OPTICAL TRANSMITTER, OPTICAL RECEIVER, OPTICAL SYSTEM AND METHOD FOR QUANTUM COMMUNICATION

An optical transmitter for quantum communication, including a QKD channel comprising at least QKD light source and configured to emit a stream of QKD encoded pulses; a reference channel including a reference light source and configured to emit a stream of reference pulses; and a control circuit connected to the QKD channel and to the reference light channel. The control circuit is configured to control the QKD channel and the reference light channel to emit the reference pulses with a predetermined time delay to the QKD encoded pulses. A difference of a wavelength of the QKD encoded pulses and a wavelength of the reference pulses is 5 nm or less. A corresponding optical receiver, an optical system and a method for quantum communication are also provided.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of the European patent application No. 22214397.6 filed on Dec. 16, 2022, the entire disclosures of which are incorporated herein by way of reference.

FIELD OF THE INVENTION

The present invention relates to an optical transmitter for quantum communication, an optical receiver, a system and a method for quantum communication.

BACKGROUND OF THE INVENTION

In order to establish long-distance secure communication links, it is expected that in the near future an important application of quantum communication will be free-space quantum key distribution (QKD) over an air-ground link or a space-ground QKD link. In these channels, atmospheric turbulences distort the wavefront of optical beams including those used for quantum communication. Since it is not possible to amplify quantum signals, a setup for free-space QKD requires a large telescope for receiving the signal. However, when the diameter of the telescope becomes larger than the coherence length of the wavefront, the so-called Fried parameter, the telescope cannot efficiently focus the received light. Therefore, distortions of the wavefront of a QKD link are even more severe than for a classical communication link.

For these systems, there is a need to correct the wavefront of a beam in the QKD link in an efficient way.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, an optical transmitter for quantum communication is provided. The optical transmitter comprises a QKD channel comprising at least QKD light source and configured to emit a stream of QKD encoded pulses; a reference channel comprising a reference light source and configured to emit a stream of reference pulses; and a control circuit connected to the QKD channel and to the reference light channel, wherein the control circuit is configured to control the QKD channel and the reference light channel to emit the reference pulses with a predetermined time delay to the QKD encoded pulses, wherein a difference of a wavelength of the QKD encoded pulses and a wavelength of the reference pulses is 5 nm or less.

According to a second aspect of the invention, an optical receiver for quantum communication is provided. The optical receiver comprises a detection unit containing one or more photon counters arranged and configured to detect QKD encoded pulses emitted by an optical transmitter, in particular by the optical transmitter according to the preceding claims; a wavefront sensor arranged to receive reference pulses emitted by an optical transmitter, in particular by the optical transmitter, and configured to measure a wavefront of the reference pulses to provide a wavefront signal; an adaptive optical element arranged upstream of the detection unit and configured to manipulate a wavefront of the QKD encoded pulses; and a receiver controller connected to the wavefront sensor, the adaptive optical element and the detection unit, wherein the receiver controller is configured to: apply a time gate to the one or more photon counters in the detection unit at the time of arrival of the QKD encoded pulses; obtain the wavefront signal indicating the measured wavefront of the reference pulses; and trigger the adaptive optical element to correct the wavefront of the QKD encoded pulses based on the wavefront signal.

According to a third aspect of the invention, an optical system for quantum communication is provided. The optical system comprises an optical transmitter according to the invention, and an optical receiver according to the invention.

According to a fourth aspect of the invention, a method for quantum communication is provided. The method comprises emitting, by an optical transmitter, a stream of QKD encoded pulses and reference pulses, wherein the reference pulses are emitted with a predetermined time delay relative to the QKD encoded pulses, wherein a difference of a wavelength of the QKD encoded pulses and a wavelength of the reference pulses is 5 nm or less; measuring, by a wavefront sensor, the wavefront of the reference pulses; applying, based on the measured wavefront, a wavefront correction to the QKD encoded pulses by an adaptive optical element; and obtaining, by a detection unit containing one or more photon counters, the QKD encoded pulses by applying a time gate based on a time of arrival of the QKD encoded pulses at the one or more photon counters in the detection unit for temporally filtering the QKD encoded pulses from the reference pulses.

