Abstract: A spectral filter includes a wavelength demultiplexer having a first output and a second output, a signal channel pump rejection filter coupled to a first output, and a herald channel pump rejection filter coupled to the second output. The spectral filter also includes a first chain of asymmetric Mach-Zehnder interferometers (MZIs) coupled to the signal channel pump rejection filter. Each of the asymmetric MZIs in the first chain of asymmetric MZIs is characterized by a decreasing free spectral range. The spectral filter further includes a second chain of asymmetric MZIs coupled to the herald channel pump rejection filter. In some embodiments, each of the asymmetric MZIs in the second chain of asymmetric MZIs is characterized by a decreasing free spectral range.

Abstract: A notch filter circuit includes a wavelength demultiplexer, a first pump rejection filter coupled to the wavelength demultiplexer, and a second pump rejection filter coupled to the wavelength demultiplexer. The notch filter circuit also includes a first notch filter arm coupled to the first pump rejection filter and including a first chain of asymmetric Mach-Zehnder interferometers (MZIs) and a second notch filter arm coupled to the second pump rejection filter and including a second chain of asymmetric MZIs.

Abstract: An example photon counting device includes one or more unit cells. Each unit cell includes one or more superconducting components. Each unit cell also includes a current dynamics control element coupled in series with the one or more superconducting components. Each unit cell further includes a transistor having a superconducting gate element that is coupled with the one or more superconducting components and a channel element that is electrically insulated from the superconducting gate element, where the channel element has a first resistance while the superconducting gate element is in a superconducting state and a second resistance while the superconducting gate element is in a non-superconducting state. The photon counting device further includes a bias current source coupled to the one or more unit cells and a waveguide optically coupled to the one or more unit cells.

Abstract: A device includes a spot size converter on a substrate. The substrate includes an optical fiber alignment structure formed thereon. The spot size converter is aligned with the optical fiber alignment structure. The spot size converter includes an oxide layer that has a first refractive index and includes a tapered section such that a first end of the spot size converter is smaller than a second end of the spot size converter. The spot size converter also includes a waveguide core in the oxide layer. The waveguide core is tapered and is smaller at the first end than at the second end of the spot size converter. The spot size converter further includes a cladding layer surrounding the oxide layer, the cladding layer having a second refractive index lower than the first refractive index of the oxide layer.

Abstract: A device includes a substrate and a dielectric layer on the substrate. The device also includes a light sensitive component in the dielectric layer and a trench having a first portion disposed in the substrate and a second portion disposed in the dielectric layer. The trench is adjacent the light sensitive component and includes an adhesion layer in the first portion and the second portion, an optical isolation layer on the adhesion layer, and a first fill material in the first portion and a second fill material in the second portion. The first fill material is characterized by a first coefficient of thermal expansion (CTE) that matches a CTE of the substrate and the second fill material is characterized by a second CTE that matches a CTE of the dielectric layer.

Abstract: An example photonic integrated circuit (PIC) includes a plurality of input ports to input light, such as a quantum state of light that comprises one or more photons, into the PIC. In addition, the PIC may include a waveguide network that includes a crossing network to combine light, and optical couplers that are coupled to the crossing network. The PIC can further include output ports to output the light.

Abstract: An example integrated generalized Mach-Zehnder Interferometer (GMZI) to process a quantum state of light is described. A quantum state of light comprising one or more photons can be received by a first coupler network in the GMZI. Using the first coupler network, the quantum state of light is distributed to one or more of a plurality of waveguide arms in the GMZI. The phase of the quantum state of light is adjusted using a plurality of phase shifters in the GMZI. The phase is adjusted for portions of the quantum state of light in one of the plurality of waveguide arms. The phase-adjusted quantum light is received by a second coupler network in the GMZI. Using the second coupler network, the quantum state of light is combined onto one or more outputs of the waveguide arms. The combined quantum light is outputted.

