Patent Description:
Any mention and/or discussion of prior art throughout the specification should not be considered, in any way, as an admission that this prior art is well known or forms part of common general knowledge in the field.

A source of polarization entangled photons is one of the basic requirements for laboratory research in quantum communication. Moreover, quantum key distribution networks require compact and robust entangled photon sources which can withstand vibrations and temperature fluctuations.

Hence, bright and stable sources of polarization entangled photon pairs are of great interest in quantum communication. The most popular way of generating such photon pairs is through spontaneous parametric down conversion (SPDC) of a laser beam in a second order nonlinear crystal. One of the main concerns for the design of such sources is the phase stability of the generated state against temperature fluctuations and vibrations. As the phase is directly related to the pump laser wavelength, the fluctuations in the central wavelength of the pump will degrade the entangled state. In critically phase matched designs [<NPL>], angular instability in the alignment of the nonlinear crystals will contribute to the phase instability of the generated state. Specifically, [<NUM>] describes a method that utilizes the two coherent SPDC processes to generate photon pairs entangled in polarization.

In quasi-phase matched designs [<NPL>] the whole process depends also on the thermal fluctuations in the temperature bath in which the periodically poled crystal is mounted. Both sources of instability are severe limitations for the application of entangled photon sources. Moreover, high bright polarization entangled photons are created by superposing photon states from two SPDC processes which are orthogonal to each other. This quantum superposition is done via interferometry which involves mirrors and beam splitters. Hence such designs have relatively larger foot print and are vulnerable to misalignments due to mechanical vibrations.

<CIT> describes, in a quantum entangled photon pair generator, selectively generating polarization entangled photon pairs and time-bin entangled photon pairs, an excitation optical pulse shaper receives a linearly polarized optical pulse, and selectively outputs either one of a polarization excitation optical pulse pair to be a seedlight pulse for a polarization entangled photon pair and a consecutive excitation optical pulse pair to be a seedlight pulse for a time-bin entangled photon pair. An optical interferometer receives the polarization excitation optical pulse pair or the consecutive excitation optical pulse pair, and outputs a correlated photon pair forming signal and idler photons through a parametric fluorescence process. A quantum entangled photon pair extractor spatially extracting wavelength components corresponding to photons of the quantum entangled photon pair to output the components as the polarization entangled photon pair or the time-bin entangled photon pair.

<CIT> describes a polarization-entangled photon source comprises a down conversion crystal having a first waveguide and a second waveguide, a dielectric spacer positioned adjacent to the down conversion crystal and configured to receive electromagnetic radiation emitted from the first waveguide, and a half-wave plate positioned adjacent to the down conversion crystal and configured to receive electromagnetic radiation emitted from the second waveguide. The polarization-entangled photon source also includes a beam displacer positioned adjacent to the dielectric spacer and the half-wave plate and configured to combine the electromagnetic radiation output from the dielectric spacer and the half-wave plate into a single beam of electromagnetic radiation.

<CIT> describes a polarization entangled-photon state source comprises a single transmission layer configured for transmitting electromagnetic radiation. The transmission layer includes a beamsplitter and a down-conversion device, both of which are configured to convert a pump beam into first and second signal beams and first and second idler beams. The transmission layer also includes a mode converter configured to invert electric and magnetic field components of both the first signal beam and the first idler beam, and a combiner configured to receive the first and second signal beams and the first and second idler beams and output the first and second signal beams and the first and second idler beams in an entangled polarization states.

<CIT> describes a quantum entangled photon-pair producing device is disclosed which includes a superposed state generating device for generating a superposed state of photon-pairs entering through N (N≧<NUM>) different incident optical paths and being composed of photons having different polarization directions, and a light-guide device for separating the photon-pairs entering through the N (e.g. N is two) incident optical paths into photons having a first polarization direction (e.g. horizontally polarized light) and those having a second polarization direction (e.g. vertically polarized light) and guiding the photons having the first polarization direction and entering through the i-th <NUM>≦i≦N) (e.g. the first) incident optical path and photons having the second polarization direction and entering through the (N-i+<NUM>) (e.g. the second) incident optical path to the i-th (e.g. the first) exit optical path through optical paths having the same optical path length. Therefore, quantum entangled photons of N channels having a quantum correlation with regard to the polarization direction can be produced.

