Single-photon spectrometer and method for determining a wavelength of at least one photon

Abstract

The invention relates to a single-photon spectrometer (10) having at least one dispersion element (40) which has a first optical dispersion, and having a photon detector (30) which is arranged to detect at least one photon (15) which has passed through the dispersion element (40), and having a time-of-flight meter (60) which is designed to automatically measure a propagation time (t) of the at least one photon (15) through the dispersion element (40), wherein the photon detector (30) is designed to automatically determine a wavelength (λ) of the at least one photon (15) from the propagation time (t).

Description

[0001] ' U > - 1 ' ^ M i

[0002]

[0003] inte! I ectua I profitty Physikalisch-Technische Bundesanstalt Lawyer's file:

[0004] Braunschweig and Berlin 0454-0335 PCT-1 Bundesallee 100

[0005] 38116 Braunschweig Date:

[0006] December 3, 2025

[0007] Single-photon spectrometer and method for determining a wavelength of at least one photon

[0008] The invention relates to a single-photon spectrometer. Furthermore, the invention relates to a method for determining the wavelength of at least one photon using a single-photon spectrometer.

[0009] Single-photon spectrometers are used to determine the wavelength of individual photons or groups consisting of a few photons, particularly fewer than 100, preferably fewer than 50. This is important, for example, in astronomy when analyzing the light from stars, from which only a few photons per second reach the detectors. From the wavelength or spectrum of the detected light, it is then possible to determine, for example, which elements are contained in the star or which are optically located between the star and the detector. Single-photon measuring instruments are also relevant for applications such as quantum communication and quantum information processing.

[0010] In known measuring devices, the wavelength is converted into a spatial distance using a prism or other dispersive elements. This spatial distance is then detected, and the wavelength is determined from it. A disadvantage of this method is that the measurement accuracy is significantly limited by the spatially restricted size of the detector.

[0011] From Bingke Shi et al., Optics Express 43805, Vol. 32, No. 25, 2 Dec 2024, a quantum two-way optical fiber connection is known that includes a biphoton source, a programmable waveformer, an optical circulator, single-mode optical fiber coils, a superconducting nanowire, a single-photon detector, and a time tagger for determining the arrival time of the single photons. From Mohsen K. Akhlaghi et al., Nature Communications 6:8233, a single-photon detector with a superconducting nanowire in an optical fiber with optimized photon efficiency is known.

[0012] From Raupach, SMF et al., Physical Review B 108, 054507 (2023) a single-photon detector is known in which photon pulses of defined mean photon number are applied to a superconducting nanowire single-photon detector and a time tagger is used to determine the photon impact events, where it was found that the afterpulse probability depends on the photon number.

[0013] The invention is based on the objective of improving single-photon spectrometry.

[0014] The invention solves the problem by a single-photon spectrometer comprising (a) a dispersion element having a first optical dispersion, and (b) a photon detector arranged to detect photons that have passed through the first dispersion element, and (c) a time-of-flight meter configured to automatically measure the transit time of at least one photon through the dispersion element, wherein (d) the photon detector is configured to automatically determine the wavelength of at least one photon from the dispersion of the dispersion element and the transit time.

[0015] The problem is further solved by a method for determining a wavelength of at least one photon using a single-photon spectrometer, in particular according to the invention, comprising the steps (a) measuring a transit time of at least one photon through a dispersion element of the single-photon spectrometer and (b) determining a wavelength of the at least one photon from the transit time.

[0016] The advantage is that no moving components are required for wavelength determination, because instead of the usual spatial division according to different wavelengths and required spatial detection, a time difference is detected.

[0017] Compared to measuring spatial distances, wavelength measurement via time is advantageous because the measurement uncertainty can be lower, as time intervals can usually be measured more accurately than spatial distances. Furthermore, the spectrometer typically requires less space than detectors that measure spatial dispersion. This allows for a more compact design and thus simpler construction, installation, and transport.

[0018] Within the scope of this description, a single-photon spectrometer is understood to mean, in particular, a device configured to determine one or more wavelengths of single photons and / or groups of a few photons comprising at most 100, and in particular at most 50, photons. Preferably, the single-photon spectrometer is configured to measure the wavelength of the individual photons with a relative measurement uncertainty of at most '4 .

[0019] A photon is understood to be a particle of light with a wavelength between 100 nm and 20 pm. It is not necessary for the single-photon spectrometer to function across this entire wavelength range.

[0020] In the context of the present invention, a photon detector is understood to be a component designed to detect at least one photon that strikes the photon detector, for example on a detector surface of the photon detector.

