input 1014, a first output 1016, and a second output 1018. The first coupler 1002 couples the first input 1012 and the second input 1014 of the interferometer to the input of the first optical path and the input of the second optical path. The second coupler 1004 combines the output of the first optical path 1006A and the output of the second optical path 1006B. The interferometer of the photonic assembly 1000 further comprises a phase modulator 1010. The passive photonic element is provided on the first portion 1020 and the phase modulator 1010 is provided on the second portion 1030 .

[0119] In the embodiment of FIG. 10, a phase modulator 1010 is provided in the first optical path 1006A. As a result, pulses entering the first optical path will be phase shifted by an amount determined by the modulation state of the phase modulator 1010. The phase modulator 1010 is configured to have its modulation state switched by an external signal. For example, the phase modulator 1010 is a multi-level electro-optic phase modulator responsive to an applied voltage. Light propagating through the phase modulator 1010 will be phase shifted by an amount that depends on the applied voltage. For example, the phase modulator may shift the phase of the received light by an amount of 0, π / 2, π, 3π / 2, or −π / 2, or any phase.

[0120] In one embodiment, a photonic assembly 1100 is shown in FIG. 11. The photonic assembly 1100 comprises an interferometer. The interferometer comprises a plurality of passive elements comprising a first coupler 1102 and a second coupler 1104. The interferometer further comprises a first optical path 1106A, a second optical path 1106B, a first input 1112, a second input 1114, a first output 1116, and a second output 1118. The first coupler 1102 couples the first input 1112 and the second input 1114 of the interferometer to the input of the first optical path and the input of the second optical path. The second coupler 1104 combines the output of the first optical path 1106A and the output of the second optical path 1106B. The interferometer of the photonic assembly 1100 further comprises a phase modulator 1110. The passive photonic elements are provided on the first portion 1120, and the phase modulator 1110 is provided on the second portion 1130. The interferometer of the photonic assembly 1100 further comprises a first phase modulator 1110A and a second phase modulator 1110B. The passive photonic elements are provided on the first portion 1120, and the first phase modulator 1110A and the second phase modulator 1110B are provided on the second portion 1130.

[0121] In the embodiment of FIG. 11, a first phase modulator 1110A is provided in the first optical path 1106A, and a second phase modulator 1110B is provided in the second optical path 1106B. As a result, pulses entering the first optical path will be phase shifted by an amount determined by the modulation state of the first phase modulator 1110A, and pulses entering the second optical path will be phase shifted by an amount determined by the modulation state of the second phase modulator 1110B. Each phase modulator is configured to have its modulation state switched by an external signal. For example, each phase modulator is a multi-level electro-optic phase modulator responsive to an applied voltage. Light propagating through each phase modulator will be phase shifted by an amount that depends on the applied voltage. For example, each phase modulator may shift the phase of the received light by an amount of 0, π / 2, π, 3π / 2, or -π / 2, or any phase.

[0122] In the illustrated embodiments of Figures 10 and 11, each of the first and second couplers is a 2x2 coupler. Thus, each interferometer is shown as having two inputs and two outputs. However, it will be understood that the first and / or second couplers may be 1x2 couplers such that a single input and / or a single output is provided to either interferometer.

[0123] Each of the interferometers of Fig. 10 and Fig. 11 operates as an intensity modulator. In each case, when a single light pulse is provided to the interferometer at the optical input of the photonic assembly, the first coupler splits the signal into two pulses, each of which propagates through the first or second optical path. Due to the presence of a phase modulator (Fig. 10) or multiple phase modulators (Fig. 11), the pulses propagating through the two optical paths will be phase shifted relative to each other depending on the modulation state of the phase modulator(s). The pulses are then recombined in the second coupler. The two pulses will then undergo interference in the second coupler, resulting in a modulation of the intensity of the light output from the second coupler.

[0124] In the embodiments of Figures 10 and 11, each coupler may be an asymmetric coupler, so that the two outputs of each coupler have different intensities. Thus, the light pulses propagating through the two optical paths may have different intensities. The asymmetric couplers may be fixed or adjustable.

