Abstract:
The present invention provides devices, systems, and methods for producing bi-photons and/or entangled photons without the need for complex alignment or source design by the user. The invention provides a scalable source of high-brightness, high-visibility, bi-photons and entangled photons that can be configured for a number of applications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application Ser. No. 61/985,375, filed on Apr. 28, 2014, the entire disclosure of which is incorporated herein by reference, including the drawings 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates the field of quantum entangled photons and ways to generate them reliably and in a commercially-viable manner for use in any of a variety of applications, including but not limited to quantum computing, quantum sensing, and quantum encryption and communication systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not submitted to describe or constitute prior art to the present invention. 
         [0004]    Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This is in sharp contrast to classical physics—where particle properties and behaviors depend purely on local conditions. Objects are said to be “entangled” when a plurality (e.g., two or more) objects interact in ways such that the quantum state of each particle cannot be described independently—instead, a quantum state must be given for the system as a whole. Examples of entangled states include position, angular momentum, spin, polarization, energy, and time. 
         [0005]    Quantum theory was developed in the early  1900 &#39;s when classical physics could not explain the behavior of atomic and sub-atomic systems or weak fields. There are many unusual properties which occur at the sub-atomic level, one of which is known as entanglement. 
         [0006]    Historically, entanglement was first recognized by Einstein, Podolsky, and Rosen (A. Einstein, B. Podolsky, and N. Rosen, “Can quantum-mechanical description of physical reality be considered complete?”, Phys. Rev. 47 777 (1935)) and Schrodinger (E. Schrodinger, “Discussion of probability relations between separated systems”, Proceedings of the Cambridge Philosophical Society, 31: 555-563 (1935); 32: 446-451 (1936)). Over the years, quantum entanglement has been recognized as a physical resource. Like energy, entanglement can be measured and transformed. The recent development of quantum information theory has shown that entanglement can have important practical applications. 
         [0007]    The first known experiment showing polarization measurements on two opposite propagating photons was published by Pryce and Ward (M. Pryce and J. Ward, “Angular correlation effects with annihilation radiation”, Nature 160, 435 (1947)). The early demonstrations of photonic entanglement were centered on annihilation processes, e.g., the decay of gamma particles and the photon emissions which followed. Later, in late 1960s, researchers began to connect the emission of optical entanglement with parametric interactions. 
         [0008]    Parametric interactions, which were first studied by Faraday and Lord Rayleigh in the nineteenth century, received renewed attention during this time as a result of their ability to be utilized as microwave amplifiers. Optical parametric interactions within a nonlinear crystal are viewed positively today, in part because they can be utilized for production of entanglement. In contrast, these interactions were viewed as being detrimental to the desired effects of the  1960 s. The sentiment changed from “optical parametric noise” to “parametric down-conversion,” largely due to the work of Burnham and Weinberg (D. Burnham and D. Weinberg, “Observation of simultaneity in parametric production of optical photon pairs”, Phys. Rev. Lett. 25, 84 (1970)). 
         [0009]    Quantum information science has only recently become a widely recognized field of scientific inquiry. Interest and developments in the field increased greatly in 1994 when Peter Shor discovered a quantum algorithm for factoring large integers in polynomial time (P. Shor, in Proceedings of the 35th Annual Symposium on Foundations of Computer Science, S. Goldwasser, ed., (IEEE Computer Society, Los Alamitos, Calif.), pp. 124-134 (1994)). This discovery sparked a new interest in the abstract notion of quantum computing originally put forth by Paul Benioff, Richard Feynman and David Deutsch in the early 1980s. 
         [0010]    The use of quantum effects for communication security were proposed around the same time, in the form of quantum key distribution (QKD) (C. Bennett and G. Brassard, “Quantum cryptography: Public-key distribution and coin tossing,” in Proceedings of IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984,(IEEE Press, 1984), pp. 175-179; C. Bennett and G. Brassard, “Quantum public key distribution,” IBM Technical Disclosure Bulletin 28, 3153-3163 (1985)). Today, commercial prototypes of many quantum technologies are on display. As the technology expands, the demand for more reliable and efficient entanglement sources has and likely will follow. 
         [0011]    Quantum entanglement is required for long distance quantum communications and large-scale quantum computing networks. One of the most promising quantum computing architectures, measurement-based quantum computation, is also particularly well-suited for optical implementation. Currently, the best way for generating optical entanglement is via parametric down-conversion, formerly known as parametric noise. The quality of an entangled photon source is commonly characterized by its brightness, that is, the number of generated pairs per mW of pump power and per nm of generated bandwidth, as well as the purity of the entangled state, or visibility. Improvements in source brightness, visibility, and fidelity are constantly being sought. 
         [0012]    Early success of parametric down-conversion for entanglement distribution came, primarily, from two major advances in methodology. These techniques, which exploit the geometry of non-collinear parametric down-conversion emissions, were both proposed and realized by Kwiat et al. (P. Kwiat, K. Mattle, H. Weinfurter, and A. Zeilinger, “New High-Intensity Source of Polarization-Entangled Photon Pairs”, Physical Review Letters 75,4337 (1995); P. Kwiat, E. Waks, A. White, I. Appelbaum, and P. Eberhard, “Ultra-bright source of polarization-entangled photons”, Physical Review A 60, 773 (1999)). 
         [0013]    Early demonstrations of polarization entanglement primarily utilized beta-Barium Borate (β-BaB 2 O 4 , hereinafter “BBO”) or Potassium Titanium Oxide Phosphate (KTiOPO 4 , hereinafter “KTP”) crystals that produced spatially-separated entangled beams. 
         [0014]    More recently, progress has been made in the collinear regime. The success of collinear parametric down-conversion is due to a crystal manufacturing procedure that yields a periodic nonlinearity to the crystal structure. Emissions within periodically-poled crystals can occur with non-critically phase-matched configurations in materials with large nonlinear coefficients. This has led to significant increases in entangled source brightness (C. Kuklewicz, M. Fiorentino, G. Messin, F. Wong, and J. Shapiro, “High-flux source of polarization-entangled photons from a periodically poled KTiOPO4 parametric down-converter”, Physical Review A 69, 013807 (2004)). Some of the brightest, high-visibility sources of polarization entangled photons demonstrated, to date, utilize periodically-poled crystals in a waveguide structure. Waveguide periodically-poled KTP allows a pair generation rate that is more than 50 times higher (or brighter) than the non-periodically poled, non-waveguide bulk crystal KTP generation rate (M. Fiorentino, S. Spillane, R. Beausoleil, T. Roberts, P. Battle, and M. Munro, “Spontaneous parametric down-conversion in periodically poled KTP waveguides and bulk crystals”, Optics Express 15, 7479 (2007)). 
