Sensors
Sandia leverages quantum mechanics to enable exquisite metrology devices, such as inertial sensors and frequency standards that go beyond the capabilities of conventional methods

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  • Quantum Sensing

    We are leveraging quantum mechanics to enable sensors that go beyond the capabilities of conventional methods. Using the quantized states of matter, it is possible to build exquisite metrology devices such as frequency standards and inertial sensors. Our efforts bridge the gap between the fundamental science that enable these new technologies and the applications that will benefit most.
  • Atomic Interferometers

    Atom interferometers have the potential to be exceptional broadband inertial sensors. However, typically such systems are designed for stable laboratory environments. We demonstrate a compact atom interferometer for simultaneously measuring acceleration and rotation, and suitable for dynamic environments.

    Our dual-axis accelerometer and gyroscope forms a building block of a six-axis inertial measurement unit. By recapturing the atoms after the interferometer sequence, we maintain a large atom number at high data-rates of 50 to 100 measurements per second. Two cold ensembles are formed in trap zones located a few centimeters apart, and are launched toward one-another. During their ballistic trajectory, they are interrogated with a stimulated Raman sequence, detected, and recaptured in the opposing trap zone. We achieve sensitivities at µg/√Hz and µrad/s/√Hz levels, making this a compelling prospect for expanding the use of atom interferometer inertial sensors beyond benign laboratory environments.


    Image description: Front view diagram of the apparatus implementing the cold ensemble exchange and dual-axis, high data-rate atom interferometer. Two MOTs are loaded 36 mm apart. Cooling beams are shown in blue, probe beams in pink, and Raman beams in yellow. The trap is turned off, and the outer and inner cooling beams are blue and red detuned, respectively, which launches the ensembles towards each other. After the experiment, atoms are recaptured in the opposite trap to facilitate loading. The vector g shows the direction of gravity, while a is the direction of the acceleration sense axis. The rotation measurement axis points out of the page.
    • Demonstration Video


      Video description: A slow motion movie demonstrating the launch and recapture technique. The ensembles are launched towards each other over 2 ms to a velocity of 2.5 m/s, with simultaneous sub-Doppler cooling. The interferometer takes place during their ballistic trajectory for ≈ 8 ms. Imaging beams are off during the interferometer to reduce photon scatter which would otherwise cause decoherence. Dashed circles represent the approximate positions of the ensembles during this stage. Atoms are then recaptured in the opposite trapping region, with additional atoms loaded from vapor for 7 ms. This process repeats here 50 times per second, but can be optimized to various rates to suit the application.
    We demonstrate matterwave interference in a warm vapor of rubidium atoms. Established approaches to light pulse atom interferometry rely on laser cooling to concentrate a large ensemble of atoms into a velocity class resonant with the atom optical pulse. In our experiment, we show that clear interference signals may be obtained without laser cooling. This effect relies on the Doppler selectivity of both the atom interferometer resonance and the probe transition. This interferometer may be configured to measure accelerations, and we demonstrate that multiple interferometers may be operated simultaneously by addressing multiple velocity classes.

    Image description: Vapor interferometer concept—not to scale. The 2-D mesh Gaussian represents the Maxwell-Boltzmann distribution in cylindrical coordinates z and ρ for warm temperature atoms in the hyperfine ground state |F=1>. The solid blue Sinc functions are two relatively narrow velocity classes centered at ± vz in |F=2> that are selected from |F=1> using the Doppler shift in the Raman transitions that comprise the light-pulse atom interferometer. The arrows indicate the directions of the Raman and probe lasers used to generate and detect the two narrow classes. Each Raman laser carries two frequencies separated by the hyperfine splitting, vhf = 6.8 GHz, plus an additional amount equal to |keff| vz/π such that a counter propagating two-photon Raman transition is simultaneously resonant with the two velocity classes. Following the LPAI, the two velocity classes are simultaneously detected with two resonant probe lasers.

    Image taken from APS website
    An atom released from an optical trap, moving through time in one of two different trajectories, shown above a metal surface for an experiment to measure the Casimir-Polder effect. When detected, the particle will collapse to one port. (Description taken from APS )

    It is generally accepted that atom interferometers can operate because the atoms interfere with themselves. Numerous demonstrations with electrons, neutrons, trapped atoms, and molecules all validate this fundamental concept. However, single particle control in a free-space atom interferometer has in the past seemed intractable. This is due in large part to the experimental challenges associated with single neutral atom trapping, control, and detection. In our work, we use a micron-scale optical tweezer to observe a single cesium atom in a light-pulse atom interferometer experiment where the wavepacket separation is 240 times larger than the coherence length. In doing so, we also introduce a technique to probe forces with high spatial resolution that inherits the absolute accuracy intrinsic to atom interferometry.

