Patent Abstract:
The present invention discloses a vacuum chamber having operating pressures in the ultra-high vacuum (UHV) range (10 −8  torr to 10 −13  torr) and incorporating transparent windows, said windows constructed from transparent materials (preferably glass), and having low helium permeability velocity under operating and storage conditions. Embodiments may also contain surface coatings on windows to reduce helium permeation. Also disclosed herein is a method for vacuum processing said chamber by heating entire chamber and exposing the inside and outside of the chamber windows to helium free environments. Methods for final sealing said chamber are also discussed. The vacuum chamber is useful as a container for optically-cooled atoms for use in quantum information and atomic clocks and as a sensor for magnetic fields, gravitational fields, and inertial effects.

Full Description:
[0001]    This application is a non-provisional application claiming the benefits of provisional application no. 61/516,758 filed Apr. 7, 2011. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N68335-10-C-0508 awarded by the Naval Air Systems Command (NAVAIR) of the U.S. Department of Defense (DoD). 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to a vacuum chamber constructed with optical windows made to have a high resistance to helium leakage, and a fabrication and processing method thereof. 
       BACKGROUND OF THE INVENTION 
       [0004]    The proposed applications that can benefit from cold and ultracold atom technology includes atom interferometry, quantum computing, nonlinear optics, atom interaction studies, atomic timekeeping, inertial navigation, magnetic sensing, and gravitational sensing. One serious obstacle to developing these applications has been the complexity and size of the vacuum systems required for ultracold atom production. Although recent scaled-down vacuum systems intended for producing Bose Einstein condensation (BEC) have begun to address this issue, there remains much that is still required in terms of miniaturization and reducing system complexity. 
         [0005]    Active vacuum pumps like sputter-ion pumps and turbo pumps have become ubiquitous in cold and ultracold atom systems. In general, these pumps provide convenience, ease of use, and the ultra-high vacuum (UHV) conditions (10 −8  torr to 10 −13  torr) required for producing optically cooled and ultracold matter. A sputter ion pump works by ionizing vacuum impurities in the volume of the pump. A high voltage accelerates the ions toward the walls of the pump where they are sequestered via burial deep into the wall or by chemical reaction with the materials that form the wall. A turbo pump is a mechanical pump that uses spinning turbine blades to create a preferred direction of flow of vacuum impurities out of the volume of the chamber. There are several drawbacks to using these active vacuum pumps and these drawbacks become more significant under miniaturization for use in sensory applications based on optically cooled atoms. 
         [0006]    Active pumps such as sputter ion pumps and mechanical turbo pumps have better pumping capability if they are large. Under miniaturization, the pumping capability of these active pumps is reduced to a point of diminishing returns. The large physical size of the active pump itself is a major limitation and dictates the ultimate size of the source. Furthermore, as cold atom vacuum chambers get smaller in size, the active pumps must be closer in proximity to the collections of cold and ultracold atoms. Stray magnetic fields from active vacuum pumps can have a detrimental effect on cold atom-based sensors. The effects of these stray fields are accentuated in smaller systems and it becomes increasingly difficult to shield the atoms from them. Elimination of such pumps can enable further miniaturization of ultracold atom sources and spur application development. 
         [0007]    Traditional vacuum systems utilized in the production of optically cooled atoms and ultracold atoms (such as BEC) are large in size. It is common for these systems to weigh between 10 kg and 50 kg with length of about 1 meter in at least one physical dimension. These systems incorporate heavy suitcase-sized active pumps such as sputter ion pumps or mechanical turbo pumps to maintain the low pressures required for producing collections of cold and ultracold atoms. The required pressures can vary from as high as about 10 −8  torr to about 10 −13  torr depending on the goal of the apparatus. The chambers contain a mechanism to dispense the atoms of interest; typically alkali atoms such as rubidium or cesium. Atoms are dispensed into the chamber to form a low-density room-temperature vapor that can be cooled and confined in a magneto-optical trap (MOT). Traditional vacuum systems for producing ultracold matter can be based on a two-chamber design where the chambers are coupled using a narrow tube or aperture. A single-chamber design can also be used for optical cooling, however, the two-chamber design gives better vacuum performance albeit, at the cost of greater system size and complexity. 
         [0008]    The present invention addresses the need for a miniature vacuum chamber with reduced reliance on active pumping to be used as a source for ultracold atoms. A UHV vacuum chamber is formed that eliminates or reduces the use of active vacuum pumps and further provides a simplified geometry for producing ultracold matter. The primary challenge in creating such a chamber is managing the permeation of substances, called vacuum impurities, into the vacuum chamber which add to the background pressure and spoil the necessary UHV conditions. Of particular concern is helium as a vacuum impurity. Helium is naturally found in the air and can readily diffuse through many window materials. Furthermore, helium is not effectively pumped by any passive getter material. The present invention provides a helium impervious window in a vacuum chamber used for atom cooling, trapping, and probing. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is a vacuum chamber for working with cold and ultracold atoms that can maintain ultra-high vacuum levels in a closed cavity with minimal or no active pumping. The chamber incorporates optical windows allowing atoms or molecules on the inside of the cavity to be addressed with light. One example of an application for which the vacuum chamber is well suited is the application of optically cooling atoms, ions, or molecules, but this invention is not limited to such applications and for one skilled in the art, it is easy to imagine other applications or fields for which this invention can be applied with minimal or no modification. The optical portions of the chamber are constructed from transparent materials having a helium permeability velocity below about 1×10 −13  cm 2 /s, the material made from ingredients preferably having an alumina content over about 4% by weight. The chamber may contain passive vacuum pumping in the form of an atom collector. Additionally, the chamber may contain a device for depositing atoms or molecules of specific species into the volume of the vacuum chamber which will be referred to in the text as a target atom injector. 