[0021] Preferably, the single-photon spectrometer is configured for detection and determination in a wavelength range between a minimum wavelength and a maximum wavelength, wherein the minimum wavelength is preferably at least 100 nm, preferably at least 250 nm, preferably at least 350 nm, preferably at least 450 nm, and / or wherein the maximum wavelength is preferably at most 20 pm, preferably at most 10 pm, preferably at most 3 pm.

[0022] A dispersion element is understood to be, in particular, a component or a group of several components that exhibits dispersion, that is, a wavelength-dependent refractive index. As a result, the transit time through the dispersion element is always different for different wavelengths.

[0023] Preferably, the dispersion element is configured such that the transit time difference for photons with a wavelength difference of 1 nm is at least 1 picosecond after the photons have passed through the dispersion element. Preferably, the transit time difference is at least 200 picoseconds per 1 nm.

[0024] According to a preferred embodiment, the photon detector is a superconducting nanostrip. Such superconducting nanostrips have high temporal resolution and high sensitivity to individual photons. The nanostrip is, for example, a nanowire. Preferably, the nanostrip has a width of at most 800 nm along a second-longest direction of extension, more preferably at most 300 nm, and more preferably at most 200 nm. More preferably, the nanostrip has a width of at least 20 nm along a second-longest direction of extension, more preferably at least 60 nm, and more preferably at least 80 nm.

[0025] Preferably, the nanostrip has a length of at most 1 cm along its longest direction of extension, more preferably at most 1 mm, more preferably at most 500 pm. Preferably, the nanostrip has a length of at least 10 pm along its longest direction of extension, more preferably at least 50 pm, more preferably at least 100 pm.

[0026] Preferably, the nanostrip has a height of at most 20 nm along its shortest direction of extension, more preferably at most 12 nm, and more preferably at most 8 nm. Preferably, the nanostrip has a height of at least 1 nm, and more preferably at least 3 nm, along its shortest direction of extension. Preferably, the height is arranged approximately parallel to the direction of photon travel upon impact with the nanostrip.

[0027] Preferably, the superconducting nanostrip is arranged and designed such that photons leaving the dispersion element strike the nanostrip and transform a locally limited area from a superconducting state to a non-superconducting state.

[0028] The nanostrip is, for example, straight, meaning that it is rectangular in both a top and a side view, consists of a single piece, and has no folds or curves. Alternatively, the nanostrip has, for example, a meandering shape, in which preferably at least two, and in particular several, sections are arranged parallel to each other. In this shape, all sections of the nanostrip are preferably located in one plane, so that the overall height, as the smallest dimension of the nanostrip shaped in this way, is a maximum of 20 nm, preferably a maximum of 12 nm, and more preferably a maximum of 8 nm. Alternatively, the nanostrip is, for example, spirally shaped. This is advantageous to reduce the influence of polarization.

[0029] Preferably, the nanostrip is designed and arranged such that the total detector surface area on which photons can strike and be detected is at least 30% of the sum of all surfaces of the nanostrip, preferably at least 40%. This makes particularly efficient use of the nanostrip surface for detection.

[0030] The dispersion element includes, for example, an optical fiber. Preferably, the length of the optical fiber along which the photons can travel is at least 10 m, preferably at least 100 m.

[0031] The dispersion element translates a wavelength difference into a transit time difference. This transit time, or transit time difference, is automatically measured by the transit time meter. A transit time meter is specifically a component or module designed to directly or indirectly measure the transit time through the dispersion element.

[0032] The transit-time meter, preferably designed as a stopwatch or time tagger, preferably has a time resolution of better than 10 picoseconds, preferably at least 5 picoseconds, and preferably at least one picosecond or less. In this way, the wavelength can be measured with low measurement uncertainty.

[0033] In the context of this description, detection time uncertainty refers in particular to the full width at half maximum (FWHM) of a distribution in which a number of the detected photons are assigned to a correct arrival time.

[0034] To achieve such or a lower detection time uncertainty, the nanostrip, for example, has the shortest possible length or differential readout at both ends to minimize geometric contributions to the measurement uncertainty of the detection time, and / or cooled readout electronics of the photon detector to minimize contributions of the thermal noise of the readout electronics.

[0035] The transit time is measured absolutely by the transit-time meter according to one embodiment, specifically at a first time point, which corresponds, for example, to a photon emission from a photon source, and at a detection time point, when the photon detector detects the photon. Alternatively, the transit-time meter can be configured to measure the relative transit time, for example, as the difference between the transit times of two photons or groups of photons, particularly, but not necessarily, by different dispersion elements. The relative transit time can also be referred to as the transit-time difference. This allows a wavelength-dependent transit-time difference to be measured precisely and cost-effectively by the dispersion element.

