High-brightness quantum source based on multi-wavelength combination via arrayed Type-0 ppKTP crystal and method of generating entangled photon pairs

A quantum source includes: a quantum source generator dividing a pump beam into a plurality of channels and generating a plurality of signals and a plurality of idlers using nonlinear crystals respectively located optical paths of the plurality of channels; and an entangled photon pair combiner outputting an enhanced signal by combining the plurality of signals and outputting an enhanced idler by combining the plurality of idlers.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2022-0041889 filed in the Korean Intellectual Property Office on Apr. 4, 2022, the entire contents of which are incorporated herein by reference.

BACKGROUND

Embodiments of the present invention relates to a high-brightness quantum source based on multi-wavelength combination via arrayed type-0 ppKTP crystal and a method of generating entangled photon pairs.

(b) Description of the Related Art

The generating of entangled photon pairs is not only a key element in fundamental research fields, such as quantum optics, but also an essential technology in real-world quantum information technologies, such as quantum communication, quantum computing, and quantum sensing. However, the realization of a global quantum network that may be implemented through links between satellites and the ground or between satellites and the realization of quantum lidar or radar capable of detecting stealth weapon systems call for the development of intensive, robust entanglement light source with a high production rate and entanglement purity.

SUMMARY

From this point of view, high-brightness quantum sources with sufficient brightness and entanglement visibility have been continuously researched and developed through various methods, and among them, spontaneous parametric down conversion (SPDC) based on nonlinear crystals with second-order nonlinear coefficients has been reported as the most appropriate method.

Embodiments of the present invention attempts to provide a high-brightness quantum source and a method of generating an entangled photo pairs, having a more stable combination method by eliminating an interference phenomenon that may occur in the same wavelength combination by using entangled photon pairs generated with different wavelengths according to temperature conditions when developing a high-brightness quantum source based on arrayed combination.

According to an exemplary embodiment, a quantum source includes: a quantum source generator dividing a pump beam into a plurality of channels and generating a plurality of signals and a plurality of idlers using nonlinear crystals respectively located optical paths of the plurality of channels; and an entangled photon pair combiner outputting an enhanced signal by combining the plurality of signals and outputting an enhanced idler by combining the plurality of idlers.

As the respective nonlinear crystals of the plurality of channels are adjusted to different temperatures, the plurality of signals may have different wavelengths and the plurality of idlers have different wavelengths.

The entangled photon pair combiner may include a polarizing beam splitter for combining the plurality of signals or the plurality of idlers.

The entangled photon pair combiner may include a plurality of reflection mirrors and a diffraction grating for combining the plurality of signals or the plurality of idlers.

The entangled photon pair combiner may include a spherical mirror and a diffraction grating for combining the plurality of signals or the plurality of idlers.

According to another exemplary embodiment, a quantum source includes: a first channel unit including a first nonlinear crystal controlled to have a first temperature and generating a signal and an idler of a first channel by injecting a pump beam to the first nonlinear crystal; a second channel unit including a second nonlinear crystal controlled to have a second temperature and generating a signal and an idler of a second channel by injecting a pump beam to the second nonlinear crystal; a signal combiner combining the signal of the first channel and the signal of the second channel; and an idler combiner combining the idler of the first channel and the idler of the second channel.

As the first nonlinear crystal and the second nonlinear crystal are adjusted to have different temperatures, the signal of the first channel and the signal of the second channel have different wavelengths and the idler of the first channel and the idler of the second channel may have different wavelengths.

The quantum source may further include: a third channel unit including a third nonlinear crystal adjusted to have a third temperature and generating a signal and an idler of a third channel by injecting a pump beam to the third nonlinear crystal, wherein the signal combiner combines the signal of the first channel, the signal of the second channel, and the signal of the third channel, and the idler combiner combines the idler of the first channel, the idler of the second channel, and the idler of the third channel.

The signal combiner may include: a polarizing beam splitter combining the signal of the first channel and the signal of the second channel; and a dichroic mirror combining combined light obtained by combining the signal of the first channel to the signal of the second channel, to the signal of the third channel.