         [0010]    A critical component of this invention is the method for processing the window material so that it has a reduced concentration of helium that is dissolved in the material. 
         [0011]    The method of vacuum processing is a key part of this invention. The process of obtaining vacuum levels with sufficiently low pressures requires the vacuum chamber to be temporarily connected to a vacuum pumping apparatus or enclosed within a processing chamber that is connected to a pumping apparatus. The temperature of the chamber is elevated to between about 50 C and 450 C to liberate vacuum impurities. These vacuum impurities may contain surface impurities with high vapor pressures which are common on materials which have been exposed to normal atmosphere. They may also contain impurities trapped within the bulk of the glass and other materials that make up the vacuum chamber. Impurities include, but are not limited to, water vapor, hydrocarbons, nitrogen, hydrogen, oxygen, CO 2 , and noble gases(for example, helium). The temperature of the pumping apparatus or the temperature of a processing chamber connected to the pumping apparatus may also be elevated during the processing procedure to increase pumping speed and as a means of indirectly heating the chamber inside the processing chamber. During pumping and heating some of these impurities contained in the bulk material migrate to the surface where they can be pumped away by the pumping apparatus. The temperature of the chamber is raised to between about 50 C and 450 C to increase the rate of impurity migration out of the vacuum chamber materials. One example of an impurity that is critical to remove from the bulk of the vacuum chamber material is helium. Helium is found in the atmosphere at a partial pressure of about 4×10 −3  torr and because it is small and chemically non-reactive, it readily permeates through many materials. For materials used in a vacuum system with minimal or no active pumping, the dissolved helium impurity leads to serious vacuum contamination. Impurities are removed from all sides of the vacuum chamber materials by subjecting the exposed inner and outer surfaces to an environment (preferably a vacuum) that contains a low concentration of helium. One method is performed by enclosing the whole vacuum chamber within a vacuum processing enclosure. An alternate method can be performed by having a separate processing apparatuses for the inside of the vacuum chamber and one for the outside of the vacuum chamber. The method for removing helium from the bulk of the material is called degassing and is performed by baking the material or complete vacuum chamber in a helium free atmosphere. The mentioned atmosphere may be composed of a vacuum, or it may be a helium free purge gas. Low concentrations of helium will generally be achieved in the processing chamber by vacuum pumps that actively remove helium atoms from the volume of the processing chamber which represents the environment to which the surface of the vacuum chamber material is exposed during processing. The duration of vacuum processing of the vacuum chamber extends from between about 3×10 1  seconds to about 3×10 7  seconds. One alternative is to use a purge system incorporating a gas that has low helium partial pressure less than about 10 −8  torr. The purge gas should be chosen as to not readily contaminate the bulk material of the chamber. As one example, high-purity nitrogen may be used as the purge gas to reduce the amount of helium located in the bulk of the material. Processing with a purge gas extends from about about 3×10 2  seconds to about 3×10 7  seconds. Purge gas processing, used on both the interior and exterior of the vacuum chamber material to reduce helium content, is followed by vacuum processing to remove the purge gas from the interior of the vacuum chamber. Purge gas can also be used on the exterior of the vacuum chamber cavity, with vacuum processing being used separately to process the interior of the cavity. Processing with a purge gas on the exterior of the cavity and vacuum processing of the interior of the cavity extends in time from about 3×10 2  seconds to about 3×10 7  seconds. 
         [0012]    One example of a vacuum cell without active pumping is formed with optical windows composed of a transparent material (preferably glass) with helium permeability velocity below about 1×10 −13  cm̂2/s. The windows can be formed into a body structure using a variety of leak-tight bonding techniques including, but not limited to glass fusing, diffusion bonding, anodic bonding or optical contacting. The body structure can be made to have a variety of shapes, including but not limited to, a tube with a round cross section, a tube with a square cross section, or an irregular geometric structure. The body structure can be attached to a metal flange by melting one edge of the glass body and bringing the molten part of the glass in contact with a metal edge. When the assembly is cooled, the interface between the glass and metal forms a leak-tight seal known in the art as a glass-to-metal seal. A target atom injector is placed within the cavity of the cell and can be affixed to the walls of the body structure or affixed to a separate metal flange that then attaches to the flange having the glass-to-metal seal. As an option, the target atom injector may be left unconstrained within the cavity. An atom collector is placed within the cavity of the cell and can be affixed to the walls of the body structure or affixed to a separate metal flange that then attaches to the flange having the glass-to-metal seal. A metal valve may be incorporated into the structure by attaching with a metal flange. The flange need not be removable. Optionally, a metal tube known in the art as a pinchoff tube can be used in place of the metal valve or in addition to the metal valve for sealing the vacuum chamber after sufficient vacuum processing as described above. The valve is closed by rotating a threaded plunger. The pinchoff tube is sealed by squeezing the tube together from the outside until a leak-tight cold weld bond is formed to and the tube is severed. 
         [0013]    Building upon this example, a vacuum cell as above can be constructed that includes a processing port which can be opened and resealed or cut off and replaced. After sufficient operating time has elapsed such that the vacuum quality of the chamber is not favorable, the cell can be “recharged” by attaching it to a vacuum pumping apparatus for reprocessing via the processing port. 
