Device and method for frequency conversion of a photon

Abstract

A photon frequency conversion device comprises a first waveguide segment and a second waveguide segment, each encased in a nonlinear medium. The device is configured to superimpose an input photon of a first wavelength with an electromagnetic driving field of a second wavelength on the input side of an optical path through the first and second waveguide segments, in order to obtain an output photon of a third wavelength on the output side of the optical path. The device includes a filter element arranged in the optical path between the first and second waveguide segments. The filter element is configured to suppress or attenuate the propagation of at least one second-order or higher-order harmonic of the driving field on the optical path.

Description

[0001] Embodiments relate to a device for frequency conversion of a photon, in particular a device for quantum information-preserving frequency conversion of a photon. Further embodiments relate to a method for frequency conversion of a photon. Some embodiments relate to a filter system for minimizing noise in frequency converters for the efficient conversion of single-photon sources.

[0002] Current research is focusing on implementing networks between quantum computers. The fluorescence wavelengths of quantum bits (qubits, the computing units of a quantum computer) are typically not within the range of telecommunications wavelengths (1260 nm - 1625 nm), at which propagation losses are particularly low during transmission via fiber optics. Therefore, a conversion step is necessary to reduce propagation losses. Frequency conversion of photons signaling quantum information places special demands on a frequency converter that are not relevant for classical frequency conversion. In contrast to classical optical communication, the information to be transported is encoded in individual photons rather than in fields of macroscopic intensity. Therefore, the data rate during transmission is reduced by each additional photon of the same wavelength introduced (i.e., from another source).These photons are called noise photons.

[0003] In previous frequency converters for the application described above, the conversion process takes place in waveguide structures in periodically poled nonlinear media (NLM) (e.g., lithium niobate LNB or potassium titanyl phosphate KTP). In addition to the radiation field to be converted ("input signal, field" with wavelength λin), another field of high average power is incident, achieving optical-optical conversion efficiencies on the order of quantum efficiency (ηQ = λout / λin). Here, λout denotes the wavelength of the converted field ("output signal, field"). To reduce noise, current systems employ narrowband filtering in the output channel. From the article “Vahid Esfandyarpour, Carsten Langrock, and Martin Fejer, Cascaded downconversion interface to convert single-photon-level signals at 650 nm to the telecom band, Opt. Lett. 43, 5655-5658 (2018)” it is also known that between two stages photons of the output channel, iethe target wavelength, which were created in the first stage.

[0004] It would be desirable to enable frequency conversion with an improved trade-off between signal-to-noise ratio and complexity.

[0005] Embodiments of the present invention are based on the idea of ​​performing a frequency conversion of an input signal, or input photon, through interaction with a driver field in a nonlinear medium, wherein the frequency conversion is performed in multiple waveguide segments, for example, in multiple conversion stages or in multiple sections. The signal to be converted is wavelength-selectively filtered between the waveguide segments in order to attenuate or suppress fields with the wavelength of the second and / or a higher harmonic of the driver field. Embodiments are based on the finding that the short-wave fields of the second or higher harmonic of the driver field can make a significant contribution to the generation of noise photons through interaction with the nonlinear medium, for example, through "spontaneous parametric down-conversion", SPDC.By filtering the second or higher harmonics of the driving field between the waveguide segments, the propagation length of these fields in the nonlinear medium can be reduced, thus increasing the efficiency of generating noise photons at the wavelength of the output photon. This increases the signal-to-noise ratio of the frequency conversion.

[0006] Embodiments of the invention provide a device for frequency conversion of a photon. The device has a first waveguide segment and a second waveguide segment, each formed in a nonlinear medium. The device is designed to superimpose an input photon (e.g. an electromagnetic field, e.g. an input field or an electromagnetic radiation, e.g. an input radiation) of a first wavelength on the input side of an optical path through the first waveguide segment and the second waveguide segment with an electromagnetic driver field (or driver radiation) of a second wavelength in order to obtain an output photon (e.g. an output field or an output radiation) of a third wavelength on the output side of the optical path. The device has a filter element arranged in the optical path between the first waveguide segment and the second waveguide segment.The filter element is designed to suppress or attenuate propagation of at least one second-order or higher-order harmonic of the driver field along the optical path. The second-order harmonic, or second harmonic, of the driver field can refer to an electromagnetic field with a wavelength corresponding to half the second wavelength.

[0007] According to embodiments, a conversion process is realized, for example, by superimposing the photons to be converted with a driver field, e.g., a powerful field, in a nonlinear medium. However, the presence of a powerful field in a nonlinear medium can cause photons to be generated in all spectral ranges of longer wavelengths and therefore also in the output channel, e.g., via SPDC. In addition to the direct generation of noise by SPDC, higher-order processes can also contribute to the noise: Through nonlinear effects, the powerful laser can generate shorter-wavelength fields, e.g., at the second harmonic (“second harmonic generation” (SHG)) or higher harmonics. These can then also generate SPDC noise photons in the output channel. Embodiments of the invention reduce the efficiency of these SPDC processes (e.g.,driven by higher harmonics) while maintaining a consistently high efficiency of the desired conversion process.

