Systems and methods for a cold atom frequency standard

Systems and methods for a cold atom frequency standard are provided herein. In certain embodiments, a cold atom microwave frequency standard includes a vacuum cell, the vacuum cell comprising a central cylinder, the central cylinder being hollow and having a first open end and a second open end; a first end portion joined to the first open end; and a second end portion joined to the second open end, wherein the first end portion, the central cylinder, and the second end portion enclose a hollow volume containing atoms, the first end portion and the second end portion configured to allow light to enter into the hollow volume. The cold atom microwave frequency standard also includes a cylindrically symmetric resonator encircling the central cylinder, wherein the resonator generates a microwave field in the hollow volume at the resonant frequency of the atoms.

BACKGROUND

Atomic frequency standards (atomic clocks) are some of the most stable frequency references available. Due to the stability offered by atomic clocks, atomic clocks are frequently used in multiple applications that demand stable frequency references. However, high performance atomic clocks have traditionally been relatively large rack mounted or table top devices. Thus, efforts are under way to reduce the size of atomic clocks such as by reducing the physics package of atomic clocks and other sensors which utilize cold atom clouds as the sensing element.

Making the physics package smaller has unique and complex challenges because the physics package must be hermetically sealed, permit the introduction of light into its interior, and be constructed of non-magnetic materials. In certain methods of manufacturing a physics package, a glass body is machined with multiple holes for placement of mirrors and windows on its exterior, and a plurality of angled borings that serve as light paths to trap, cool, and manipulate the cold atomic sample.

In general, a laser cooled atomic clock operates by trapping and manipulating atoms with light beams from one or more lasers and magnetic confining fields generated by one or more conductive bodies. The physics package defines a vacuum sealed chamber that holds the atoms that are manipulated and measured. The atoms within the physics package are trapped within the volume such that the multiple light paths intersect with the atoms from different angles. Developing a small volume physics package which allows for large optical beams and added-flexibility of a multi-beam configuration is important to the development of high performance miniature atomic physics packages. However, smaller size requirements for atomic clocks present challenges for current building techniques.

SUMMARY

Systems and methods for a cold atom frequency standard are provided herein. In certain embodiments, a cold atom microwave frequency standard includes a vacuum cell, the vacuum cell comprising a central cylinder, the central cylinder being hollow and having a first open end and a second open end; a first end portion joined to the first open end; and a second end portion joined to the second open end, wherein the first end portion, the central cylinder, and the second end portion enclose a hollow volume containing atoms, the first end portion and the second end portion configured to allow light to enter into the hollow volume. The cold atom microwave frequency standard also includes a cylindrically symmetric resonator encircling the central cylinder, wherein the resonator generates a microwave field in the hollow volume at the resonant frequency of the atoms.

DETAILED DESCRIPTION

Systems and methods for a cold atom frequency standard (atomic clock) are provided. As described below, cold atom atomic clocks function by trapping atoms within a vacuum sealed chamber and measuring characteristics of the atoms. For example, spectroscopic measurements of the trapped atoms can be used as a reference frequency in a timing system. As described in the present disclosure, a vacuum cell is mounted within a cylindrically symmetric resonator, where the vacuum cell admits laser beams that cool/trap atoms at particular locations within the vacuum cell. In at least one implementation, faceted windows on the vacuum cell permit the introduction of laser beams into the vacuum cell that fits within the resonator. Further, the resonator encircles the vacuum cell and, when operating, generates a persistent microwave field through the middle of the vacuum cell. As described above, the systems and methods apply to cold atom atomic clocks. The embodiments described above and further below also generally apply to cold atom sensors.

FIG. 1is a drawing illustrating multiple views of a vacuum cell100. In certain embodiments, the vacuum cell100includes a central cylinder104, a first end portion102and a second end portion106. The central cylinder104is a hollow open-ended cylinder that encircles an enclosed volume108. In at least one implementation, the central cylinder104is fabricated from a rigid material that does not impede the transmission of radio frequency (RF) energy through the material. For example, the central cylinder104may be fabricated from glass, silicon, ceramic, and the like.

In contrast to the central cylinder104, the first end portion102and the second end portion106are fabricated from a material that is transparent and does not strongly absorb light as it passes through the first end portion102and the second end portion106. For example, the first end portion102and the second end portion106may be fabricated from glass or other material that allows for the passage of light through the material. In at least one implementation, portions of the first end portion102and the second end portion106that allow light to pass through are polished to further reduce the possibility that material in the first end portion102and the second end portion106interact with light that directly passes through the first end portion102and the second end portion106. In at least one implementation, one of the first end portion102and the second end portion106is fabricated as part of the central cylinder104.