         [0014]    Helium permeation rates can alternatively be reduced by coating the vacuum chamber walls with a material having low helium permeability. Possible mat  1  s include, but are not limited to, graphene and aluminum oxide. The coating may be applied to the inside of the chamber, the outside of the chamber, or both. The coating may be applied prior to, subsequent to, or as part of the vacuum processing described above. 
         [0015]    To complete the process of vacuum processing the vacuum chamber, a final vacuum-tight seal must be made before disconnecting the vacuum chamber from the vacuum pumping apparatus. Prior art shows that a vacuum chamber may be sufficiently sealed using a pinchoff tube that, when squeezed together using a tool, forms a leak-tight cold weld. During the pinch process, the pinchoff tube is severed and the vacuum chamber is freed from the vacuum processing apparatus. In prior art a commercial vacuum valve can be used for a final or intermediate vacuum seal. The vacuum valve consists of a threaded plug that, when turned, seats against a sealing surface to form a vacuum seal. The use of a pinchoff tube and a vacuum valve can be used on vacuum chambers as described above that incorporate a glass-to-metal seal. 
         [0016]    Another way to seal the vacuum chamber is by placing a plug over an evacuation port in the vacuum chamber and then producing a leak-tight bond at the interface between the vacuum chamber and the plug. The bond can be formed using optical contacting. For optical contacting, the plug and a mating surface on the vacuum chamber must have a flatness ranging from about lambda/5 to about lambda/50 and are preferably made from the same material. The surface of the plug and mating surface of the vacuum chamber are thoroughly cleaned prior to processing. After evacuation, the seal is formed by bringing the plug in contact with the mating surface of the vacuum chamber. The movement of the plug relative to the vacuum chamber is accomplished using a vacuum compatible position translator. Heat may be applied at the interface to strengthen the bond. A leak-tight seal is formed. 
         [0017]    If the plug is made of silicon or metal, the seal may be performed using anodic bonding. For anodic bonding, the plug and a mating surface on the vacuum chamber must have a flatness ranging from about lambda/5 to about lambda/50. The surface of the plug and mating surface of the vacuum chamber are thoroughly cleaned prior to processing. A seal based on anodic bonding is performed by bringing the plug in contact with the mating surface of the vacuum cell. The interface is heated using an internal heater to a temperature ranging from about 50 C to about 450 C. Using internal electrodes, a high voltage is applied to the silicon ranging from about 500 volts to about 2000 volts relative to the glass. A leak-tight seal is formed. 
         [0018]    A seal may also be formed using a thin film of indium between the plug and the mating surface of the vacuum cell. After evacuation of the vacuum cell, the plug is brought into contact with the mating surface of the vacuum chamber. The indium film is sandwiched between the plug and mating surface of the vacuum chamber. Heat is applied to the bond interface with an internal heater a leak-tight seal is formed. 
         [0019]    Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  Measurement results of helium permeation for different types of material. 
           [0021]      FIG. 2   a  Glass chamber with plug. 
           [0022]      FIG. 2   b  Glass chamber with plug and divider. 
           [0023]      FIG. 3  Cell with valve. 
           [0024]      FIG. 4  Glass cell with electrically actuated injector and collector. 
           [0025]      FIG. 5   a  (Prior art) Cross-section of window exposed to air on one side and vacuum on the other. 
           [0026]      FIG. 5   b  Cross-section of the window during the present invention degassing method using a vacuum. 
           [0027]      FIG. 5   c  Cross-section of the window during the present invention degassing method using a purge gas. 
           [0028]      FIG. 5   d  Cross-section of the window during the present invention degassing method using a combination of vacuum and purge gas. 
           [0029]      FIG. 6   a  Proposed schematic drawing of vacuum chamber processing, degasification and sealing setup. 
           [0030]      FIG. 6   b  Proposed schematic drawing of anodic bonding method of sealing plug. 
           [0031]      FIG. 7  Proposed alternate schematic drawing of vacuum chamber processing and degasification setup. 
           [0032]      FIG. 8  Proposed alternate schematic drawing of vacuum chamber processing and degasification setup. 
           [0033]      FIG. 9  Miniature cold atom vacuum chamber. 
           [0034]      FIG. 10  Optically cooled atom instrument. 
           [0035]      FIG. 11  Permeation of helium into processed atom chamber of various materials. 
           [0036]      FIG. 12  Helium barrier coatings on vacuum chamber. 
       
    
    
       [0037]    Before explaining the disclosed embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    Referring first to  FIG. 1  the horizontal component represents reciprocal of the absolute temperature in Kelvin times 1000 of the test glass under vacuum in the range of 10 −3  torr to 10 −10  torr. The following prior art glasses were used to construct bulbs and the bulbs were used to characterize the helium permeation velocity (the vertical component) of the materials: one borosilicate glass bulb, one fused silica glass bulb, and three aluminosilicate glass (ASG # 1 , ASG # 2 , ASG # 3 ) bulbs. The size of the bulbs were about spherical in shape with an outer dimension of about 17 mm to about 20 mm with wall thickness of about 0.2 mm to about 1.5 mm. ASG # 1  is Corning(R) 1720 ignition tube glass having 59.2% SiO 2 , 4.6% B 2 O 3 , 22.2% AL 2 O 3 , 4.4% CaO, and 9.9% MgO. ASG # 2  is Schott(R) 8252 aluminosilicate glass. ASG # 3  is Schott(R) 8436 sapphire sealing glass. These aluminosilicate glasses have common uses in constructing high-temperature combustion tubes, producing glass thermometers, and sealing to metal electrodes and flanges as in the case of high temperature halogen light bulbs. 