[0125] In one embodiment, a photonic assembly 1200 is provided as illustrated in FIG. 12. The photonic assembly 1200 comprises a Sagnac interferometer. The Sagnac interferometer comprises an optical path 1206, an input 1212, and an output 1214. The Sagnac interferometer comprises a coupler 1202, which is a passive photonic element, disposed on a first portion 1220. The coupler 1202 is a 2x2 coupler that couples the input of the interferometer to a first end of the optical path 1206 and a second end of the optical path 1206. The Sagnac interferometer further comprises a phase modulator 1210 disposed on a second portion 1230.

[0126] The output and input of the phase modulator 1210 are coupled to the coupler 1202 by the coupled waveguides described above.

[0127] When an optical pulse is received at the input of coupler 1202, coupler 1202 outputs a pulse at each output of coupler 1202. Thus, a first pulse will propagate clockwise around optical path 1206 and a second pulse will propagate counterclockwise around optical path 1206. Phase modulator 1210 is disposed in optical path 1206 and is disposed closer to the first output of coupler 1202 than to the second output of coupler 1202. Thus, pulses traveling around optical path 1206 in one direction will arrive at the phase modulator at a different time than pulses traveling around optical path 1206 in the opposite direction.

[0128] The phase modulator 1210 is configured to have its modulation state switched by an external signal. For example, the phase modulator 1210 is a multi-level electro-optic phase modulator responsive to an applied voltage. The phase modulator 1210 is configured to switch its modulation state to selectively phase-shift each of the clockwise traveling pulses and the counterclockwise traveling pulses. For example, the modulation state of the phase modulator 1210 is switched between two arrival times of two pulses traversing the optical path 1206. As a result, each of the clockwise and counterclockwise traveling pulses will be phase-shifted by a different amount. Thus, when the pulses combine in the coupler 1202, the pulses will interfere. Thus, the Sagnac interferometer of FIG. 12 can operate as an intensity modulator.

[0129] The passive components further include an isolator 1240 disposed between the interferometer input 1212 and the input to the coupler 1202. The isolator allows light to pass from the interferometer input 1212 to the coupler 1202, but prevents light from passing in the reverse direction. Thus, when the coupler 1202 receives clockwise and counterclockwise pulses, the isolator 1240 suppresses the output of the coupler that would otherwise pass back to the interferometer input 1212.

[0130] The Sagnac interferometer of the photonic assembly 1200 may be used to encode a quantum state. When two time-separated pulses are provided to the interferometer input 1212, the coupler 1202 outputs a pulse pair at each output of the coupler 1202. Thus, the first pulse pair propagates clockwise through the optical path 1206, and the second pulse pair propagates counterclockwise through the optical path 1206. The phase modulator first receives, in order, a clockwise traveling front pulse, a clockwise traveling back pulse, a counterclockwise traveling front pulse, and a counterclockwise traveling back pulse. The phase modulator 1210 may be configured such that the modulation state is switched in time to selectively phase-shift the back pulse of each pulse pair by the same amount φ. When the pulse pairs are recombined at the coupler 1202, |ψ enc >|0>+e iφ Any chosen final state |1> can be encoded.

[0131] In a variation of the above embodiment, each photonic assembly may incorporate a polarizing element as part of the interferometer. For example, each coupler may be a polarization splitter and combiner, the optical path may be configured to maintain polarization, and the interferometer may include one or two polarization rotators. The polarizing element is provided to reduce losses in the photonic assembly.

[0132] One embodiment is illustrated in Figure 13. Figure 13 shows a photonic assembly 1300. The photonic assembly 1300 comprises elements as described above in connection with the embodiment of Figure 4. The configuration and operation of the photonic assembly 1300 are similar to those described above in connection with Figure 4 and will not be repeated here. In addition to the aforementioned components of the photonic assembly 400, the photonic assembly 1300 comprises a polarization rotator 460 disposed in the second optical path 406B. Furthermore, in this embodiment, the first coupler 402P is a polarization splitter and combiner, and the second coupler 404P is a polarization splitter and combiner.