    For many applications of atom interferometry, a bulk atom interferometer approach is well suited. However, a significant advantage of implementing an atom interferometer using a single atom in an optical tweezer is that the atom itself can be highly localized in space. Of particular interest at this length scale is the ability to probe, with absolute accuracy, forces that are very near to surfaces such as Casimir-Polder forces as well as hypothetical forces that result in non-relativistic deviations from Newtonian gravitation. We show that our technique is sensitive to forces at the level of 3.2 E-27 N with a spatial resolution at the micron scale.



    Image description: Emergence of the interference fringe. The plots show the cumulative number of Cs atoms (per phase) detected in |F=3> after N independent single atom experiments for (a) N=1, (b) N=2, (c) N=17, and (d) N=813. Note the individual scaling of the vertical axes. To generate the fringe, the phase of the atom is mapped out by scanning the phase of the Raman coupling field after the first beam-splitter pulse.
  • Optically Pumped Atomic Magnetometers

    Magnetoencephalography (MEG) is a clinical technique for measuring the magnetic fields produced by the human brain. As a brain scanning method, MEG gives millisecond temporal resolution and sub-centimeter spatial resolution. However, current technology requires liquid helium, which makes instruments large and expensive, so the availability of MEG is limited. We are developing a small, low-cost MEG system based on optically pumped atomic magnetometers (OPAMs). In addition to small size and low cost, OPAMs offer to possibility of reconfigurable arrays of sensors to allow the accommodation of different head sizes. Since the signal from a dipolar magnetic source decays as 1/r3, where r is the distance from the source to the sensor, positioning the OPAMs as close as possible to the head is critical. Other applications of ultra-sensitive magnetometers range from locating unexploded ordnance and underground structures to the detection of other biomagnetic fields, such as human heart activity.


    Image description: A prototype OPAM sensor has detected MEG signals from auditory stimulation in a magnetically shielded room. The sensor has 4 spatially separated channels. With our current, improved OPAM sensor, we have achieved a sensitivity better than 10 fT/Hz1/2 over a frequency range of 5-100 Hz.


    Image description: MEG signals from auditory stimulation are measured with the prototype OPAM sensor. Pure 1-kHz tones are played in both ears, and approximately 300 trials are averaged and bandpass filtered from 2 to 55 Hz. Noise is cancelled using a signal space projection technique. As expected, a strong M100 brain response is visible near 100 ms.


    Image description: Sandia project leader Peter Schwindt demonstrates the use of the prototype OPAMs for collection of MEG data from a human subject.


    Image description: A cut-away view of a concept drawing showing an OPAM sensor array installed in a human-sized magnetic shield for MEG. This complete MEG system will have 36 magnetometer channels to provide partial head coverage for localizing magnetic sources in the human brain. The magnetic shield is similar in size to a standard MRI tube. The inset shows how the array can be moved to different positions on the head.

    Acknowledgements
    This work is done in collaboration with The Mind Research Network, The Wright State Research Institute, and Candoo Systems.

  • Atomic Clocks

    Atomic clocks are no longer just the room-sized systems used as national time standards. Modern atomic clocks have made it possible for today’s Global Positioning System to be used almost everywhere by anyone with a cellphone. Sandia has helped to extend the reach of these devices in recent years by enabling further miniaturization.
    Sandia provided Vertical Cavity Surface Emitting Lasers (VCSELs) for the development of the Chip Scale Atomic Clock (CSAC). The VCSELs proved to be a critical component for achieving both the reduced size and the reduced power requirements of the clock. This development, funded by DARPA, led to CSAC becoming a commercial product now manufactured by MicroSemi Corp.


    This figure shows the integrated package at the heart of the CSAC.
    Miniature Ytterbium Ion Clock

    Sandia has developed a highly miniaturized trapped-ion clock by probing the 12.6 GHz hyperfine transition in the 171Yb+ ion. The goals of this DARPA-funded project are to develop a clock that consumes 50 mW of power, has a size of 5 cm3, and has a long-term fractional frequency instability of 10-14 at one month. Trapped ion systems are an excellent candidate for such extreme miniaturization because ions are well isolated from the environment independent of the size of the trap. Significant miniaturization has already been demonstrated with the 199Hg+ trapped ion clock developed at the Jet Propulsion Laboratory. New technologies are needed for the development of the miniature Yb ion clock. We have worked to develop low-power-consumption lasers at 369 nm and 935 nm, miniaturized optical systems including a MEMS shutter, custom control electronics, and highly miniaturized vacuum packages with an integrated ion trap and miniaturized Yb sources. The vacuum packages have a titanium body with sapphire windows and contain an RF Paul trap in a linear quadrupole configuration. The trapped ions are buffer gas cooled with a few microtorr of helium, and the vacuum is maintained within the vacuum package with a non-evaporable getter. Electrical signals are passed into the vacuum package using either standard electrical feedthroughs or a high temperature co-fired ceramic (HTCC) substrate with electrical vias to form one wall of the vacuum package. The HTCC substrate is used in our smallest, 1 cm3 vacuum package. The atomic clocks made with the miniature vacuum packages demonstrate a fractional frequency instability of 2 × 10-11/𝝉1/2 and integrate down to the 10-14 stability range with long averaging times.