         [0039]    The present invention begins with an aluminosilicate material including, but not limited to those evaluated above for use as an optical window into a vacuum chamber. The window may alternately use soda lime glass. Referring next to  FIG. 2   a , vacuum chamber  20  has size desired from about 100 mm in length to about 1 mm in length and the shape of a cube. Other embodiments could be rectangular with size about 150 mm by about 25 mm as displayed in the figure. The walls  21  can be glass as shown. The walls  21  can be used as an optical window. An aluminosilicate material or soda lime glass as noted above is used as glass  21 . The wall thickness d 1  can range from about 0.1 mm to about 20 mm. Hole  22  is used to evacuate the cavity  26 . The plug  23  is made from a material that can be bonded to glass  21  and can be made from materials including, but not limited to glass, silicon, or metal. Some applications may contain a target atom injector  25 . This target atom injector  25  emits a specified plurality of atoms into the cavity  26 . The specified atom may include, but is not limited to alkali metal atoms or alkali-earth atoms. Some applications may also contain an atom collector  24 . 
         [0040]    The atom collector  24  may be a passive device for collecting impurity atoms in cavity  26 . Prior art atom collectors include devices known in the art as getters. Getters can be of the evaporable or non-evaporable nature. An example of an evaporable getter is a titanium or gold film formed by high temperature vapor deposition. An example of a non-evaporable getter (NEG) is a piece of carbon such as activated charcoal or various forms of graphite. NEG Pumps may also contain a blend of sintered metal powders including zirconium, vanadium, and iron such is commercially sold by SAES(R). Atom collector  24  and target atom injector may be unattached to glass  21 , and left unconstrained in cavity  26 . Atom collector  24  and target atom injector  25  could also be pre-fastened to plug  23 . Atom collector  24  and target atom injector  25  could be films on glass  21  and could be vapor deposited onto glass  21  through hole  22 . 
         [0041]    Referring next to  FIG. 2   b , vacuum chamber  220  incorporates a divider  28  that separates the cavity into cavity  26  and cavity  27 . Divider  28  may have fluid communication channel  29  from cavity  26  to cavity  27 . An alternate embodiment (not shown) may have multiple dividers  28 , cavities  26  and  27 , and fluid communications  29  among all of the cavities. Atom collector  24  and atom injector  25  may reside in one or all of the cavities and may also be in separate cavities. 
         [0042]    Referring next to  FIG. 3 , vacuum chamber  30  has a different means of sealing its top. A metal flange  32  has an opening  33 . Flange  32  is attached to glass  21  at a glass-to-metal interface  31 . Prior art glass-to-metal seals include a metal cylinder sealed to a glass cylinder. In the prior art, the metals can be chosen to have similar expansion coefficients to that of glass  21 . Port  37  of valve  39  is connected to flange  32 . The valve  39  is constructed to have a threaded plug  34 . Turning threaded plug  34  causes a sealing surface  35  to meet sealing surface  36  forming a vacuum tight seal. Evacuation port  40  exhausts to vacuum source V through pinchoff tube  38 . Pinchoff tube  38  can be sealed by squeezing with a tool. It can later be reopened or replaced to re-apply vacuum to cavity  26 . Atom collector  24  and target atom injector  25  could be optionally fastened to flange  32 . A second lower flange (not shown) could comprise the bottom of the vacuum chamber  30  and support the atom collector  24  and target atom injector  25 . 
         [0043]    Referring next to  FIG. 4 , the vacuum chamber  40  has holes in glass  21  suited to seal in wires  44  for the target atom injector  43  and the wires  42  for the atom collector  41 . As is well known in the art, all embodiments of the atom collector and target atom injector may contain electrical wires for electrical activation and operation. 
         [0044]    The shape of the chambers  20 ,  220 ,  30 , and  40  could have a regular cross-section such as a square, pentagon, hexagon, octagon or circle. Additionally the chambers may have non-regular cross-sections incorporating multiple windows at various angles and may not be prismatic in shape. 
         [0045]    Referring next to  FIG. 5   a , vacuum impurities  50  are distributed in the glass  21 . By vacuum impurity we mean impurities that can migrate out of the glass  21  when exposed to a vacuum V, further contaminating cavity  26 . Most dangerous to contaminating the cavity  26  are vacuum impurities  50  consisting of helium atoms which have atomic radius of about  31  picometers and are chemically inert. Materials exposed to air during storage or manufacturing will contain helium atoms  50 . For a sealed and evacuated vacuum cavity  26 , a helium atom  52  from the outside of the chamber OC can migrate through to the inside cavity  26 . The pattern of helium migration through  21  forms a column C of high concentration helium  50  at the air-glass boundary. From column C individual helium atoms  50  can permeate through glass  21  into cavity  26 . 
         [0046]    Referring next to  FIG. 5   b , the present invention degassing method is shown. Glass  21  could be an entire container or a segment such as a window for a vacuum chamber. Glass  21  is heated by heat H to a temperature range of about 50 C to about 450 C. Heat H could be applied by radiative heating such as using a laser or a nearby hot body, or by direct contact with a hot body. Helium impurities  50  diffuse out of glass  21 . Vacuum V is simultaneously applied to both sides of the glass thus removing helium  50  from glass  21 . Vacuum V can range from about 10 −4  torr to about 10 −13  torr. This process is continued for between about 3×10 2  seconds to about 3×10 7  seconds. After degassing, a final partial pressure of helium impurities  50  less than about 10 −4  torr is obtained at both of the surfaces of glass  21 . 