[0036] Preferably, the time-of-flight meter is configured to transmit the measured transit time to the photon detector and / or a computing module. The time-of-flight meter is connected to the photon detector and / or the computing module, for example, mechanically, electronically, or optically. A computing module is understood to be, in particular, a device comprising a processor and a digital memory. A program is stored in the computing module, particularly the digital memory, which is either erasable or non-erasable. This program is executed automatically and calculates the wavelength of the photon from the transit time and the refractive index of the at least one dispersion element. It is possible, but not necessary, for the wavelength to be calculated in a unit of length. It is also possible for the wavelength to be calculated in a unit from which the wavelength can be uniquely determined.

[0037] Particularly through the interaction of a time-tagger, as a possible implementation of a high-precision, space-saving transit-time meter, together with a photon detector in the form of a superconducting nanostrip, which exhibits high temporal resolution and high sensitivity to individual photons, it is advantageously achieved that the wavelength of one or a few photons can be determined with particularly low measurement uncertainty while simultaneously requiring little space. Preferably, the single-photon spectrometer includes a computing module configured to calculate a wavelength Z from the transit time. For example, the computing module determines the wavelength Z from the transit time t measured by the transit-time meter and a wavelength-dependent refractive index n^A) of the dispersion element.

[0038] The dependence of the refractive index

[0039]

[0040] The wavelength Z is stored in the processing module, for example, as a function or as an interpolable table. From the measured refractive index

[0041]

[0042] The wavelength Z is calculated in this way. It is also possible that the corresponding wavelength for each measured transit time t is stored in the computing module, for example as a function or as an interpolable table. The computing module preferably has at least one processor and at least one memory. The computing module is, for example, a computer.

[0043] Preferably, the computing module is configured to determine an optical path length, for example, from a provided refractive index value and a provided geometric path length. Preferably, the computing module is configured to determine the wavelength from the optical path length and the measured transit time.

[0044] Preferably, the single-photon spectrometer has a photon source configured to emit at least one photon into the dispersion element.

[0045] A photon source is understood to be a component or a group of components designed to emit a photon or a group of several photons such that these enter the dispersion element and pass through it, preferably along a longest direction of extension of the dispersion element.

[0046] The photon source serves, for example, as the sole photon source or as a reference photon source in addition to a primary photon source, which may be external, meaning that it does not necessarily have to be a component of the spectrometer. If the spectrometer has a reference photon source, the wavelength of the photons emitted by the reference photon source is preferably known, and the photon detector is preferably configured to determine the wavelength of the primary photon source from a time-of-flight difference between the photons of the reference photon source and the primary photon source.

[0047] Preferably, the photon source is configured to emit a maximum of 1000 photons per unit of time, preferably a maximum of 100 photons, and preferably a maximum of 10 photons. The unit of time corresponds to a time constant or time resolution of the photon detector or, if the spectrometer has an optical loop or optical ring, to a minimum transit time of a photon passing through the dispersion element once. The unit of time is, for example, between 1 picosecond and 100 nanoseconds. For example, the photon source is pulsed with a preferably approximately constant repetition rate, wherein the pulse intervals between temporally adjacent pulses preferably vary by a maximum of 5% of the pulse intervals. Preferably, the pulse duration is shorter than the transit time of a photon through the dispersion element in a single pass, and more preferably shorter than the transit time.

[0048] Preferably, the time-of-flight meter is configured to measure the transit time in the form of the time difference between a first photon emission time from the photon source and a detection time at which the photon detector detects the photon. For example, at the first time, the time-of-flight meter receives a signal via a trigger or an electrical output from the photon source, causing it to start its measurement. Preferably, the time-of-flight meter receives a signal from the photon detector at the detection time, causing it to stop its measurement.

[0049] It is possible that the spectrometer is calibrated, for example, with respect to delays that influence the time difference between a reference signal, such as a trigger signal, and the detection time. When using a single dispersion element, self-referencing of the photon source without requiring a reference signal such as a trigger signal is possible, in particular by having each photon traverse the same dispersion element multiple times, for example, by means of an optical loop or an optical ring. The photon detector is preferably configured to determine the actual, effective optical path length for the photon from the temporal distribution of detections after the first, second, ..., up to the nth traverse.However, this limits the rate at which subsequent photons can be fed into the dispersion element, since the previous photon must always first be absorbed by the dispersion element and the dispersion element must have returned to its initial energy state.

