Manipulating Quantum Systems An Assessment of Atomic, Molecular, and Optical Physics in the United States (2020) / Chapter Skim
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6 Precision Frontier and Fundamental Nature of the Universe
6 Precision Frontier and Fundamental Nature of the Universe
Pages 181-222
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From page 181... ...
We now live fully in the world of the quantum. The extraordinary advances in control of quantum matter and light discussed in previous chapters have generated transformative tools for precision measurement.
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The fantastic achievement of detecting gravitational waves was also enabled by quantum tech nologies. Moreover, the first prototype of a new type of gravitational wave detec tor based on matter waves has just been approved for construction.
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CRISIS OF MODERN FUNDAMENTAL PHYSICS We live in an exciting time for fundamental physics. All the particles predicted by the so-called Standard Model (see Figure 6.1)
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scale, such as those carried out at the LHC, new physics may very well be observed via low-energy precision measurements based on AMO science. In many theories that have been proposed to go beyond the Standard Model (BSM)
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Here, the committee fo cuses on new prospects in clock development, as these are particularly important for probing fundamental physics questions. Systematic uncertainties of 1 × 10−18 have now been demonstrated with clocks based on both neutral atoms trapped in an optical lattice and on single trapped ions, as shown in Figure 6.2.
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The advances in atomic clocks over the past decade have already enabled improved tests of the constancy of the fine-structure constant and proton to-electron-mass ratio, several orders of magnitude improved tests of Lorentz invariance, and new applications of clocks to DM detection as described in the section later in this chapter, "Searches for New Physics Beyond the Standard Model." Considering that the gravitational redshift is 1 × 10−19 for a 1 mm height change on Earth, it is tantalizing to think of the future scientific possibilities when we improve clock performance to, say, the 1 × 10−21 precision level, when the gravitational redshift on clock atoms separated by a mere 10 μm can be detected. Such capabilities will be invaluable for relativistic geodesy and testing quantum mechanics in curved spacetime.
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From page 187... ...
However, it has been known for several de cades that quantum mechanics in principle allows access to significantly improved performance if a suitably quantum correlated state is used for the sensor. In this case, particles no longer behave as statistically independent entities; rather, the quantum states of particles (e.g., the two atomic states comprising an atomic clock)
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Atom Interferometric Inertial Sensors Atom interferometers exploit the quantum mechanical wave properties of matter to realize sensors and instruments analogous to optical interferometers. Progress in this field has been fueled by development of cold atomic sources, which offer precise control over the velocity, hence wavelength (via the de Broglie rela tion)
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From page 189... ...
Related instruments have been recently used to make new measurements of the fine-structure constant through indirect observation of the momentum recoil of atomic wave-packets induced by photon scattering. Atom interferometric inertial sensors derive a performance advantage com pared to existing state-of-the-art sensors from the fact that atoms are superbly isolated from spurious non-inertial forces, and from the fact that precision laser sources are used to manipulate and control atomic wave-functions.
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If laser light, with its exquisite precision, is to be used as the meter stick for measuring the spacing between the test mass, then the test masses must also double as mirrors. Quantum Engineering for Gravitational Wave Detectors In Chapter 2, the committee introduced squeezed states of light -- specially engineered quantum states where the noise is redistributed between the amplitude and phase properties of the light.
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Earth based detectors like LIGO are sensitive to GWs of frequencies between 10 Hz and 10 kHz, typically radiated by compact objects that have a few times the mass of our Sun. Much heavier objects, such as supermassive black holes at the centers of galaxies, would radiate GWs at much lower frequencies, in the 10 to 100 mHz range.
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To reach this precision, LIGO uses many techniques of precision measurement and quantum optics that were developed as part of the extensive, diverse, and growing AMO science toolkit. The imperative to continue to explore the universe with GWs, searching for new unknown objects, and gaining a deeper understanding of what we already see, is driving exploration and design of a new generation of laser interferometric GW detectors that will be capable of mapping out all merging black hole binaries in the observable universe.
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Following the spectacular success of the Pathfinder mission, which dem onstrated that the spacecraft can be controlled to meet the stringent requirements on acceleration, LISA is now slated for launch in 2034. Matter Wave Interferometry for Future Gravitational Wave Detection Laser interferometry is the most mature technology for GW detection, owing in no small part to decades of intellectual and financial investments.