The idler combiner may include: a polarizing beam splitter combining the idler of the first channel and the idler of the second channel; and a dichroic mirror combining combined light obtained by combining the idler of the first channel to the idler of the second channel, to the idler of the third channel.

The signal combiner may include: a first diffraction grating; and a plurality of mirrors for injecting the signal of the first channel, the signal of the second channel, and the signal of the third channel to the first diffraction grating, wherein the first diffraction grating generates an enhanced signal by combining the signal of the first channel, the signal of the second channel, and the signal of the third channel.

The idler combiner may include: a second diffraction grating; and a plurality of mirrors injecting the idler of the first channel, the idler of the second channel, and the idler of the third channel injected into the second diffraction grating, wherein the second diffraction grating generates an enhanced idler by combining the idler of the first channel, the idler of the second channel, and the idler of the third channel.

The signal combiner may include: a first diffraction grating; and a spherical mirror injecting the signal of the first channel, the signal of the second channel, and the signal of the third channel to the first diffraction grating, wherein the first diffraction grating generates an enhanced signal by combining the signal of the first channel, the signal of the second channel, and the signal of the third channel.

The idler combiner may include: a second diffraction grating; and a spherical mirror injecting the idler of the first channel, the idler of the second channel, and the idler of the third channel to the second diffraction grating, wherein the second diffraction grating generates an enhanced idler by combining the idler of the first channel, the idler of the second channel, and the idler of the third channel.

According to another exemplary embodiment, a method of generating entangled photon pairs includes: dividing a pump beam into a plurality of channels; adjusting nonlinear crystals respectively located in optical paths of the plurality of channels to different temperatures; separating signals and idlers for each of a plurality of channels from photon pairs generated by the nonlinear crystals respectively located in the plurality of channels; and generating an enhanced signal by combining the signals for each of the plurality of channels, and generating an enhanced idler by combining the idlers for each of the plurality of channels.

The signals for each of the plurality of channels have different wavelengths, and the idlers for each of the plurality of channels may have different wavelengths.

Using the fact that photon pairs are generated with different wavelengths depending on the temperature through the SPDC of Type-0 ppKTP nonlinear crystal, several crystals are arranged in an arrayed type under different temperature conditions, and signal photons and idler photons generated at different wavelengths are combined to generate an enhanced signal and idler, thereby configuring a high-brightness quantum source. In addition, a high-brightness quantum source of a more stable combination method may be provided by eliminating an interference phenomenon that may occur in the same wavelength combination.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings so that they may be easily implemented by one of ordinary skill in the art. As those skilled in the art would realize, the described exemplary embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

FIG.1is a block diagram illustrating a high-brightness quantum source based on multi-wavelength combination according to an exemplary embodiment of the present invention.

Referring toFIG.1, a high-brightness quantum source10based on multi-wavelength combination according to an exemplary embodiment of the present invention may include a quantum source generator100and an entangled photon pair combiner200.

The quantum source generator100may divide a pump beam into a plurality of channels and generate a plurality of signals S1, S2, and S3and a plurality of idlers Id1, Id2, and Id3. At this time, the quantum source generator100may adjust nonlinear crystals of each of the plurality of channels to have different temperatures, so that the signals S1, S2, and S3of the plurality of channels may have different wavelengths and the idlers Id1, Id2, and Id3of the plurality of channels may have different wavelengths.

The nonlinear crystal may be type-0 periodically poled potassium titanyl phosphate, ppKTP (periodically poled KTiOPO4) that causes spontaneous parametric down conversion (SPDC) and has a periodic polling structure. The nonlinear crystal causes SPDC when a continuous wave laser is injected, and one injected photon of the continuous wave laser may create two photons (a photon pair) having a converted wavelength.

The quantum source generator100may transfer the plurality of signals S1, S2, and S3and idlers Id1, Id2, and Id3of each channel to the entangled photon pair combiner200through an optical fiber.