[0008] In examples, the frequency conversion of photons, e.g., into the infrared spectral range relevant for telecommunications (1260 nm to 1625 nm), is realized by one or more second-order nonlinear conversion processes (three-wave mixing) in a nonlinear medium with a waveguide structure. For example, periodically poled crystals or bulk crystals can be used. To achieve a conversion efficiency on the order of quantum efficiency, a powerful field ("driver, radiation, field") with the second wavelength λdrive can be spatially superimposed on the input radiation. The frequency conversion can be performed in several conversion stages or sections, making it possible to insert wavelength-dependent filters between these conversion stages or sections.Here, fields at the higher (second and / or higher) harmonics of the powerful field are filtered, which can then no longer contribute to the generation of noise in the output channel.

[0009] According to one embodiment, the device is designed to obtain the output photon by means of one or more nonlinear conversion processes, e.g., second-order, in the first and / or the second waveguide segment. For example, the device can be designed to obtain the output photon by means of a three-wave mixing process based on the input photon and involving a photon of the wavelength of the driver field. Alternatively, the conversion process can be multi-stage, i.e., involve a photon of a further wavelength, e.g., an intermediate wavelength, wherein the different stages can take place in the different waveguide segments.

[0010] According to one embodiment, the device is designed to superimpose the input photon with the electromagnetic driver field in the first waveguide segment in order to obtain a photon of a fourth wavelength, e.g., a wavelength shorter than the third wavelength, on the output side of the first waveguide segment. According to this embodiment, the device is further designed to superimpose the photon of the fourth wavelength with the electromagnetic driver field or with a further electromagnetic driver field in the second waveguide segment, e.g., to obtain the output photon, or to obtain a photon of a further wavelength, which is fed to a further waveguide segment.

[0011] In other words, the conversion process can be multi-stage, and the first waveguide segment can be configured for a first conversion stage and the second waveguide segment for a second conversion stage, with filtering of the second and / or higher harmonics of the driver field being performed between the conversion stages. For example, a driver field of the second wavelength or a driver field of a further wavelength can be used for the second conversion stage.

[0012] According to one embodiment, the first waveguide segment is configured to generate the photon of the fourth wavelength by means of three-wave mixing from the input photon and with the participation, e.g., with the generation or annihilation, of a photon of the wavelength of the electromagnetic driver field. According to this embodiment, the device is further configured to generate the output photon by means of three-wave mixing from the photon of the fourth wavelength and with the participation, e.g., with the generation or annihilation, of a photon of the wavelength of the electromagnetic driver field or another electromagnetic driver field. The conversion process can therefore be two-stage. This offers a good compromise between complexity, conversion efficiency, and signal-to-noise ratio.

[0013] According to one embodiment, the nonlinear medium of the first waveguide segment and the nonlinear medium of the second waveguide segment are periodically poled. The periodicity of the polarity of the first waveguide segment differs from the periodicity of the polarity of the second waveguide segment. A periodic polarity of the nonlinear media can result in a phase match between the input field and the driver field, or between the photon of the fourth wavelength and the driver field. This increases the spatial superposition of the fields and improves the conversion efficiency. The periodicity of the polarity can be adjusted according to the wavelengths of the fields involved in the conversion process, so that different periodicities can be advantageous in a multi-stage conversion process.

[0014] According to one embodiment, the device is designed to provide the further electromagnetic driver field in the second waveguide segment, wherein the further electromagnetic driver field has the second wavelength. Alternatively, the further electromagnetic driver field can have a further wavelength. The provision of the further driver field can ensure high performance of the driver field in the second waveguide segment. Furthermore, the multi-stage conversion process can be flexibly defined with regard to the energy distribution between the conversion stages, which enables a selection of the size of the individual conversion steps, e.g. with regard to the conversion efficiency or with regard to the selection of nonlinear media.

[0015] According to one embodiment, the first waveguide segment and the second waveguide segment are each configured to generate the output photon by means of three-wave mixing based on the input photon and involving a photon of the wavelength of the electromagnetic driver field, e.g., by generating or annihilating a photon of the wavelength of the electromagnetic driver field. The conversion process can thus be implemented in a single stage, whereby a propagation path in the nonlinear medium can be divided into several waveguide segments, between which the second and / or higher harmonics can be filtered to minimize their propagation length in the nonlinear medium.

[0016] According to one embodiment, the nonlinear medium of the first waveguide segment and the nonlinear medium of the second waveguide segment are periodically poled, with the periodicity of the polarity of the first waveguide segment and the periodicity of the polarity of the second waveguide segment being identical. Thus, the two waveguide segments can be configured, for example, for the same conversion process.