         [0015]    Source performance also becomes better as new engineering techniques and models emerge, e.g. determination of optimal focusing techniques to enable better fiber coupling (R, Bennink, Y. Liu, D. Earl, and W. Grice. “Spatial distinguishability of photons produced by spontaneous parametric down-conversion”, Physical Review A 74, 023802 (2006)). System design can help the performance as well, e.g. improved mounting of a non-linear crystal by encapsulating it within an optically clear material (P. Kwiat, PhD Thesis; “Nonclassical effects from spontaneous parametric down-conversion: adventures in quantum wonderland.”). 
         [0016]    Patents related to packaging an entangled photon source include U.S. Pat. No. 6,897,434, “All-fiber photon-pair source for quantum communications,” issued May 24, 2005 to Kumar. Kumar describes a source and/or method of generating quantum-entangled photon pairs using parametric fluorescence in a fiber whose dispersion zero is close to that of the pump wavelength, and specifically, a Sagnac loop at wavelengths around 1550 nm, with detectors in “that window (1000-1600 nm).” A commercial product (EPS-1000) by the company NuCrypt, LLC, claims to practice the teachings of this patent. 
         [0017]    Another patent, U.S. Pat. No. 6,424,665 to Kwiat, “Ultra-bright source of polarization-entangled photons,” describes a polarization entangled source using spontaneous parametric down-conversion in a multi-crystal geometry. 
         [0018]    Emerging applications for quantum technology create an increasing demand for ever more stable, efficient, high-quality sources of entangled photons. There is therefore a need for a source that can be readily configured and provided to an end-user to produce a rugged, bright, and flexible source to serve the quantum sensing, quantum cryptography, and quantum computing fields. 
       SUMMARY OF THE INVENTION 
       [0019]    It is an object of the present invention to provide systems and methods for producing bi-photons and/or entangled photons. As described hereinafter, the present invention provides a scalable system with tunable efficiency without the need for complex alignment or source design by the user. 
         [0020]    In a first aspect, the present invention provides systems/devices for generating bi-photons and/or entangled photons. These systems comprise:
       a nonlinear crystal element comprising a first face, a second face parallel to the first face, and a first axis perpendicular to the first and second faces;   a photon source configured to provide a pump beam traversing a first optical path to a first face of the nonlinear crystal element along the first axis, the nonlinear crystal element providing downconversion of a subset of photons in the pump beam to provide downconverted bi-photons and non-downconverted photons;   a beam splitter positioned in the first optical path between the photon source and the non-linear crystal, the beam splitter configured to direct photons reflected from the first face of the nonlinear crystal element onto a second optical path along a second axis;   a visualization element configured to receive photons traversing the second optical path from the beam splitter and to provide therefrom an image of the first face of the nonlinear crystal element;   a focusing optic configured to provide adjustment of the position of the pump beam relative to the first axis; and   an optical element comprising a dichroic reflector configured to receive photons exiting a second face of the nonlinear crystal element along the first axis and to direct downconverted bi-photons to a third optical path along the first axis, and to direct non-downconverted photons to a fourth optical path along a third axis.       
 
         [0027]    The term “bi-photon” refers to the simultaneous production of two photons in such a way as they are distinguishable, meaning they have optical properties (i.e. polarization, wavelength, etc.) that are related to some other distinguishing and measureable variable. 
         [0028]    The term “entangled” refers to the simultaneous production of two photons in such a way as they are indistinguishable, meaning they have one or more physical properties (e.g., polarization, wavelength, etc.) that are not related to any distinguishing and/or measureable variable. 
         [0029]    In certain preferred embodiments, the systems and devices of the present invention comprise a phase shifting wave plate positioned in the first optical path between the photon source and the first face of the nonlinear crystal element. 
         [0030]    The phase shifting wave plate is a variable wave plate positioned in the first optical path between the photon source and the first face of the non-linear crystal. The skilled artisan understands that a variable wave plate (or retarder) is an optical device that slows the propagation of polarized light travelling through it along a defined axis. Variable wave plates may be constructed according to a variety of methods and from a variety of materials readily known to persons of ordinary skill in the art. In a typical example, a variable wave plate may be constructed from a liquid crystal device that allows the index of refraction of the wave plate material to be adjusted electronically. Such devices can comprise a transparent cell filled with a solution of liquid crystal (LC) molecules. Two parallel faces of the cell wall are coated with a transparent conductive film so that a voltage can be applied across the cell. The orientation of the LC molecules is determined by the alignment layer in the absence of an applied voltage. When an AC voltage is applied, the molecules will change from their default orientation based on the applied rms value of the voltage. Hence, the phase offset in a linearly-polarized beam of light can be actively controlled by varying the applied voltage. 
         [0031]    In certain preferred embodiments, the systems and devices of the present invention can comprise a polarization rotating pre-crystal wave plate positioned in the first optical path between either the phase shifting wave plate, if present, and the first face of the non-linear crystal element, or the photon source, if the phase shifting wave plate is not present, and the first face of the nonlinear crystal. The polarization rotating pre-crystal wave plate may be a half-wave plate, quarter-wave plate, or variable wave plate. The skilled artisan understands that a wave plate (or retarder) is an optical device that can alter the polarization state of a light wave travelling through it. Two common types of fixed wave plates are the half-wave plate, which shifts the polarization direction of linearly polarized light, and the quarter-wave plate, which converts linearly polarized light into circularly polarized light and vice versa. Wave plates are typically constructed out of a birefringent material such as quartz or mica. As noted previously, the polarization rotating pre-crystal wave plate may also be an electronically variable wave plate, constructed and utilized in the manner previously discussed. In a preferred embodiment, the polarization rotating pre-crystal wave plate is a half wave plate configured manually to modify the polarization state of the pump beam, thereby altering the efficiency of downconverted bi-photon production by the system. 
         [0032]    In preferred embodiments, the photon source is a laser. For ease of alignment, the photon source is preferably optically coupled to an optical fiber at a first end of the optical fiber, wherein the pump beam exits the optical fiber at a second end thereof, and wherein the second end of the optical fiber is positioned within a translating mount to provide the beam to focusing optic. This translating mount allows the optical axis of the photons exiting the photon source to be adjustable relative to the optical components in the beam path. By way of example, translation of the optical fiber can be used to align the pump beam along first axis such that it impinges on the first face of the nonlinear crystal element in a precise orientation (e.g., approximately centered on, and perpendicular to, the first face of the nonlinear crystal element). Once aligned during manufacture, the translating mount may be “locked down” so that no end user adjustment is necessary. 
