Abstract: An optical module for an atomic oscillator uses a quantum interference effect. The optical module includes a light source adapted to emit light including a fundamental wave having a center wavelength, and sideband waves of the fundamental wave, a wavelength selection section receiving the light from the light source, and adapted to transmit the sideband waves out of the light input, a gas cell encapsulating an alkali metal gas, and irradiated with light transmitted through the wavelength selection section, and a light detection section adapted to detect an intensity of light transmitted through the gas cell. The wavelength selection section includes an etalon and a temperature control section adapted to control temperature of the etalon.

Abstract: Techniques for obtaining a frequency standard using the crystal field splitting frequency of nitrogen vacancy center in diamond are disclosed. In certain exemplary embodiments, a microwave field is applied to the diamond and optically exciting the diamond under green light. The photoluminescent response of the diamond is measured by a photodetector. The intensity of the photoluminescent response can be used to determine the phase shift between the microwave and the crystal field splitting frequency. The microwave field frequency can be adjusted until the phase shift is below a predetermined threshold, and the microwave frequency can then be output for use as a standard.

Abstract: The invention relates to a radiofrequency oscillator which incorporates: a spin-polarized electric current magnetoresistive device (6) for generating an oscillating signal at an oscillation frequency on an output terminal (10), and a terminal (18) for controlling the frequency or amplitude of the oscillating signal, and a feedback loop (44) comprising an amplifier (46) provided with: an input connected to the output terminal (10) of the magnetoresistive device (6) so as to amplify the portion of an oscillating signal detected at the output terminal, and an output connected to the control terminal (18) so as to inject onto said control terminal the amplified portion of the oscillating signal which is phase-related to the oscillating signal generated at the output terminal.

Abstract: A device for an atomic clock, including: a laser source (102) generating a laser beam; a quarter-wave plate (105) modifying the linear polarization of the laser beam into a circular polarization and vice versa; a gas cell (106) placed on the laser beam having a circular polarization; a mirror (107) sending the laser beam back toward the gas cell; a first photodetector (108a); means (103, 101a, 107) for diverting the reflected beam of the laser source (102), and a second photodetector (109) placed behind the mirror (107), the mirror being semitransparent and allowing a portion of the laser beam to pass therethrough, the second photodetector (109) being used for controlling the optical frequency of the laser and/or for controlling the temperature of the cell (106).

Abstract: A device for an atomic clock, including: a laser source (102) that generates a laser beam; a splitter (101) that makes it possible to divert and allow a portion of the laser beam to pass therethrough in accordance with a predefined percentage; a quarter-wave plate (105) that modifies the linear polarization of the laser beam into circular polarization and vice versa; a gas cell arranged on the circular polarization laser beam; a mirror (107) sending the laser beam back toward the gas cell (106); a first photodetector (108a), and a polarizer (103) arranged between the laser beam outlet and the splitter in order to protect the laser source from the retroreflections emitted by different optical elements constituting the device.

Abstract: In one embodiment, a silicon-based atomic clock for use in an electronic device includes a single-isotope silicon crystal and energy level transitions within the silicon are used as a frequency resonance of the clock.

Abstract: An atomic oscillator includes a base, a unit portion including a gas cell and the like, and a support member that supports the unit portion against the base. The support member includes a plurality of leg portions and a connecting portion that connects the plurality of leg portions to each other. A connection portion between each leg portion and the base is separated from the unit portion in plan view from a direction in which the base and the unit portion overlap each other.

Abstract: An atomic oscillator includes: a gas cell which includes two window portions having a light transmissive property and in which metal atoms are sealed; a light emitting portion that emits excitation light to excite the metal atoms in the gas cell; a light detecting portion that detects the excitation light transmitted through the gas cell; a heater that generates heat; and a connection member that thermally connects the heater and each window portion of the gas cell to each other.

Abstract: A quantum interference device causing electromagnetically induced transparency in an alkali metal atom includes: a light source generating first and second resonant lights with frequency differences ??; a magnetic field generator applying a magnetic field to the atom; a light detector detecting intensities of the first and second resonant lights passing through the atom; and a controller causing a frequency difference between specified first and second resonant lights to equal a frequency difference corresponding to an energy difference between two ground levels of the atom based on the detected light. The controller causes the frequency ?? or magnetic field intensity to satisfy 2×?×n=?? or ??×n=2×?. The frequency ? corresponds to an energy difference between two Zeeman split levels differentiated by one magnetic quantum number and generated in the two ground levels of the atom by energy splitting.