The entangled photon pair combiner200may combine the plurality of signals S1, S2, and S3having different wavelengths input from different channels, and may combine the plurality of idlers Id1, Id2, and Id3having different wavelengths input from different channels. The entangled photon pair combiner200may combine the plurality of signals S1, S2, and S3to output an enhanced signal S_out, and combine the plurality of idlers Id1, Id2, and Id3to output an enhanced idler Id_out. The enhanced signal S_out and the enhanced idler Id_out finally output from the entangled photon pair coupler200may be used as signals and idlers for a pixel imaging device, a quantum communication device, and the like.

By combining the plurality of signals S1, S2, and S3having different wavelengths and combining the plurality of idlers Id1, Id2, and Id3having different wavelengths, interference that may occur in the same wavelength combination may be eliminated and the efficiency of the quantum source may be improved.

FIG.2is a block diagram illustrating a quantum source generator according to an exemplary embodiment of the present invention.

Referring toFIG.2, the quantum source generator100may include a pump laser110, a beam splitter120, and a plurality of channel units CH1, CH2, and CH3.

The pump laser110may generate a pump beam P and inject the generated pump beam P to the beam splitter120. The pump beam P may be a continuous wave laser having a wavelength of 405 nm.

The beam splitter120may include a plurality of half wave plates (HWPs)121,122, and123and a plurality of polarizing beam splitters (PBSs)124,125, and126, and may divide a pump beam P into a plurality of channels using the plurality of HWPs121,122, and13and the plurality of PBSs124,125, and126. The first half wave plate121, the first polarizing beam splitter124, the second half wave plate122, the second polarizing beam splitter125, the third half wave plate123, and the third polarizing beam splitter126may be arranged in order along an optical path of the pump beam P.

The first polarizing beam splitter124, the second polarizing beam splitter125, and the third polarizing beam splitter126may serve to divide the pump beam P into a plurality of channels to transmit the same. The first half wave plate121, the second half wave plate122, and the third half wave plate123may serve to appropriately adjust polarization of the pump beam P to equalize the intensity of the pump beam P transmitted to a plurality of channels for each channel.

The pump beam P may be injected into the first channel unit CH1through the third polarizing beam splitter126, the pump beam P may be injected into the second channel unit CH2through the second polarizing beam splitter125, and the pump beam P may be injected into the third channel unit CH3through the first polarizing beam splitter124.

Each of the first channel unit CH1, the second channel unit CH2, and the third channel unit CH3may include half wave plates131,132, and133, first lenses141,142, and143, nonlinear crystals151,152, and153, second lenses144,145, and146, first dichroic mirrors161,162, and163, second dichroic mirrors164,165, and166, and reflection mirrors171,172, and173.

The half wave plates131,132, and133of each channel may set the pump beam P injected into each channel to be vertically polarized. Vertical polarization is polarization required for the nonlinear crystals151,152and153to interact with the pump beam P.

The first lenses141,142, and143of each channel may focus the vertically polarized pump beam P toward the center of the nonlinear crystals151,152, and153, and the second lenses144,145, and146of each channel may horizontally collimate light formed through the nonlinear crystals151,152, and153.

The nonlinear crystals151,152, and153of each channel may be type-0 ppKTPs having a periodic polling structure. The three nonlinear crystals151,152and153may be arranged side by side in an array. When the pump beam P is injected, the nonlinear crystals151,152, and153of each channel may cause SPDC to generate a photon pair.

The nonlinear crystals151,152, and153of each channel may operate under different temperature conditions. To this end, each of the channel units CH1, CH2, and CH3may include temperature controllers156,157, and158, and each of the temperature controllers156,157, and158may adjust the nonlinear crystal151,152, and153of each channel to have different temperatures. The type-0 ppKTP with a periodic poling structure of 3.425 μm generates photon pairs having a central wavelength of 810 nm at 30.2° C. for a wavelength of 405 nm. Because polarizations of the generated signal and idler photons are the same, the type-0 ppKTP may generate signal and idler photon pairs having different wavelengths under temperature conditions of 30.2° C. or higher. For example, the first temperature controller156may maintain the first nonlinear crystal151of the first channel unit CH1at 32° C., the second temperature controller157may maintain the second nonlinear crystal152of the second channel unit CH2at 35° C., and the third temperature controller158may maintain the third nonlinear crystal153of the third channel unit CH3at 39° C.