         [0033]    In certain embodiments, the systems and devices of the present invention can comprise a filter positioned in the first optical path between the photon source and the beam splitter. This filter is preferably configured to remove undesired wavelengths of light from reaching the beam splitter. 
         [0034]    As noted above, the systems and devices of the present invention comprise a visualization element configured to receive photons traversing the second optical path from the beam splitter and to provide therefrom an image of the first face of the nonlinear crystal element. This advantageously allows visualization of the pump beam impinging on the first face of the nonlinear crystal element, thereby facilitating adjustment of the translation of the optical fiber for alignment of the pump beam in a precise orientation. Examples of suitable visualization elements include a CCD image sensor, a CMOS image sensor, an NMOS image sensor, an active pixel sensor, and an oversampled binary image sensor. This list is not meant to be limiting. In certain embodiments, the visualization element can be removed following manufacture and lockdown of the translating mount following alignment. Thus, a system/device of the invention shipped for use by an end user may lack the visualization element. 
         [0035]    The nonlinear crystal element of the present invention may be made of a variety of materials known in the art, including but not limited LBO, CLBO, BBO, KTP, KDP, AGS, AGSE, SBN, BSO, Li 0   3 , and LiNb 0   3 . In certain embodiments, the nonlinear crystal element is a composite of materials. By way of example, a nonlinear crystal element can comprise two non-linear crystals separated by a half wave plate, or a plurality of non-linear crystals, each having a crystal axis oriented ninety degrees relative to an adjacent non-linear crystal. The nonlinear crystal element is selected to produce Type 0, I, or II downconversion of a portion of photons passing through the crystal matrix. A nonlinear crystal splits photons into pairs of photons that, in accordance with the law of conservation of energy, have combined energies and momenta equal to the energy and momentum of the original photon, are phase-matched in the frequency domain, and have correlated polarizations. Spontaneous parametric down-conversion (SPDC) in a certain low percentage of photon pairs are created at random times. 
         [0036]    The nonlinear crystal is preferably potted in potting material having an index of refraction lower than the index of refraction of the non-linear crystal. Suitable materials are preferably optically transparent at the desired wavelengths. Suitable materials include silicone-based rubbers such as NuSil LS-6941 and LS-6140, Sylgard® Silicone Elastomer (Dow Corning), and ELASTOSIL® Solar 2202 (Wacker). 
         [0037]    In a preferred embodiment, after exiting the second face of the nonlinear crystal, photons may be separated by a dichroic mirror into downconverted bi-photons along a first path, and non-downconverted photons along a second optical path. Downconverted bi-photons traveling along the first optical path exit the dichroic minor and, in certain preferred embodiments, would travel through a polarization rotating post-crystal wave plate. The polarization rotating post-crystal wave plate may be a half-wave plate, quarter-wave plate, or variable wave plate. In a preferred embodiment, the polarization rotating post-crystal wave plate is a half wave plate configured manually to modify the polarization state of the downconverted photons. 
         [0038]    In certain embodiments, a second beam splitter may be positioned along the first optical path and configured to receive the downconverted bi-photons and direct individual bi-photons in a bi-photon pair along separate optical paths. These separate optical paths may comprise, for example, separate optical fibers. 
         [0039]    As described in additional detail hereinafter, the non-downconverted photons directed by the dichroic minor down the second path may be “recycled” and used in the pump beam of the same device as part of the photon source, or may be used as the photon source for a pump beam in a second device of similar or identical configuration as the first device. 
         [0040]    In certain preferred embodiments, an optical element is configured to focus the non-downconverted photons onto an end of a second optical fiber which emits the pump beam as indicated above, to the pump beam of the same device as part of the photon source, in which case the second optical fiber is one of the “2,” or split, ends of a 2×1 fiber optic beam combiner that combines the recycled beam with the pump beam, in which case, the other end of the second optical fiber is the “1,” or combined end of the 2×1 optic beam combiner and is preferably positioned within a translating mount to provide the combined beam to the focusing optic. In this embodiment, the photon source is be optically coupled to the other “2” or split end of the 2×1 optic beam combiner. A 2×1 optic beam combiner can be readily obtained from commercial sources, such as Thorlabs. 
         [0041]    Optionally, as indicated above, the second optical fiber directs the non-downconverted photons to a second device, in which case the other end of the second optical fiber is preferably be positioned in a translating mount to facilitate focusing of the recycled beam on the non-linear crystal in the second device. 
         [0042]    In certain preferred embodiments, the non-downconverted photons exiting the dichroic minor travel through a polarization rotating recycling wave plate located between the dichroic mirror and the translating mount. The polarization rotating recycling wave plate may be a half-wave plate, quarter-wave plate, or variable wave plate. In a preferred embodiment, the polarization rotating recycling wave plate is a half wave plate configured manually to reverse the polarization modifications made to the pump beam photons by the polarization rotating pre-crystal wave plate. 
         [0043]    Because the efficiency of downconversion is low, there is little loss of intensity in the recycled photon beam. The “recycled” pump beam can be provide to a second device which can also provide a “recycled” pump beam to a third device, which can provide a “recycled” pump beam to a fourth device, etc. This allows for scalability of the systems described herein, meaning one or more devices can be provided as a linked array, all receiving an initial “pump” from a single photon source at the first device, and other devices in the chain being linked to this single photon source through the recycled non-downconverted photons collected from the previous device in the series. 
         [0044]    Furthermore, as described in additional detail hereinafter, by adjusting the polarization of the pump beam relative to the first axis individually in each of dev 1  through dev, (where dev 1  is device one and dev, is the nth, or final, device in the series) one is able to modify the efficiency of downconverted bi-photon production by each device in the series to be approximately identical. 
         [0045]    In related aspects, the present invention provides methods of manufacturing, aligning, and tuning a scalable photon source. These methods comprise:
       providing devices dev 1  through dev, as described herein, wherein n=at least 2, wherein dev 1  comprises a laser as the photon source and devs 2 to n  utilize non-downconverted photons from the fourth optical path of dev 1  as the photon source; and   adjusting the polarization of the pump beam relative to the first axis in each of dev 1  through dev n  to modify the efficiency of downconverted bi-photon production by each device, preferably to adjust the efficiency of downconverted bi-photon production in each of dev 1  through dev n  to produce an approximately equal output of bi-photons in each device.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]    The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings. Other embodiments of the invention will be apparent from the following detailed description, figures, and claims. 