[0133] When unpolarized light is received at the first coupler 402P, the light output to the first optical path 406A will be polarized in a first polarization state, and the light output to the second optical path will be polarized in a second polarization state. The second coupler 404P is configured to couple the light in the first polarization state to a first output and the light in the second polarization state to a second output. The first optical path and the second optical path are polarization-maintaining, for example, each optical path may be formed by a polarization-maintaining waveguide. The light traveling through the second optical path is received by the polarization rotator 460. The polarization rotator 460 rotates the polarization from the second polarization state to the first polarization state. Thus, the light received at the second coupler from the first optical path 406A and the second optical path are each in the same polarization state, and all the light will be coupled to the same output of the coupler 404P. If the photonic assembly 1300 is configured to have a single output, using the polarizing elements described above means that more light can be output to that single output, thus reducing losses. Alternatively, only the second coupler 404P is a polarizing coupler. In this alternative embodiment, the light entering the photonic assembly 1300 at input 412 and / or input 414 is polarized with some components in a first polarization state and some components in a second polarization state.

[0134] Optionally, as an alternative to or in addition to the first polarization rotator, a second polarization rotator may be provided in the first optical path. The beam splitters and combiners of the polarizations and the polarization rotators may be fixed or adjustable. Since the beam splitters and combiners of the polarizations and the polarization rotators do not need to be switchable at a fast speed, these components may be provided on the first part. In an alternative to the above, the beam splitters and combiners of the polarizations and the polarization rotators may be provided on the second part.

[0135] In a variation of the above-described embodiment, the plurality of passive photonic elements comprises at least one optical reflector. The optical reflector may be located in any portion of the optical path and coupled between other passive photonic components. For example, in each of the embodiments of Figures 1-5, 10, and 11, the interferometer may be configured as a Michelson interferometer, in which the plurality of passive photonic elements comprises a first optical reflector in a first optical path and a second optical reflector in a second optical path. One embodiment is illustrated in Figure 14. Figure 14 shows a photonic assembly 450. The photonic assembly 450 comprises elements as described above in connection with the embodiment of Figure 4. The configuration and operation of the photonic assembly 450 is similar to that described above in connection with Figure 4 and will not be repeated here. In addition to the aforementioned components of photonic assembly 400, photonic assembly 450 includes a first reflector 440A in the first optical path 406A and a second reflector 440B in the second optical path 406B. The inclusion of reflectors in the interferometer assembly allows for more flexible placement of components and therefore allows for more compact integration of photonic elements.

[0136] In the above-described embodiments, individual phase modulators are provided as part of the interferometer described. In variations of these embodiments, the interferometer may be implemented with different phase modulator arrangements. For example, at least one additional phase modulator may be provided. Figure 15A illustrates different exemplary phase modulator arrangements.

[0137] In one example, each phase modulator in the above embodiments may be a single electro-optic phase modulator 1510A, configured such that the modulation state is switched by an external signal in the manner described above.

[0138] Alternatively, the phase modulator may comprise a first electro-optic phase modulator 1510A and a second phase modulator 1510B provided at the output of the first phase modulator 1510A. Each phase modulator may be independently controlled to switch between modulation states. Providing two phase modulators allows for more precise modulation of the phase of the incident light.

[0139] Alternatively, the phase modulator comprises a Mach-Zehnder modulator comprising a first electro-optic phase modulator 1510C and a second electro-optic phase modulator 1510D.

[0140] Alternatively, the phase modulator may comprise a Mach-Zehnder Modulator (MZM) comprising a first electro-optic phase modulator 1510A and a first electro-optic phase modulator 1510C and a second electro-optic phase modulator 1510D.

[0141] The above phase modulator arrangements are merely examples. Further implementations may include more than one MZM connected in series and any number of phase modulators (electro-optic phase modulators or MZM modulators as described) connected in series.

[0142] Additionally, the above-described embodiments may further comprise a further MZM disposed at one of the interferometer input and output, the further MZM being disposed on the second portion. The further MZM is configured as an intensity modulator for modulating the intensity of the light pulse entering or exiting the interferometer. Such a modulator may be used, for example, to attenuate the intensity of the light pulse, so as to reduce the average photon number to less than one.

[0143] The phase modulator arrangement comprises passive photonic elements (Mach-Zehnder modulator waveguides and couplers) and active photonic elements (electro-optic modulators). In one embodiment, all photonic elements of the phase modulator arrangement are provided on the second part. Alternatively, the passive photonic elements are provided on the first part and the active photonic elements are provided on the second part, and the components are coupled to each other by waveguide coupling as described above.