Abstract: An atomic oscillator includes an alkali metal cell encapsulating an alkali metal atom; a light source that emits laser light; a light detector that detects light which has passed through the alkali metal cell; and a polarizer arranged between the alkali metal cell and the light detector. A modulation frequency in the light source is controlled, according to a coherent population trapping resonance which is a light absorption characteristic of a quantum interference effect for two kinds of resonant lights, by modulating the light source to generate sidebands and injecting laser lights with the sidebands into the alkali metal cell. A magnetic field is applied on the alkali metal cell in a direction parallel to a propagating direction of the laser light, and the laser light entering the alkali metal cell has a linear polarization, which is not parallel to a polarization direction of the polarizer.

Abstract: An optical module for an atomic oscillator using a quantum interference effect includes a light source adapted to emit light including a fundamental wave having a predetermined wavelength, and sideband waves of the fundamental wave, a wavelength selection section receiving the light from the light source, and adapted to transmit the sideband waves out of the light input, a gas cell encapsulating an alkali metal gas, and irradiated with light transmitted through the wavelength selection section, and a light detection section adapted to detect an intensity of light transmitted through the gas cell, and the wavelength selection section includes a fiber Bragg grating, and a voltage application section adapted to apply a voltage to the fiber Bragg grating.

Abstract: An oscillator is provided including a nanopillar and current injector for injecting a power supply current through the nanopillar, the nanopillar including at least one pattern including first and second layers made from a ferromagnetic material separated from each other by an intermediate layer made from a non-magnetic material. Each of the first and second ferromagnetic layers is prepared such that its remanent magnetic configuration corresponds to a vortex configuration and the polarity of the vortex core of the first layer is opposite the polarity of the vortex core of the second layer. The intermediate layer can allow repellant magnetic coupling between the two vortices of the first and second layers, for a zero intensity of the power supply current and a zero amplitude of the outside magnetic field.

Abstract: A radiofrequency oscillator comprises: a free layer (4), a current injector (6) for injecting spin-polarized current into the free layer, this injector having a spin-polarized current injection face (16) directly in contact with the free layer, a magnetoresistive contact (8) having a measurement face (26) directly in contact with the free layer, in order to form, in combination with the free layer, a tunnel junction for measuring the precession of the magnetization of the free layer, a conducting pad (30) directly in contact with the free layer in order to make an electrical current flow through the injector without passing through the magnetoresistive contact. At least part of the measurement face (26) and part of the injection face (16) are placed facing each other on each side of the free layer (4).

Abstract: An atomic oscillator includes an atom that generates interaction with first and second lights in accordance with an energy level of a three-level system; and a light source that emits the first light having a first plurality of lights of a first plurality of frequency components different from each other and the second light having a second plurality of lights of a second plurality of frequency components different from each other, wherein when the first and second lights are irradiated to the atom, an electromagnetically induced transparency phenomenon occurs in accordance with one of the first plurality of lights and one of the second plurality of lights.

Abstract: An oscillator and a method of operating the same are provided, the oscillator may include a free layer, a pinned layer on a first surface of the free layer, and a reference layer on a second surface of the free layer. The free layer may have a variable magnetization direction. The pinned layer may have a pinned magnetization direction. The reference layer may have a magnetization direction non-parallel to the magnetization direction of the pinned layer.

Abstract: Disclosed is a surface emitting laser device, including a substrate; a lower reflecting mirror provided on the substrate; an active layer provided on the lower reflecting mirror; an upper reflecting mirror provided on the active layer, including an emitting region, laser light being emitted from the emitting region, the upper reflecting mirror being formed by alternately laminating dielectrics, refracting indices of the dielectrics being different from each other; and an adjusting layer formed of semiconductor, provided in the emitting region between the active layer and the upper reflecting mirror, a shape of the adjusting layer in a plane parallel to a surface of the substrate including shape anisotropy in two mutually perpendicular directions.