<CIT> describes a non-degenerate, polarization-entangled photon source comprises a half-wave plate that outputs both a first pump beam and a second pump beam, and a first beam displacer that directs the first pump beam into a first transmission channel and the second pump beam into a second transmission channel. A down-conversion device converts the first pump beam into first signal and idler photons and converts the second pump beam into second signal and idler photons. A second beam displacer directs both the first signal and idler photons and the second signal and idler photons into a single transmission channel. A dichroic mirror directs the first and second signal photons to a first fiber optic coupler and the first and second idler photons to a second fiber optic coupler.

Embodiments of the present invention seek to address at least one of the above problems.

In accordance with a first aspect of the present invention, there is provided a method of converting position or momentum correlation of correlated photon pairs to a polarization entangled photon pair, as defined in claim <NUM>.

In accordance with a second aspect of the present invention, there is provided a module for converting position or momentum correlation of correlated photon pairs to a polarization entangled photon pair, as defined in claim <NUM>:.

Embodiments of the present invention provide a method to use the inherent position/momentum correlations of photons to generate polarization entangled photons. In an example embodiment a compact design offers a robust, bright and compact source of polarization entangled photons. As the position and momentum correlations are inherent for the two photon state, the method according to example embodiments can be used irrespective of the generation process. Compared to existing polarization entangled photon sources, embodiments of the present invention can advantageously offer smaller footprint and robustness against mechanical vibrations and fluctuations in temperature or laser wavelength.

As embodiments of the present invention can use limited resources, they can be an ideal source for basic research involving entanglement. On the other hand, the compact, robust fiber-in fiber-out source according to an example embodiment can be useful in quantum communication networks distributing entanglement.

Example embodiments of the present invention convert the inherent position/momentum correlations to polarization entanglement. Advantageously, embodiments of the present invention can be used to convert any two photon state to an entangled state and hence can be used in any type of critical or non-critical phase matched designs. Moreover, the embodiments presented here can be extended to any system that can produce twin photons, i.e. photon pairs, with position or momentum correlation. Thus, embodiments of the present invention can be used to generate entangled photons with atomic linewidth, for example if applied to a heralded single photon generated by four wave mixing.

Explicit stipulations of example embodiments preferably are:.

A position correlated state of photons generated via SPDC is given by <MAT> where x denotes the position coordinate. Discretizing the continuous position variable as <MAT> the state becomes, <MAT>.

The generated photons will be co-polarized in type I or type <NUM> SPDC. So the combined position-polarization state of the photon is <MAT>.

By applying a position controlled polarization NOT gate, the state transforms into <MAT> which is a GHZ state. Here, H and V denote the linear horizontal and vertical polarization states. The position information can be erased, for example, by coupling the photons into a single mode fiber, and the state becomes the famous polarization Bell state <MAT>.

Conversely, if we use the momentum correlation <MAT>, we can generate the <MAT>.

Example embodiments for the conversion of position correlations (can be used for momentum correlations also) to polarization entanglement are described herein. Two example embodiments involve interferometers while one example embodiment is a fiber-in fiber-out compact design involving spatially variant waveplates.

In embodiment <NUM>, SPDC photons are created using critical or quasi phase matched nonlinear crystals (NLC). In embodiment <NUM>, a periodically poled Potassium titanyl phosphate (ppKTP) is used as the NLC <NUM> which generates photon pairs, each entangled in momentum and position. With reference to <FIG>, two lenses <NUM>, <NUM> are used such that the generation plane of the SPDC is imaged on to a wedge mirror <NUM> using 4f configuration. The wedge mirror <NUM> splits the photon pairs according to their generation position within the NLC <NUM>. In this configuration the photon pairs are correlated in position, i.e. signal photons born in one position are correlated to the idler photons generated at the same position, with one signal photon and its correlated idler photon constituting the correlated photon pair, as given in Eq. (<NUM>). With the 4f imaging system, the wedge mirror <NUM> essentially discretizes the continuous position states in the NLC <NUM> resulting the state given in Eq. <NUM>. It is noted that in another embodiment the wedge mirror can be used to split the photons on the basis of momentum as well. This can be done by keeping the wedge mirror after a lens (not shown) which collimates the SPDC output.