A fundamental concept of the invention is to use the very small duty cycle, i.e., the time period between two successive QKD encoded pulses, to emit a reference pulse in between. The reference pulse has much more energy than a QKD encoded pulse. It is therefore easier to measure the wavefront of the reference pulses than directly measure the wavefront of the QKD encoded pulses. Having a corrected wavefront, the QKD encoded pulses can be better focused on the photon counter, which then reduces the Qubit error rate (QBER) and thus improves the data rate.

In order to temporarily place a reference pulse between two successive QKD encoded pulses, a predetermined time delay is applied between the reference pulses and the preceding QKD encoded pulses. For this, the repetition rate of the reference pulses is the same or a multiple of the repetition rate of the QKD encoded pulses. Since the duty cycle of the QKD stream comprising the QKD encoded pulse is very small, e.g., 100 ps (picosecond) pulses every 10 ns (nanoseconds), i.e., about 1%, the predetermined time delay may be relatively large. This allows the two different pulses to be discriminated with a high degree in the time domain.

The control circuit generates the predetermined time delay that is applied to the reference pulses. The controller thus has some sort of time base that is used to generate electric pulses that are split according to the number of the light sources in the transmitter. A delay line or circuit within the controller may be applied to generate the predetermined time delay or time shift of the electric pulses prior to their arrival at the respective light source.

At the receiver, the reference pulses can easily be analyzed by a wavefront sensor, which can be a Shack-Hartmann sensor or any other suitable sensor. The measured wavefront is corrected by an adaptive optical element, which can be a deformable mirror, a spatial light modulator or any other device suitable for this task.

A particular advantage in the solution according to an aspect of the invention is that by measuring the wavefront of the reference pulses, measurement at the same wavelength of the QKD signal, i.e., the QKD encoded pulses, becomes possible with this scheme. By transmitting the pulses through the same communication channel, this scheme ensures a close to optimal wavefront correction.

Furthermore, since the reference pulses are emitted in a similar amount as the QKD encoded pulses, this scheme advantageously allows for quickly measure changes of wavefront of the QKD encoded pulses. Therefore, it is possible to quickly adapt the wavefront of the QKD encoded pulses thus taking into account changes in the atmospheric path of the QKD channel, which is regularly occurring, e.g., in a space/air-to-ground link.

As within this application, a QKD encoded pulse is a pulse intended and suitable for quantum communication, typically containing on average less than one photon and carrying information in the form of a quantum state. Classically, such a quantum state may be polarization. In the most common BB84 scheme, polarization states of 0°, 90°, +45° and −45° are used. However, other quantum states such as, e.g., the orbital angular momentum, and other QKD schemes, such as the E91 scheme or decoy states may be applied to the QKD encoded pulses within this invention.

According to some further aspects of the optical transmitter according to the invention, the control circuit comprises a pulse generator, wherein the control circuit is configured to drive the reference light source by pulses generated by the pulse generator. The light source is configured to convert the electrical pulses into optical pulses. This may be achieved using a common laser diode, DFB laser diode, VCSEL, or other lasers allowing such pulsed operation mode. This represents a simple implementation to generate the reference pulses.

According to some further aspects of the optical transmitter according to the invention, the optical transmitter further comprises an amplitude modulator connected to the control circuit. Such an amplitude modulator may, e.g., be a Mach-Zehnder based modulator having an optical delay that is controlled by the control circuit. The reference light source is configured as a cw (continuous wave) light source. This allows an employment of a cw light source, such as a diode laser, or another laser, such as a gas laser. The amplitude modulator is arranged in the reference channel downstream of the reference light source. The amplitude modulator is configured to modulate emitted cw light of the reference light source to generate the reference pulses. In this way, the reference pulses may be generated in a flexible and efficient way.

According to some further aspects of the optical transmitter according to the invention, the control circuit comprises a reference delay line configured to delay an electric signal to the reference channel to generate the predetermined time delay. The reference delay line is connected to the reference channel and enables the accurate generation of the predetermined time delay between the QKD encoded pulses and the reference pulses.

According to some further aspects of the optical transmitter according to the invention, the optical transmitter further comprises a spatial mode converter, which is configured to modulate a spatial mode of the QKD encoded pulses and/or the reference pulses. Such a spatial mode converter could be a phase mask or a spatial light modulator, e.g., based on liquid crystals. This allows further discrimination of the reference pulses from the QKD encoded pulses, thus avoiding crosstalk between the reference channel and the QKD channel.