As the nonlinear crystals151,152, and153of each channel are adjusted to have different temperatures, the photon pairs of each channel may have different wavelengths. That is, the signals S1, S2, and S3of the plurality of channels may have different wavelengths, and the idlers Id1, Id2, and Id3of the plurality of channels may have different wavelengths.

The first dichroic mirrors161,162, and163of each channel may reflect the pump beam P having a wavelength of 405 nm transmitted through the nonlinear crystals151,152, and153and allow the photon pairs to be transmitted therethrough. For example, a cut-off wavelength of the first dichroic mirrors161,162, and163may be 650 nm. The pump beam P reflected by the first dichroic mirrors161,162, and163of each channel is discarded as a dump, and the photon pairs transmitted through the first dichroic mirrors161,162, and163are injected into the second dichroic mirrors164,165, and166.

The second dichroic mirrors164,165, and166of each channel may separate the injected photon pairs into signals S1, S2, and S3and idlers Id1, Id2, and Id3. That is, the second dichroic mirrors164,165, and166of each channel may reflect the signals S1, S2, and S3and allow the idlers Id1, Id2, and Id3to be transmitted therethrough. The second dichroic mirrors164,165, and166of each channel may have different cut-off wavelengths corresponding to the photon pairs having different wavelengths for each channel.

The signals S1, S2, and S3reflected by the second dichroic mirrors164,165, and166of each channel may be collected to optical fibers by optical fiber couplers (or collimators)181,183, and185, respectively. In addition, each of the idlers Id1, Id2, and Id3transmitted through the second dichroic mirrors164,165, and166of each channel may be reflected by the reflection mirrors171,172, and173and then collected to the optical fibers by collimators182,184, and186.

FIG.3is a block diagram illustrating an entangled photon pair combiner according to an exemplary embodiment of the present invention.

Referring toFIG.3, the entangled photon pair combiner200may include a signal combiner210and an idler combiner220.

The signal combiner210may be connected to a plurality of optical fibers that transfer the plurality of signals S1, S2, and S3for each channel in the quantum source generator100, and the plurality of signals S1, S1and S3for each channel may be injected into the signal combiner210through the plurality of optical fibers. The signal combiner210may be configured to output an enhanced signal S_out by combining the signals S1, S2, and S3for each channel.

As a component for combining the plurality of signals S1, S2, and S3for each channel, the signal combiner210may include a reflection mirror211, a plurality of half wave plates212and213, a polarizing beam splitter214, and a dichroic mirror215.

The reflection mirror211injects the signal S1of the first channel into one side of the polarizing beam splitter214. The signal S2of the second channel may be injected into the other side of the polarizing beam splitter214, and the polarizing beam splitter214may combine the signal S1of the first channel and the signal S2of the second channel and output the same. The plurality of half wave plates212and213may be disposed in optical paths of the signal S1of the first channel and the signal S2of the second channel to equalize the intensity of the beam injected into the polarizing beam splitter214.

The dichroic mirror215may reflect combined light of the signal S1of the first channel and the signal S2of the second channel combined by the polarizing beam splitter214and output, and allow the signal of the third channel S3to be transmitted therethrough so that the combined light and the signal S3of the third channel are combined. The enhanced signal S_out obtained by combining the signal S1of the first channel, the signal S2of the second channel, and the signal S3of the third channel may be collected to a multi-mode fiber (MMF) or a single-mode fiber (SMF) through the collimator216.