Abstract: Synthetic antiferromagnetic (SAF) and synthetic ferrimagnetic (SyF) free layer structures are disclosed that reduce Ho (for a SAF free layer), increase perpendicular magnetic anisotropy (PMA), and provide higher thermal stability up to at least 400° C. The SAF and SyF structures have a FL1/DL1/spacer/DL2/FL2 configuration wherein FL1 and FL2 are free layers with PMA, the coupling layer induces antiferromagnetic or ferrimagnetic coupling between FL1 and FL2 depending on thickness, and DL1 and DL2 are dusting layers that enhance the coupling between FL1 and FL2. The SAF free layer may be used with a SAF reference layer in STT-MRAM memory elements or in spintronic devices including a spin transfer oscillator. Furthermore, a dual SAF structure is described that may provide further advantages in terms of Ho, PMA, and thermal stability.

Abstract: A gas cell unit includes a gas cell in which a gaseous alkali metal atom is sealed, a first heater to heat the gas cell, and a second heater which is provide to face the first heater across the gas cell and heats the gas cell. The first heater includes a first heating resistor which generates heat by energization, and the second heater includes a second heating resistor through which a current flows in the same direction as the direction of a current flowing through the first heating resistor and which generates heat by energization. Between the first heating resistor and the second heating resistor, a magnetic field generated by the energization to the first heating resistor and a magnetic field generated by the energization to the second heating resistor are mutually cancelled or weakened.

Abstract: A surface emitting laser element includes a lower DBR formed on a substrate; an active layer formed above the lower DBR; an upper DBR formed on the active layer. The upper DBR includes a dielectric multilayer that is formed as a result of dielectrics having different refractive indexes being alternately laminated and formed, a light shielding part is formed above the upper DBR, and the light shielding part has an opening at a central area for emitting light.

Abstract: A method of forming a physics package for an atomic sensor comprises providing an expendable support structure having a three-dimensional configuration, providing a plurality of optical panels, and assembling the optical panels on the expendable support structure such that edges of adjacent panels are aligned with each other. The edges of adjacent panels are sealed together to form a physics block having a multifaced geometric configuration. The expendable support structure is then removed while leaving the physics block intact.

Abstract: An atomic oscillator includes: a light-receiving element including a light-receiving section; a cell layer that is laminated on the light-receiving element and includes a cavity having an opening above the light-receiving section; gaseous alkali metal atoms sealed in the cavity; a transparent member to close the opening; and a light-emitting element to emit resonance light to the light-receiving section through the transparent member and the alkali metal atoms.

Abstract: A spin torque oscillator and a method of making same. The spin torque oscillator is configured to generate microwave electrical oscillations without the use of a magnetic field external thereto, the spin torque oscillator having one of a plurality of input nanopillars and a nanopillar having a plurality of free FM layers.

Abstract: Some demonstrative embodiments include devices, systems and/or methods of generating a frequency reference using a solid-state atomic resonator formed by a solid-state material including an optical cavity having color centers. A device may include a solid-state atomic clock to generate a clock frequency signal, the solid-state atomic clock including a solid state atomic resonator formed by a solid-state material including au optical cavity having color centers, which are capable of exhibiting hyperfine transition, wherein the solid-state atomic clock may generate the clock frequency signal based on a hyperfine resonance frequency of the color centers.

Abstract: A vacuum chamber assembly includes a vacuum chamber containing a reactive material, an inlet fill tube fixedly attached to the vacuum chamber, and an outlet fill tube fixedly attached to the vacuum chamber. The inlet fill tube has a first vacuum tight seal and the outlet fill tube has a second vacuum tight seal.

Abstract: A spin-torque oscillator with antiferromagnetically-coupled free layers has at least one of the free layers with increased magnetic damping. The Gilbert magnetic damping parameter (?) is at least 0.05. The damped free layer may contain as a dopant one or more damping elements selected from the group consisting of Pt, Pd and the 15 lanthanide elements. The free layer damping may also be increased by a damping layer adjacent the free layer. One type of damping layer may be an antiferromagnetic material, like a Mn alloy. As a modification to the antiferromagnetic damping layer, a bilayer damping layer may be formed of the antiferromagnetic layer and a nonmagnetic metal electrically conductive separation layer between the free layer and the antiferromagnetic layer. Another type of damping layer may be one formed of one or more of the elements selected from Pt, Pd and the lanthanides.