[0050] Preferably, the single-photon spectrometer comprises a second dispersion element which (i) has a second optical dispersion that differs from the first optical dispersion and / or (ii) has a second optical path length that differs from a first optical path length of the first dispersion element. The feature that the second optical dispersion differs from the first optical dispersion is understood, in particular, to mean that the wavelength-dependent refractive index of the first dispersion element differs from the wavelength-dependent refractive index of the second dispersion element.

[0051] If the time at which a photon entered one of the two dispersion elements is known, the photon's travel time can be used to determine which dispersion element it passed through to reach the photon detector, provided the optical path lengths of the two dispersion elements differ sufficiently. Preferably, the optical path lengths differ so much that, for photons of any wavelength, the path through one of the dispersion elements takes longer than the path through the other.

[0052] In this case, the computing module is preferably configured to automatically (i) determine which dispersion element the photon has passed through to the photon detector from the transit time, and (ii) determine the wavelength of the photon from the transit time and the dispersion of the dispersion element through which the photon has passed to the photon detector.

[0053] Preferably, the single-photon spectrometer includes a beam splitter arranged to divide the light path of a photon emitted from the photon source between the first dispersion element and the second dispersion element. A beam splitter is understood to be a component or group of components that deflects at least a portion of the photons striking the beam splitter such that, with a probability of at least 30%, preferably at least 40%, preferably at most 70%, and preferably at most 60%, the photons exit the beam splitter at a first angle that differs from at least a second angle at which the remaining photons exit the beam splitter.

[0054] Preferably, the single-photon spectrometer includes a beam combiner arranged to combine the light path of a photon emitted from the photon source through the first dispersion element and the second dispersion element in the photon propagation direction downstream of the dispersion elements. The photon detector is arranged in the light path downstream of the beam combiner.

[0055] A beam aggregator is understood to be a component or group of components that is designed to guide, break up and / or deflect the photons exiting the first dispersion element and the second dispersion element and striking the beam aggregator in such a way that all photons exit the beam aggregator along a single path and propagate further.

[0056] Preferably, the photon detector is configured to detect at least one photon that has passed through the first dispersion element and / or the second dispersion element. The "and" case is only possible if at least two photons are detected, or if a transit time is recorded for several successive measurement intervals, in each of which at least one photon is detected. The computing module preferably determines a transit time difference or a frequency distribution, for example, a histogram of the measurement intervals. Preferably, additional contributions to the transit time, such as optical path lengths in front of the dispersion elements, remain constant within the measurement uncertainty across the measurement intervals.

[0057] In the event that at least one first photon has passed through the first dispersion element and at least one second photon, having the same wavelength as the first photon, has passed through the second dispersion element, and both photons have been detected by the photon detector, an absolute transit time is not required to obtain the wavelength X. Instead, it suffices for the transit-time meter to detect a transit-time difference At. From the transit-time difference At, the photon detector preferably determines the wavelength X of the photon directly, together with a predefined mapping between transit-time differences At and each wavelength X.Alternatively, it is possible for the photon detector, together with given values ​​for the speed of light in a vacuum c and the dispersion element length s, to determine a refractive index difference An = n1- n2, whereby the wavelength difference AX is calculated from the wavelength dependence of the refractive index, for example on the basis of a calibration curve A(An), which may be provided and / or obtained by means of a calibration, and which is stored, for example as a formula or as a table, in the photon detector, in particular the computing module.

[0058] The advantage of the two dispersion elements is that, using cost-effective and space-saving means, photons of the same wavelength can have different transit times for a first path through the first dispersion element than for a second path through the second dispersion element. The wavelength can then be precisely determined from this transit time difference. A further advantage is that triggers or other activation mechanisms for a transit-time meter are unnecessary. For example, the transit-time meter starts when the first photon hits the photon detector and stops when the second photon hits it. Alternatively, the photon detector can be configured to determine the transit-time difference from detection times at which at least one photon hits the photon detector.

[0059] Alternatively, if the single-photon spectrometer has a beam splitter as described above, it may not have a beam combiner, but instead the photon detector may have a first and second sub-detector. The first sub-detector is arranged to detect photons from the first light path through the first dispersion element, and the second sub-detector is arranged to detect photons from the second light path through the second dispersion element. The photon detector is then configured to measure a transit time in the form of the transit time difference between the first transit time of photons that have passed through the first dispersion element and the second transit time of photons that have passed through the second dispersion element.