A half wave plate <NUM> at <NUM> degree converts the polarization of photon pairs with state |X', H〉 to |X', V〉. The state |X, H〉 passes through a half wave plate <NUM> oriented at <NUM> degrees which does not change its state. That is, the polarization of the first correlated photon pair group is rotated such that the polarization of the first correlated pair group in one path is at <NUM> degrees relative to the polarization of the second correlated photon pair group in the other path. The half wave plate <NUM> is introduced just to compensate the path lengths in embodiment <NUM>. The two paths are combined in a polarizing beam splitter (PBS) <NUM>.

The combined beam <NUM> passes through a spatial filter (SF) <NUM> which erases the spatial information to form the polarization entangled state at numeral <NUM>. The spatial filtering of the combined beam can be done by a pinhole or by collecting it to a single mode fiber. The spatial overlap of the two beams <NUM>, <NUM> is important for the entanglement quality. Therefore, optionally a dove prism <NUM> can be used to flip the spatial distribution of one path to overlap the two beams <NUM>, <NUM> perfectly in which case there is no need to do any kind of spatial filtering, i.e. SF <NUM> can be omitted. To compensate the additional path length due to dove prism, a glass block <NUM> of equal length can be inserted in the other arm.

<FIG> shows schematic drawings illustrating the overlap of the two correlated photon pair beams that can be achieved when using the SF <NUM> (<FIG>) and when flipping the spatial distribution, respectively. Specifically, illustrated on the left is that the photon pairs generated in the upper semicircle <NUM> of the generation plane of the NLC <NUM> (<FIG>) partially overlap the photon pairs generated at the lower semicircle <NUM> of the generation plane, providing a reduced overlap region <NUM> after filtering using SF <NUM> (<FIG>). Preferably, when flipping one of the spatial distributions, here <NUM>*, maximum overlap can be achieved. In the practical case, both configurations are fine according to example embodiments when the photons are collected with a small collection focus (compared to the total beam distribution), for example when the photons are collected using a single mode fiber with a collection focus full width half maximum of <NUM> micrometres, which is much smaller than the total beam distribution shown in <FIG>.

Referring again to <FIG>, one of the mirrors <NUM> in the interferometer <NUM> is attached to a piezoelectric actuator (not shown) for the active stabilization of the interferometer <NUM>. The pump laser <NUM> itself can be used to lock the interferometer so that it does not introduce any phase fluctuations. This can e.g. be done by locking into the destructive interference produced by the pump laser <NUM> at the other end of the PBS <NUM> after projecting to a diagonal polarization. The photons collected e.g. into a single mode fiber at numeral <NUM> will be entangled in polarization. The entangled signal photons and the entangled idler photons can be separated later on by, for example, a wavelength division multiplexer (WDM) or a dichroic beam splitter for output of respective polarization entangled photon pairs. The relative phase between |H, H〉 and |V, V〉 states advantageously depends only on the path length difference in the interferometer <NUM> and the pump <NUM> wavelength <MAT>.

That is, the causes of instability in the phase is only due to the change in the pathlength difference in the interferometer <NUM> and the fluctuations in the pump <NUM> laser wavelength. Notably, unlike existing polarization entangled sources, the phase does not arise from the dispersion of photons in the non-linear material <NUM>. That is, negligible wavelength dependent phase difference can advantageously be achieved.