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In the atom interferometric approach, gravitational radiation is sensed through the gravitational wave-induced phase shifts on the propagation of laser beams between two spatially separated, inertially isolated, laser-cooled atomic ensembles. Momentum recoil associated with the interactions between the laser and atomic ensembles results in the concomitant interference of atomic wave packets.
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As GW detectors get more sensitive, scientists can learn about the properties of these still largely mysterious sources, which in turn can increase understanding of how the universe we see came about, how stars live and die, what is the structure of neutron stars and black holes, how galaxies form, and much more. In addition to astrophysical discoveries, GW detections also serve to teach us more about the fundamental nature of gravity, such as the speed and dispersion of GWs, the mass and spin of the graviton, whether gravitational waves perma nently alter the spacetime of a region through which they have passed, and more.
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In particular, for the scope of this chapter, sensitive measurement of magnetic fields plays a crucial role in the testing of fundamental symmetries. For example, magnetometers are employed in setting limits on the electron or other particle's EDM, on particular forms of violation of Lorentz invariance, and on spin-dependent forces that might be mediated by axions.
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On the other hand, one can work in a shielded environment FIGURE 6.5 This figure shows schematically, for a variety of approaches for the measurement of magnetic fields, the trade-space of two significant metrics -- namely, the resolution at which one can resolve an object versus the sensitivity with which one can detect its magnetic field. The points in the diagram show the approximate required spatial resolution versus needed magnetic sensitivity in order to detect a single proton, a single electron, a biological cell by NMR, a neuron, and to do a (brain)
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One then measures the Larmor precession frequency, which is proportional to the total magnetic field. This allows DC magnetic field measurements with femtotesla-level resolution.
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Nevertheless, many exciting applications are emerging using NV-diamond, including noninvasive sensing and imaging of biomagnetism in living cells and whole animals with submicron resolution; mapping of magnetic materials within primitive meteorites and early Earth rocks with micron resolution, which is already providing advances in the understanding of the formation of the solar system and Earth's geodynamo; and imaging patterns of nanoscale magnetic fields in a wide variety of advanced materials, allowing development of smart materials. Somewhere in between for both sensitivity and spatial resolution are trapped cold atoms and even Bose-Einstein condensates (BECs)
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This breadth of applications and approaches highlights the role of magnetometry as one of the AMO-based tools that enable precision measurement, both as an enabler for studies of fundamental physics and as sensors in applications in other areas of basic and applied science, and for industrial and military use. Some of these are discussed below in the context of searches for BSM physics, and others in Chapter 7 as impacts on other areas.
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They are used for evaluation of blackbody radiation shifts and other systematic uncertainties in optical atomic clocks; design of highly charged ion clocks; analysis of experiments for new physics searches; creation of new state-insensitive cooling and trapping schemes for studies of degenerate quantum gases; and atom and light shift modeling used to understand and control alkaline atoms held with optical tweezers, in optical lattices for quantum simulation, and for many other applications. Moreover, the development of several ab initio relativistic methods of increasing accuracy allows for strategies to accurately estimate uncertainties of theoretical predictions for which no experimental data are available.
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Considering the discovery potential, such collaborations should be encouraged and strengthened, for example with joint research funding. SEARCHES FOR NEW PHYSICS BEYOND THE STANDARD MODEL Searches for Permanent Electric Dipole Moments Remarkably, table-top-scale experiments using methods of AMO physics can be sensitive to new forces and particles, similar to those sought at the largest particle colliders.
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For example, in theories where new particles have masses corre sponding to energies above the TeV scale -- that is, 10 times larger than the mass of the Higgs boson -- the size of predicted EDMs is typically within the limits current AMO experiments could detect. Since this exceeds the scale directly accessible to current and near-future particle colliders, these AMO-based EDM experiments have a high potential for discovering new particles and forces not accessible in any other way.
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The structure of its energy levels makes it possible to easily re verse its internal E-field, not only by reversing the laboratory field that polarizes the molecules but also by preparing different internal quantum states. ThO also hold two electrons with unpaired spins: one feels a particularly large internal E-field while the other serves to cancel the magnetic moment of the first.
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Colored regions are predictions from various theories of particle physics. The standard model of particle theory predicts a value ~9 orders of mag nitude smaller than the current best bound from the ACME experiment.
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Modulate the values of the fundamental constants of nature, inducing changes in atomic transition frequencies and the local gravitational field. As a result, all of the AMO precision tools discussed in this chapter -- atomic clocks, interferometers, and magnetometers -- can be used as DM detectors.