According to some further aspects of the optical transmitter according to the invention, the optical transmitter further comprises combiner optics arranged and configured to combine the QKD encoded pulses and the reference pulses to a combined output beam. This combining may be realized by a beam splitter or by fiber optics employing a waveguide coupler. This allows the reference pulses to propagate exactly on the same path as the QKD encoded pulses. In this way, a wavefront measurement of the reference pulses allows to correct the wavefront of the QKD encoded pulses with improved, i.e., high, quality.

According to some further aspects of the optical transmitter according to the invention, the QKD channel comprises a plurality of QKD light sources, which are configured as single photon sources, wherein the QKD light sources preferably are configured as VCSELs. This allows having a high data rate of the QKD channel, thus also a high repetition rate of the reference pulses and allows, with a suitable receiver, to quickly adapt their wavefronts.

According to some further aspects of the optical transmitter according to the invention, the control circuit comprises a QKD delay line for each of the QKD light sources, wherein the control circuit is configured to trigger the plurality of QKD light sources to substantially emit QKD encoded pulses simultaneously. This avoids a time jitter within the QKD encoded pulses, thereby preventing or at least reducing crosstalk between the reference pulses and the QKD encoded pulses. The QKD delay line further ensures that all the QKD light sources emit at the same time with respect to the time base. This ensures that an eavesdropper cannot understand which light source has emitted a QKD encoded pulse or photon, by observing its arrival time with respect to the time base.

According to some aspects of the optical transmitter according to the invention, the predetermined time delay is approximately half of the time period of two successive QKD encoded pulses. The exact time delay might be longer than half of the time period, to take into account the asymmetry of the electrical pulses generated by the photon counters, e.g., the tail of the pulse is typically longer than the rise time. This predetermined time delay represents the optimum predetermined time delay for discriminating or separating the reference pulse from the QKD encoded pulses since the temporal distance is at maximum. Therefore, crosstalk between the reference channel and the QKD channel is reduced.

According to some further aspects of the optical transmitter according to the invention, the control circuit comprises an internal clock, wherein the internal clock is configured to provide a time base for the predetermined time delay of the reference pulse to the preceding QKD encoded pulses. This ensures an accurate time delay, thus avoiding a crosstalk between the QKD channel and the reference channel. It further provides a local time base, which enables independency of external time bases. Alternatively, a time base can be provided by an external clock, for example, derived from an atomic clock, or it could be extracted from an According to some further aspects of the optical receiver according to the invention, the optical receiver further comprises a dichroic filter. The dichroic filter is arranged and configured to substantially spectrally filter, i.e., separate, the QKD encoded pulses from the reference pulses, when a difference between the wavelength of the QKD encoded pulses and the wavelength of the reference pulses is between 1 nm and 5 nm. Thus, the dichroic filter is able to substantially discriminate the reference pulses and the QKD encoded pulses in case they have a wavelength difference between 1 nm and 5 nm. Preferably, this difference is between 2 nm and 3 nm. In addition to the time gating, this further reduces crosstalk and improves the isolation of the pulses at the receiver side.

According to some further aspects of the optical receiver according to the invention, the adaptive optical element is arranged upstream of the wavefront sensor and configured to manipulate the wavefront of the reference pulses. The receiver controller is configured to correct a spatial mode of the reference pulses based on the measured wavefront. This allows the application of higher spatial modes to the reference pulses. The higher spatial mode can then be spatially filtered by a demultiplexer, e.g., a phase plate, from the QKD encoded pulses.

This further improves the isolation of the reference pulses from the QKD encoded pulses.

In the figures of the drawing, elements, features and components which are identical, functionally identical and of identical action are denoted in each case by the same reference designations unless stated otherwise.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG.1shows a schematic illustration of an optical transmitter1for quantum communication according to an embodiment of the invention.

The optical transmitter1shown inFIG.1is an optical transmitter1for quantum communication. The optical transmitter1comprises a QKD channel2and a reference channel5. The QKD channel2is configured as a quantum communication channel using Quantum Key Distribution, QKD. The QKD channel2comprises at least one QKD light source3and is configured to emit a stream of QKD encoded pulses4. In this embodiment, the QKD light sources3are configured as a single photon source emitting, on average, less than one photon per pulse. The reference channel5comprises a reference light source6and is configured to emit a stream of reference pulses7. The QKD channel2and the reference channel5are configured to emit QKD encoded pulses4and reference pulses7, which are directed in a parallel direction.