         [0047]    Example 1: We used an ASG Corning(R) 1720 glass tube about 25 mm in diameter with a material thickness of about 1.5 mm. The tube was attached at both ends to a glass-to-metal seal. Each seal contained a CF1.33 vacuum flange. A SAES(R) rubidium dispenser model RB/NF/7/25 FT10+10 was used as an atom injector. A SAES(R) non-evaporable getter model ST172/HI/7-6/150C was used as an atom collector. Both the injector and collector were spot welded to a vacuum electrical feedthru on a CF1.33 vacuum flange. The injector/collector assembly was attached to the tube assembly by bolting the CF1.33 flanges together. A vacuum valve from VAT(R) model 54024-GE02 was attached by the CF1.33 flange to the other side of the tube assembly. Furthermore a copper pinchoff tube connected the vacuum valve to the processing pumping apparatus. The apparatus consisted of a UHV turbo pump and a UHV ion pump. The vacuum chamber, not including the processing pumping apparatus, had a volume of about 104 cc. The chamber was heated directly using resistive heating cord and infrared heaters to a temperature of about 250 C for about 4 weeks while the pumping apparatus continued to remove impurities from the chamber. The chamber was then allowed to cool to about 25 C after which, the vacuum valve was closed. The pinchoff tube was then squeezed which had the effect of severing the vacuum chamber from the processing pumping apparatus. Atoms were laser cooled and trapped in the chamber by first applying electrical current of about 2.5 A to the atom injector and then applying three sets of orthogonal pairs of antiparallel laser beams at 780 nm to the volume of the vacuum chamber. A magnetic field gradient was added to the volume using a set of external coils in an anti-Helmholtz configuration. The fluorescence of the atoms was monitored on a photodiode and on a near infrared video camera. The laser cooling was maintained without active pumping. 
         [0048]    Referring next to  FIG. 5   c  shows an alternate present invention degassing method. Glass  21  could be an entire container or a segment such as a window for a vacuum chamber. Glass  21  is heated by heat H to a temperature range of about 50 C to about 450 C. Heat H could be applied by radiative heating such as using a laser or a nearby hot body, by direct contact with a hot body, or by convective heating with a hot gas. Helium impurities  50  diffuse out of glass  21 . A low helium concentration environment is achieved by applying a high-purity purge gas  51 , as one example nitrogen, simultaneously applied to both sides of the glass thus removing helium  50  from glass  21 . Purge gas  51  must have a helium concentration such that the helium partial pressure in purge gas  51  is less than about 10 −4  torr. The process of applying heat H and purge gas  51  is continued for between about 3×10 2  seconds to about 3×10 7  seconds. After degassing, a final partial pressure of helium impurities  50  less than about 10 −4  torr is obtained at both of the surfaces of glass  21 . 
         [0049]    Referring next to  FIG. 5   d  shows an alternate present invention degassing method. Glass  21  could be an entire container or a segment such as a window for a vacuum chamber. Glass  21  is heated by heat H to a temperature range of about 50 C to about 450 C. Heat H could be applied by radiative heating such as using a laser or using a nearby hot body, by direct contact with a hot body, or by convective heating with a hot gas. Helium impurities  50  diffuse out of glass  21 . A low helium concentration environment is achieved in cavity  26  by applying vacuum V thus removing helium  50  from glass  21  from the inside of glass  21 . A low helium concentration environment is achieved on the outside of glass  21  by applying a high-purity purge gas  51 , as one example nitrogen, to the outside of the glass  21  thus removing helium  50  from the outside of glass  21 . Purge gas  51  must have a helium concentration such that the helium partial pressure in purge gas  51  is less than about 10 −4  torr. The process of applying heat H, vacuum V and purge gas  51  is continued for between about 3×10 2  seconds to about 3×10 7  seconds. After degassing, a final partial pressure of helium impurities  50  less than about 10 −4  torr is obtained at both of the surfaces of glass  21 . 
         [0050]    Referring next to  FIG. 6   a , a proposed degassing and sealing apparatus  60   a  is shown. We start with a vacuum enclosure for processing  62  which has an access panel  67  to enable the placement of a vacuum chamber  20  therein. Vacuum enclosure  62  is preferably constructed of stainless steel or aluminum. Panel  67  may incorporate an inspection window (not shown). Panel  67  could also be a gate valve that separates the vacuum enclosure for processing  62  from a sample load-lock enclosure (not shown). A platform  64  anchors vacuum chamber  20  for processing. A plug  23  is removably connected to the plug holder  64 . The plug holder  64  also incorporates a heating element  69 . The plug holder  64  can be moved up and down (see arrows Up, Down) by a position actuator  66  which powers a motional feedthru  65 . Position actuator could be a manually driven screw, an electric motor, or pneumatic actuator. Alternately, plug holder  64  could be manipulated by a electrically actuated translation stage (not shown) internal to the processing chamber. Vacuum pump V has a fluid communication  61  with the process chamber  63 . For processing, the vacuum at  63  ranges from about 10 −6  torr to about 10 −13  torr. The aluminosilicate glass  21  is from about 0.1 mm to about 10 mm in thickness and we predict a processing time interval ranging from about 3 days to about 3 months. During this vacuum processing time interval the vacuum chamber  20  is heated by heater  69  in the platform  64 . Alternately the vacuum enclosure for processing  62  could be heated externally (not shown). The temperature of vacuum chamber  20  is maintained at a value ranging from about 50 C to about 450 C, during the processing time interval. At the end of the helium impurity  50  evacuation process, the plug  23  sealing operation begins. 