[0017] According to one embodiment, the filter arrangement is designed to reduce the propagation of the second harmonic of the driver field on the optical path by a factor of at least 10 3 , or at least 10 5 , or at least 10 6 to suppress. The greater the suppression, the more the efficiency of generating noise photons by the second harmonic in the second waveguide segment is reduced.

[0018] According to one embodiment, the filter element includes one or more of the following elements: a reflective element, an absorbing element, a refractive element, a scattering element, a diffractive element, an interfering element.

[0019] According to one embodiment, the filter element is formed in a waveguide. This makes it possible to provide the optical path entirely through waveguides, for example.

[0020] According to one embodiment, the first waveguide segment, the second waveguide segment, and the filter element are formed on an optical chip. Optical chips enable a compact implementation of optical structures or circuits.

[0021] According to one embodiment, the filter element is designed as an optical fiber. An optical fiber enables flexible and efficient coupling of the two waveguide segments while integrating the filter element.

[0022] According to one embodiment, the optical path between the first waveguide segment and the second waveguide segment comprises a free-space segment. The filter element is arranged in the free-space segment. A free-space segment allows for considerable flexibility in the implementation of the filter element.

[0023] According to one embodiment, the second wavelength is longer than the first wavelength, and the second wavelength is longer than the third wavelength. Thus, the wavelength of the driver field is the longest wavelength, resulting in low noise at the output wavelength, i.e., the third wavelength.

[0024] According to one embodiment, the third wavelength is longer than the first wavelength. The conversion process is therefore a down-conversion process, which can provide high conversion efficiency.

[0025] According to one embodiment, the third wavelength lies in a range between 1.2 µm and 1.7 µm. These wavelengths enable low-loss transmission in optical fibers. At wavelengths in this range, SPDC can generate a significant rate of noise photons, particularly from higher harmonics of the driver field. Filtering these photons can thus improve the signal-to-noise ratio, particularly in this wavelength range.

[0026] According to one embodiment, the filter element is configured to allow an electromagnetic field of the second wavelength to pass along the optical path. This allows the driver field of the first waveguide segment to be used in the second waveguide segment. This is particularly advantageous in embodiments with a single-stage conversion process in multiple sections or in multi-stage conversion processes based on driver fields of the same wavelength.

[0027] According to one embodiment, the filter element is designed to couple electromagnetic radiation exiting the first waveguide segment at the output side, e.g., radiation that passes through the filter element and is therefore not filtered by it, into the second waveguide segment. In other words, the filter element can be designed to optically image an output of the first waveguide segment onto an input of the second waveguide segment. The filter element can thus provide part of the optical path and / or be designed to keep an electromagnetic field propagating along the optical path on the optical path, while simultaneously performing the filtering function.

[0028] Further embodiments of the invention provide a method for frequency conversion of a photon, comprising: coupling an input photon and an electromagnetic driver radiation into an optical path through a first waveguide segment and a second waveguide segment to generate an output photon by superimposing the input photon with the electromagnetic driver radiation; filtering a signal propagating on the optical path between the first waveguide segment and the second waveguide segment to suppress propagation of at least a second harmonic of the driver field on the optical path.

[0029] Examples of the invention are described below with reference to the accompanying figures. They show: Fig. 1 illustrates a device for frequency conversion of a photon according to an embodiment, Fig. 2 illustrates a device for frequency conversion of a photon according to an embodiment with multi-stage conversion, Fig. 3 illustrates a device for frequency conversion of a photon according to an embodiment with several conversion sections, Fig. 4 illustrates a device for frequency conversion of a photon according to an embodiment with a waveguide-integrated filter element, Fig. 5A, B illustrate devices for frequency conversion of a photon according to embodiments with a filter element integrated into an optical fiber, Fig. 6 illustrates a device for frequency conversion of a photon according to an embodiment with a free beam segment, Fig. 7 shows a block diagram of a method for frequency conversion of a photon according to an embodiment.

[0030] Examples of the present disclosure are described in detail below using the accompanying descriptions. Many details are described in the following description to provide a more thorough explanation of examples of the disclosure. However, it will be apparent to those skilled in the art that other examples may be implemented without these specific details. Features of the various described examples may be combined with one another unless features of a corresponding combination are mutually exclusive or such a combination is expressly excluded.

[0031] It should be noted that identical or similar elements, or elements having the same functionality, may be provided with identical or similar reference symbols or be designated alike. Repeated descriptions of elements that are provided with identical or similar reference symbols or are designated alike are typically omitted. Descriptions of elements that have identical or similar reference symbols or are designated alike are interchangeable.