    Image description: Picture of the ion trap vacuum packages developed for trapping and containing Yb ions for use in an atomic clock.


    Picture of an integrated trapped Yb ion atomic clock. The 3 cm3 vacuum package is combined with integrated lasers and optics, a magnetic shield, C-field coil, a signal detection photomultiplier tube, and custom electronics.


    The 1 cm3 vacuum package in use in an atomic clock after it has been sealed with a copper pinch-off. (Right) The internal components of the 1 cm3 vacuum package.

    Acknowledgements

    We thank our collaborators the Jet Propulsion Laboratory and Microsemi Corp. for contributing to the success of this work.

    This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions, and/or findings contained in this article are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

  • Neutral Atom Entanglement

    Controlling quantum entanglement between parts of a many-body system is the key to unlocking the power of quantum information processing for applications such as quantum computation, high-precision sensing, and simulation of many-body physics. Spin degrees of freedom of ultracold neutral atoms in their ground electronic state provide a natural platform given their long coherence times and our ability to control them with magneto-optical fields, but creating strong coherent coupling between spins has been challenging. We demonstrate for the first time a strong and tunable Rydberg-dressed interaction between spins of individually trapped cesium atoms with energy shifts of order 1 MHz in units of Planck's constant. This interaction leads to a ground-state spin-flip blockade, controlled solely with microwave-frequency fields, whereby simultaneous hyperfine spin flips of two atoms are blockaded due to their mutual interaction. We employ this spin-flip blockade to directly produce Bell-state entanglement between atoms in the long-lived hyperfine clock states.
    Image Description: Entanglement Sequence. To achieve both a strong ground-state atom-atom interaction and high-fidelity signal detection we perform these steps at different interatomic spacings. Two Cs atoms are initially 6.6 microns apart, and held by optical tweezers. After qubit-state preparation, the two trapped atoms translate toward each other in the moving tweezers. At the target distance, the Rydberg-dressing laser at 319 nm and the Raman laser illuminate the two atoms simultaneously. The Rydberg-dressing laser causes the interaction by coupling the qubits to highly sensitive Rydberg states, so sensitive in fact that the presence of another nearby atom can cause a significant energy shift. The Raman laser drives transitions among the interacting qubit states to produce Bell states. During the interaction the atoms are momentarily released from the traps to avoid optical perturbation. The two atoms then translate back to the original positions for state detection.

    Collaboration with University of New Mexico
    We collaborate with Faculty in the University of New Mexico, Department of Physics on topics in quantum information science. This often involves hosting on-site graduate research leading to advanced degrees.

    Acknowledgements
    We thank our collaborators in the group of Prof. Deutsch at the University of New Mexico Department of Physics for contributing to the success of this work.

  • Quantum Communication

    We are exploring the combination of atom chips with microfabricated optical cavities to enable strong atom-photon coupling in a scalable system. We have demonstrated a scalable micro-mirror suitable for atom chip based cavity quantum electrodynamics (cQED) applications (see Figure 1). A very low surface roughness of 2.2 Angstroms rms on the silicon cavity mirrors is achieved using chemical dry etching along with plasma and oxidation smoothing. Our Fabry-Perot cavity comprised of these mirrors currently demonstrates the highest finesse, F = 64,000, using microfabricated mirrors. We compute a single atom cooperativity for our cavities of more than 200, making them promising candidates for detecting individual atoms and for quantum information applications on a chip (see Figure 2). The fabrication process used here is compatible with production techniques for atom chips which allow for intercavity transport and precision positioning of laser cooled neutral atoms. The combination of these technologies may enable devices delivering large arrays of atom-cavity systems for use in quantum information applications. We are currently fabricating a two-layer atom chip with integrated micro-mirrors.

    Figure 1. A scanning electron microscope image of two Si micro-mirrors. The foreground mirror is cleaved along its diameter revealing that the mirror is 9.75 µm deep with an opening diameter of 70.5 µm.


    Figure 2. Calculated single atom cooperativity, C1, based on RoC measurements for 87Rb as a function of cavity length. The solid curve shows C1 given the average value of the Finesse and RoC while the scattered points are the values for each individual measurement.
  • Quantum Information

    Quantum bits, or qubits, are the basic information storage elements of quantum computers, which perform quantum information processing and offer the opportunity to efficiently solve problems that are numerically challenging for classical computers. Quantum computers, therefore, may someday augment conventional classical computers through the application of some of the unusual properties of quantum systems to speed up computation.
  • Solid State Quantum Devices


    Figure 1: (a) scanning electron microscope image of Sandia's dual quantum dot structure fabricated in silicon (the dots suggest single electrons confined by the surrounding gates); (b) schematic cross section of the quantum dot structure showing the position of the single electron locations; and (c) schematic representation of spin manipulation using rotation and precession of two different spins.