In certain embodiments, the first end portion102and the second end portion106are joined to opposite ends of the central cylinder104such that the combination of the central cylinder104, the first end portion102, and the second end portion106fully enclose the enclosed volume108within the vacuum cell100. Further, when the first end portion102and the second end portion106are joined to the central cylinder104, the first end portion102and the second end portion106are bonded to the central cylinder104such that the enclosed volume108is hermetically sealed within the vacuum cell100. For example, the first end portion102and the second end portion106are joined to the central cylinder104using a frit. Generally, the first end portion102and the second end portion106may be joined to the central cylinder104by a material or bonded using a method that is able to form a hermetic seal and maintain the hermetic seal during the operation of the atomic clock. In at least one implementation, a getter is formed in a getter tube that is attached to the vacuum cell100, where the getter tube helps preserve the vacuum after fabrication. Getter tubes are shown in U.S. patent application Ser. No. 13/231,438 entitled “SYSTEMS AND METHODS FOR GETTERING AN ATOMIC SENSOR”, which is incorporated herein by reference.

In certain embodiments, when the vacuum cell100functions as part of a cold atom sensor, the enclosed volume108contains atoms that are cooled by laser light that is introduced into the enclosed volume108. For example, the enclosed volume108may contain rubidium atoms or another atom that has the characteristics that allow the atom to function in an atomic clock. To introduce the light into the enclosed volume108, the light is introduced through the first end portion102, the second end portion106, or both the first end portion102and the second end portion106. In certain embodiments, the light sources are arranged about the vacuum cell such that light introduced into the enclosed volume108at light ports110will intersect each other at ninety degree angles. The light that is introduced into the enclosed volume108collects atoms from a background vapor or other source of hot atoms, then cools and compresses the collected atoms to form a sample whose properties are to be measured for the operation of the atomic clock. In at least one implementation, the light is extinguished during the period of time during which the properties of the atoms are measured.

FIG. 2is a drawing illustrating multiple views of a resonator200. The resonator200provides a persistent microwave field within a resonant cavity. In certain implementations, the resonator200is a loop gap resonator. When the resonator200is a loop gap resonator, the resonator200comprises supports204, electrodes208, shield206, and gaps202. For example, the resonator200includes a metallic slotted loop inside a cylindrical shield206, where the loop is formed by electrodes208that are separated along the loop by gaps202, where the loop is supported by supports204that connect the electrodes208to the shield206. Within the resonator200, the resonator200is able to create a microwave field. Further, the shape of the resonator200determines the frequency of the microwave fields that resonate. For example, the size of the gaps202, the length of the supports204, the circumference of the shield206, and the size of the electrodes208determine the frequency of resonant microwave fields.

In certain implementations, as shown inFIG. 3, the vacuum cell100may be placed within the resonator200to form a physics package300. As shown inFIG. 3, the resonator200is an open ended resonator, where the resonator200encircles the central cylinder104of the vacuum cell100. The resonator200generates a microwave field within the vacuum cell100that probes the atoms that are within the vacuum cell100. Further, the resonator200allows the first end portion102and the second end portion106to be exposed to multiple lasers at different angles to trap atoms inside the vacuum cell100. Further, because the physics package is implemented as a cold atom sensor, the resonator200produces a microwave field with uniform, linear polarization along the axis of the resonator and homogenous spatial phase throughout the volume within the resonator200and within the vacuum cell100In at least one implementation, where the atoms within the vacuum cell are rubidium atoms, the resonator200generates a microwave field having a frequency of 6.835 GHz within the vacuum cell100, where the generated frequency is the resonant frequency of the atoms' ground state hyperfine transition, commonly used as a clock transition in microwave clocks.

FIGS. 4A-4Cillustrate several views of the fabrication of a vacuum cell400. For example,FIG. 4Aillustrates an exploded view of a vacuum cell400, where vacuum cell400is similar to vacuum cell100inFIG. 1. Vacuum cell400includes a first end portion402, a central cylinder404, and a second end portion406that function in a similar manner to first end portion102, central cylinder104, and second end portion106inFIG. 1. As illustrated, the first end portion402is bonded to a side of the central cylinder404through a first frit seal403. The second end portion406is likewise bonded to an opposite side of the central cylinder404through a second frit seal405. During processing, the first frit seal403and the second frit seal405are heated such that the first frit seal403and the second frit seal405melt and hermetically seal the first end portion402and the second end portion406to opposite sides of the central cylinder404. In an alternative implementation, the first end portion402and second end portion406are bonded to the central cylinder404through means other than a frit seal, such as anodic bonding and the like.