           [0049]      FIG. 1  depicts a schematic of a design of an exemplary system to generate quantum entangled photons in the quantum entangled “triplet state.” 
           [0050]      FIG. 2  depicts a schematic of a design of an exemplary system to generate quantum entangled photons in the quantum entangled “triplet state,” incorporating an optional phase shifting wave plate. 
           [0051]      FIG. 3  depicts a schematic of a design of an exemplary system to generate quantum entangled photons in the quantum entangled “triplet state,” incorporating an optional polarization rotating pre-crystal wave plate, and an optional polarization rotating recycling wave plate. 
           [0052]      FIG. 4  depicts a schematic of a design of an exemplary system to generate quantum entangled photons in the quantum entangled “triplet state,” incorporating an optional polarization rotating post-crystal wave plate. 
           [0053]      FIG. 5  depicts a schematic of a design of an exemplary system to generate quantum entangled photons in the quantum entangled “triplet state,” incorporating an optional phase shifting wave plate, an optional polarization rotating pre-crystal wave plate, an optional polarization rotating post-crystal wave plate, and an optional polarization rotating recycling wave plate. 
           [0054]      FIG. 6  depicts an exemplary alignment of one sub-assembly of the system to generate quantum entangled photons—referred to herein as “the source side.” 
           [0055]      FIG. 7  depicts an exemplary alignment of a second sub-assembly of the system to generate quantum entangled photons—referred to herein as “the receiver side.” 
           [0056]      FIG. 8  depicts an exemplary array used to generate quantum entangled photons which are being excited by the same pump beam. In the depicted example, the outputs of the two systems are combined for the purpose of producing the quantum entangled photons in the quantum entangled “singlet state.” 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]      FIG. 1  depicts a schematic of a design of a system to generate quantum entangled photons  10 . The system comprises a source side and a receiver side. The source side comprises a laser pump  11  that is delivered to the system through a fiber optic cable. In a preferred embodiment, the fiber optic cable is a singlemode, polarization-maintaining optical fiber, but can also be a multimode or singlemode non-polarization-maintaining fiber. The optical fiber is connected to the system through an optical fiber focusing device  14 . The focusing device comprises one or more lens(es) to focus the fiber output. The focusing device  14  is mounted with a first translating mount  17 . The first translating mount  17  allows the x-translation, y-translation, tip, tilt, and focal point placement of the fiber output to be precisely adjusted. In a preferred embodiment, adjustments are made through alignment screws in the mount and can be locked down with one or more set screws once all alignments are finalized. 
         [0058]    The pump light is then directed through an optical filter  20  that eliminates any light not at the laser&#39;s target wavelength. A person of ordinary skill in the art would readily apprehend that the optical filter  20  can be a laser notch filter, bandpass filter, or long or short pass filter. 
         [0059]    The focused laser light continues through a pellicle beamsplitter  23 . The pellicle beamsplitter  23  splits off a first portion of the incident laser light, that first portion then being directed away from a camera  26  located nearby. 
         [0060]    The remaining second portion of the incident laser light continues through the pellicle beamsplitter  23  and converges onto and into the non-linear crystal  32 . The non-linear crystal has a first face  33  and a second face  34 . 
         [0061]    In a preferred embodiment, the non-linear crystal  32  is housed in a fixed mount  35  such that its alignment along the optical axis is precise to within 0.1° and stable. Preferably, the non-linear crystal  32  is potted in potting material having an index of refraction lower than the index of refraction of the non-linear crystal. Most preferably, the potting material has a low optical absorption coefficient (&lt;0.01%/cm) to ensure minimal destructive heating when accidental misalignment of the laser occurs. 
         [0062]    In a preferred embodiment, the crystal is potted in a cylindrical optical mount that is designed to interface with two precision, removable end-caps. The mount interfaces with each end-cap in such a way that the front region of the crystal (approximately 1 mm long) is captured by the first end-cap while the back region of the crystal (approximately 1 mm long) is captured by the second end-cap. This results in the crystal being carefully aligned and secured relative to the optical mount and creates a sealed mold that potting material can then be injected into. The potting compound is then poured into the mold through an injection port in the first end-cap and heat cured. The mold end-caps are removed once the final assembly has cured. The result is a very stable and precisely aligned crystal. 
         [0063]    Light reflected from the first face  33  of the non-linear crystal  32  is reflected back toward the pellicle beamsplitter  23 . However, as a person of ordinary skill in the art would readily apprehend the pellicle beamsplitter  23 , directs the reflected light towards the camera  26 . In a preferred embodiment, this back-reflected light facilitates the focus of the laser on the first face  33  of the non-linear crystal  32  to be imaged by the camera  26 . This camera image is viewed and used in the final alignment of the focused laser light into the non-linear crystal  32 . In a preferred embodiment, the camera  26  provides a real-time method for monitoring the centering of the focused laser beam onto and into the first face of the non-linear crystal. 
         [0064]    As a person of ordinary skill in the art would readily apprehend, the non-linear crystal  32  may be comprised of a single crystal constructed from bulk non-linear materials (i.e. BBO, KTP, etc.), periodically-poled crystals (i.e., periodically-polled KTP (or “PPKTP”)), or waveguide-structured crystals. The crystals may be fabricated to produce either Type 0, Type I, or Type II downconversion. In such embodiments, the polarization of the incident laser light would correspond with the axis of the crystal to result in bi-photon production. 
         [0065]    In other preferred embodiments, the non-linear crystal may be comprised of more than one component non-linear crystal, wherein the crystal axes of each component non-linear crystal are oriented ninety degrees relative to one another in sequence. 
         [0066]    In other preferred embodiments, the non-linear crystal may be comprised of two non-linear crystals separated by an intermediary half wave plate specified for the degenerate down-converted wavelength. In this arrangement, the crystal axes of each non-linear crystal are aligned. The intermediary half-wave plate would rotate the polarization of any down-conversion in the first crystal relative to the polarization of the down-conversion in the second crystal. The intermediary wave plate would have no impact on the polarization of the pump beam, which is half the wavelength of the down-conversion making the intermediary half-wave plate appear to be a whole wave-plate at the pump wavelength. 
         [0067]    The non-linear crystal(s)  32  are mounted in a fixed mount  35  that rigidly positions the crystal  32  relative to the optical axis. The crystal will output photons that include both the laser&#39;s photon wavelength as well as the down-converted output from the non-linear crystal  32 , which will generally be twice the wavelength of the laser. In a preferred embodiment, an optional electric heater  83  allows the temperature of the system to be increased and maintained at a given temperature, permitting the down-converted photon wavelength to be made degenerate or non-degenerate. 