Abstract: An atom cell module includes an atom cell in which atoms are enclosed, a heating unit that heats the atom cell by generating heat when a current flows, and a magnetic field generator that generates a magnetic field inside the atom cell. A magnetic field at a predetermined position inside the atom cell generated by the magnetic field generator includes a magnetic field component in an opposite direction to a magnetic field at the predetermined position generated on the basis of a current flowing through the heating unit.

Abstract: A manufacturing method of an atom oscillator (an example of a quantum interference device) includes assembling the atom oscillator (an example of the quantum interference device) by respectively disposing a gas cell, a semiconductor laser, a light detector, ICs of a circuit unit, heaters, and coils at desired locations, and adjusting at least one of currents which flow through the coils, and positions and shapes of the coils such that a frequency-temperature characteristic of a pair of resonance light beams becomes approximately flat.

Abstract: The embodiments herein relate to a magnetic field feedback based spintronic microwave oscillator driven by DC current. The microwave oscillator works based on a magnetic tunnel junction structure connected to a feedback waveguide. Any fluctuation in the magnetization direction of free magnetization layer of MTJ drives an oscillating current through the feedback waveguide which in turn exerts an oscillating magnetic field on the free layer and amplifies the magnetization fluctuations. If the DC current passing through the MTJ is more than a critical value, continuous processing states of the magnetization are possible. The critical current is independent of the thickness and magnetization of the free layer. A MTJ can be driven into spontaneous oscillations with DC current and magnetic field feedback circuit and can act as a spintronic microwave oscillator.

Abstract: In an example, a chip-scale atomic clock physics package is provided. The physics package includes a body defining a cavity having a base surface and one or more side walls. The cavity includes a first step surface and a second step surface defined in the one or more side walls. A first scaffold mounted to the base surface in the cavity. One or more spacers defining an aperture therethrough are mounted to the second step surface in the cavity. A second scaffold is mounted to a first surface of the one or more spacers spans across the aperture of the one or more spacers. A third scaffold is mounted to a second surface of the one or more spacers in the cavity and spans across the aperture of the one or more spacers. Other components of the physics package are mounted to the first, second, and third scaffold.

Abstract: A system for distributing a reference oscillator signal includes a clock having a reference oscillator and a femtosecond laser stabilized by the reference oscillator. The system also includes at least one beamsplitter configured to split the femtosecond laser. The system further includes one or more remote nodes that are spaced from the clock. The remote nodes are configured to generate reference signals based on the split femtosecond laser.

Abstract: A quantum interference device includes: gaseous alkali metal atoms; and a light source for causing a resonant light pair having different frequencies that keep a frequency difference equivalent to an energy difference between two ground states of the alkali metal atoms, the quantum interference device causing the alkali metal atoms and the resonant light pair to interact each other to cause an electromagnetically induced transparency phenomenon (EIT), wherein there are a plurality of the resonant light pairs, and center frequencies of the respective resonant light pairs are different from one another.

Abstract: A low power microfabricated atomic clock generates a Coherent Population Trapping resonance. An absorption cell is disposed within a resonator cavity of a Fabry-Perot (FP) resonator or an optical ring resonator to enhance a modulation term of a transmittance. A modulated laser source, external to the resonator, is configured to excite the resonator and the absorption cell with a laser beam passing therethrough. A detector then determines a frequency associated with the CPT resonance of laser light exiting the resonator, and a frequency controller is coupled to the detector to adjust the modulated laser source based on the determined frequency. First and second quarter wave plates are positioned adjacent to respective first and second sides of the resonator.

Abstract: An optical module for an atomic oscillator using a quantum interference effect includes a light source adapted to emit light including a fundamental wave having a predetermined wavelength, and sideband waves of the fundamental wave, a wavelength selection section receiving the light from the light source, and adapted to transmit the sideband waves, a gas cell encapsulating an alkali metal gas and irradiated with light transmitted through the wavelength selection section, and a light detection section adapted to detect an intensity of light transmitted through the gas cell, and the wavelength selection section including an optical filter section adapted to transmit light having a wavelength within a predetermined wavelength range, and an optical filter characteristic control section adapted to vary the wavelength range of light to be transmitted by the optical filter section.

Abstract: A gas cell unit has a gas cell, inside which a gaseous alkali metal atom is sealed, a heater that heats the gas cell. The heater includes a heating resistor including a plurality of band-like portions so as to be parallel to each other. By making the directions of electric current flowing through two band-like portions adjacent to each other opposite to each other, it is possible to mutually offset or alleviate the magnetic fields generated along with the electric conduction to the plurality of band-like portions.