    A critical challenge in building a quantum information processing system is the need to couple and manipulate tiny qubits in the form of a quantum circuit that produces a useful function. Sandia researchers are focused on basic questions related to the feasibility of manufacturing a simple qubit and simple quantum circuits – a task that includes demonstrating a silicon qubit, integrating the qubit with classical complementary metal–oxide–semiconductor (CMOS) technology, and designing quantum error correction circuits that are tuned to the physical qubit’s unique properties.

    Sandia’s approach is to physically encode quantum information in the spin state of an electron that is confined in a silicon quantum dot. Although gallium arsenide quantum dots have been demonstrated, quantum dots made from silicon are expected to have longer decoherence times and improved integration with silicon-based classical circuitry. A significant challenge is to engineer the Si qubit and the surrounding electronics all operating at ~ 0.1K (0.1 degrees above absolute zero).

  • Microfabricated Ion Traps

    At Sandia National Laboratories, numerous teams and disciplines are engaged in attempts to move significantly closer to the actualization of quantum information processing. Trapped ions are among the most promising physical realizations of quantum bits or qubits. Sandia is developing micro-traps for the electrostatic confinement (trapping) of ions. Sandia’s work applies the device physics principles and engineering techniques for fabricating microelectromechanical systems (MEMS) and CMOS devices utilizing Sandia’s Microsystems and Engineering Sciences Applications (MESA) microfabrication facilities. For trapped ions to be a suitable platform for quantum information processing, a scalable in principle technique for trap fabrication must be demonstrated. The surface geometry is the most amenable to microfabrication, but it poses challenges related to the low trap depth and difficulty in making a working shuttling junction. To address the low trap depth issue, planar electrode configurations with overhung electrode structures were designed to allow over-coating of the micro-trap electrodes and the chip surface. The micro-traps also include a through-hole to allow for loading of ions and for 3D application of the laser light necessary for the cooling and manipulation of the trapped ions. Sandia has demonstrated a microfabricated surface electrode trap that addresses this first challenge, and given its consistency of fabrication and trap performance, it can be used to create more sophisticated structures with similarly repeatable performance. Sandia has also successfully demonstrated an integrated optical system for collecting the fluorescence from a trapped ion.


    SEM image showing the metal overhang
    from the supporting oxide.

    In developing Sandia’s ion traps, particular emphasis was placed on the design principle of minimizing the line of sight access to the ion from exposed dielectrics, thereby reducing the impact of stray electric charges. To realize this design principle, the top metal layers of these traps (comprising electrodes, their leads, and outside grounded regions) overhang their supporting oxide pillars by 5 μm. The oxide pillars are grown through multiple layers of plasma- enhanced chemical vapor deposition, and are between 9 and 14 μm thick. The overhang allows for vertical deposition of metal on top of the aluminum electrode layer without shorting DC control or radio frequency (RF) electrodes.

    The lateral separation between electrically isolated top metal layers (such as between neighboring electrodes) is set to be 7 microns, and the lateral dimensions of the electrodes can be arbitrarily determined. A hole through the Si substrate of the trap chip runs the entire length of the trapping region to allow for loading of ions from the backside of the trap. This prevents shorting of the trap electrodes by the atoms, which can occur when loading ions from the side. DC rails inside the RF rails allow for additional principle axis rotation and compensation. The back side of the chip is evaporated with gold at a small off-normal angle to coat the exposed vertical edges of the silicon substrate and the platform supporting the electrodes. This prevents charge buildup by pinning the backside of the chip to ground.


    The efficient collection of fluorescence photons from a single ion is an essential ingredient for trapped ion-based quantum information processing. Sandia has successfully demonstrated an integrated optical system for collecting the fluorescence from a trapped ion. The system, consisting of an array of transmissive, dielectric micro-optics and an optical fiber array, has been incorporated into the ion trapping chip without negatively impacting trapping performance. Considerations such as choice of epoxies, vacuum feedthrough, and optical component materials did not degrade the vacuum environment. Additionally, Sandia has demonstrated light detection as well as ion trapping and shuttling behavior similar to trapping chips without integrated optics, with no modification to the control voltages of the trapping chip.


    Completed ion trap with optics integrated.
    Detail of fiber/chip assembly in chamber.

R&D

The Research and Development website is currently under development.

Publications, Patents, and Related Work

Atom Interferometer Publications, Patents, and Related Work

Atomic Magnetometer Publications and Patents

Atomic Clock Publications, Patents, and Related Work

Neutral Atom Entanglement Publications and Related Work

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