         [0068]    In a preferred embodiment, the output from the non-linear crystal  32  exits from the second face  34  and is directed toward a dichroic minor  38 . The dichroic minor  38  separates the remaining laser pump beam from the down-converted photons. The laser pump beam reflects from the dichroic minor  38  onto other parts of the system, as discussed further hereinafter. 
         [0069]    The down-converted photons pass through the dichroic mirror  38  and are directed to a tilted window  41 . The tilted window  41  is preferably designed to compensate for the axial shift in the optical axis introduced by the dichroic mirror, creating compensated down-converted photons. In a preferred embodiment, the tilted window is composed of quartz, but could be any variety of materials, including but not limited to Sapphire, BK7, and Magnesium Fluoride. 
         [0070]    Optionally, in a preferred embodiment, the compensated down-converted photons continue through an optional compensating crystal  44  which has opposite birefringence properties compared to the non-linear crystal  32 . The compensating crystal  44  can be composed of any variety of materials, including rotated KTP, LiTaO 3 , and YVO 4  but is most preferably rotated KTP. 
         [0071]    Also optionally, and in a preferred embodiment, the compensated down-converted photons then pass through a refining optical filter  47 . The refining optical filter  47  is comprised of one or more optical filters and removes any remaining laser light or other unwanted light and noise from the signal. The refining optical filter  47  can be a laser notch filter, bandpass filter, or long or short pass filter, but is most preferably a bandpass filter. 
         [0072]    Optionally, and in a preferred embodiment, an optional beamsplitter  50  splits the compensated down-converted bi-photons into two separate paths. In a preferred embodiment for applications involving post-selection of the down-converted photons, the optional beamsplitter  50  is a non-polarizing beamsplitter designed to operate at twice the wavelength pump laser. However, for other applications, including but not limited to the construction of heralded photons, decoherence free subspaces, and external combinations with other entangled source, the optional beamsplitter  50  would be a polarizing beamsplitter. For other applications, the beamsplitter  50  would be eliminated altogether, resulting in the collection and focusing of both down-converted photons into a single output optical fiber. 
         [0073]    One output from the optional beamsplitter  50  directs light into an optional signal focusing element  62 . In embodiments comprising the optional beamsplitter  50 , there is preferably a signal focusing element  62 , comprised of one or more lenses. In embodiments comprising the signal focusing element  62 , it is preferably mounted in a signal 5-axis alignment mount  65  which facilitates its alignment relative to a fiber optic connector The signal focusing element  62  concentrates and focuses the light toward the attached fiber optic connector, creating a signal beam  68  that can be efficiently coupled into an attached single or multimode optical fiber. 
         [0074]    The other output from the optional beamsplitter  50  directs light into an idler focusing element  53 . In embodiments in which there is no optional beamsplitter  50 , the photons from the non-linear crystal are directed to the idler focusing element  53 . The idler focusing element  53  is comprised of one or more lenses mounted relative to a fiber optic connector and mounted in an idler 5-axis alignment mount  56 . The idler focusing element  53  concentrates and focuses the light toward the attached fiber optic connector, creating an idler beam  59  that can be efficiently coupled into an attached single or multimode optical fiber. 
         [0075]    In a preferred embodiment, the pump beam exiting the non-linear crystal  32  exits from the second face  34 , is directed toward a dichroic mirror  38 , and reflects down a separate optical path, creating a raw recycling beam. 
         [0076]    The raw recycling beam is directed into a recycling optical focusing assembly  74 . The recycling optical focusing assembly  74  is comprised of one or more optical lenses. The recycling optical focusing assembly  74  is preferably mounted in a second translating mount  77  that allows the focus to be carefully adjusted. In a preferred embodiment, the x-translation, y-translation, tip, tilt, and distance of the focus can all be adjusted through six alignment screws in the mount, and the final alignment can be locked in place through set screws. 
         [0077]    The recycling optical focusing assembly  74  focuses the raw recycling beam, concentrating it into an optical fiber, creating a focused recycling beam  80 . The focused recycling beam can optionally be used to excite the non-linear crystal  33  a second time, creating a temporal uncertainty that is useful for various forms of quantum entanglement, or optionally can be directed to a second system to generate additional photons, in which case that process can be repeated many times, greatly extending the application of the laser source. In other embodiments, the recycled beam could be used to monitor phase relations between the system and one or more other systems. 
         [0078]    The first translating mount  17 , optical filter  20 , pellicle beamsplitter  23 , camera  26 , fixed mount  35 , dichroic mirror  38 , tilted window  41 , optional compensating crystal  44 , refining optical filter  47 , optional beamsplitter  50 , and idler 5-axis alignment mount  56  are held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a first set of four bars that span the system at different points, one of which is indicated at  39 . 
         [0079]    The second translating mount  77  is also held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a second set of four bars that span the system at different points, one of which is indicated at  78 . 
         [0080]    If present, the signal 5-axis alignment mount  65  is also held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a third set of four bars that span the system at different points, one of which is indicated at  63 . 
         [0081]    Most preferably, the first, second and third cages are also held in place by rigid mounting of the components to a plate  86 . A light-tight housing encompasses the entire assembly and mounts to the plate  86 . 
         [0082]      FIG. 2  depicts a schematic of a design of a system, incorporating an optional phase shifting wave plate. 
         [0083]    In the embodiment illustrated in  FIG. 2 , the focus of the laser is directed through an optional phase shifting wave plate  18 . In a preferred embodiment shown in  FIG. 2 , the optional phase shifting wave plate  18  is located between the first translating mount  17  and the optical filter  20 . However, the optional phase shifting wave plate  18  could be placed anywhere between photon source  11  and the first face of the non-linear crystal  33 . In embodiments in which the optional phase shifting wave plate  18  is placed in between the pellicle beamsplitter  23  and the first face  32  of the non-linear crystal  32 , the light reflected from the first face  32  passes through the optional phase shifting wave plate  18  on its way back to the pellicle beamsplitter  23 . 
         [0084]    In a preferred embodiment, the optional phase shifting wave plate  18  is a variable wave plate, preferably an electronically variable wave plate constructed from a liquid crystal device. The optional phase shifting wave plate is made of a material (such as a liquid crystal) that has a polarization dependent index of refraction that can be adjusted electrically. The optional phase shifting wave plate is oriented such that its optical fast axis is aligned with the polarization axis of the laser pump  11 . 