Abstract: A nano-oscillator magnetic wave propagation system has a group of aggregated spin-torque nano-oscillators (ASTNOs), which share a magnetic propagation material. Each of the group of ASTNOs is disposed about an emanating point in the magnetic propagation material. During a non-wave propagation state of the nano-oscillator magnetic wave propagation system, the magnetic propagation material receives a polarizing magnetic field. During a wave propagation state of the nano-oscillator magnetic wave propagation system, each of the group of ASTNOs initiates spin waves through the magnetic propagation material, such that a portion of the spin waves initiated from each of the group of ASTNOs combine to produce an aggregation of spin waves emanating from the emanating point. The aggregation of spin waves may provide a sharper wave front than wave fronts of the individual spin waves initiated from each of the group of ASTNOs.

Abstract: In an example, a chip-scale atomic clock physics package is provided. This chip-scale atomic clock physics package includes a body defining a cavity, and a first scaffold mounted in the cavity. A laser is mounted on the first surface of the first scaffold. A second scaffold is also mounted in the cavity. The second scaffold is disposed such that the first surface of the second scaffold is facing the first scaffold. A first photodetector is mounted on the first surface of the second scaffold. A vapor cell is mounted on the first surface of the second scaffold. A waveplate is also included, wherein the laser, waveplate, first photodetector, and vapor cell are disposed such that a beam from the laser can propagate through the waveplate and the vapor cell and be detected by the first photodetector. A lid is also included for covering the cavity.

Abstract: A gas cell unit has a gas cell, inside which gaseous metallic atoms are sealed a first heater that heats the gas cell, and a second heater that is provided to face the first heater via the gas cell and heats the gas cell. The heaters include a first heating resistor and a second heating resistor which are provided so as to face each other, are heated by electric current flow, and mutually offset the magnetic fields generated along with the electric current flow.

Abstract: The frequency of an atomic clock may be stabilized against C-field variation by applying a rf magnetic field perpendicular to the C-field to cause a coherent population transfer between Zeeman states that compensates for quadratic frequency shift of transitions of the clock. The cancellation, provided by a feed-forward mechanism, is exact. The invention can be implemented in any atomic clock by including an electrode in the clock generating a magnetic field perpendicular to the C-field, and providing an electronic circuit to send rf signals to the electrode.

Abstract: A disclosed surface-emitting laser element includes a lower DBR formed on a substrate, an active layer formed on the lower DBR, an upper DBR formed on the active layer, a wavelength-adjusting layer formed above the active layer, and a plurality of surface-emitting lasers configured to emit respective laser beams having different wavelengths by changing a thickness of the wavelength-adjusting layer. In the surface-emitting laser element, the wavelength-adjusting layer includes one of a first film having alternately layered GaInP and GaAsP and a second film having alternately layered GaInP and GaAs, the thickness of the wavelength-adjusting layer being changed by partially removing each of the alternating layers of a corresponding one of the first and second films.

Abstract: A synthesizer includes a second frequency-synthesizing stage comprising a radiofrequency oscillator configured to oscillate at a frequency ?fo when it is synchronized with a signal s0(t), where ? is a rational number different from one such that ?f0=ft. The radiofrequency oscillator has a magnetoresistive device within which there flows a spin-polarized electrical current to generate a signal st(t) oscillating at the frequency ft on an output electrode connected to the rendering terminal. This device is formed by a stack of magnetic and non-magnetic layers, a synchronization terminal for synchronizing the frequency of the oscillating signal st(t) with the frequency of the signal received at the synchronization terminal. The synchronization terminal being connected to the output terminal of the first stage to receive the signal s0(t).

Abstract: A spin torque oscillator and a method of making same. The spin torque oscillator is configured to generate microwave electrical oscillations without the use of a magnetic field external thereto, the spin torque oscillator having one of a plurality of input nanopillars and a nanopillar having a plurality of free FM layers.

Abstract: An oscillator using spin transfer torque includes i) a pinned magnetic layer having a fixed magnetization direction, ii) a non-magnetic layer located on the pinned magnetic layer, and iii) a free magnetic layer located on the non-magnetic layer. The pinned magnetic layer includes i) a first part of the fixed magnetic layer and ii) a second part of the fixed magnetic layer located thereon. The first part of the fixed magnetic layer includes i) a first interface in contact with the second part of the fixed magnetic layer and ii) a second surface exposed to an outside while surrounding the first interface.