[0032] Fig. 1 illustrates a device 10 for frequency conversion of a photon according to one embodiment. The device 10 can also be referred to as a frequency converter. The device 10 has a first waveguide segment 31 and a second waveguide segment 32. The first waveguide segment 31 and the second waveguide segment 32 are each formed in a nonlinear medium, i.e., for example, a medium in which nonlinear optical effects occur. For example, the first waveguide segment 31 and the second waveguide segment can be formed in a common nonlinear medium. Alternatively, the first waveguide segment and the second waveguide segment can each be formed in a respective nonlinear medium, i.e., the first waveguide segment in a first nonlinear medium and the second waveguide segment in a second nonlinear medium.

[0033] The device 10 further includes an optical path 20 comprising the first waveguide segment 10 and the second waveguide segment 20. In other words, the optical path 10 may extend through the first waveguide segment 31 and the second waveguide segment 32. In other words, the first waveguide segment 31 and the second waveguide segment 32 may each provide a portion of the optical path 20.

[0034] The device 10 is configured to superimpose an input photon 22 of a first wavelength on an input side of an optical path 20 through the first waveguide segment 31 and the second waveguide segment 32 with an electromagnetic driver field 24 of a second wavelength in order to obtain an output photon 28 of a third wavelength on the output side of the optical path. For example, the device 10 can be configured to provide the input photon 22 and the driver field 24 on the input side of the optical path 20 in order to superimpose the input photon 22 with the driver field 24 on the optical path 20.

[0035] The device further comprises a filter element 50 arranged in the optical path 20 between the first waveguide segment 31 and the second waveguide segment 32. The filter element is designed to suppress or attenuate propagation of at least one second-order or higher-order harmonic of the driver field along the optical path. The second-order harmonic of the driver field can refer to an electromagnetic field with a wavelength corresponding to half the wavelength of the driver field.

[0036] In other words, the filter element 50 can be configured to wavelength-selectively filter an electromagnetic field propagating along the optical path 20, suppressing at least the propagation of an electromagnetic field of a wavelength corresponding to a second-order or higher harmonic of the driver field. The suppression can be achieved, for example, by absorption, reflection, diffraction, refraction, scattering, or interference.

[0037] For example, the filter element 50 is configured to suppress propagation of photons of a wavelength of the second harmonic and / or one or more higher order harmonics of the driver field on the propagation path.

[0038] The filter element 50 may, for example, be designed to filter photons of the first wavelength and / or the third wavelength and / or an intermediate wavelength (e.g. as described later with respect to Fig. 2) unfiltered, ie essentially unattenuated, on the optical path 20.

[0039] By superimposing the input photon 22 with the driver field 24 in a nonlinear medium, a conversion process can take place based on nonlinear optical effects in the nonlinear medium, in which conversion process the output photon 28 or a photon of an intermediate wavelength is generated by annihilating the input photon 22 and under the influence of the driver field 24. In other words, the input photon 22 can be converted into the output photon or a photon of an intermediate wavelength during the conversion process. As described in detail below, the first waveguide segment and the second waveguide segment can be configured for similar conversion processes, i.e., both the first and the second waveguide segment can be configured to generate a photon of the third wavelength, i.e., the output photon 28, by annihilating the input photon 22.In this case, the output photon 28 is generated either in the first or the second waveguide segment. Alternatively, the first waveguide segment 31 can be designed for a first conversion process in which a photon of a fourth wavelength, for example referred to as an intermediate wavelength, is generated by annihilating the input photon 22 and under the influence of the driver field 24, and the second waveguide segment 32 can be designed for a second conversion process in which the output photon 28 or a further intermediate photon, e.g. a photon of a fifth wavelength, from which the output photon is generated in a further conversion process, is generated by annihilating the photon of the fourth wavelength and under the influence of the driver field 24 or a further driver field.

[0040] The conversion processes (taking place in the first and second waveguide segments, for example) can therefore be three-wave mixing processes, such as differential frequency generation. The ratio of the first, second, and third wavelengths can result from energy conservation.

[0041] For example, in the conversion process of the first waveguide segment 31, a photon of the wavelength of the driver field and either the output photon 28 or a photon of the intermediate wavelength can be generated by annihilating the input photon 22.

[0042] In some examples, the output photon can be generated by a down-conversion process.

[0043] According to embodiments, the device 10 is thus designed to obtain the output photon 28 by means of one or more nonlinear conversion processes, e.g. second order, in the first and / or the second waveguide segment.

[0044] The waveguide structures can cause a local limitation of the fields propagating on the optical path, thereby increasing the probability of the conversion process and thus its efficiency.

[0045] For example, the input photon may be part of an input signal or input field having the first wavelength. Device 10 may therefore be configured to provide the input signal or input field of the first wavelength on the input side of the optical path in order to convert it into an output field of the third wavelength.

[0046] For example, the input field may consist of single photons of the first wavelength. This means, for example, that the input signal has a rate of less than 1 MHz or less than 100 kHz.

[0047] The device 100 can thus, for example, be designed to convert single photons in a quantum information-preserving manner.