         [0085]    In a preferred embodiment, the optional phase shifting wave plate can be electronically adjusted to introduce a varying phase shift to the incoming pump beam. This arrangement allows the phase of the pump beam to be controlled relative to other systems, a process that is useful when the outputs from multiple systems are combined. 
         [0086]    The optional phase shifting pre-crystal variable wave plate  18  is held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a first set of four bars that span the system at different points, one of which is indicated at  39 . 
         [0087]      FIG. 3  depicts a schematic of a design of a system, incorporating an optional polarization rotating pre-crystal wave plate, and an optional polarization rotating recycling wave plate. 
         [0088]    In a preferred embodiment illustrated by  FIG. 3 , the light passes through an optional polarization rotating pre-crystal wave plate  29  before converging onto and into the non-linear crystal. The optional polarization rotating pre-crystal wave plate  29  can either be configured manually (i.e. through a manual rotation) or electronically (i.e. through a liquid crystal medium or other optical phase manipulating media). The optional polarization rotating pre-crystal wave plate allows the polarization of the pump beam to be modified, having the result of reducing the down conversion efficiency production of the non-linear crystal  32 . 
         [0089]    In a preferred embodiment shown in  FIG. 3 , the light passes through an optional polarization rotating pre-crystal wave plate  29 . The optional polarization rotating pre-crystal wave plate  29  is preferably located, as illustrated in  FIG. 3 , between the pellicle beamsplitter  23  and the first face  33  of the non-linear crystal  32 . However, optional polarization rotating pre-crystal wave plate  29  is could be placed anywhere between photon source  11  and the first face of the non-linear crystal  33 . 
         [0090]    In a preferred embodiment illustrated by  FIG. 3 , light reflected from the first face  33  of the non-linear crystal  32  is reflected back toward the pellicle beamsplitter  23 , and passes through the optional polarization rotating pre-crystal wave plate  29  on its way to the pellicle beamsplitter  23 . 
         [0091]    In other preferred embodiments, the non-linear crystal  32  may be comprised of more than one component non-linear crystal, wherein the crystal axes of each component non-linear crystal are oriented ninety degrees relative to one another in sequence. In these embodiments, the polarization of the incident light is preferably modified by an optional polarization rotating pre-crystal wave plate  29  to be at a forty-five degree angle to each of the component non-linear crystal axes. 
         [0092]    In other preferred embodiments, the non-linear crystal  32  may be comprised of two non-linear crystals, separated by an intermediary half wave plate specified for the degenerate down-converted wavelength. In these embodiments, the crystal axes of each non-linear crystal are aligned. The optional polarization rotating pre-crystal wave plate  29  would be a half-wave plate oriented  45  degrees relative to the crystal axes. The intermediary half-wave plate would rotate the polarization of any down-conversion in the first crystal relative to the polarization of the down-conversion in the second crystal. The intermediary wave plate would have no impact on the polarization of the pump beam, which is half the wavelength of the down-conversion making the intermediary half-wave plate appear to be a whole wave-plate at the pump wavelength. 
         [0093]    In a preferred embodiment illustrated in  FIG. 3 , the pump beam exiting the non-linear crystal  32  exits from the second face  34 , is directed toward a dichroic mirror  38 , and reflects down a separate optical path. The laser pump beam photons are reflected by the dichroic minor toward an optional polarization rotating recycling wave plate  71 . The optional polarization rotating recycling-wave plate  71  is preferably selected in accordance with the laser wavelength, and can be used to reverse the effect of any polarization rotation produced in the optional polarization rotating pre-crystal wave plate  23 , creating a polarized raw recycling beam. 
         [0094]    The polarized raw recycling beam is directed into a recycling optical focusing assembly  74 . The recycling optical focusing assembly  74  is comprised of one or more optical lenses. The recycling optical focusing assembly  74  is preferably mounted in a second translating mount  77  that allows the focus to be carefully adjusted. In a preferred embodiment, the x-translation, y-translation, tip, tilt, and distance of the focus can all be adjusted through six alignment screws in the mount, and the final alignment can be locked in place through set screws. 
         [0095]    The recycling optical focusing assembly  74  focuses the polarized raw recycling beam, concentrating it into an optical fiber, creating a focused recycling beam  80 . In a preferred embodiment, the focused recycling beam  80  can optionally be used to excite the non-linear crystal  33  a second time, creating a temporal uncertainty that is useful for various forms of quantum entanglement, or optionally can be directed to a second system to generate additional photons, in which case that process can be repeated many times, greatly extending the application of the laser source. In other embodiments, the recycled beam could be used to monitor phase relations between the system and one or more other systems. 
         [0096]    The optional polarization rotating pre-crystal wave plate  29  is held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a first set of four bars that span the system at different points, one of which is indicated at  39 . 
         [0097]    The polarization rotating recycling wave plate  71  and second translating mount  77  are also held rigidly in place. In a preferred embodiment, that is accomplished in part by a cage design consisting of a second set of four bars that span the system at different points, one of which is indicated at  78 . 
         [0098]      FIG. 4  depicts a schematic of a design of a system, incorporating an optional polarization rotating post-crystal wave plate  45 . 
         [0099]    In a preferred embodiment illustrated by  FIG. 4 , the light passes through an optional polarization rotating post-crystal wave plate  45  before being focused onto and into the idler focusing element  53 . In embodiments in which the optional beamsplitter  50  is incorporated, the light passes through the optional polarization rotating post-crystal wave plate  45  prior to the optional beamsplitter  50 . Preferably, optional polarization rotating post-crystal wave plate  45  is placed between the titled window  41  (or the optional compensating crystal  44 , if present) and the refining optical filter  47 , as illustrated in  FIG. 4 . However, the optional polarization rotating post-crystal wave plate  45  may be placed anywhere between the second face  34  of the non-linear crystal  32  and the optional beamsplitter  50  (if present) or the idler focusing element  53  (if the optional beamsplitter  50  is not present). 
         [0100]    The optional polarization rotating post-crystal wave plate  45  can either be fixed or variable, and may be configured manually (i.e. through a manual rotation) or electronically (i.e. through a liquid crystal medium or other optical phase manipulating media). 
         [0101]    The optional polarization rotating post-crystal wave plate  45  rotates the polarization of the compensated down-converted photons. The optional polarization rotating post-crystal wave plate can be rotated either manually or through electronic means and media in a number of ways readily-comprehensible to persons of ordinary skill in the art, some of which are discussed above in connection with other wave plates. In a preferred embodiment, the optional polarization rotating post-crystal wave plate  45  is an electronically-adjustable liquid crystal device placed after the non-linear crystal  32 . 