         [0051]    Plug  23  is preferably made out of the same type of material as the glass  21 . The mating surfaces  68  of the vacuum chamber  20  and the plug  23  have been prepared to a flatness ranging from about lambda/5 to about lambda/50 where lambda is 633 nm prior to placement into degassing and sealing apparatus  60   a . The surfaces of plug  23  and mating surfaces  68  may be cleaned with an acid such as HF or a base such as KOH prior to placement into degassing and sealing apparatus  60   a . Plug  23  is lowered against the mating surface  68  of vacuum chamber  20  using the position actuator  66 . The heater  69  in plug holder  64  is actuated to a temperature of about 50 C to about 450 C. The heater  69  has the effect of raising the temperature of the glass  21  at mating surface  68  to a value ranging between 50 C and 450 C. The temperature is sustained for a time ranging between about 3 hours to about 3 days. At that point the vacuum chamber  20  can be removed and used for application including, but not limited to, optical cooling of atoms, magnetic sensing, gravitational sensing, and quantum information. It is estimated that a commercially viable vacuum chamber  20  may range from about 1 cubic mm to about 1 liter. After vacuum processing, the outside of glass  21  (of chamber  20 ) may be machined to reshape or true the outer surfaces using common glass grinding and polishing techniques known in the art. 
         [0052]    Referring next to  FIG. 6   b , the processing and degassing apparatus  60   b  has a positive charged conductor  71  that passes through the insulator  70  to the plug holder  64  which is electrically conductive. A conductive wire  72  at a lower potential such as ground may pass through an insulator  70  and connect to an electrode  73  that is in physical contact with glass  21  on or adjacent to mating surface  68 . Alternately, the insulator  70  can be eliminated and conductive wire  72  may connect to directly to the inside of vacuum enclosure  62  if the potential on vacuum enclosure  62  is maintained near about ground (not shown). Plug  23 S is preferably made from silicon. The mating surfaces  68  of the vacuum chamber  20  and the plug  23 S have been prepared to a flatness ranging from about lambda/5 to about lambda/50 prior to placement into degassing and sealing apparatus  60   b . The surfaces of plug  23 S and mating surfaces  68  may be cleaned with an acid such as HF or a base such as KOH prior to placement into degassing and sealing apparatus  60   b . When the plug  23 S is pressed into contact with the surface  68 , the circuit is completed wherein a current flows from the plug holder  64  through the glass  21  to the electrode  73  and back through the conductive wire  72 . Heat is applied through heater  69  raising the mating surface  68  to a temperature ranging from about 100 C to about 450 C. The voltage applied to conductor  71  ranges from about 500 volts to about 2000 volts. It is estimated that a time period ranging between about 5 minutes and about 1 hour is needed to complete a leak-tight bond. This process is known as anodic bonding. See US patent number U.S. Pat. No. 7,807,509 B2 incorporated herein by reference. See especially  FIG. 2   a.    
         [0053]    Referring next to  FIG. 6   c , an indium film  601  is deposited onto mating surface  68 . The indium film  601  may be in the form of an indium foil ranging in thickness from about 0.020 mm to about 1 mm. Indium film  601  may also be applied previously to mating surface  68  using vapor deposition or by electroplating (both techniques being known in the art) with indium thickness ranging from about 0.005 mm to 0.1 mm. For vapor deposition or electroplating, a metal base layer may be used composed from a combination of metals, including, but not limited to Cu, Ag, Au, Cr, Mo, and W. The process of degassing material  21  is the same as described above. Plug  23  is lowered against the indium film  601  on mating surface  68  of vacuum chamber  20  using the position actuator  66 . The heater  69  in plug holder  64  is actuated to a temperature of about 50 C to about 250 C. The heater  69  has the effect of raising the temperature of the glass  21  at mating surface  68  to a value ranging between 50 C and 250 C. The temperature is sustained for a time ranging between about 10 minutes to about 10 hours. 
         [0054]    Referring next to  FIG. 7 , shown is the present invention vacuum processing and degassing apparatus  80 . Vacuum chamber embodiment  30  as described above is incorporated into vacuum processing apparatus  80 . We start with a vacuum enclosure  81  for processing which has an access panel  67  to enable the placement of a vacuum cell  30  therein. Vacuum enclosure  81  is preferably constructed of stainless steel or aluminum and is heated from the outside by a heater (not shown). Panel  67  may incorporate an inspection window (not shown). At least one vacuum pump V has fluid connection  83  through vacuum port  82  to pinchoff tube  38 . Vacuum pump V has fluid connection  84  to process vacuum space  63 . Vacuum pump V could be bifurcated or two separate pumps. The procedure for degassing glass  21  is the same as described above in  FIG. 6   a  and  FIG. 5   b . The aluminosilicate glass  21  is from about 0.1 mm to about 10 mm in thickness and we predict a processing time interval ranging from about 3 days to about 3 months. During this vacuum processing time interval the vacuum chamber  20  is heated indirectly by heating the vacuum enclosure  81  from the outside with a resistive or radiative heater (not shown) or a flame. The temperature of the enclosure  81  is maintained at a value ranging from about 50 C to about 450 C, during the processing time interval. Enclosure  81  heats apparatus  30  indirectly from heat conducted through pinchoff tube  38  and heat radiated inward from the walls of enclosure  81 . At the end of the helium impurity  50  evacuation process, the outer volume  63  is returned to ambient pressure. The access panel  67  is removed. The valve  39  is closed. The pinchoff tube  38  is closed. The apparatus  30  is removed and ready for service. 