[0144] The above-described embodiments include couplers as part of the interferometer. These couplers are illustrated as 2x2 couplers, such as multimode interferometers (MMIs). The 2x2 couplers can be symmetric couplers, where equal light intensities are sent to each output of the coupler. Alternatively, the couplers can be asymmetric, providing different output intensities to each output of the coupler. For example, the 2x2 asymmetric coupler can be configured as a Mach-Zehnder interferometer (MZI) illustrated in FIG. 15B. The MZI includes a first coupler 1502A, a second coupler 1502B, a first phase modulator 1502C between a first output of the first coupler 1502A and a first input of the second coupler 1502B, and a second phase modulator 1502D between a second output of the first coupler 1502A and a second input of the second coupler 1502B. The phase modulator may be configured to impart different phases to adjust the intensity output at each of the outputs of the second coupler. The different phases may be fixed or adjustable.

[0145] The above embodiment provides a photonic assembly with an interferometer having at least one input and at least one output. At least one input of the interferometer may be coupled to one or more inputs of the photonic assembly to receive a signal external to the photonic assembly. Alternatively, at least one input to the interferometer is coupled to other elements on the photonic assembly, such as a light source, according to the above-mentioned embodiment. At least one output of the interferometer may be coupled to one or more outputs of the photonic assembly. Alternatively, at least one output of the interferometer may be coupled to other elements on the photonic assembly, such as a photodetector, according to the above-mentioned embodiment.

[0146] Thus, a photonic assembly may be provided with at least one input and / or at least one output.Figures 16A-16F illustrate exemplary optical connections to a photonic assembly.

[0147] In the embodiment of Fig. 16A, a photonic assembly 1600A is provided. The photonic assembly 1600A comprises a first portion 1620A and a second portion 1630A. According to the above-mentioned embodiment, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630A, and a passive photonic element 1640A is provided on the first portion 1620A.

[0148] At least one input to the photonic assembly takes the form of at least one input port on the second portion 1630A. At least one output of the photonic assembly takes the form of at least one output port on the first portion 1620A. A single input port 1650A is coupled to a single input fiber, and multiple output ports 1670A and 1660A are coupled to a fiber array 1610A of output fibers. In FIG. 16A, only one input port 1650A is shown, but more than one input port may be provided and coupled to a fiber array. Similarly, multiple output ports 1670A and 1660A are shown, but more output ports may be provided or only one output port may be provided and coupled to a single fiber output.

[0149] In the embodiment of Fig. 16B, a photonic assembly 1600B is provided. The photonic assembly 1600B comprises a first portion 1620B and a second portion 1630B. According to the above-mentioned embodiment, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630B, and a passive photonic element 1640B is provided on the first portion 1620B.

[0150] At least one input to the photonic assembly takes the form of at least one input port on the first portion 1620B. At least one output of the photonic assembly takes the form of at least one output port on the second portion 1630B. A plurality of input ports 1660B and 1670B are coupled to the fiber array 1610B, and the output port 1650B is coupled to a single output fiber. In FIG. 16B, only one output port 1650B is shown, but more than one output port may be provided and coupled to the fiber array. Similarly, a plurality of input ports 1670B and 1660B are shown, but more input ports may be provided or only one input port may be provided and coupled to a single fiber output.

[0151] In the embodiment of Fig. 16C, a photonic assembly 1600C is provided. The photonic assembly 1600C comprises a first portion 1620C and a second portion 1630C. According to the above-mentioned embodiments, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630C, and a passive photonic element 1640C is provided on the first portion 1620C.

[0152] At least one input to the photonic assembly takes the form of at least one input port on the second portion 1630C. At least one output of the photonic assembly 1600C takes the form of at least one output port on the second portion 1630C. The input port 1650C and the output port 1660C are coupled to a fiber array 1610C comprising input and output fibers. Although only one input port 1650C and only one output port 1660C are shown in FIG. 16C, more than one input port and more than one output port may be provided and coupled to the fiber array.

[0153] In the embodiment of Fig. 16D, a photonic assembly 1600D is provided. The photonic assembly 1600D comprises a first portion 1620D and a second portion 1630D. According to the above-mentioned embodiment, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630D, and a passive photonic element 1640D is provided on the first portion 1620D.

[0154] At least one input to the photonic assembly 1600D takes the form of at least one input port on the first portion 1620D. At least one output of the photonic assembly 1600D takes the form of at least one output port on the first portion 1620D. The input port 1650D and the output port 1660D are coupled to a fiber array 1610D comprising input and output fibers. In FIG. 16D, only one input port 1650D and only one output port 1660D are shown, but more than one input port and more than one output port may be provided and coupled to the fiber array.