The optical transmitter1further comprises a control circuit8, which is connected to the QKD channel2and to the reference light channel5, more concretely to the respective QKD light source3and the reference light source6. The control circuit8is configured to control the QKD channel2and the reference channel5to emit the reference pulses7with a predetermined time delay T to the preceding QKD encoded pulses4. Although inFIG.1, the time delay T shows a distance between a QKD encoded pulse4and the successive a reference pulse7, it can be understood that the pulse distance inFIG.1relates to the delay time T by the speed of light (i.e., about 3*108 m/s in vacuum). SinceFIG.1is a schematic drawing, a scale indicating the value of the emission frequency or the repetition rate of the pulses cannot be derived fromFIG.1.

A difference of a wavelength of the QKD encoded pulses4and a wavelength of the reference pulses7is 5 nm or less. Therefore, the wavelength of the two pulses is very close. Optionally, the wavelength of the QKD encoded pulses4and a wavelength of the reference pulses7are substantially the same, i.e., less than 5 nm. In this way, the reference pulse7approximately experiences the same distortions when propagating through the atmospheric turbulence, as described further below.

In the embodiment shown inFIG.1, the QKD encoded pulses4and the reference pulse7are not combined and transmitted separately through different apertures (not shown). In further embodiments as described below, the QKD channel2and the reference channel5are combined prior to transmitting a combined beam through a single aperture.

FIG.2shows a schematic illustration of an optical receiver10for quantum communication according to an embodiment of the invention.

FIG.2shows an optical receiver10for quantum communication. The optical receiver10comprises a detection unit11containing one or more photon counters arranged and configured to detect QKD encoded pulses4. The photon detection unit11May comprise one or more photon-counter, and the optics required to split the photons of the QKD encoded pulses4based on their state, e.g., their polarization. The photon detection unit11May also contain optics to correct the polarization state of the photons, e.g., waveplates. The optics inside the photon detection unit11might be discrete optical components or they might be implemented as a photonic chip, e.g., a photonic integrated chip (PIC). In the case of the BB84 scheme, the detection unit11contains four photon counters arranged using three beam splitter and a half waveplate to detect four different linear polarization states (horizontal 0°, vertical 90°, plus +45°, minus −45°). The optical receiver10further comprises a wavefront sensor12, which is arranged to receive reference pulses7. The wavefront sensor12is a Shack-Hartmann sensor in this embodiment. The wavefront sensor12is configured to measure a wavefront13of the reference pulses7incident on the wavefront sensor12to provide a wavefront signal. The separation of the pulses4,7is performed by a dichroic filter17, which separates a common receiver channel R into a first receiver channel R1directed to the detection unit11and a second receiver channel R2directed to the wavefront sensor12. In the ideal case, the QKD encoded pulses4propagate on the first receiver channel R1and the reference pulses7on the second receiver channel R2. In the present embodiment, a difference between the wavelength of the QKD encoded pulses4and the wavelength of the reference pulses7is larger than about 1 nm, e.g., between 1 nm and 5 nm, preferably between 2 nm and 3 nm. In this case, the dichroic filter17is arranged and configured to substantially filter the QKD encoded pulses4from the reference pulses7by spectral separation. This can be achieved, e.g., by a notch filter or a narrow band-pass filter, which are commercially available items.

In further embodiments, the wavelength of the QKD encoded pulses4and the reference pulses7are substantially the same. In this case, instead of the dichroic filter17, a beam splitter may be employed.

The optical receiver10further comprises an adaptive optical element14, which is arranged upstream of the detection unit11containing one or more photon counters and configured to manipulate the wavefront15of the QKD encoded pulses4. The measured wavefront13of the reference pulses7is thus corrected by the adaptive optical element14, which is configured as a deformable mirror in this embodiment. In further embodiments, a spatial light modulator is applied for this task.

Not shown inFIG.2is an aperture arranged on the common receiver channel R1and configured to receive QKD encoded pulses4and reference pulses7emitted by an optical transmitter1, in particular by an optical transmitter1, described above.

The optical receiver10comprises a receiver controller16, which is connected to the wavefront sensor12, the adaptive optical element14and the detection unit11. The receiver controller16is configured to apply a time gate G to the one or more photon counters inside the detection unit11at the time of arrival of the QKD encoded pulses4. The time gate G can be configured as a gain switch in the photon counter inside the detection unit11such that it is only capable to detect and count an incident photon during the time of the applied time gate G. Data about the QKD encoded pulses4are thus collected only during the application of the time gate G. Such time gate G has a width W, which is larger than a pulse width WQKD of the QKD encoded pulses4. The time gate G is applied periodically to the one or more photon counters according to the repetition rate of the QKD encoded pulses4.