         [0055]    Apparatus  30  can reevacuated at a later time. The used pinchoff tube  38  is removed and replaced. Apparatus  30  is replaced into the processing and degassing apparatus  80  through access panel  67 . The open end of the pinchoff tube  38  is sealed to port  82 . Vacuum V is activated, the valve  39  is opened, and access panel  67  is closed. The evacuation process is repeated as described above. 
         [0056]    Next referring to  FIG. 8 , apparatus  30  is placed into a purge gas enclosure  95 . Enclosure  95  incorporates a port  91 . A purge gas supply GAS, injects a desired purge gas  51  into volume  92  via inlet tube  93 . Excess purge gas  51  and helium impurities  50  exhaust through port  91 . Purge gas  51  may be a number of gases including, but not limited to nitrogen, argon, CO2, as anyone skilled in the art may select. Purge gas  51  must be pure of helium. All other process variables are as described in  FIG. 7 . 
         [0057]    Referring next to  FIG. 9   a , a bar  100  composed primarily of glass is made from a base  101 , spacer  102 , spacer  103  and a top  104 . Base  101 , spacer  102 , spacer  103  and a top  104  can range in length from about 5 mm to about 200 mm. Base  101 , spacer  102 , spacer  103  and a top  104  can range in thickness from about 0.1 mm to about 20 mm. Base  101 , spacer  102 , spacer  103 , top  104  may be composed exclusively or in part by glass  21 . Spacer  102  and spacer  103  are placed on base  101 . Top  104  is placed on spacer  102  and spacer  103  forming a channel  105 . Adjoining surfaces  130  are bonded together. One of a variety of bonding techniques can be used including, but not limited to, optical contact bonding, chemically assisted optical contact bonding, anodic bonding, or direct diffusion bonding. To one skilled in the art of glass bonding, these are all known techniques. Adjoining surfaces  130  and cavity surfaces  131  are previously prepared before bonding to be flat ranging from about lambda/5 to about lambda/50 using standard glass polishing techniques known in the art. Lambda is 633 nm. 
         [0058]    Referring next to  FIG. 9   b , a bar  100  is cut along line  104  forming in  FIG. 9   c , a slice  110 . Exposed front and back surfaces  106 , which have a normal vector along the direction of channel  105 , are polished to a flatness ranging from about lambda/5 to about lambda/50. 
         [0059]    Referring next to  FIG. 9   d , a cap  111  is bonded to one exposed surface  106  to form chamber  140 . Cap  111  may be composed exclusively or in part by glass  21 . One of a variety of bonding techniques can be used including, but not limited to, optical contact bonding, chemically assisted optical contact bonding, anodic bonding, or direct diffusion bonding. 
         [0060]    Referring next to  FIG. 9   e , assembly  140  is placed into a processing apparatus such as degassing and sealing apparatus  60   a  or  60   b  as shown in  FIGS. 6   a ,  6   b . A plug  23  is removably connected to the plug holder  64 . Plug  23  may be composed exclusively or in part by glass  21 . The plug holder  64  can be moved up and down by a motional feedthru  65 . Vacuum processing, degassing, and sealing is performed the same as described in  FIG. 6   a  or  FIG. 6   b . One way of forming the embodiment  220  of  FIG. 2   b  would be to us a plug  23  (not shown) that has a hole in it which would form fluid communication channel  29 . Next another chamber  140  would be attached over the plug  23  with the hole to form the multiple chamber embodiment  220  of  FIG. 2   b.    
         [0061]    Referring next to  FIG. 9   f , chamber  140  is removed from the processing apparatus such as degassing and sealing apparatus  60   a  or  60   b . Chamber  120  is machined along cut lines CL 1 , CL 2 , CL 3 , and CL 4  using standard glass machining techniques such as glass sawing or glass grinding which are known in the art. Additional cut lines (not shown) may also be performed. Rough surfaces such as those along cut lines CL 1 , CL 2 , CL 3 , and CL 4  may be polished to an optical flatness ranging from about lambda/1 to about lambda/50 using standard glass polishing techniques known in the art. Other surfaces of chamber  140  may also be polished. 
         [0062]    Referring next to  FIG. 9   g , executing the cut lines as noted in  FIG. 9   f  results in the vacuum chamber  150 . Cavity  151  may have a volume as small as about 0.001 mm 3  or as large as about 8 cm 3 . The wall thickness may be as small as about  0 . 001  mm or may be as large as about 200 mm. Applications can include optical wavelength references, a container for optically cooled atoms, inertial sensors, gravity sensors, magnetic field sensors, atomic clocks, and atomic oscillators. 