[0060] Preferably, the single-photon spectrometer includes a cooling device configured to cool the photon detector, which preferably comprises a superconducting nanostrip, preferably to a temperature below the transition temperature of the superconducting nanostrip. This brings the nanostrip into the superconducting state and keeps it there. For example, the nanostrip has a transition temperature between 3 Kelvin and 5 Kelvin.

[0061] The cooling device is preferably configured to cool the nanostrip such that when a photon strikes the nanostrip, the nanostrip is locally converted into a non-superconducting state. For example, the nanostrip then exhibits finite resistance in a region with a diameter of less than 1 pm around the point of impact of the photon. The cooling device is also preferably configured to cool the nanostrip such that, after a photon strike, it returns to the superconducting state across its entire volume, preferably within less than 100 nanoseconds, and preferably within less than 2 nanoseconds. This enables rapid re-detection of another photon and thus precise time-of-flight difference measurement.

[0062] Preferably, the photon detector, if it has a superconducting nanostrip, has an electronic circuit configured to apply a direct current to the nanostrip such that the superconducting state of the nanostrip is maintained and preferably such that a photon striking the nanostrip at an impact point and having a wavelength within a predetermined wavelength measurement range destroys the superconducting state at the impact point, so that a finite electrical resistance exists at the impact point for a limited time.

[0063] In a further embodiment, the first dispersion element is preferably designed as an optical loop, such that in at least two measurement runs, preferably at least five measurement runs, preferably at least 10 measurement runs, at least one photon is guided through the dispersion element before it hits the photon detector, wherein the number of revolutions of the photon through the dispersion element differs at least once in the measurement runs.

[0064] An optical loop is understood to be a dispersion element, in particular an optical fiber, which is configured such that a photon is guided through the dispersion element once, twice, three times, or n times, where n is a natural number greater than zero and describes the number of revolutions of a photon through the dispersion element, the number of revolutions for a photon being essentially random. For example, the optical loop is at least partially annular, such that a photon passing through the optical loop either traverses one ring circumference or two, three, or n ring circumferences.

[0065] The advantage of an optical loop is that, without a start time and therefore also with external photon sources, an absolute runtime can be determined by the dispersion element.

[0066] Preferably, the time-of-flight meter is configured to automatically measure the transit time t of the at least one photon through the dispersion element for each measurement run. Preferably, the photon detector is configured to automatically determine a wavelength X of the at least one photon from the at least two transit times t. In particular, the time-of-flight meter measures exactly as many different transit times as there are different numbers of photon revolutions. In other words, for example, in ten measurement runs, in each of which a photon is guided through the optical loop, the number of revolutions is two in three measurement runs, one in two measurement runs, three in two measurement runs, and five in the third measurement run. Thus, the time-of-flight meter measures four different transit times, namely for a single revolution, two revolutions, three revolutions, and five revolutions.From the difference in the respective measured transit times, the photon detector, in particular the processing module, preferably determines an absolute transit time through the optical loop for a single revolution. From this absolute transit time of the single revolution, the photon detector determines the wavelength of the photons, as described above.

[0067] According to a further aspect, the invention relates to a method. Preferably, the method comprises the steps (a) providing a database of values ​​or a table that preferably assigns a wavelength Z to each transit time t, and (b) reading out such a wavelength Z that is assigned to the measured transit time t. Alternatively, the method comprises the step of calculating a refractive index n from the detected transit time t, the length of the dispersion element s, and the speed of light in a vacuum c, for example, using the formula:

[0068] ct

[0069] n = —

[0070] s

[0071] Preferably, the method additionally or alternatively includes, in addition to the calculation, the step of providing a database of values ​​or a table and reading out a wavelength Z corresponding to the calculated refractive index n. Preferably, the method also includes the step of determining the wavelength Z from the calculated refractive index n and the provided assignment.

[0072] Preferably, the method also includes the step of outputting the specified wavelength Z.

[0073] Preferably, the method also includes the step of starting a time-of-flight timer if the single-photon spectrometer has a time-of-flight timer. For example, the single-photon spectrometer has a trigger input for this purpose, such as for connection to the trigger output of a photon source or an optical shutter.

[0074] As an alternative to using a time-of-flight meter, the photon detector, in particular the computing module, is preferably configured to perform the following steps: (a) Determining timestamps that encode detection times at which photons strike the photon detector, and optionally encoding reference times based on the trigger input; (b) Determining time differences between the detection times encoded by the timestamps and / or reference times; (c) Determining a frequency distribution, in particular a histogram, of the time differences.

[0075] Preferably, the computing module is configured to perform the following further steps: (d) Determining at least one transit-time maximum of the frequency distribution, which is preferably a local maximum, (e) Calculating a transit-time or transit-time difference from the transit-time maximum, preferably from a smallest transit-time maximum, (f) Determining a wavelength of the photons from the transit-time or transit-time difference.