The receiver controller16is further configured to obtain a wavefront signal indicating the measured wavefront13of the reference pulses7. The related data of the wavefront13of the reference pulses7are thus provided by the wavefront sensor12and transmitted to the receiver controller16.

The receiver controller16is further configured to trigger the adaptive optical element14to correct the wavefront15of the QKD encoded pulses4based on the wavefront signal. Having a corrected wavefront, the QKD encoded pulses4can be better focused on the photon counters inside the detection unit11, which reduces the Qubit error rate (QBER) and thus improves the data rate.

FIG.3shows a schematic illustration of an optical system20for quantum communication comprising an optical transmitter1and an optical receiver10according to an embodiment of the invention.

The optical system20for quantum communication comprises an optical transmitter1. The shown optical system20is compatible with the optical transmitter1described above in relation toFIG.1. The optical system20further comprises an optical receiver10, and is compatible with the optical receiver10described above in relation toFIG.2.

The optical transmitter1is arranged and configured to emit an output beam21towards the optical receiver10. The output beam21comprises the QKD encoded pulses4and the reference pulses7, which are delayed by a predetermined time delay T to the preceding QKD encoded pulses4. The optical receiver10is arranged and configured to receive the output beam21of the optical transmitter1.

In the optical system20shown inFIG.3, the output beam21passes through the atmosphere22creating turbulences on the wavefront13of the reference pulses7and the wavefront15of the QKD encoded pulses4. Initially at the optical transmitter1, both wavefronts13,15are plane waves. However, after passing through the atmosphere22, both wavefronts13,15experience distortions that are originated by turbulences in the atmosphere22causing refractive index variations due to pressure and/or temperature differences in the air.

FIG.4shows a schematic illustration of a reference channel5and a QKD channel2for quantum communication according to an embodiment of the invention.

FIG.4depicts the QKD encoded pulses4in the QKD channel2and the reference pulses7in the reference channel5on a time/distance axis40. Although the axis40relates to a physical distance, since the pulses4,7are emitted constantly with the same repetition rate, axis40can also be regarded as a time axis as it is proportional to the distance, by introducing the speed of light.

In this embodiment, the predetermined time delay T is approximately half of the time period P of two successive QKD encoded pulses4. It results that the predetermined time delay T from a QKD encoded pulse4to a preceding reference pulse7is similar to the predetermined time delay T from a reference pulse7to a preceding QKD encoded pulse4. This ensures the best isolation of the two pulse streams, which is particularly useful at the optical receiver10, where the pulses4,7are separated. The QKD encoded pulses4are covered by the time gate G that has a width W, which substantially covers to entire a QKD encoded pulse4. A detector, such as the one or more photon counters in the detection unit11, is gain switched such that a gain is high at the time when the gate is “open” at the arrival of the QKD encoded pulse4.

FIG.5shows a schematic illustration of an optical transmitter1for quantum communication according to a further embodiment of the invention.

In the embodiment ofFIG.5, the QKD channel2of the optical transmitter1comprises four QKD light sources3arranged in four QKD subchannels2a,2b,2c,2d.The QKD light sources3are configured as single photon sources, and configured to emit in average less than one photon per pulse, e.g., 0.5 photons per pulse. The QKD light sources3are configured as VCSELs, thus being possible to function as a single photon source. In further embodiments, the QKD light sources3are configured as other types of laser diode.

In this embodiment, the control circuit8comprises a power supply50to power the electronic devices such as the light sources3,6. The control circuit8comprises a pulse generator51, which is connected to the power supply50. The pulse generator51is configured as a current pulse generator51for generating current pulses that are used in this embodiment to drive the QKD light sources3and the reference light source6. In further embodiments, the pulse generator51is configured as voltage pulse generator51and an additional circuit is used to convert the voltage pulse to a current pulse to drive the light sources3,6.