Embodiment <NUM> is conceptually similar to embodiment <NUM> where the photon pairs are split according to their position of origin using imaging lenses and a wedge mirror. However, in embodiment <NUM> a folded Mach Zehnder interferometer <NUM> is used in which the photon pairs hit the same mirrors, as shown in <FIG>. This can advantageously increase the phase stability as the pathlength does not change due to the thermal expansion of the mirrors. With proper boxing up, the phase can be made stable for sufficiently large time interval and embodiment <NUM> advantageously doesn't require any active stabilization. In embodiment <NUM> also, the optional image inversion with a dove prism <NUM> and glass block <NUM> can be applied.

In embodiment <NUM> SPDC photon pairs are again created using critical or quasi phase matched nonlinear crystals (NLC). In embodiment <NUM>, similar to embodiment <NUM>, a periodically poled Potassium titanyl phosphate (ppKTP) is used which generates photon pairs entangled in momentum and position. Two lenses <NUM>, <NUM> are again used such that the generation plane of the SPDC is imaged on to a wedge mirror <NUM> using 4f configuration. The wedge mirror <NUM> splits the photon pairs according to their position of generation. In this configuration the photon pairs are correlated in position, i.e. signal photons born in one position are correlated to the idler photons generated at the same position as given in Eq. (<NUM>). The wedge mirror <NUM> essentially discretizes the continuous position states resulting the state given in Eq. <NUM>. It is again noted that in another embodiment the wedge mirror can be used to split the photon pairs on the basis of momentum as well. This can be done by keeping the wedge mirror after a lens (not shown) which collimates the SPDC output.

A half wave plate <NUM> at <NUM> degree converts the polarization of photon pairs with state |X',H〉 to lX', V〉. The state |X',H〉 passes through a half wave plate <NUM> oriented at <NUM> degrees which does not change its state. That is, the polarization of the first correlated photon pair group is rotated such that the polarization of the first correlated pair group in one path is at <NUM> degrees relative to the polarization of the second correlated photon pair group in the other path. The half wave plate <NUM> is introduced just to compensate the path lengths in embodiment <NUM>. The two paths are combined in a polarizing beam splitter (PBS) <NUM>, with one of the beams reflected at the wedge mirror <NUM> towards the PBS <NUM>.

The combined beam <NUM> passes through a spatial filter (SF) <NUM> which erases the spatial information to form the polarization entangled state at numeral <NUM>. The spatial filtering of the combined beam <NUM> can be done by a pinhole or by collecting it to a single mode fiber. The overlap of the two beams <NUM>, <NUM> is important for the entanglement quality. Therefore, optionally the dove prism <NUM> can be used to flip the spatial distribution of one path to overlap the two beams <NUM>, <NUM> perfectly in which case there is no need to do any kind of spatial filtering, i.e. SF <NUM> can be omitted. To compensate the additional path length due to dove prism, the glass block <NUM> of equal length can be inserted in the other arm.

Again, negligible wavelength dependent phase difference can advantageously be achieved.

Embodiment <NUM> is a compact fiber-in fiber-out design which could be useful for the field deployment of entangled photons in quantum networks. Figure 3A shows a top view of embodiment <NUM>, in which the pump laser from a single mode fiber <NUM> is focussed to the nonlinear crystal <NUM> using a focusing lens <NUM>. At the other end of the crystal <NUM>, a custom waveplate <NUM> is introduced. The custom waveplate <NUM> is formed by two half wave plates (HWP) <NUM>, <NUM> (for the SPDC) bonded together side by side. The fast axis orientation of one HWP <NUM> is at <NUM> degrees with respect to the optical axis, while the other HWP <NUM> is oriented at <NUM> degrees. This will convert the polarization of the correlated photon pairs generated in one semicircle (here the left) of the generation plane into vertical while the polarization of photon pairs generated at the other semicircle (here the right) is unaffected. That is, the polarization of the first correlated photon pair group is rotated such that the polarization of the first correlated pair group in one path is at <NUM> degrees relative to the polarization of the second correlated photon pair group in the other path.