[0155] In the embodiment of Fig. 16E, a photonic assembly 1600E is provided. The photonic assembly 1600E comprises a first portion 1620E and a second portion 1630E. According to the above-mentioned embodiment, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630E, and a passive photonic element 1640E is provided on the first portion 1620E.

[0156] At least one input to the photonic assembly 1600E takes the form of at least one input port on the first portion 1620E. At least one output of the photonic assembly 1600E takes the form of at least one output port on the first portion 1620E. The input port 1650E and the output port 1660E are coupled to a fiber array 1610E comprising input and output fibers. In FIG. 16E, only one input port 1650D and only one output port 1660E are shown, but more than one input port and more than one output port may be provided and coupled to the fiber array. The input port 1650E is coupled to a second portion 1630E, which is then coupled to a passive photonic element 1640E. The output port 1660E is coupled to the passive photonic element 1640E.

[0157] In the embodiment of Fig. 16F, a photonic assembly 1600F is provided. The photonic assembly 1600F comprises a first portion 1620F and a second portion 1630F. According to the above-mentioned embodiment, an active photonic element, such as one or more phase modulators, is provided on the second portion 1630F, and a passive photonic element 1640F is provided on the first portion 1620F.

[0158] At least one input to the photonic assembly 1600F takes the form of at least one input port on the first portion 1620F. At least one output of the photonic assembly 1600F takes the form of at least one output port on the first portion 1620F. Three ports 1650F, 1660F, and 1670F are coupled to a fiber array 1610F comprising three fibers. Each of the three ports 1650F, 1660F, and 1670F can function as an input port or an output port. For example, the photonic assembly may have two input ports and one output port, or one input port and two output ports, depending on the function of the system. In FIG. 16F, three ports are shown, but more input / output ports may be provided and coupled to the fiber array. The port 1650F is coupled to a second portion 1630F, which is then coupled to a passive photonic element 1640F. The remaining ports 1660F and 1670F are coupled to the passive photonic element 1640F.

[0159] As mentioned above, the embodiments herein may be implemented as part of a quantum encoder / quantum transmitter or a quantum decoder / quantum receiver. In one embodiment, a quantum state encoder is provided that includes a photonic assembly of any one of FIGS. 1-2, 4-7, and 12 and is configured to encode a quantum state from an incoming light pulse. In one embodiment, the quantum encoder further includes a photonic assembly of any one of FIGS. 10-12 configured to provide intensity modulation to the light pulse used for quantum encoding. The intensity modulation may be used, for example, to generate a decoy state. In another embodiment, a quantum state decoder is provided that includes a photonic assembly of any one of FIGS. 3-5, and 8-9 configured to decode a quantum state.

[0160] In a quantum encoder, high propagation losses in the photonic elements can be mitigated by increasing the intensity of the light received from the light source. However, propagation losses in the photonic elements in a quantum decoder result in information being lost, especially in quantum regimes where the average number of photons in a received pulse is less than one. The hybrid photonic assemblies for use in phase decoders described herein mitigate this propagation loss and therefore improve the efficiency of the phase decoder and the quantum communication system of which the decoder may form a part.

[0161] In Fig. 17, a further embodiment is illustrated in which a quantum communication system 1700 is illustrated comprising a quantum encoder 1710 according to the above-mentioned embodiment, a quantum decoder 1750 according to the above-mentioned embodiment, and at least one quantum communication channel 1780 connecting the quantum encoder 1710 and the quantum decoder 1750. The quantum encoder 1710 may comprise an encoding photonic assembly 1740 (e.g., the photonic assembly of Fig. 6 or Fig. 7) comprising a light source. Alternatively, the quantum encoder 1710 may comprise an encoder photonic assembly without a light source, but coupled to a light source 1720 separately implemented on the quantum encoder 1710. The quantum encoder may comprise an intensity modulator 1730. The intensity modulator may be implemented on an active photonic assembly as described above. The quantum decoder comprises a decoder photonic assembly 1760 (e.g., the photonic assembly of Fig. 8 or Fig. 9) comprising a photodetector. Alternatively, the quantum decoder comprises a decoder photonic assembly that does not have a photodetector, but is coupled to at least one photodetector 1770 that is separately implemented on the quantum decoder 1750.