For driving the QKD light sources3by pulses generated by the pulse generator51, the control circuit8comprises a QKD delay line54, which is connected to the pulse generator51and to the QKD light sources3in each QKD subchannel2a,2b,3c,2d.The QKD delay lines54are configured to generate a time delay to each generated pulse of the pulse generator51. By triggering a pulse generation in the pulse generator51, the plurality of QKD light sources3can substantially emit QKD encoded pulses4simultaneously. It is understood that only the relevant QKD light source/s3are switched on depending on the encoded information.

A synchronized current pulse56is depicted downstream the QKD delay line54in each of the QKD channels2a,2b,2c,2d.These synchronized current pulses56drives the respective QKD light source3to emit a pulse simultaneously.

In this embodiment, a polarizer58is used to encode information on the pulses emitted by the QKD light source3after their emission. The polarizer58is configured as a micro polarizer. In this setup, fiber optics60are used to combine the four QKD channels2a-dby using fiber coupling optics61for coupling the QKD encoded pulses4into an optical fiber62, which, in this case, is a single mode and polarization maintaining fiber. Fiber couplers63are used to combine the QKD encoded pulses4. A fiber collimator64is connected to emit the QKD encoded pulses4combined from all four QKD channels2ato2dto free space.

The control circuit8comprises an internal clock52, which is connected to the pulse generator51. The internal clock52is configured to provide a time base for the predetermined time delay T of the reference pulse7to the preceding QKD encoded pulses4. In further embodiments, instead of the internal clock52shown inFIGS.5and6, a time base is provided by an external clock, e.g., derived from an atomic clock. The control circuit8further comprises a reference delay line55connected to the internal clock52and in between a driver53and the reference light source6. The driver53is configured to drive the reference light source6.

The reference delay line55is configured to delay an electric signal from the internal clock52to the reference channel5to generate the predetermined time delay T to be applied to the reference light channel5. This reference delay line55may generally be less accurate than the QKD delay lines54used for the QKD channel2.

A current reference pulse57is shown inFIG.5. The reference light6inFIG.5is a fiber coupled light source. The reference pulses7emitted by the reference light source6are collimated into free space by a fiber collimator64arranged in the reference channel5.

The emitted reference pulse7is first directed to a mirror65where it is redirected to combiner optics66. The combiner optics66is arranged and configured to combine the QKD encoded pulses4emitted on the QKD channel2and the reference pulses7emitted on the reference channel5to a combined output beam21using a beam combiner66. By this beam combining, the QKD encoded pulses4and the reference pulses7are directed substantially in the same direction to propagate on a common optical axis A. In this embodiment, the beam combiner66is a regular beam splitter or half-mirror. In further embodiments, the beam combiner66is configured as a dichroic mirror17with a sharp spectral edge on reflection/transmission between the wavelength of the QKD encoded pulses4and the reference pulses7. In further embodiments, the combination of the QKD channel2and the reference channel5is performed by fiber optics60employing fiber couplers63. In such embodiments, the entire optical setup may be performed by fiber optics. In further embodiments, the QKD encoded pulses4and the reference pulses7are transmitted through separate apertures as in the embodiment shown inFIG.1.

In the above embodiments, we refer to fiber optics60for simplicity. It is understood that waveguides inside a photonic chip could also be used, when convenient, to further reduce the size of the optical transmitter1.

FIG.6shows a schematic illustration of an optical transmitter1for quantum communication according to a further embodiment of the invention.

The embodiment shown inFIG.6is based on the embodiment inFIG.5and has only a few modifications. In this embodiment, instead of directly driving the reference light source6with a time delayed current reference pulse57, the reference light source6is configured as a cw, i.e., continuous wave, light source emitting light with continuous optical power. The optical transmitter1further comprises an amplitude modulator59connected to the control circuit8. The amplitude modulator59is arranged in the reference channel5and configured to modulate emitted light of the reference light source6to generate the reference pulses7. The control circuit8comprises a reference delay line55configured to generate the predetermined time delay T applied to the reference light channel5, in this case to the amplitude modulator59to generate the reference pulses7. In this embodiment, the amplitude modulator59is a Mach-Zehnder Modulator. In further embodiments, the amplitude modulator59May be any different type of modulator suitable for this task.

InFIG.5andFIG.6, the QKD channel2and the reference channel5are implemented in a single structural unit. In further embodiments, the QKD channel2and the reference channel5May be implemented as sub-modules, arranged in the same or in two separate housings, connected via cable or wirelessly.

FIG.7shows a schematic illustration of an optical system20for quantum communication comprising an optical transmitter1and an optical receiver10according to a further embodiment of the invention.