This can also be done with a single half wave plate with fast axis orientated <NUM> degrees to the beam axis acting only on one half of the SPDC distribution, noting that any added constant phase difference can be compensated later on. The correlated photon pairs are collected to a single mode fiber <NUM> using a lens <NUM>. It is noted that other spatial filtering methods can be used as well in different embodiments. It is further noted that the fiber coupled laser at the input can be replaced to a laser diode in different embodiments. Pump, signal and idler photons at the output can be separated by a WDM or a dichroic beam splitter, or can be split by their momentum distribution, for example by using a wedge mirror. Again, negligible wavelength dependent phase difference can advantageously be achieved.

As embodiment <NUM> does not involve an interferometer, good phase stability can be achieved. Unlike existing Sagnac interferometer based sources, in the embodiments described above the phase does not depend on the nonlinear crystal temperature. The reason for the inherent phase stability stems from the fact that the photon pairs are generated during the same process within the same nonlinear medium, and that the pairs (from left and right semicircle) upon leaving the crystal are virtually indistinguishable from each other in the temporal domain. Thus the embodiments described above do not need unrealistic temperature stabilities. In embodiment <NUM>, as ΔL =<NUM> (here it means that the path length difference for |H, H〉 and |V, V〉is zero), the source will be stable against wavelength fluctuations or mode hopes of the pump laser. The custom waveplate <NUM> (or the single waveplate) is situated within the Rayleigh range of the generated correlated photon pairs. This is achieved by producing adequate pump and collection focal sizes and overlapping them by adjusting the corresponding lens positions. Also the effective coupling of the |H, H〉and |V, V〉photons have to be ensured, for example by using translation stages on lenses and/or fibers or by fixing lenses and fibers at the appropriate distances and positions. Unlike existing polarization entangled photon sources, in the embodiments described above, the phase does not depend on the angular stability of the crystals used.

<FIG> shows a schematic diagram illustrating an example implementation of a method and system according to embodiment <NUM>. As the custom waveplate <NUM> introduces scattering at the centre of the SPDC photons, it can be difficult to couple both |HH> and |VV> photons into a single mode fiber. The example implementation presented here combines the two photon pair paths after the custom waveplate <NUM> using a birefringent crystal <NUM> of sufficient length.

Specifically, <FIG> is a top view of the compact source. A collimated pump <NUM> is employed to generate SPDC photons. This can be launched through a fiber or a collimated laser diode. The SPDC photons, as soon as they exit the crystal <NUM>, pass through the custom wave plate <NUM>. The dashed line <NUM> represents the distinction between correlated photon pairs generated at the left (401a) and right (401b) part of the ppKTP crystal <NUM> in this example implementation. Filled and unfilled circles represent horizontally and vertically polarized photons, respectively. It will be appreciated that the generation of respective correlated photon pairs at the left and right parts 401a, 401b follow a statistical distribution, hence the representation in <FIG> is not intended to suggest simultaneous generation.

A WDM <NUM> splits the photons of the photon pairs according to their wavelength into signal (s) and idler (i). The polarization state of the photon pairs e.g. 409a generated at the left part 401a is converted to vertical (as indicated by the change from filled circled to unfilled circles at the custom waveplate <NUM>) while the photon pairs e.g. 409b generated at the right part 401b remain unaffected.

The two polarization states are superposed using a birefringent crystal <NUM> via the spatial walk off. In this example implementation a Beta Barium Borate (BBO) crystal <NUM> with a cut angle of <NUM> degrees and a length of <NUM> is used. However, any other birefringent crystals such as Quartz or Calcite can be used in different implementations. The length of the birefringent crystal <NUM> is chosen such that it introduces a displacement that is half the pump <NUM> beam width. The superposed photons are then collected using the single mode fiber based WDM <NUM> which separates signal and idler photons for output of respective polarization entangled pairs, indicated at numerals <NUM> and <NUM>.