[0076] The frequency distribution typically exhibits at least two local maxima. The transit-time maximum is, in particular, the local maximum associated with the smallest time interval and preferably does not correspond to a photon emission time interval, nor to any integer multiple thereof, nor to a sum of the photon emission time interval, or any integer multiple thereof, and a transit time, or any integer multiple thereof, when the photon source is pulsed and emits photons at an approximately constant photon emission rate. The photon emission time interval is understood to be, in particular, a time interval that is the reciprocal of the photon emission rate.

[0077] If the photon source is not pulsed or not pulsed regularly, the transit-time maximum is in particular the maximum that corresponds to the smallest time differences.

[0078] If the spectrometer has at least one optical loop, further local maxima typically occur at integer multiples of the transit-time maximum. The local maximum associated with the smallest time interval is therefore usually associated with the transit time required for a single revolution through the dispersion element.

[0079] In other words, the computing module is preferably designed to calculate the transit time for a single pass or revolution through the dispersion element from the positions of the local maxima of the frequency distribution within a photon emission time interval.

[0080] Preferably, the device for determining the transit-time difference from a frequency distribution comprises an optical loop, an optical ring, and / or a second dispersion element. This has the advantage that no start time needs to be measured or recorded, thus enabling the use of external photon sources, such as chemical combustion or stars. The invention is explained in more detail below with reference to the accompanying figure.

[0081] Figure 1a shows a single-photon spectrometer according to a first embodiment of the invention.

[0082] Figure 1b shows a single-photon spectrometer according to a second embodiment of the invention.

[0083] Figure 1c shows a single-photon spectrometer according to a third embodiment of the invention.

[0084] Figure 1a shows a single-photon spectrometer 10 with a photon detector 30 and a first dispersion element 40, which has a first optical dispersion n^A). The photon detector 30 is configured to detect at least one photon 15 that has passed through the first dispersion element 40 and to determine a wavelength Z of the at least one photon 15 from the dispersion n^A of the dispersion element 40 and the transit time t. The transit time t is measured by the transit-time meter 60 and preferably transmitted to the photon detector 30 or a computing module 32.

[0085] The transit time t is, for example, a time interval At between a start time tstart, for example, the emission of photon 15 by photon source 50, and a stop time tstopp, for example, the time of detection of the photon on photon detector 30. Photon source 50 can be part of single-photon spectrometer 10, as shown here; alternatively, photon source 50 can also be located outside of single-photon spectrometer 10.

[0086] The transit time t is recorded by the transit time meter 60, which is preferably a time tagger and / or which preferably has a measurement uncertainty of less than 50 picoseconds.

[0087] For example, the time-of-flight meter starts measuring the transit time at the start time tstart and stops measuring it at the stop time tstopp. The start time tstart is determined, for example, by transmitting an electrical signal, such as from a trigger, to the photon source 50. Based on preliminary investigations, the time between the trigger signal to the photon source 50 and the emission of the photon 15 is known. Preferably, the photon source 50 is configured such that the distribution of the time interval between the application of the trigger signal to the photon source 50 and the emission of the photon 15 has a temporal variance that is less than 10 picoseconds, preferably less than 1 / 4 of the intrinsic temporal variance of the detector.

[0088] If the single-photon spectrometer 10 has a photon source 50, the start time tstart can correspond to the time when a photon leaves the photon source 50, for example, via a trigger output of the photon source. The stop time tstopp is determined, for example, by the photon 15 striking the photon detector 30.

[0089] The computing module 32 is preferably configured to calculate the wavelength Z of the at least one photon 15 from the dispersion n^A) of the dispersion element 40, which was determined, for example, in preliminary experiments, and the measured transit time t. For this purpose, the transit time meter 60 preferably transmits the recorded transit time t to the computing module 32.

[0090] Preferably, the photon detector 30 comprises at least one superconducting nanostrip 34. Preferably, the superconducting nanostrip 34 is arranged and configured such that photons leaving the dispersion element strike the nanostrip and convert a locally limited area into the non-superconducting state.

[0091] Preferably, the single-photon spectrometer 10, if the photon detector 30 has at least one superconducting nanostrip 34, has a cooling device 36 configured to cool the nanostrip 34 to a temperature below the transition temperature of the nanostrip 34, so that it is in a superconducting state.