[0048] The driver field 24 may, for example, be a high-power field, ie, have a higher power than the input field. For example, the power of the driver field may be increased by a factor of 10 4 or 10 6 or 10 8 be greater than the power of the input field. A high power of the driver field achieves high conversion efficiency.

[0049] For example, the efficiency of the conversion process refers to the power ratio of the output field to the input field.

[0050] Through the interaction of the driver field with the nonlinear medium in which the first or second waveguide segment is formed, photons can be generated that have a longer wavelength than the driver field, for example, including photons of the output wavelength (third wavelength). This can result in photons of the third wavelength, which arise from the conversion of input photons into output photons. Since these photons are indistinguishable from the output photons based on their wavelength, they represent noise photons. For example, these noise photons can be generated through spontaneous parametric downconversion (SPDC).In addition to the direct generation of noise by SPDC, higher-order processes can also contribute to noise: Due to nonlinear effects in the nonlinear medium, the powerful driver field can generate shorter-wavelength fields, for example, at the second harmonic generation (SHG) or higher harmonics. These can then also generate noise photons in the output channel via SPDC.

[0051] Filtering, i.e., suppressing, the second or higher harmonic of the driver field by means of the filter element 50 prevents propagation of photons of the second or higher harmonic generated in the first waveguide segment 31 in the second waveguide segment, and thus prevents generation of noise photons based on these photons of the second or higher harmonic in the second waveguide segment 32. In other words, filtering the photons of the second or higher harmonic shortens their propagation length in the nonlinear medium as viewed on the optical path 20. Since the probability of generating a noise photon based on a photon of the second or higher harmonic is proportional to its propagation length in the nonlinear medium, filtering the photons of the second or higher harmonic between the first and second waveguide segments significantly reduces the generation of noise photons.Accordingly, the generation of noise photons can be reduced by suppressing the propagation of second or higher harmonic photons.

[0052] According to embodiments, the optical path has one or more further waveguide segments, wherein a filter element is arranged in the optical path between two adjacent waveguide segments in order to suppress photons of the second and / or higher harmonics of a driver field used in the respective preceding waveguide segment, e.g. the driver field 24.

[0053] The more filter elements the optical path has, the shorter the propagation time of generated second or higher harmonic photons on the optical path, so that a higher number of waveguide segments with intermediate filter elements can reduce the number of noise photons.

[0054] According to one embodiment, the nonlinear medium of the first waveguide segment and the nonlinear medium of the second waveguide segment (e.g., the same or another nonlinear medium) are periodically poled.

[0055] By periodically polarizing nonlinear media, a phase adjustment of the two input fields of a conversion process, e.g. the input photon and the driver radiation 24, can be achieved, whereby the efficiency of the conversion process can be increased.

[0056] According to one embodiment, the filter arrangement 50 is designed to reduce the propagation of the second harmonic of the driver field on the optical path by a factor of at least 10 3 , or at least 10 5 , or at least 10 6 to suppress.

[0057] According to one embodiment, the filter arrangement 50 is designed to reduce the propagation of the second and / or third harmonic of the driver field on the optical path by a factor of at least 10 3 , or at least 10 5 , or at least 10 6 to suppress.

[0058] According to one embodiment, the filter element 50 is configured to suppress the propagation of the second and / or third harmonic of the driver field on the optical path by reflection or absorption or refraction or scattering or diffraction or interference.

[0059] For example, the filter element 50 contains one or more of the following elements: - a reflective element, - an absorbing element, - a breaking element, - a scattering element, - a diffractive element, - an interfering element.

[0060] According to one embodiment, the second wavelength is longer than the first wavelength and the second wavelength is longer than the third wavelength.

[0061] The wavelength of the output photon can therefore be the longest wavelength in a three-wave mixing process.

[0062] According to one embodiment, the third wavelength is longer than the first wavelength.

[0063] For example, the third wavelength is in a range between 1.2 µm and 1.7 µm, e.g. in a range between 1260 nm and 1625 nm.

[0064] The first wavelength can, for example, be in a range between 500 nm and 800 nm.

[0065] According to one embodiment, the filter element 50 is designed to couple electromagnetic radiation exiting the first waveguide segment 31 on the output side, e.g., radiation that passes through the filter element 50, i.e., is not filtered by it, into the second waveguide segment 32. In other words, the filter element 50 can be designed to optically image an output of the first waveguide segment 31 onto an input of the second waveguide segment.

[0066] For example, a surface region of the filter element facing the first waveguide segment 31 and / or a surface region of the filter element facing the second waveguide segment 32 can be shaped to optically map the output of the first waveguide segment 31 onto the input of the second waveguide segment, e.g., be spherically curved. In another example, the filter element 50 can be formed in a volume such that the output of the first waveguide segment 31 is mapped onto the input of the second waveguide segment 32, e.g., have a refractive index gradient.