Embodiments <NUM> & <NUM> can have a drawback of losses due to multiple reflections. With ideal focusing and collection conditions, embodiment <NUM> can offer brightness as high as the generated photon pair rate (single mode). The state generated is maximally entangled when the tip of the wedge mirror (in embodiments <NUM> & <NUM>) or the boundary between the different axis orientations in the custom waveplate (in embodiment <NUM>) is placed exactly at the centre of the SPDC photon distribution. Moving it off-centre, will create non-maximally entangled states. It is noted that if a single waveplate is used in embodiment <NUM>, the state generated is maximally entangled when one edge of the waveplate is placed exactly at the centre of the SPDC photon distribution.

The state generated in all embodiments does not critically depend on the pump coherence. The reason for the inherent phase stability stems from the fact that pairs are generated during the same process within the same nonlinear medium, and that the pairs (here from left and right portion of the crystal) upon leaving the crystal are virtually indistinguishable from each other in the temporal domain. This translates in a constant phase difference between pairs generated on either side of the crystal.

Similar designs can be used to convert the momentum correlations of photons as well according to example embodiments. For the momentum correlation, the photons can be split according to their momenta by discretizing the state in the far field, for example by using a wedge mirror (similar to embodiments <NUM> and <NUM>) or by using a segmented HWP (similar to embodiment <NUM>). The state generated will be <MAT> as the momenta of SPDC photons are anti-correlated. Alternatively, one can use a single lens that collimates the SPDC photons and use the wedge prism or segmented HWP after that.

Applications of embodiments of the present invention can include:.

Embodiments of the present invention can have one or more of the following features and associated benefits/advantages:.

Aspects of the systems and methods described herein such as the active stabilization of the interferometer may be implemented as functionality programmed into any of a variety of circuitry, including programmable logic devices (PLDs), such as field programmable gate arrays (FPGAs), programmable array logic (PAL) devices, electrically programmable logic and memory devices and standard cell-based devices, as well as application specific integrated circuits (ASICs). Some other possibilities for implementing aspects of the system include: microcontrollers with memory (such as electronically erasable programmable read only memory (EEPROM)), embedded microprocessors, firmware, software, etc. Furthermore, aspects of the system may be embodied in microprocessors having software-based circuit emulation, discrete logic (sequential and combinatorial), custom devices, fuzzy (neural) logic, quantum devices, and hybrids of any of the above device types. Of course the underlying device technologies may be provided in a variety of component types, e.g., metal-oxide semiconductor field-effect transistor (MOSFET) technologies like complementary metal-oxide semiconductor (CMOS), bipolar technologies like emitter-coupled logic (ECL), polymer technologies (e.g., silicon-conjugated polymer and metal-conjugated polymer-metal structures), mixed analog and digital, etc..

The above description of illustrated embodiments of the systems and methods is not intended to be exhaustive or to limit the systems and methods to the precise forms disclosed. While specific embodiments of, and examples for, the systems components and methods are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the systems, components and methods, as those skilled in the relevant art will recognize. The teachings of the systems and methods provided herein can be applied to other processing systems and methods, not only for the systems and methods described above.

The elements and acts of the various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the systems and methods in light of the above detailed description.

In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and the claims, but should be construed to include all processing systems that operate under the claims. Accordingly, the systems and methods are not limited by the disclosure, but instead the scope of the systems and methods is to be determined entirely by the claims.

Claim 1:
A method of converting position or momentum correlation of correlated photon pairs to a polarization entangled photon pair, the method comprising:
generating the correlated photon pairs in a crystal (<NUM>, NCL, <NUM>, <NUM>) using a single pump beam;
a conversion step of separating the correlated photon pairs into first and second groups based on generated positions of the first and second groups, respectively, in first and second portions of a photon generation distribution of the single pump beam in the crystal (<NUM>, NCL, <NUM>, <NUM>) or directions of the first and second groups, respectively about the propagation axis and rotating a polarization of the first correlated photon pair group such that the polarization of the first correlated pair group is at <NUM> degrees relative to the polarization of the second correlated photon pair group; and
a combining step of combining the first and second correlated photon pairs such that at least respective portions of respective spatial distributions of the first and second photon pair groups overlap without wavelength dependent phase difference.