         [0063]    Example 2: We constructed micro vacuum chambers from fused silica glass. We started with four pieces; a bottom, a top, and two spacers. The bottom piece was 100 mm wide, 40 mm long, and 2 mm thick. The top piece had the same dimensions. The two spacers were each 4 mm wide, 40 mm long, and 2 mm thick. The pieces were polished to better than lambda/10 at surfaces that were to undergo bonding. Two different bonding methods were used. In one case we used optical contact bonding. The pieces were cleaned thoroughly using solvents, acetone, IPA, and a basic cleaning agent KOH, and water. The spacers were stacked onto the bottom piece leaving a uniform gap between the spacers. An optical contact bond was formed by applying pressure by hand at the bond joint. The top was then stacked onto the spacers forming a bar with an open channel. Once again, an optical contact bond was formed. In another case we repeated these steps, but substituted optical contact bonding with diffusion bonding. The stacks were aligned and placed into an oven. In the oven, pressure of about 6 to ten PSI was applied to the stack while the temperature was raised to about 1100 C for 6 Hours. This temperature was chosen to be about near the strain point of the glass and not exceed the softening point of the glass. After bonding, the bar was cut into slices that were 2 mm thick. The front and back faces of the slices were polished to a flatness of about lambda/10. Flat windows measuring 6 mm, by 8 mm, by 2 mm thick were bonded over the channel using both bonding techniques forming a sealed cavity. In the case of those that were bonded using diffusion bonding, a vacuum was created inside the cavity due to expansion and contraction of the gas that filled the cavity during heating and cooling. The assembly was cut along planes parallel to surfaces that form the cavity. The cut planes were then polished. The final micro vacuum chambers measured 4 mm×4 mm×4 mm, with an internal cavity that measured 2 mm×2 mm×2 mm. 
         [0064]    Referring next to  FIG. 10 , a cold atoms instrument  1000  consists of crossed laser beams  1001  that meet about near the center a vacuum chamber such as  20  shown in  FIG. 2 . The crossed laser beams  1001  can have a typical intensity ranging from about 1 mW/cm 2  to about 100 mW/cm 2 . Vacuum chamber  20  can be substituted with vacuum chamber  30 , vacuum chamber  40 , or vacuum chamber  140  as described earlier. Target atom injector  25  is actuated to dispense target atoms into the volume of the vacuum chamber  20 . A set of magnetic coils (not shown) provide a magnetic field gradient ranging from about 1 Gauss/cm to about 100 Gauss/cm. A collection of cooled atoms  1003  forms where the crossed laser beams  1001  intersect. The collection of cooled atoms  1003  can be adjusted from about 1 atom to about 10 10  atoms by varying the intensity of the crossed laser beams  1001  and the magnetic field gradient at the location of the atoms. Background atom collector  24  helps to maintain low operating pressures. A probe laser  1002  addresses the collection of cooled atoms  1003 . A detector  1004  is used to sense light emitted or absorbed by the collection of cooled atoms  1003 . The temperature of the collection of cooled atoms  1003  can range from about 1 millikelvin to about 0.1 nanokelvin. Various properties of the collection of cooled atoms  1003  can be measured including, but not limited to, the number of atoms, the atom temperature, the atoms&#39; quantum state, the pressure in the vacuum chamber  20 , magnetic energy shifts, energy shifts related to inertial excitement of the atoms, quantum phase, and energy of atomic levels. An example is shown that the flux density of magnetic field B may be measured. 
         [0065]    Referring next to  FIG. 11 , the horizontal component represents a time delay in days. The graph represents the accumulation of helium in a closed glass vacuum chamber as a function of time. The helium partial pressure is plotted on the vertical component for glass vacuum chambers where the glass has been properly degassed as described above. The calculation is performed for glass vacuum chambers constructed from three different glass types and having a size of 3.5 cm×3.5 cm×5 cm and window thickness of 2. Dashed line  1101  shows helium partial pressure inside the chamber as a function of time for a vacuum chamber made from borosilicate glass. Dotted line  1102  shows helium partial pressure inside the chamber as a function of time for a vacuum chamber made from soda-lime silica glass. Solid line  1103  shows helium partial pressure inside the chamber as a function of time for a vacuum chamber made from aluminosilicate glass. A typical pressure at which optical cooling begins to fail is about 10 −8  torr. Setting this as our failure criteria, dashed line  1101  shows that the present invention constructed from borosilicate glass would have a device lifetime of about 10 days. Dotted line  1102  shows that the present invention constructed from soda-lime silica glass would have a device lifetime of about 450 days. Solid line  1103  shows that the present invention constructed from aluminosilicate glass would have a device lifetime of about 4500 days. 
         [0066]    Referring next to  FIG. 12 , a vacuum chamber  1200  may be constructed from a variety of types of glass  1203 , including those that have a high helium permeability. Helium permeation rates may be reduced by depositing a coating  1201 , with low helium permeability, onto the outside of glass  1203  and plug  23 . Coating  1202 , with low helium permeability, may be deposited to the inside of glass  1203  and plug  23 . Possible coating materials for coatings  1201  and  1202  include, but are not limited to, graphene and aluminum oxide. The coatings may be applied to the inside of the vacuum chamber  1200 , to the outside of the vacuum chamber  1200 , or to both. The coating may be deposited prior to, subsequent to, or as part of the vacuum processing described above. Coatings  1201  and  1202  can range in thickness from 0.335 nm to 0.1 mm. The coatings  1201  and  1202  may be deposited using wet chemistry techniques such as dip coating which is known in the art. Alternately, the coatings may be deposited using vapor deposition which is known in the art. The evacuation and sealing method noted above would remain the same to utilize vacuum chamber  1200 . 
         [0067]    Although the present invention has been described with reference to the disclosed embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred. Each apparatus embodiment described herein has numerous equivalents.

Technology Classification (CPC): 2