[0162] Although 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. The novel devices and methods described herein may be embodied in other various forms, and various omissions, substitutions, and changes may be made in the form of the devices, methods, and products described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover any such forms or modifications that fall within the scope and spirit of the invention.

Claims

1. a first portion comprising a first substrate; a second portion comprising a second substrate; an interferometer comprising a plurality of passive photonic elements and a phase modulator; Equipped with the phase modulator is provided on the second portion and made of the same material as the second portion; the second portion includes an active material, the second material exhibiting an electro-optic effect that changes the refractive index in response to an applied electric field in a telecommunications wavelength band; the plurality of passive photonic elements are provided on the first portion and made of the same material as the first portion; the first portion includes a passive material, the first material having a larger band gap than the second material, the first material having a lower photon absorption coefficient in the telecommunications wavelength band than the second portion; the first material includes a material having a band gap larger than the energy of photons propagating within the material; A photonic assembly, wherein the second material has a band gap corresponding to a pulse frequency used in a quantum key distribution scheme.

2. The photonic assembly of claim 1 , wherein the phase modulator is an electro-optic phase modulator.

3. 2. The photonic assembly of claim 1, wherein the plurality of passive photonic elements comprises one or more waveguides, and the interferometer further comprises one or more additional waveguides provided on the second portion, at least one of the one or more waveguides being coupled to at least one of the one or more additional waveguides.

4. the interferometer comprises a first optical path and a second optical path; the plurality of passive photonic elements comprises a first coupler, a second coupler, and a delay line; the first coupler couples an input of the interferometer to an input of the first optical path and to an input of the second optical path; the second coupler combines an output of the first optical path with an output of the second optical path; The delay line is provided in the first optical path or in the second optical path. The photonic assembly of claim 1 .

5. The photonic assembly of claim 4 , wherein the phase modulator is provided at the output of the second coupler.

6. 5. The photonic assembly of claim 4, wherein the phase modulator is disposed in the first optical path, and the interferometer further comprises a further phase modulator disposed on the second portion, the further phase modulator being disposed in the second optical path.

7. Further comprising a light source; an output of the light source is coupled to the input of the interferometer; the light source is provided on the second portion, or the light source is provided on the third portion; 6. The photonic assembly of claim 5.

8. The photonic assembly of claim 4 , wherein the phase modulator is provided at the input of the first coupler.

9. Further comprising at least one photodetector; the at least one photodetector is coupled to an output of the interferometer; the at least one photodetector is provided on the second portion, or the at least one photodetector is provided on a third portion; 7. The photonic assembly of claim 6.

10. the interferometer comprises a first optical path and a second optical path; the plurality of passive photonic elements comprises a first coupler and a second coupler; the first coupler couples an input of the interferometer to an input of the first optical path and to an input of the second optical path; the second coupler combines an output of the first optical path with an output of the second optical path; The phase modulator is provided in the first optical path. The photonic assembly of claim 1 .

11. 5. The photonic assembly of claim 4, wherein the second coupler is a polarization splitter and combiner, and the interferometer further comprises a polarization rotator, the polarization rotator being provided in the first optical path or the second optical path.

12. The photonic assembly of claim 4 , wherein the plurality of passive photonic elements comprises a first optical reflector in the first optical path and a second optical reflector in the second optical path.

13. 2. The photonic assembly of claim 1, wherein the interferometer comprises an optical path, the plurality of passive photonic elements comprises a coupler, the coupler coupling an input of the interferometer to a first end of the optical path and to a second end of the optical path, and the phase modulator is disposed in the optical path.

14. 10. The photonic assembly of claim 1, wherein the passive material is optically passive for light having wavelengths within a first wavelength range, and the active material is optically active for light having wavelengths within the first wavelength range.

15. The photonic assembly of claim 1 , wherein the first substrate comprises a silicon or nitrogen-containing material.

16. The photonic assembly of claim 1 , wherein the second substrate comprises a material comprising a III-V or II-VI semiconductor material.

17. A quantum state encoder comprising the photonic assembly of claim 5.

18. A quantum state decoder comprising the photonic assembly of claim 8.

19. A quantum communication system comprising a quantum state encoder according to claim 17 and a quantum state decoder according to claim 18.

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