[0067] Fig. 2 illustrates a further embodiment of the device 10. According to the example of Fig. 2, the device 10 is designed to superimpose the input photon 22 with the electromagnetic driver field 24 in the first waveguide segment 31 in order to obtain a photon 26 of a fourth wavelength on the output side of the first waveguide segment 31, e.g. by means of a conversion process in the first waveguide segment 31. Furthermore, according to this example, the device is designed to superimpose the photon 26 of the fourth wavelength with the electromagnetic driver field 24 or with a further electromagnetic driver field in the second waveguide segment 32.

[0068] For example, the second waveguide segment 32 can be configured to generate the output photon 28 by means of a conversion process based on the photon 26 of the fourth wavelength. Alternatively, the second waveguide segment 32 can be configured to obtain a photon of a further wavelength by means of a conversion process based on the photon 26 of the fourth wavelength, which photon is fed to a further waveguide segment arranged downstream of the second waveguide segment 32 in the optical path 20.

[0069] For example, the fourth wavelength may be shorter than the third wavelength.

[0070] As already mentioned, the conversion processes in the first and second waveguide segments can each be three-wave mixing processes.

[0071] In other words, the Fig. Figure 2 shows an example in which the conversion from the input photon 22 to the output photon 28 takes place in multiple stages, e.g., in two stages, in the sense that at least two conversion processes take place to convert the input photon into the output photon. Further embodiments include more than two conversion stages.

[0072] According to examples, the first waveguide segment 31 is configured for converting the input photon 22 into the photon 26 of the fourth wavelength, and the second waveguide segment 32 is configured for converting the photon 26 into the output photon 28 or another photon of a further wavelength. For example, the periodicity of the polarity of the nonlinear media or the configuration of the waveguide segments is adapted to the wavelengths participating in the conversion process.

[0073] For example, the nonlinear medium of the first waveguide segment 31 and the nonlinear medium of the second waveguide segment 32 are periodically poled, wherein a periodicity of the polarity of the first waveguide segment is different from a periodicity of the polarity of the second waveguide segment.

[0074] In examples, the device 10 is configured to provide the driver field 24 in the first and second waveguide segments. For example, the filter element 50 may be transparent to the driver radiation 24. In other words, the same driver radiation may be used in both or more conversion processes of the multi-stage conversion process.

[0075] According to embodiments, the device may be configured to provide a further electromagnetic driver field 24' in the second waveguide segment 32. The further electromagnetic driver field 24' may have the second wavelength.

[0076] Alternatively, the further electromagnetic driver field 24' may have a further wavelength.

[0077] The second waveguide segment 32 can thus be designed for a conversion of the photon 26 with a photon of the further driver field 24' into the output photon 28 or a photon of a further intermediate wavelength.

[0078] Fig. 3 illustrates a further embodiment of the device 10. According to the example of Fig. 3, the first waveguide segment 31 and the second waveguide segment 32 are each configured to generate the output photon 28 by three-wave mixing based on the input photon 22 and a photon of the electromagnetic driving field 24.

[0079] The conversion of the input photon 22 into the output photon 28 can thus take place in a single step, for example, a single three-wave mixing process. The conversion then takes place either in the first waveguide segment 31 or the second waveguide segment 32, for example, depending on a probability of the conversion process or an efficiency of the conversion process, and depending on the length of the first waveguide segment 31.

[0080] In other words, according to the embodiment of the Fig. 3, a waveguide designed for converting the input photon 22 into the output photon 28 can be divided into several sections, e.g. two or more than two sections, between each of which a filter element 50 is arranged to suppress photons of the second and / or a higher harmonic of the driver field 24.

[0081] According to the embodiment of Fig. 3, the first and second waveguide segments can be designed for the same conversion process.

[0082] For example, the nonlinear medium of the first waveguide segment and the nonlinear medium of the second waveguide segment are periodically poled, wherein a periodicity of the polarity of the first waveguide segment 31 and a periodicity of the polarity of the second waveguide segment 32 are identical.

[0083] Fig. 4 illustrates a further embodiment of the device 10, according to which the filter element 50 is formed in a waveguide.

[0084] For example, as in Fig. 4, the waveguide in which the filter element 50 is formed can be arranged adjacent to the first waveguide segment 31 on the output side with respect to the optical path and adjacent to the waveguide segment 32 on the input side.

[0085] As in the Fig. As shown in Figure 4, the first waveguide segment 41 may be formed in a first nonlinear medium 41, and the second waveguide segment 42 may be formed in a second nonlinear medium 42. The first and second nonlinear media may be identical or different.

[0086] According to one embodiment, the first waveguide segment 31, the second waveguide segment 32 and the filter element 50 are formed on an optical chip.

[0087] For example, the optical path 20 may be provided exclusively by waveguides.

[0088] For example, the device 10 can be as shown in Fig. 4 may represent an optical chip or a part of an optical chip.