FIG. 4Billustrates the mounting of vacuum tubes of vacuum cell400. In certain implementations, vacuum cell400may include a first vacuum tube408and a second vacuum tube410. The first vacuum tube408and second vacuum tube410aid in evacuating the air from the vacuum cell400and also in maintaining the vacuum within the vacuum cell400. Further, one of the first vacuum tube408and the second vacuum tube410may function as a fill tube for introducing an atomic source into the evacuated vacuum cell400. For example, when a vacuum tube functions as a fill tube, the vacuum tube may contain an atomic source or reservoir within the vacuum tube. Upon evacuation of air from the vacuum cell through the vacuum tube that does not contain the atomic source, the fill tube may be crushed to introduce the atomic source into the vacuum cell. The introduced atomic source may then release atoms, where the released atoms are then trapped and manipulated by light beams within the vacuum cell400. In at least one embodiment, the first vacuum tube408is bonded to the first end portion402through a first vacuum frit seal401. Likewise, the second vacuum tube410is bonded to the second end portion406through a second vacuum frit seal407.FIG. 4Cillustrates the vacuum cell400when the different components of vacuum cell400are bonded to one another. As illustrated, the vacuum cell400connects to two vacuum tubes, however, in alternative implementations; the vacuum cell400may connect to larger quantity of vacuum tubes. Vacuum tubes are further described in U.S. patent application Ser. No. 12/263,186 entitled “METHODS FOR INTRODUCTION OF A REACTIVE MATERIAL INTO A VACUUM CHAMBER”, which is incorporated herein by reference.

FIG. 5illustrates one exemplary implementation of a structure500that is used to hold the physics package300. For example, the structure500includes a rigid frame502having a vacuum connection506. The vacuum connection506connects to a vacuum tube that extends away from the vacuum cell100to pump air out of the vacuum cell. Further, frame502includes functional fixtures504that control light and signals that are provided to the physics package300. For example, functional fixtures504include laser beams, beam splitters, reflectors, support electronics, and the like. For example, the functional fixtures504direct the light beams (such as lasers) into the vacuum cell100such that the light beams intersect at right angles within the physics package and trap the atoms at the location where the light beams intersect. Also, the functional fixtures504may include a coupling loop, and possible tuning elements, which drive the resonator200to provide the desired microwave field within the physics package300as described above. In a further embodiment, the structure500may include a coupling element that secures the location of the vacuum cell100in relation to the resonator200.

FIG. 6is a flow diagram of a method600for fabricating a physics package according to at least one embodiment. Method600proceeds at602, where a vacuum cell is fabricated. For example, the vacuum cell is fabricated by forming two separate end portions and bonding the end portions to a hollow central cylinder, where the hollow central cylinder has two open ends on opposite sides of the cylinder and the end portions are bonded to the open ends of the central cylinder using a hermetic seal. The bonding of the end portions to the central cylinder encloses an enclosed volume within the vacuum cell. Further, when the central cylinder and the end portions are bonded to each other, the central cylinder and the end portions are in the presence of inert gasses. In at least one implementation, the inert gasses are pumped out by a vacuum pump connected to the cell via tubes such as vacuum connection506inFIG. 4. Atoms may then be liberated into the cell from a reservoir attached to the cell. In at least one implementation, the vacuum connection is left attached to the vacuum cell when the vacuum cell is used as an atomic clock.

Method600proceeds at604, where a resonator is fabricated. The resonator is a device, which, when driven by a source of microwave energy, establishes a persistent microwave field within the resonator's interior volume. In at least one implementation, the resonator is a loop gap resonator that has been tuned to provide a homogenous microwave field at the resonant frequency of an electromagnetic transition between internal energy levels of the species of atoms contained within the vacuum cell. Method600then proceeds to606, where a vacuum cell is mounted within the resonator. For example, the vacuum cell is mounted such that the resonator encircles the central cylinder and light sources are positioned such that laser light enters through the end portions of the vacuum cell to trap the atoms in the vacuum cell at the location where the lasers intersect within the vacuum cell. Further, the microwave fields generated by the resonator are able to probe the trapped atoms to provide measurements that are used when providing the reference frequency.