The optical system20shown inFIG.7is based on the previously described embodiments of the invention, in particular, in the optical system20shown inFIG.3. In this embodiment of an optical system20, the optical transmitter1further comprises a spatial mode converter70that is configured to modulate a spatial mode72of the reference pulses7to generate a converted spatial mode72, which is orthogonal to a spatial mode73of the QKD encoded pulses4. In further embodiments, the spatial mode converter70is configured to modulate the spatial mode73of the QKD encoded pulses4. The mode converter could convert a fundamental mode, e.g., a Gaussian mode TEM00 or a L01 mode from a step-index fiber, into a higher order converted spatial mode72, such as a TEM01 or a LP11 mode.

Similar to the previous embodiments ofFIGS.5and6, the optical transmitter1comprises a mirror65to deflect the reference pulses7onto the combiner optics66arranged and configured to combine the QKD encoded pulses4and the reference pulses7to a combined output beam21. In this case, the QKD encoded pulses4and the reference pulses7propagate on the same optical axis A but may experience a different divergence due to the different spatial modes.

The optical receiver10in this embodiment comprises a spatial demultiplexer71, which is configured in this embodiment as a phase plate. The spatial demultiplexer71is arranged on a receiver channel R and configured to substantially spatially filter the QKD encoded pulses4from the reference pulses7on respective receiver channels R1and R2. In this embodiment, the spatial demultiplexer71is configured to transmit the converted spatial mode72of the reference pulses7to the first receiver channel R1to be incident onto the wavefront sensor12. The demultiplexer is configured to reflect the spatial mode73of the QKD encoded pulses4to a second receiver channel R2to be incident onto the detection unit11.

The optical receiver10comprises an adaptive optical element14that is here arranged upstream of the wavefront sensor12and configured to manipulate the wavefront13of the reference pulses7. Due to the converted mode of the reference pulses7, the receiver controller16is configured to correct a spatial mode72of the reference pulses7based on the measured wavefront13, taking into account the converted mode at the optical transmitter1. By correcting the wavefront13of the receiver pulse7towards the converted spatial mode72of the reference pulses7, the wavefront15of the QKD encoded pulses4is also corrected, thus enabling optimal focusing.

In further embodiments, an additional dichroic filter17is used to spectrally filter the QKD encoded pulses4and the reference pulses7when a difference between the wavelength of the QKD encoded pulses4and the wavelength of the reference pulses7is between 1 nm and 5 nm, preferably between 2 nm and 3 nm. This wavelength difference makes it possible to further isolate the QKD encoded pulses4and the reference pulses7.

FIG.8shows a schematic illustration of a method for quantum communication according to an embodiment of the invention.

FIG.8shows a method for quantum communication. The method comprises the step of emitting S1, by an optical transmitter1, a stream of QKD encoded pulses4and reference pulses7, wherein the reference pulses7are emitted with a predetermined time delay T to the preceding QKD encoded pulses4. A difference of a wavelength of the QKD encoded pulses4and a wavelength of the reference pulses7is 5 nm or less. The method further comprises measuring S2, by a wavefront sensor12, the wavefront13of the reference pulses7. An additional step of applying S3based on the measured wavefront13a wavefront correction to the QKD encoded pulses4by an adaptive optical element14is provided. The method further includes the step of obtaining S4, by a detection unit11containing one or more photon counters, the QKD encoded pulses4by applying a time gate G based on a time of arrival of the QKD encoded pulses4to the one or more photon counters in the detection unit11for temporally filtering the QKD encoded pulses4from the reference pulses7.

In the detailed description above, various features have been combined in one or more examples in order to improve the rigorousness of the illustration. However, it should be clear in this case that the above description is of merely illustrative but in no way restrictive nature. It serves to cover all alternatives, modifications and equivalents of the various features and exemplary embodiments.

Many other examples will be immediately and directly clear to a person skilled in the art on the basis of his knowledge in the art in consideration of the above description.

The exemplary embodiments have been chosen and described in order to be able to present the principles underlying the invention and their application possibilities in practice in the best possible way. As a result, those skilled in the art can optimally modify and utilize the invention and its various exemplary embodiments with regard to the intended purpose of use. In the claims and the description, the terms “including” and “having” are used as neutral linguistic concepts for the corresponding terms “comprising”.

LIST OF REFERENCE SIGNS