         [0102]      FIG. 5  depicts a schematic of a design of a system, incorporating an optional phase shifting wave plate, an optional polarization rotating pre-crystal wave plate, an optional polarization rotating post-crystal wave plate, and an optional polarization rotating recycling wave plate. In this preferred embodiment, an optional phase shifting wave plate  18 , an optional polarization rotating pre-crystal wave plate  29 , an optional polarization rotating post-crystal wave plate  45 , and an optional polarization rotating recycling wave plate  71  are all utilized and discussed and taught above. 
         [0103]    An aspect of the present invention relates to the manner of aligning the system. In a preferred embodiment, alignment of the system  10  is achieved through a three-stage process. 
         [0104]    The alignment process can be understood with reference to  FIG. 6 , which depicts a first sub-assembly  90 , also referred to as the “source side” sub-assembly. The first sub-assembly comprises the optical fiber focusing device  14 , the first translating mount  17 , the optical filter  20 , and the pellicle beamsplitter  23 , and a beam profiler  93 . The beam profiler  93  can be readily obtained from commercial sources, including Thorlabs. Optionally, as discussed above, the first sub-assembly may comprise one or more of the following: an optional phase shifting wave plate  18 , and/or an optional polarization rotating pre-crystal wave plate  29 . 
         [0105]    The beam profiler  93  is rigidly, but temporarily, affixed to the other components using a portion of the first set of bars  39  described earlier and is placed such that its detection surface is at the precise focal length at which the mid-point between the first face  33  and the second face  34  of the non-linear crystal  32  will be located in the assembled system. 
         [0106]    Light from a fiber optic laser  96  of the same wavelength that will be used for the pump beam  11  is directed at the same location on the surface of the optical fiber focusing device  14  of the first sub-assembly  90  which to produce a focused spot of light that is detected by the beam profiler  93 . As is readily appreciated by persons of ordinary skill in the art, the beam profiler  93  can then determine the location, diameter, optical power, and divergence of the spot, as partially schematically illustrated as the beam profiler output  100 . 
         [0107]    Although the beam profiler output  100  is schematically illustrated in  FIG. 2 , persons of ordinary skill in the art will readily apprehend that beam profilers can and do output data in a variety of formats, providing, as noted above, a great deal more information than location and diameter. The schematic illustration is provided for graphic illustration and is not intended to convey the full output available nor limit the teachings of the present invention in any manner. 
         [0108]    The output from the beam profiler  93  is then monitored as alignment adjustments are made to the translating mount  17  and used to exactly focus the light from fiber optic laser  96  so that the light will be maximally-focused through the optical fiber focusing device  14 , the optional phase shifting pre-crystal variable, if present  18 , the optical filter  20 , the pellicle beamsplitter  23 , and the optional polarization rotating pre-crystal wave plate  29 , if present, at the focal distance to be utilized when the beam profiler  93  is removed and replaced by the non-linear crystal  32  in its mounting  35 . 
         [0109]    The second phase of the alignment and assembly process can be understood with reference to  FIG. 7 .  FIG. 7  depicts a second sub-assembly  110 , also referred to as the “receiver-side” sub-assembly. The second sub-assembly comprises: the beam profiler  93 , the dichroic mirror  38 , the recycling optical focusing assembly  74 , the second translating mount  77 , the tilted window  41 , idler focusing element  53 , and the idler 5-axis alignment mount  56 , all affixed, directly or indirectly as discussed above, by portions of the first set of bars  39 . Optionally, as also discussed above, the second sub-assembly may comprise one or more of the following: an optional compensating crystal  44 , an optional polarization rotating post-crystal wave plate  45 , an optional refining optical filter  47 , an optional beamsplitter  50 , an optional signal focusing element  62 , the optional the signal 5-axis alignment mount  65 , and/or the optional polarization rotating recycling wave plate  71 , which may be affixed, as discussed above, using portions of the first, second, and/or third set of bars  39 ,  78 , and/or  63 . 
         [0110]    To assemble and align the second sub-assembly  110 , the beam profiler  93  is rigidly, but temporarily affixed to the dichroic minor  38  at a distance such that its detection surface is exactly the length from the dichroic minor  38  that the mid-point between the first face  33  and second face  34  of the non-linear crystal  32  will be in the final assembly when the non-linear crystal  32  is affixed where the beam profiler  93  is shown in  FIG. 7 . 
         [0111]    The idler focusing element  53  is aligned. Idler focusing element tuning light  112  is introduced. The idler focusing element tuning light  112  is light from a laser with a wavelength equal to the down-converted photons to be produced, introduced at the spot on the idler focusing element  53  that will emit the down-converted photons in the final assembly. That light will pass through the assembled elements and a portion will eventually reach the beam profiler  93 . As explained previously, the beam profiler will produce and display data regarding the location, diameter, optical power, and divergence of the spot, as partially schematically illustrated as the beam profiler output  102 . Adjustments to the idler 5-axis alignment mount  56  can be made to focus the beam onto the beam profiler  93  and optical axis of the second sub-assembly  110 . 
         [0112]    If present, the signal focusing element  62  is aligned. Signal focusing element tuning light  114  is then introduced. The signal focusing element tuning light  114  is light from a laser with a wavelength equal to the down-converted photons to be produced, introduced at the spot on the signal focusing element  62  that will emit the down-converted photons in the final assembly. That light will pass through the assembled elements and a portion will eventually reach the beam profiler  93 . As explained previously, the beam profiler will produce and display data regarding the location, diameter, optical power, and divergence of the spot, as partially schematically illustrated as the beam profiler output  102 . Adjustments to the signal 5-axis alignment mount  65  can be made to focus the beam onto the beam profiler  93  and optical axis of the second sub-assembly  110 . 
         [0113]    The recycling optical focusing assembly  74  is aligned. Recycling focusing element tuning light  116  is then introduced. The recycling focusing element tuning light  116  is laser light of the same wavelength that will be used for the pump beam  11 . That light will pass through the assembled elements and a portion will eventually reach the beam profiler  93 . As explained previously, the beam profiler will produce and display data regarding the location, diameter, optical power, and divergence of the spot, as partially schematically illustrated as the beam profiler output  102 . Adjustments to the second translating mount  77  can be made to focus the beam onto the beam profiler  93  and optical axis of the second sub-assembly  110 . 