The idler coupler220is connected to a plurality of optical fibers that transfer the plurality of idlers Id1, Id2, and Id3for each channel in the quantum source generator100, and the plurality of idlers Id1, Id1, and Id3for each channel may be injected into the idler coupler220through the plurality of optical fibers. The idler combiner220may be configured to output an enhanced idler Id_out by combining the plurality of idlers Id1, Id2, and Id3for each channel.

As a component for combining the plurality of idlers Id1, Id2, and Id3for each channel, the idler combiner220may be configured to be substantially the same as the signal combiner210. In other words, the idler coupler220may include a reflection mirror221, a plurality of half wave plates222and223, a polarizing beam splitter224, and a dichroic mirror225.

The reflection mirror221injects the idler Id1of the first channel into one side of the polarizing beam splitter224. The idler Id2of the second channel may be injected into the other side of the polarizing beam splitter224, and the polarizing beam splitter224may combine the idler Id1of the first channel and the idler Id2of the second channel and output the same. The plurality of half wave plates222and223may be disposed in the optical path of the idler Id1of the first channel and the idler Id2of the second channel to equalize the intensity of a beam injected into the polarizing beam splitter224.

The dichroic mirror225may reflect combined light of the idler Id1of the first channel and the idler Id2of the second channel which is combined by the polarizing beam splitter224and output, and allow the third idler Id3of the third channel to be transmitted therethrough so that the combined light and the idler (Id3) of the third channel are combined. The enhanced idler Id_out in which the idler Id1of the first channel, the idler Id2of the second channel, and the idler Id3of the third channel are combined may be collected to multi-mode optical fiber (MMF) or single-mode fiber (SMF) through the collimator226.

Hereinafter, generation of photon pairs by a nonlinear crystal ppKTP having a periodic polling structure will be described with reference toFIGS.4to7.

FIG.4is a diagram illustrating a nonlinear crystal having a periodic polling structure according to an exemplary embodiment.FIG.5illustrates a momentum relationship inside a nonlinear crystal having a periodic poling structure.

Referring toFIGS.4and5, an exemplary embodiment of the present invention proposes the high-brightness quantum source10based on multi-wavelength combination by forming a type-0 ppKTP crystal having a high second-order nonlinear coefficient in an array form. The ppKTP having a periodic polling structure has a structure in which a sign of a nonlinear coefficient of a crystal is periodically inverted, and an inverse period is referred to as a polling period, and the photon pair is generated toward satisfying a quasi-phase matching condition.

For specific analysis of SPDC, it is necessary to approach through quantum mechanical analysis, which may be expressed by Equation 1. The Hamiltonian for a phase matching condition is represented by an equation of the pump, signals, and idler.

Here, Ep, Es, Eirepresent electric field operators of the pump, signal, and idler, respectively, and HC represent the Hermitian conjugate operator. A mathematical expression of the electric field operator is shown in Equation 2.

μp(ω)=exp[-(ω-ω_p)22⁢(Δ⁢ωp)2]
denotes a frequency envelope (spectral envelope) of the pump,

εj=h⁢ω_j2⁢ϵ0⁢n⁢V
denotes a constant value of an electric field.

Therefore, an interaction Hamiltonian may be summarized as in Equation 3.

A wave function of the photon pair generated from the interaction Hamiltonian obtained above is shown in Equation 4.

In a sinc function, Δk is the degree of phase mismatching and represents a change amount of momentum in a traveling direction (x-axis), and L represents a length of the crystal in the traveling direction. As the degree of phase mismatching increases, the value of the sinc function decreases, and thus, downconversion occurs less. Also, when the conservation of momentum is completely satisfied, the value of the sinc function is maximized as 1.

A momentum relationship inside the nonlinear crystal is as shown inFIG.5, and the degree of phase mismatching may be expressed as in Equation 5.
Δk=kp−kscos θ−kicos ϕ  (Equation 5)

Here, as for θ and ϕ, in a situation in which the pump beam travels in the x-axis direction of the nonlinear crystal and the signal and idler photon pair travels in an x-y plane, an angle formed by the pump beam and the signal beam is defined as θ and an angle between the pump beam and the idler beam is defined as ϕ.