[0089] In other words, according to one embodiment, all conversion stages are designed on a single optical chip. Optical filters integrated into the waveguide on the chip can be inserted between the conversion stages. These filters can spectrally separate the higher harmonics (e.g., the second and / or one or more higher harmonics) of the driver field. Reflective, absorbing, refractive, scattering, diffractive, and / or interfering structures, materials, or layer systems can be used as filter elements.

[0090] Fig. 5A and Fig. 5B illustrate further embodiments of the device 10, according to which the filter element 50 is formed as an optical fiber or in an optical fiber.

[0091] The optical fiber may, for example, comprise at least partially reflecting, absorbing, refractive, scattering, diffracting, and / or interfering structures or materials designed to suppress the propagation of one or more higher harmonics of the driver field.

[0092] According to the embodiment of the Fig. 5A, the optical fiber is connected to an output side 34 of the first waveguide segment 31 and to an input side 36 of the second waveguide segment 32, e.g., permanently connected or directly connected. This means, for example, that the optical path between the waveguide segments and the optical fiber does not have a free-space section.

[0093] According to the embodiment of the Fig. 5B, the device 10 has a coupling optic 61 for coupling photons emerging from the first waveguide segment 31 into the optical fiber, and a further coupling optic 62 for coupling photons emerging from the optical fiber into the second waveguide segment 32.

[0094] Fig. Fig. 6 illustrates a further embodiment of the device 10, according to which the optical path 20 between the first waveguide segment 31 and the second waveguide segment 32 comprises a free-beam segment, and wherein the filter element is arranged in the free-beam segment 54, as in Fig. 6 the filter element 50a.

[0095] For example, the device 10 may include an output coupling unit 51 for collecting or collimating an electromagnetic field emerging from the output side of the waveguide segment 31 onto the free-beam segment 54 of the optical path. Furthermore, the device 10 may include an input coupling unit 52 for coupling an electromagnetic field of the free-beam segment 54 into the second waveguide segment 32.

[0096] The Fig. The filter element 50a shown in Figure 6 can be an absorbing filter element. Alternatively to the filter element 50a, a reflective filter element 50b or a refractive filter element 50c can be arranged in the free jet segment 54. As further alternatives, a scattering, a diffractive, or an interfering filter element can be arranged in the free jet segment 54.

[0097] As already described, the embodiments according to the Fig. 4, Fig. 5 and Fig. 6 the filter element 50 may be formed from one or more of a reflecting, absorbing, refractive, scattering, diffracting and interfering element, structures, materials or layer systems.

[0098] The Fig. The embodiments of the filter element 50 or the coupling between the first and the second waveguide segment shown in Figures 4 to 6 are equally applicable between further waveguide segments in embodiments that have more than two waveguide segments, e.g. embodiments that have more than two conversion stages and / or more than two conversion sections.

[0099] Fig.7 shows a block diagram of a method 100 for frequency conversion of a photon according to an embodiment. The method 100 includes coupling 101 an input photon and electromagnetic driver radiation into an optical path through a first waveguide segment and a second waveguide segment to generate an output photon by superimposing the input photon with the electromagnetic driver radiation. Furthermore, the method 100 includes filtering 102 a signal propagating on the optical path between the first waveguide segment and the second waveguide segment to suppress propagation of at least a second harmonic of the driver field on the optical path.

[0100] Although some aspects of the present disclosure have been described as features associated with a device, it is clear that such a description may also be considered a description of corresponding method features. Although some aspects have been described as features associated with a method, it is clear that such a description may also be considered a description of corresponding features of a device or the functionality of a device.

[0101] In the foregoing Detailed Description, various features have been grouped together in examples in order to streamline the disclosure. This manner of disclosure should not be interpreted as intending that the claimed examples include more features than are expressly recited in each claim. Rather, as the following claims reflect, the subject matter may lie in fewer than all of the features of a single disclosed example. Accordingly, the following claims are hereby incorporated into the Detailed Description, with each claim being capable of standing as its own separate example.While each claim may stand as its own separate example, it should be noted that although dependent claims in the claims refer to a specific combination with one or more other claims, other examples also include a combination of dependent claims with the subject matter of any other dependent claim or a combination of any feature with other dependent or independent claims. Such combinations are intended to be encompassed unless it is stated that a specific combination is not intended. Furthermore, it is intended to encompass a combination of features of a claim with any other independent claim, even if that claim is not directly dependent on the independent claim.