EXAMPLE EMBODIMENTS

Example 1 includes a cold atom microwave frequency standard comprising: a vacuum cell, the vacuum cell comprising: a central cylinder, the central cylinder being hollow and having a first open end and a second open end; a first end portion joined to the first open end; and a second end portion joined to the second open end, wherein the first end portion, the central cylinder, and the second end portion enclose a hollow volume containing atoms, the first end portion and the second end portion configured to allow light to enter into the hollow volume; and a cylindrically symmetric resonator encircling the central cylinder, wherein the resonator generates a microwave field in the hollow volume at the resonant frequency of the atoms.

Example 2 includes the cold atom frequency standard of Example 1, wherein the central cylinder has a circumference that is at least one of: circular; and polygonal.

Example 3 includes the cold atom frequency standard of any of Examples 1-2, wherein the vacuum cell further comprises at least one vacuum tube hermetically bonded to the vacuum cell, the at least one vacuum tube configured for at least one of: introducing solid material into the vacuum cell; and connecting to a vacuum, such that air is evacuated from the vacuum cell through the at least one vacuum tube.

Example 4 includes the cold atom frequency standard of any of Examples 1-3, wherein the resonator is a loop gap resonator.

Example 5 includes the cold atom frequency standard of any of Examples 1-4, further comprising a getter configured to evacuate gas from within the hollow volume.

Example 6 includes the cold atom frequency standard of any of Examples 1-5, wherein the resonator and vacuum cell are mounted within a frame, the frame comprising at least one light source to introduce a plurality of light beams into the hollow volume, wherein the plurality of light beams intersect within the hollow volume to trap the atoms.

Example 7 includes the cold atom frequency standard of any of Examples 1-6, wherein the atoms are rubidium atoms.

Example 8 includes a method for fabricating a frequency standard, the method comprising: fabricating a vacuum cell, wherein the vacuum cell comprises a hollow volume hermetically enclosing atoms; fabricating a resonator, wherein the resonator is configured to generate a microwave field at the resonant frequency of the atoms within an interior volume of the resonator; and mounting the vacuum cell within the resonator, wherein the vacuum cell is placed within the interior volume of the resonator such that a plurality of light beams may enter into the hollow volume to trap the atoms.

Example 9 includes the method of Example 8, wherein fabricating a vacuum cell comprises: fabricating a central cylinder, wherein the central cylinder encircles the hollow volume, the central cylinder having a first open end and a second open end opposite the first open end; fabricating a first end portion; fabricating a second end portion; and bonding the first end portion to the first open end and the second end portion to the second open end, wherein the atoms are hermetically sealed within the hollow volume.

Example 10 includes the method of Example 9, wherein the central cylinder is fabricated from a material that does not impede the transmission of radio frequency energy through the material.

Example 11 includes the method of any of Examples 9-10, wherein the first end portion and the second end portion are fabricated from a transparent material.

Example 12 includes the method of any of Examples 9-11, further comprising: fabricating a getter tube on the vacuum cell; and activating the getter.

Example 13 includes the method of any of Examples 9-12, wherein the first end portion is hermetically bonded to the first open end and the second end portion is hermetically bonded to the second open end in the presence of an inert gas.

Example 14 includes the method of any of Examples 9-13, wherein bonding the first end portion to the first open end and the second end portion to the second open end comprises anodically bonding the first end portion to the first open end and the second end portion to the second open end.

Example 15 includes the method of any of Examples 8-14, wherein the resonator is a loop gap resonator.

Example 16 includes the method of any of Examples 8-15, further comprising evacuating gas from within the hollow volume.

Example 17 includes the method of any of Examples 8-16, wherein mounting the vacuum cell within the resonator comprises mounting the vacuum cell and resonator on a frame that maintains the vacuum cell within the resonator.

Example 18 includes a frequency standard comprising: a vacuum cell, the vacuum cell comprising: a central cylinder, the central cylinder being hollow and having a first open end and a second open end; a first end portion joined to the first open end; and a second end portion joined to the second open end, wherein the first end portion, the central cylinder, and the second end portion hermetically enclose a hollow volume containing atoms, the first end portion and the second end portion configured to allow light to enter into the hollow volume; a resonator configured to generate a microwave field in the hollow volume at the resonant frequency of the atoms within an interior volume of the resonator; a frame configured to secure the vacuum cell within the resonator such that the central cylinder is encircled by the resonator; and at least one light source mounted to the frame, wherein the light source introduces a plurality of light beams that orthogonally intersect to trap the atoms within the hollow volume.

Example 19 includes the frequency standard of Example 18, wherein the resonator is a loop gap resonator.

Example 20 includes the frequency standard of any of Examples 18-19, wherein the first end portion and the second end portion are fabricated from a transparent material.