Abstract: An optical device includes an optical fiber and an optical die connected to the optical fiber. The optical fiber includes a core layer, a cladding layer formed on the core layer, and a first portion of a fiber alignment structure formed on the cladding layer. The optical die includes a die main body, an optical waveguide located in the die main body, and a second portion of the fiber alignment structure formed on the die main body and mated to the first portion of the fiber alignment structure so as to align the core layer of the optical fiber with the optical waveguide.

Abstract: An optical device includes a substrate, a dielectric layer on the substrate, a waveguide within the dielectric layer, a light sensitive component (e.g., a photodetector) in the dielectric layer and coupled to the waveguide, and a plurality of light isolation structures in at least one of the substrate or the dielectric layer and configured to prevent stray light from reaching the light sensitive component. In some embodiments, a light isolation structure in the plurality of light isolation structures includes two opposing sidewalls and a filling material between the two opposing sidewalls. The two opposing sidewalls include an optical isolation layer. The filling material is characterized by a coefficient of thermal expansion (CTE) matching a CTE of at least one of the substrate or the dielectric layer.

Abstract: A method of resolving a number of photons received by a photon detector includes optically coupling a waveguide to a superconducting wire having alternating narrow and wide portions; electrically coupling the superconducting wire to a current source; and electrically coupling an electrical contact in parallel with the superconducting wire. The electrical contact has a resistance less than a resistance of the superconducting wire while at least one narrow portion of the superconducting wire is in a non-superconducting state. The method includes providing to the superconducting wire, from the current source, a current configured to maintain the superconducting wire in a superconducting state in the absence of incident photons; receiving one or more photons via the waveguide; measuring an electrical property of the superconducting wire, proportional to a number of photons incident on the superconducting wire; and determining the number of received photons based on the electrical property.

Abstract: A notch filter circuit includes a wavelength demultiplexer, a first pump rejection filter coupled to the wavelength demultiplexer, and a second pump rejection filter coupled to the wavelength demultiplexer. The notch filter circuit also includes a first notch filter arm coupled to the first pump rejection filter and including a first chain of asymmetric Mach-Zehnder interferometers (MZIs) and a second notch filter arm coupled to the second pump rejection filter and including a second chain of asymmetric MZIs.

Abstract: A method includes fabricating a device including a first dielectric layer, an optical waveguide in the first dielectric layer, and a superconducting circuit in the first dielectric layer and on the optical waveguide. The method also includes forming a sacrificial structure on the first dielectric layer, the sacrificial structure aligned with the superconducting circuit, depositing a second dielectric layer on the sacrificial structure, and cutting an opening in the second dielectric layer to expose the sacrificial structure. The method further includes wet etching the sacrificial structure through the opening and sealing the opening in the second dielectric layer with a third dielectric layer to form a micro-channel between the first dielectric layer and the second dielectric layer.

Abstract: A photon counting device includes unit cells, a bias current source coupled to the unit cells, and a waveguide coupled to the unit cells. Each unit cell includes photodetector(s). Each photodetector includes superconducting component(s) and a transistor. The transistor includes a superconducting gate that is coupled in parallel with the photodetector(s), and a channel that is electrically isolated from the superconducting gate. For each unit cell, a photodetector is optically coupled to the waveguide. A superconducting component is configured to transition from the superconducting state to the non-superconducting state in response to a photon being incident upon the superconducting component while the superconducting component receives at least a portion of bias current output from the bias current source.

Abstract: A method of resolving a number of photons received by a photon detector includes optically coupling a waveguide to a superconducting wire having alternating narrow and wide portions; electrically coupling the superconducting wire to a current source; and electrically coupling an electrical contact in parallel with the superconducting wire. The electrical contact has a resistance less than a resistance of the superconducting wire while at least one narrow portion of the superconducting wire is in a non-superconducting state. The method includes providing to the superconducting wire, from the current source, a current configured to maintain the superconducting wire in a superconducting state in the absence of incident photons; receiving one or more photons via the waveguide; measuring an electrical property of the superconducting wire, proportional to a number of photons incident on the superconducting wire; and determining the number of received photons based on the electrical property.