In the case of the nonlinear crystal having a periodic polling period Λ, such as ppKTP, the nonlinear coefficient χ(2)treated as a constant in the Hamiltonian appears as a function of distance, an exponential nonlinear coefficient related to momentum has a form of a square wave whose sign is periodically flipped, and thus, it may be expressed as Equation 6 through a Fourier series.

When the wave function is recalculated in consideration of the periodic nonlinear term, it may be rearranged as in Equation 7.

Here, by defining a phase matching condition considering even the polling period as a quasi-phase matching condition, the degree of phase mismatch may be expressed as in Equation 8.

Since the term of interest in the wave function of Equation 7 is a term after SPDC, only the second term may be considered, while ignoring a vacuum term. In addition, as for sums in sigma represented by the Fourier series, only one appropriate m value has a great influence on the result of the formula, and for other values, the sinc function is sufficiently close to 0, so only one m value survives as a result. The degree of phase mismatching in Equation 8 is determined by many factors, such as the type of SPDC, the type of crystal, a size of the polling period, and a wavelength of the pump.

FIG.6is a graph illustrating simulation results of a relationship between the generation wavelength of an entangled photon pair according to temperature of a type-0 ppKTP crystal.

Referring toFIG.6, simulation parameters are shown in Table 1.

When a pump beam having a wavelength of 405 nm is injected into type-0 ppKTP, entangled photon pairs that match the temperature conditions of the crystal are generated by SPDC. When simulation is performed under the conditions of Table 1, it can be seen that an entangled photon pair of 810 nm is generated at 30.2° C., and, it can be seen that, as the temperature increases, the wavelengths of the generated signal and idler photon pair become gradually distant.

FIG.7is a graph illustrating a relationship between a photon pair generation wavelength and efficiency according to a phase matching temperature.

Referring toFIG.7, the relationship between a photon pair generation wavelength and efficiency according to a phase matching temperature is obtained from the sinc function of the wave function. As described above, it can be seen that wavelengths of photon pairs generated according to different phase matching temperatures are different. Each corresponding line width may be different from the actual simulation, but even considering a case in which a line width is made to be very wide, each wavelength may be separated by significantly changing a temperature or using an optical filter based on the center wavelength of the photons to be generated.

Hereinafter, another exemplary embodiment of the entangled photon pair combiner200will be described with reference toFIGS.8and9.

FIG.8is a block diagram illustrating an entangled photon pair combiner according to another exemplary embodiment of the present invention.

Referring toFIG.8, at least one of a signal combiner210and an idler combiner220of an entangled photon pair combiner200may be configured as follows.

The signal combiner210or the idler combiner220may include a plurality of reflection mirrors231,232,233,234,235, and236and a diffraction grating237. The plurality of reflection mirrors231,232,233,234,235, and236serve to inject a signal S1(or an idler Id2) of the first channel, a signal S2(or an idler Id2) of the second channel, and a signal S3(or an idler Id3) of the third channel into the diffraction grating237.

A pair of reflection mirrors231and232may inject the signal S1(or the idler Id2) of the first channel into the diffraction grating237, another pair of reflection mirrors233and234may inject the signal S2(or the idler Id2) of the second channel into the diffraction grating237, and the other pair of reflection mirrors235and236may inject the signal S3(or the idler Id3) of the third channel into the diffraction grating237.

The diffraction grating237may combine the signal S1(or the idler Id2) of the first channel, the signal S2(or the idler Id2) of the second channel, and the signal S3(or the idler Id3) of the third channel to generate an enhanced signal S_out (or an enhanced idler Id_out). The enhanced signal S_out (or enhanced idler Id_out) may be collected to a multimode fiber (MMF) or a single mode fiber (SMF) through a fiber coupler238.

FIG.9is a block diagram illustrating an entangled photon pair combiner according to another exemplary embodiment of the present invention.