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. The ADMX experiment exploits the strong coupling of the QCD axion to the electromagnetic field in a microwave cavity, to convert axions to micro wave photons in the presence of a strong magnetic field.
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Proposals for broadband detection with LC circuits are based on the axion-photon coupling that effectively modifies Maxwell's equations -- DM axions and ALPs generate an oscillating current density in the presence of a magnetic field. In some new physics models, the initial random distribution of the scalar field in the early universe leads to the formation of domain-wall networks as the universe expands and cools.
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DM permeating our galaxy exhibits coherence, behaving like a wave. Its coupling to the SM particles in atomic clocks leads to oscillations of fundamental constants and, therefore, to oscillations in clock transition frequencies, causing persistent time-varying signals that are localized in frequency determined by the DM mass.
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Remarkable advances in AMO measurement techniques over the past decade, coupled with new ideas emerg ing from theoretical particle physics, have reinvigorated AMO precision searches for forces beyond these four. For example, DM may have new interactions, whose discovery would lead to a much wider range of possible experiments for detection of the DM.
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Dark Matter and New Force Searches with Future Gravitational Wave Detectors Proposed atom interferometric GW detectors are sensitive to new physics that perturbs atomic trajectories or an atom's internal energy levels. For example, DM can lead to time-dependent signals in these detectors, enabling a unique probe of its existence.
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The solid red curve is the SM prediction. The table-top Cs APV result is supplemented with data from large-scale particle physics experiments whose data points are labelled.
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The bound tests encompass a wide variety of simple atoms and molecules, molecular ions, highly charged ions, and exotic atoms such as positronium, antiprotonic He, and so on. Only a few examples are highlighted.
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Precision Tests of Fundamental Interactions and Determination of Fundamental Constants Using Highly Charged Ions The comparison of experimental measurements with SM theory calculations for the magnetic moment or g-factor of the bound electron in hydrogen-like ions allows further tests of QED. In this vein, the most accurate value of the electron mass, with a relative precision at the 10−11 level, is obtained by a comparison of state-of-the-art bound-state QED calculations and precise measurements of the g-factor of the single bound electron in a trapped 12C5+ ion, done at Mainz (see Figure 6.9)
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into the ALPHATRAP Penning-trap setup. This way, unique measurements be come feasible, such as the determination of the isotopic effect in heavy highly charged ions, which gives direct and unobstructed access to nuclear effects, as well as the measurement of specifically weighted g-factor differences of hydrogen- and lithium-like heavy HCI.
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ion clocks improved that result by another factor of 100. Once again, precision measurement tools, specifi cally quantum-information-based sensors, demonstrate the remarkable potential to test fundamental physics postulates.
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The new SI officially went into effect on World Metrology Day, May 20, 2019, which means that now anyone, anywhere can realize the SI units in terms of these values combined with appropriate measurements and equations derived from the laws of nature as we presently understand them. SUMMARY, DISCOVERY POTENTIAL, AND GRAND CHALLENGES Summary The past decade brought forth a plethora of new table-top AMO experiments aimed at discovery of new physics.
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FIGURE 6.3.1 The NIST-4 Kibble balance, where the weight of a test mass is offset by a force produced when an electrical current is run through a coil of wire immersed in a surround ing magnetic field. As of May 20, 2019, it uses the defining fundamental constants that form the foundation of the International System of Units (SI)
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clocks, nitrogen vacancy centers in diamond for magnetic field measurements, atomic interferometers for many types of precision measurements, gravitational wave detectors, and so on.
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6. The discovery of gravitational waves from merging black holes and neutron stars using terrestrial laser interferometers has provided new opportunities for understanding the internal properties and cosmic populations of these objects.
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At the same time, demonstrate promising alternative technologies for the use of atomic sensors to probe gravitational physics, including proof-of-principle large-scale systems for gravitational wave detection. FINDINGS AND RECOMMENDATIONS Finding: Rapid advances in the precision and capabilities of AMO technologies have dramatically increased the potential of AMO-based techniques to discover new physics beyond the Standard Model.
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projects. Recommendation: Funding agencies should establish funding structures for continued support for collaborative efforts of atomic, molecular, and optical theory and experiment with particle physics and other fields, in cluding joint projects, joint summer schools, dedicated annual conferences, and so on.
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