[0092] Preferably, the photon detector 30, when it has a superconducting nanostrip 34, has an electronic circuit 37 which is configured to apply a direct current to the nanostrip 34 such that the superconducting state of the nanostrip 34 is maintained and preferably such that a photon which strikes the nanostrip 34 at an impact point and which preferably has a wavelength Z within a predetermined wavelength measurement range, destroys the superconducting state at the impact point, so that a finite electrical resistance exists at the impact point for a limited time.

[0093] Figure 1b shows a single-photon spectrometer 10 with a first dispersion element 40a and a second dispersion element 40b, which has a second dispersion n2( ) that is different from the first dispersion n^A). The single-photon spectrometer 10 also includes a beam splitter 72, which consists of two components 72a, 72b and is configured to split the light path into a first light path 72a through the first dispersion element 40a and a second light path 72b through the second dispersion element 40b. Here, for example, the beam splitter 72 includes a non-polarizing beam splitter 7a, such as a 50:50 beam splitter, and a mirror 72b.

[0094] Furthermore, the single-photon spectrometer 10 has a beam combiner 78, which here comprises two components 78a and 78b. The beam combiner 78 is arranged and configured to combine the first light path 72a and the second light path 72b such that the photons emitted from the two dispersion elements 40a, 40b are guided along the same path to the photon detector 30 after passing through the beam combiner 78.

[0095] The beam combiner 78 here, for example, has a mirror 78b and a plate beam splitter 78b, which is preferably arranged at a 45° angle to the light paths 78a, 78b and is coated in such a way that with at least 90% probability a photon which from the first or second light path 78a, 78b hits the plate beam splitter 78b is directed to the photon detector 30.

[0096] Due to the two light paths 72a, 72b and the different dispersions of the dispersion elements 40a, 40b, it is possible in this example for the time-of-flight meter 60 to detect a time-of-flight difference At between the arrival of at least two photons on the photon detector 30. Therefore, absolute time-of-flight measurement is not required.

[0097] The optical path length wi of the first dispersion element 40a advantageously differs from a second optical path length W2 of the second dispersion element 40b to such an extent that the fastest travel time of a photon 15 through the first dispersion element 40a is shorter than the slowest travel time of a photon 15 through the second dispersion element 40b. The processing module 32 records the travel time t and determines from it which dispersion element 40a, 40b the photon 15 has passed through. From the corresponding dispersion, the processing module 32 then calculates the wavelength X. For this purpose, the processing module 32 has, for example, a digital memory in which the dispersions ^(A), n2(A) are stored as a function of the wavelength A, for example in the form of a parameterized function with associated parameters, in the form of a database of values, or in a table.

[0098] Alternatively, a frequency distribution of the mere difference in transit times through the first and second dispersion elements is generated. In particular, if a pulsed single-photon source emits photons at a constant rate (so that a comparison across different repetition intervals is meaningfully possible by subtracting the repetition interval or its multiples), or if several photons are simultaneously emitted into the spectrometer—in the case of a single-photon source, for example, by superimposing photons of different repetition intervals through a suitable, advantageously switched, optical delay line, which advantageously allows a change in polarization to avoid physical indistinguishability of the photons—and the dispersion elements pass through simultaneously, no external reference point is required.In this case, to ensure uniqueness, it is only required that the dependence of the transit time on the wavelength in the dispersion elements is different and monotonically increasing or monotonically decreasing without intersecting.

[0099] Figure 1c shows a single-photon spectrometer 10, which has a first light path 72a and a second light path 72b as in Figure 1b, but without a beam combiner 78. Instead, the single-photon spectrometer 10 has a first photon detector 30a and a second photon detector 30b. The first photon detector 30a is arranged and configured to detect photons from the first light path 72a. The second photon detector 30b is arranged and configured to detect photons from the second light path 72b.

[0100] The photon detectors 30a and 30b measure the respective stop times tstopp, in particular the detection times, using the time-of-flight meter 60. Preferably, the processing module 32 calculates a time-of-flight difference At from the stop times tstopp, and, for example, additionally from start times to, and determines the wavelength X from the time-of-flight difference At. (Reference symbol list)