Abstract: An atomic oscillator includes: a gas cell in which a gaseous metal atom is sealed; first and second heaters heating the gas cell; an exciting light source exciting the metal atom; a light detector detecting the exciting light; a substrate including a temperature controlling circuit for the heaters; a first wiring coupling the first heater and the substrate; a second wiring coupling the second heater and the substrate; and a third wiring coupling the first heater and the second heater. In the atomic oscillator, the gas cell includes a cylinder and windows sealing both ends of the cylinder and constituting an incident surface and an emitting surface on an optical path of the exciting light. The first and second heaters are respectively formed on the windows at an incident surface side and an emitting surface side and are made of transparent heating materials.

Abstract: Some demonstrative embodiments include devices, systems and/or methods of generating a frequency reference using a solid-state atomic resonator formed by a solid-state material including an optical cavity having color centers. A device may include a solid-state atomic clock to generate a clock frequency signal, the solid-state atomic clock including a solid state atomic resonator formed by a solid-state material including an optical cavity having color centers, which are capable of exhibiting hyperfine transition, wherein the solid-state atomic clock may generate the clock frequency signal based on a hyperfine resonance frequency of the color centers.

Abstract: An apparatus includes a chip-scale atomic clock (CSAC) alkali vapor cell seated on a silicon substrate that is suspended in a package by a metalized Parylene strap having Parylene anchors embedded in a silicon frame, the Parylene strap comprising an extended rigidizing structure, and a plurality of electrical pins extending into an interior of the package, the plurality of electrical pins in electrical communication with the CSAC cell through the metalized Parylene strap, where the CSAC cell is mechanically connected to the package and thermally insulated from the package.

Abstract: According to one embodiment, a magnetic oscillator includes a layered film and a pair of electrodes. The layered film includes a first ferromagnetic layer, an insulating layer stacked on the first ferromagnetic layer, and a second ferromagnetic layer stacked on the insulating layer. The pair of electrodes is configured to apply a current to the layered film in a direction perpendicular to a film surface of the layered film. Regions having different resistance area products are provided between the first ferromagnetic layer and the second ferromagnetic layer.

Abstract: A physical section of an atomic oscillator includes: a gas cell in which gaseous metal atoms are sealed, and the gas cell includes a first window having optical transparency; a light source that emits excitation light toward the metal atoms through the first window; a first heating unit that disposes at the first window and that is located between the first window and the light source; and a Peltier element that is stacked on the first heating unit, that is located between the first heating unit and the light source, and that decreases a temperature of a side of the Peltier element facing the light source than a temperature of an opposite side of the Peltier element facing the gas cell.

Abstract: An oscillator including two groups of elementary junctions having giant magnetoresistance effect traversed by electric currents, the junctions of each of the two groups being in series and energized by respective main currents and the voltages across the terminals of the groups being added together to provide a voltage on an output of the oscillating circuit. The voltage across the terminals of one or more junctions of a first group is applied to a first input of a phase comparator and the voltage across the terminals of one or more junctions of the other group is applied to another input of the phase comparator, the phase comparator providing on two outputs secondary currents of the same amplitude and of opposite signs, which are dependent on the mean phase difference between the voltages applied to the inputs, the secondary currents each being added to a respective main current.

Abstract: An optical module of an atomic oscillator using a quantum interference effect includes a light source to generate first light including a fundamental wave having a center wavelength, and including a first sideband wave and a second sideband wave having wavelengths that are different from each other, a wavelength selection unit that emits second light by selecting the first sideband wave and the second sideband wave of the first light and by allowing them to pass through, a gas cell in which an alkali metal gas is sealed and to which the second light is irradiated, and a light detection unit that detects an intensity of the second light passing through the gas cell.

Abstract: A method for reducing or eliminating clock bias in an atomic clock is provided. The method comprises cooling a population of atoms collected in the atomic clock using a laser locked at a predetermined frequency, turning off the laser, performing atomic clock spectroscopy, turning on the laser after the atomic clock spectroscopy, and relocking the frequency of the laser to an external reference cell. The population of atoms that are in each of two ground hyperfine levels is then probed using laser light that is on or near-resonant with a selected atomic transition.