[0102] The above-described embodiments are merely illustrative of the principles of the present disclosure. It is understood that modifications and variations of the arrangements and details described herein will be apparent to others skilled in the art. Therefore, it is intended that the disclosure be limited only by the scope of the following claims and not by the specific details presented in the description and explanation of the embodiments herein. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Vahid Esfandyarpour, Carsten Langrock, and Martin Fejer, Cascaded downconversion interface to convert single-photon-level signals at 650 nm to the telecom band, Opt. Lett. 43, 5655-5658 (2018

[0003]

Claims

[1] Device (10) for frequency conversion of a photon, comprising: a first waveguide segment (31) and a second waveguide segment (32), each formed in a non-linear medium, wherein the device is designed to superimpose an input photon (22) of a first wavelength on an optical path (20) through the first waveguide segment (31) and the second waveguide segment (32) on the input side with an electromagnetic driver field (24) of a second wavelength in order to obtain an output photon (28) of a third wavelength on the output side of the optical path, wherein the device comprises a filter element (50) arranged in the optical path (20) between the first waveguide segment (31) and the second waveguide segment (32), wherein the filter element is designed to suppress propagation of at least one second or higher order harmonic of the driver field on the optical path. [2] The device according to claim 1, wherein the device is configured to obtain the output photon (28) by means of one or more non-linear conversion processes in the first and / or the second waveguide segment. [3] Device according to claim 1 or 2, wherein the device is designed to superimpose the input photon (22) with the electromagnetic driver field (24) in the first waveguide segment (31) in order to obtain a photon (26) of a fourth wavelength on the output side of the first waveguide segment (31), and to superimpose the photon (26) of the fourth wavelength with the electromagnetic driver field (24) or with a further electromagnetic driver field (24') in the second waveguide segment (32). [4] Device according to one of claims 1 to 3, wherein the first waveguide segment (31) is designed to generate the photon (26) of the fourth wavelength by means of three-wave mixing from the input photon (22) and with the participation of a photon of the wavelength of the electromagnetic driver field, and wherein the second waveguide segment (32) is designed to generate the output photon (28) by means of three-wave mixing from the photon (26) of the fourth wavelength and with the participation of a photon of the wavelength of the electromagnetic driver field or a further electromagnetic driver field. [5] Device according to one of claims 1 to 4, wherein the non-linear medium of the first waveguide segment (31) and the non-linear medium of the second waveguide segment (32) are periodically poled, wherein a periodicity of the polarity of the first waveguide segment (31) is different from a periodicity of the polarity of the second waveguide segment (32). [6] Device according to one of claims 1 to 5, wherein the device is configured to provide the further electromagnetic driving field (24') in the second waveguide segment (32), wherein the further electromagnetic driving field has the second wavelength, or wherein the further electromagnetic driving field has a further wavelength. [7] The device of claim 1, wherein the first waveguide segment (31) and the second waveguide segment are each configured to generate the output photon (28) by means of three-wave mixing based on the input photon (22) and involving a photon of the wavelength of the electromagnetic driving field. [8] The device according to any one of claims 1, 2 or 7, wherein the non-linear medium of the first waveguide segment and the non-linear medium of the second waveguide segment are periodically poled, wherein a periodicity of the polarity of the first waveguide segment and a periodicity of the polarity of the second waveguide segment are identical. [9] Device according to one of the preceding claims, wherein the filter element (50) is designed to reduce the propagation of the second harmonic of the driver field on the optical path by a factor of at least 10 3 , or at least 10 5 , or at least 106 to suppress. [10] Device according to one of the preceding claims, wherein the filter element (50) contains one or more of the following elements: - a reflective element, - an absorbing element, - a breaking element, - a scattering element, - a diffractive element, - an interfering element. [11] Device according to one of the preceding claims, wherein the filter element (50) is formed in a waveguide. [12] The device according to claim 11, wherein the first waveguide segment (31), the second waveguide segment and the filter element (50) are formed on an optical chip. [13] Device according to one of claims 1 to 10, wherein the filter element (50) is designed as an optical fiber. [14] Device according to one of claims 1 to 10, wherein the optical path (20) between the first waveguide segment and the second waveguide segment comprises a free-beam segment (54), and wherein the filter element (50a, 50b, 50c) is arranged in the free-beam segment. [15] Apparatus according to any one of the preceding claims, wherein the second wavelength is longer than the first wavelength and wherein the second wavelength is longer than the third wavelength. [16] Device according to one of the preceding claims, wherein the third wavelength is longer than the first wavelength. [17] Device according to one of the preceding claims, wherein the third wavelength is in a range between 1.2 µm and 1.7 µm. [18] Device according to one of the preceding claims, wherein the filter element (50) is designed to allow an electromagnetic field of the second wavelength to pass on the optical path (20). [19] Device according to one of the preceding claims, wherein the filter element (50) is designed to couple electromagnetic radiation emerging from the output side of the first waveguide segment (31) into the second waveguide segment (32). [20] Method (100) for frequency conversion of a photon, comprising: Coupling (101) an input photon (22) and an electromagnetic driver radiation (24) into an optical path (20) through a first waveguide segment (31) and a second waveguide segment (32) in order to generate an output photon by superimposing the input photon (22) with the electromagnetic driver radiation, Filtering (102) a signal propagating on the optical path (20) between the first waveguide segment (31) and the second waveguide segment (32) in order to suppress propagation of at least a second harmonic of the driver field on the optical path (20).

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