[0101] 10 single-photon spectrometers 15 photon

[0102] 20 procedures

[0103] 30 photon detector

[0104] 32 Computing module

[0105] 36 Cooling device

[0106] 37 electronic circuit

[0107] 40 Dispersion element

[0108] 50 photon source

[0109] 60 runtime meters

[0110] 70 beam splitters

[0111] 72a first light path

[0112] 72b second light path

[0113] 78 blasting cleaners

[0114] c Speed ​​of light in a vacuum X wavelength

[0115] n refractive index

[0116] n^A) Dispersion of the first dispersion element

[0117] n2(A) Dispersion of the second dispersion element

[0118] s Length of the dispersion element t Runtime

[0119] to start time

[0120] tstopp Stop time

[0121] At runtime difference

[0122] w optical path length

Claims

Gramm Lins intellectual property Physikalisch-Technische Bundesanstalt Lawyer's file: Braunschweig and Berlin 0454-0335 PCT-1 Bundesallee 100 38116 Braunschweig Date: December 3, 2025 Patent claims 1. Single-photon spectrometer (10) with (a) at least one dispersion element (40) having a first optical dispersion, and (b) a photon detector (30) arranged to detect at least one photon (15) that has passed through the dispersion element (40), and (c) a transit-time meter (60) configured to automatically measure the transit time (t) of at least one photon (15) through the dispersion element (40), (d) wherein the photon detector (30) is configured to automatically determine a wavelength (X) of the at least one photon (15) from the transit time (t).

2. Single-photon spectrometer (10) according to claim 1, characterized in that the photon detector (30) has at least one superconducting nanostrip (34).

3. Single-photon spectrometer (10) according to claim 1 or 2, characterized in that the dispersion element (40) comprises at least one glass fiber which is preferably at least 10 m long, preferably at least 100 m.

4. Single-photon spectrometer (10) according to one of the preceding claims, characterized in that The time-of-flight meter (60) has a time measurement uncertainty of at most 50 picoseconds.

5. Single-photon spectrometer (10) according to one of the preceding claims, characterized by a computing module (32) which is set up to calculate a wavelength (λ) of the photon (15) from the transit time (t) and a refractive index (n) of the at least one dispersion element (40, 40b).

6. Single-photon spectrometer (10) according to one of the preceding claims, characterized by (a) a photon source (50) configured to emit at least one photon (15) into the dispersion element (40), and (b) wherein the transit time meter (60) is preferably configured to measure the transit time (t) in the form of the time between a time of photon emission from the photon source (50) and a detection time at which the photon detector (30) detects the at least one photon (15).

7. Single-photon spectrometer (10) according to one of the preceding claims, characterized by (a) a second dispersion element (40b) having a second optical dispersion n2(λ) that differs from the first optical dispersion n1(λ), (b) a beam splitter (70) arranged to split a light path of photons (15) emitted from a photon source (50) into a first light path (72a) through the first dispersion element (40a) and a second light path (72b) through the second dispersion element (40b), and (c) a beam combiner (78) arranged to combine the first light path (72a) and the second light path (72b) in the photon propagation direction behind the dispersion elements (40a, 40b), (d) wherein the photon detector (30) is configured to detect at least one photon (15) that has passed through the first dispersion element (40a) and / or the second dispersion element (40b).

8. Single-photon spectrometer (10) according to one of claims 1 to 6, characterized by (a) a second dispersion element (40b) having a second optical dispersion n2(λ) that differs from the first optical dispersion n1(λ), (b) a beam splitter (70) arranged to split a light path of a photon (15) emitted from the photon source (50) into a first light path (72a) through the first dispersion element (40a) and a second light path (72b) through the second dispersion element (40b), and (c) wherein the photon detector (30) comprises a first sub-detector (30a) and a second sub-detector (30b) (d) and wherein the runtime meter (60) is configured to measure a runtime (t) in the form of a runtime difference (At) between a first transit time (t) of at least one first photon (15a) that has passed through the first dispersion element (40a), and a second runtime (t) at least a second photon (15b) that has passed through the second dispersion element (40b).

9. Single-photon spectrometer (10) according to one of the preceding claims, characterized in that (a) the first dispersion element is designed as an optical loop such that in at least two measurement runs at least one photon (15) is guided through the dispersion element (40) before it hits the photon detector (30), wherein the number of revolutions of the photon (15) through the dispersion element (40) differs at least once in the measurement runs, (b) the transit-time meter (60) is configured to automatically measure one transit time (t) of the at least one photon (15) through the dispersion element (40) for each of the measurement runs, and (c) the photon detector (30) is designed to automatically determine a wavelength (X) of the photons (15) from the at least two transit times (t).

10. Method (20) for determining a wavelength (X) of at least one photon (15) using a single-photon spectrometer (10) according to one of the preceding claims, wherein the procedure (20) comprises the steps: (a) Detecting at least one photon (15) that has passed through a first dispersion element (40), (b) Measuring a transit time (t) of the at least one photon (15) through the first dispersion element (40) and (c) Determining the wavelength (X) of at least one photon (15) from the travel time (t).

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