         [0114]    The third phase of alignment and assembly is then performed. The beam profiler  93  is removed from the sub-assembly(ies). The fixed mount  35  containing the non-linear crystal  32  is then affixed between the first and second sub-assemblies as illustrated in  FIG. 1 , where the beam profiler  93  had been during the assembly and alignment of the first and second sub-assemblies. The camera  26  is also attached to the assembly, and optionally, the electric heater  83 . 
         [0115]    Light at the pump wavelength  11  is brought into the system  10  as discussed above and focused onto the non-linear crystal  32 . The laser light is visible on the first face  33  and is translated in the x and y axis, using x and y adjustments in the translating mount  17 , but not the focal axis, to perfectly center the focal spot onto the first face  33  of the non-linear crystal. 
         [0116]    The idler beam  59 , signal beam  68 , and/or focused recycling beam  80 , are then collected into optical fibers and connected to detectors. The x and y translation of the focusing elements can then be tuned. The x and y translation of the idler focusing element  53  can be optimized by adjusting the x and y translation in the idler 5-axis alignment mount  56  to optimize the idler beam as desired. The x and y translation of the signal focusing element  65  can be optimized by adjusting the x and y translation in the signal 5-axis alignment mount  65  to optimize the signal beam as desired. The x and y translation of the recycling optical focusing assembly  74  can be optimized by adjusting the x and y translation in the second translating mount  77  to optimize the focused recycling beam as desired. 
         [0117]    In a preferred embodiment, the idler beam  59  and the signal beam  68  are directed to a coincidence detector, most preferably by fiber optic couplers. The coincidence detector can determine when photons have arrived simultaneously at these two ports. The coincidence rate of the two fiber optic outputs can then be monitored and maximized through x and y translation adjustments on the idler 5-axis alignment mount  56  and/or the signal 5-axis alignment mount  65 . 
         [0118]    Most preferably, when fully aligned, the mounts are locked in place through set screws on each mount&#39;s alignment screws. The housing for the assembly is then positioned in place and attached to the systems base plate  86 , preferably with screws. The systems base plate  86  and housing serve to protect the optics and preserve the alignment within while also eliminating unwanted outside light from being collected into the fiber optics. 
         [0119]    Using the system and methods disclosed herein permits production of bi-photons at a rate as high as  1  million pairs per second per mW of pump power from two 20 mm long PPKTP crystals. Using the system and methods disclosed herein permits a single system to produce entangled photons in the quantum entangled triplet state using post selection. 
         [0120]    As illustrated schematically in  FIG. 8 , two systems for generating polarization entangled photons in the “triplet state” can be driven from the same pump beam to create a combined output of photons in the quantum polarization entangled “singlet state” without the need for post-selection. In such embodiments, a first system  210  is assembled in accordance with the present invention. As discussed previously, that system may be system  10 , as illustrated in  FIG. 1  with or without the optional elements, or may be a system with one or more of the optional elements illustrated by the examples of  FIGS. 2 through 4 , inclusive, or could be a system as illustrated in  FIG. 5 . Most preferably, system  210  is a system as illustrated in  FIG. 5 , without the optional beamsplitter  50 , the signal focusing element  62 , or optional fiber optic alignment mount  65 . Similarly, in such embodiments, a second system  200  is also utilized. System  200  may be system  10 , as illustrated in  FIG. 1  with or without the optional elements, or may be a system with one or more of the optional elements illustrated by the examples of  FIGS. 2 through 4 , inclusive, or could be a system as illustrated in  FIG. 5 , but is most preferably a system as illustrated in  FIG. 5 , without the optional beamsplitter  50 , the signal focusing element  62 , or optional fiber optic alignment mount  65 . 
         [0121]    In a preferred embodiment, a pump laser  203  provides light through a polarization-maintaining optical fiber  206  to a polarization-maintaining fiber optic splitter  209 . The fiber optic splitter  209  produces two copies of the pump beam, each of which is connected through optical fibers to each of the quantum entanglement generating systems  210 ,  200 . In a preferred embodiment, both systems  200  and  210  are identical. Most preferably, both systems  200  and  210  comprise an optional phase shifting wave plate  18 , and an optional beamsplitter  50 . Most preferably, the systems  200  and  210  do not have the optional beamsplitter  50 , the signal focusing element  62 , or optional fiber optic alignment mount  65  and hence, do not generate a separate signal beam  6868 . 
         [0122]    In this embodiment, the pair of bi-photons produced by a single unit&#39;s non-linear crystal would have opposite polarization and would be coupled into a single outgoing optical fiber. The bi-photons emitted by system  210  would be coupled into fiber  230 . The bi-photons emitted by system  200  would be coupled into fiber  233 . The output fibers  230  and  233  would be combined at a  2 x 2  fiber optic polarization beamsplitter  236 . The outputs from the fiber optic polarization beamsplitter&#39;s output fibers  242  and  239  would contain the polarization entangled photons in the “singlet state” (outputs  245  and  248 ). 
         [0123]    In order to maintain a stable output of the “singlet state,” the phase difference between the two pump beams&#39; exciting systems must be monitored and maintained. To do this, the recycled outputs of the pump laser beam from systems  210  and  200  are preferably coupled into single-mode polarization-maintaining optical fibers  218  and  224 . These fibers direct the recycled outputs to a phase monitoring and control system  221 . In a preferred embodiment, the phase monitoring control system is a Mach-Zehnder interferometer, such as one commercially-available from Thorlabs, that would provide a measurement of the phase difference between the pump beams of the two systems  200  and  210 . In a preferred embodiment, the output from the phase monitoring control system would be fed back through an electrical coaxial cable  227  into the optional phase shifting wave plates  18  of either system  210  (as illustrated in  FIG. 8 ) or system  200 . Most preferably, the optional phase shifting wave plate  18  is electronically adjustable to compensate for any fluctuation in phase difference between the photons generated by system  200  and system  210 . 
         [0124]    While the invention has been described and exemplified in sufficient detail for those skilled in this art to make and use it, various alternatives, modifications, and improvements should be apparent without departing from the spirit and scope of the invention. The examples provided herein are representative of preferred embodiments, are exemplary, and are not intended as limitations on the scope of the invention. Modifications therein and other uses will occur to those skilled in the art. These modifications are encompassed within the spirit of the invention and are defined by the scope of the claims. 
         [0125]    It will be readily apparent to a person skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. 
         [0126]    All patents and publications mentioned in the specification are indicative of the levels of those of ordinary skill in the art to which the invention pertains. All patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. 
         [0127]    The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein. Thus, for example, in each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of&#39; may be replaced with either of the other two terms. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. 
         [0128]    Other embodiments are set forth within the following claims.