Abstract: A photonic switch includes a first waveguide including a first region extending between a first coupler section and a second coupler section and a second region extending between the second coupler section and a third coupler section. The photonic switch also includes a second waveguide including a first portion extending between the first coupler section and the second coupler section, the first portion including at least two first compensation sections each having a different waveguide width, and a second portion extending between the second coupler section and the third coupler section, the second portion including at least two second compensation sections each having a different waveguide width. The photonic switch further includes at least one variable phase-shifter disposed in at least one of the first waveguide or the second waveguide.

Abstract: A method of resolving a number of photons received by a photon detector includes optically coupling a waveguide to a superconducting wire having alternating narrow and wide portions; electrically coupling the superconducting wire to a current source; and electrically coupling an electrical contact in parallel with the superconducting wire. The electrical contact has a resistance less than a resistance of the superconducting wire while at least one narrow portion of the superconducting wire is in a non-superconducting state. The method includes providing to the superconducting wire, from the current source, a current configured to maintain the superconducting wire in a superconducting state in the absence of incident photons; receiving one or more photons via the waveguide; measuring an electrical property of the superconducting wire, proportional to a number of photons incident on the superconducting wire; and determining the number of received photons based on the electrical property.

Abstract: A photonic switch includes a first waveguide including a first region extending between a first coupler section and a second coupler section and a second region extending between the second coupler section and a third coupler section. The photonic switch also includes a second waveguide including a first portion extending between the first coupler section and the second coupler section, the first portion including at least two first compensation sections each having a different waveguide width, and a second portion extending between the second coupler section and the third coupler section, the second portion including at least two second compensation sections each having a different waveguide width. The photonic switch further includes at least one variable phase-shifter disposed in at least one of the first waveguide or the second waveguide.

Abstract: A method of resolving a number of photons received by a photon detector includes optically coupling a waveguide to a superconducting wire having alternating narrow and wide portions; electrically coupling the superconducting wire to a current source; and electrically coupling an electrical contact in parallel with the superconducting wire. The electrical contact has a resistance less than a resistance of the superconducting wire while at least one narrow portion of the superconducting wire is in a non-superconducting state. The method includes providing to the superconducting wire, from the current source, a current configured to maintain the superconducting wire in a superconducting state in the absence of incident photons; receiving one or more photons via the waveguide; measuring an electrical property of the superconducting wire, proportional to a number of photons incident on the superconducting wire; and determining the number of received photons based on the electrical property.

Abstract: A Mach-Zehnder interferometer (MZI) filter comprising one or more passive compensation structures are described. The passive compensation structures yield MZI filters that are intrinsically tolerant to perturbations in waveguide dimensions and/or other ambient conditions. The use of n+1 waveguide widths can mitigate n different sources of perturbation to the filter. The use of at least three different waveguide widths for each Mach-Zehnder waveguide can alleviate sensitivity of filter performance to random width or temperature variations. A tolerance compensation portion is positioned between a first coupler section and a second coupler section, wherein the tolerance compensation portion includes a first compensation section having a second width, a second compensation section having a third width and a third compensation section having a fourth width, wherein the fourth width is greater than the third width and the third width is greater than the second width.

Abstract: Techniques disclosed herein relate generally to photonic integrated circuits working at cryogenic temperatures. In one example, a device includes a substrate, a dielectric layer on the substrate, an optical waveguide in the dielectric layer, a superconducting circuit in the dielectric layer and coupled to the optical waveguide, and a micro-channel in the dielectric layer and adjacent to the superconducting circuit. The micro-channel is aligned with the superconducting circuit and is configured to conduct a liquid at a cryogenic temperature to locally cool the superconducting circuit.