Referring toFIG.9, at least one of the signal combiner210and the idler combiner220of the entangled photon pair combiner200may be configured as follows.

The signal coupler210or the idler coupler220may include a spherical mirror241and a diffraction grating242.

The spherical mirror241may inject the signal S1(or the idler Id2) of the first channel, the signal S2(or the idler Id2) of the second channel, and the signal S3(or the idler Id3) of the third channel into the diffraction grating242.

The diffraction grating242may combine the signal S1(or the idler Id2) of the first channel, the signal S2(or the idler Id2) of the second channel, and the signal S3(or the idler Id3) of the third channel to generate an enhanced signal S_out (or an enhanced idler Id_out). The enhanced signal S_out (or the enhanced idler Id_out) may be collected to a multimode fiber (MMF) or a single mode fiber (SMF) through the fiber coupler243.

The exemplary embodiments ofFIGS.8and9have the advantage of reducing a size of an optical setup because the signals S1, S2, and S3or the idlers Id1, Id2, and Id3) of multiple channels may be combined by one diffraction grating237or242. However, since distortion may occur in a spatial mode of a beam after reflection of the diffraction gratings237and242, optical fiber coupling efficiency may decrease. Accordingly, light combination may be configured by selecting the polarizing beam splitters214and224or the diffraction gratings237and242according to a configuration of the high-brightness quantum source.

FIG.10is a flowchart illustrating a method of generating an entangled photon pair according to an exemplary embodiment of the present invention.

Referring toFIG.10, the quantum source10according to an exemplary embodiment of the present invention may divide a pump beam P into a plurality of channels using a plurality of half wave plates121,122, and123and a plurality of polarizing beam splitters124,125, and126(S110). Nonlinear crystals151,152, and153are located in optical paths of each of the plurality of channels.

The quantum source10may adjust the nonlinear crystals151,152, and153located in each of the plurality of channels to have different temperatures using the temperature controllers156,157, and158(S120). The nonlinear crystals151,152, and153of each channel may be type-0 ppKTPs having a periodic polling structure. The three nonlinear crystals151,152and153may be arranged side by side in an array. The nonlinear crystals151,152, and153of each channel may generate a photon pair by causing SPDC when the pump beam P is injected. As the nonlinear crystals151,152, and153of each channel are adjusted to have different temperatures, photon pairs of each channel may have different wavelengths.

The quantum source10removes the pump beam P transmitted through the nonlinear crystals151,152, and153using the first dichroic mirrors161,162, and163located in each channel, and the signals S1, S2, and S3for each channel and the idlers Id1, Id2, and Id3may be separated using the second dichroic mirrors164,165, and166(S130). The second dichroic mirrors164,165, and166of each channel may have different cut-off wavelengths corresponding to photon pairs having different wavelengths for each channel.

The quantum source10generates an enhanced signal S_out by combining the signals S1, S2, and S3having different wavelengths of a plurality of channels and generate an enhanced idler Id_out by combining the idlers Id1, Id2, and Id3having different wavelengths of a plurality of channels (S140). As illustrated inFIG.3, the quantum source10may combine the signals S1, S2, and S3having different wavelengths of a plurality of channels by using the polarizing beam splitters214and224and the dichroic mirrors215and225and may combine the idlers Id1, Id2, and Id3having different wavelengths of a plurality of channels by using the polarizing beam splitters214and224and the dichroic mirrors215and225. Alternatively, as illustrated inFIGS.8and9, the quantum source10may combine the signals S1, S2, and S3having different wavelengths of a plurality of channels or the idlers Id1, Id2, and Id3having different wavelengths of a plurality of channels by using the diffraction gratings237and242.

The drawings referred to and the detailed descriptions of the present invention are merely illustrative and have been used to describe the present invention but not intended to limit the scope of the present invention described in claims. Thus, a person skilled in the art may easily select therefrom to replace the same. Thus, the scope of the present invention should be determined by claims and the equivalent, rather than by the exemplary embodiment described herein.