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Putting the Squeeze on Axions Karl Van Bibber, Konrad Lehnert, and Aaron Chou Putting the squeeze on axions Karl van Bibber, Konrad Lehnert, and Aaron Chou Citation: Physics Today 72, 6, 48 (2019); doi: 10.1063/PT.3.4227 View online: https://doi.org/10.1063/PT.3.4227 View Table of Contents: https://physicstoday.scitation.org/toc/pto/72/6 Published by the American Institute of Physics ARTICLES YOU MAY BE INTERESTED IN When condensed-matter physics became king Physics Today 72, 30 (2019); https://doi.org/10.1063/PT.3.4110 Helium users are at the mercy of suppliers Physics Today 72, 26 (2019); https://doi.org/10.1063/PT.3.4181 Paul Dirac and the Nobel Prize in Physics Physics Today 72, 46 (2019); https://doi.org/10.1063/PT.3.4342 Commentary: Basic research in a time of crisis Physics Today 72, 10 (2019); https://doi.org/10.1063/PT.3.4194 Ernest Lawrence’s brilliant failure Physics Today 72, 32 (2019); https://doi.org/10.1063/PT.3.4162 John Wheeler’s H-bomb blues Physics Today 72, 42 (2019); https://doi.org/10.1063/PT.3.4364 Karl van Bibber is a professor of nuclear engineering and associate dean for research of the College of Engineering at the University of California, Berkeley. Konrad Lehnert is a professor of physics and JILA Fellow at the University of Colorado, Boulder, and the National Institute of Science and Technology. Aaron Chou is a senior scientist at the Fermi National Accelerator Laboratory in Batavia, Illinois. Putting the squeeze ON AXIONS Karl van Bibber, Konrad Lehnert, and Aaron Chou Microwave cavity experiments make a quantum leap in the search for the dark matter of the universe. ixty years ago Norman Ramsey and collaborators 1 part in 10 billion—just by dumb luck. asserted that the neutron’s electric dipole moment Or not? In 1977 Stanford University physicists Roberto Peccei and Helen (EDM)—a measure of the separation of its positive Quinn conceived a minimal and ap- and negative electric charge—was consistent with pealing theory by which θ would be S promoted to a dynamical variable. Just 1 zero. More precisely, their experiment bounded below some large energy scale, θ the neutron’s EDM at less than 5 × 10−20 e·cm. Today, that limit is would assume a random value at each 3×10−26 e·cm, and experiments under development may push it point in space. But in the low-energy limit of the theory, the nontrivial “wash- lower by a factor of 100. board” potential of the QCD vacuum would drive θ to the CP-conserving In the parlance of fundamental symmetries, the strong in- minimum. That would be nice and tidy. teraction is seemingly protected from CP-violating effects, where However, within a few months, Steven Weinberg and Frank CP is the product operator of charge conjugation C and parity Wilczek independently realized that the remnant sloshing of P. In the 1950s theorists had no compelling reason to expect CP the θ field around that minimum implied the existence of an violation—indeed, Tsung-Dao Lee and Chen Ning Yang did elementary particle called the axion—the smoking gun of Pec- not believe that a nonzero neutron EDM would ever be found, cei and Quinn’s theory—whose mass is possibly a trillion times though it was a worthy experimental question. However, with lighter than an electron. Even such a light particle could, if suf- the advent of quantum chromodynamics (QCD) in the 1970s, ficiently abundant, constitute the 27% of the mass–energy of a major problem loomed: The theory unavoidably includes a the universe that is dark mater. CP-violating angle θ associated with topological configurations The story of the axion and its connection to dark mater is of the QCD gluon field. For any generic value of θ, the neutron delightfully told with deep physical intuition in the form of a EDM should be a whopping 10−16 e·cm. The value implied by fable in which graduate students play snooker on Mars (see the experiment is highly improbable: It’s as if you spun a roulete article by Pierre Sikivie, PHYSICS TODAY, December 1996, page 22). wheel, and it came to rest at the winning number to within In the current article, we focus on the ongoing experimental 48 PHYSICS TODAY | JUNE 2019 Axion receiver for the HAYSTAC experiment. JUNE 2019 | PHYSICS TODAY 49 AXIONS Preamplifier hunt for the axion more than two decades later. To date, the most sensitive Δff/~10−6 searches rely on the fact that the Magnet Cavity Magnet axion couples to two photons. However, one may represent an POWER external magnetic field as a sea of fmch2 virtual photons, and as Pierre =/a Sikivie realized in 1983, a massive γ FREQUENCY axion can be converted into a sin- FIGURE 1. THE SEARCH FOR AXIONS, gle real photon in an external a in principle magnetic field with the same total B and practice. In this schematic, an axion can scatter from a virtual photon associated with the magnetic energy. What’s more, the axion– γ* field B and produce in the cavity (green) a single photon conversion can be reso- real photon that can be read out and amplified. The nantly enhanced in a high-Q cav- resulting power spectrum illustrates how the signal ity.2 (See the article by van Bibber × would appear after the conversion of axions into and Leslie Rosenberg, PHYSICS m f photons; a is the mass of an axion and is its TODAY, August 2006, page 30.) frequency. Listening to the radio Sikivie’s proposed scheme was simplicity itself, and the exper- below, where T is the physical temperature) and the noise ac- iments of today closely resemble larger and more technologi- cruing from the amplifier, TA: cally sophisticated incarnations of his first experiments 30 years kT hν 1 1 kT ago. The new experiments boast a state-of-the-art low-noise BN=++.h/kTν BA ()e−1B 2 amplifier that is coupled to a tunable microwave cavity inserted in the bore of a powerful superconducting solenoid magnet,3 Half of the irreducible single quantum of noise hν comes from as shown in figure 1. the vacuum fluctuations of the cavity, even at zero temperature, The cavity is tuned in small steps. At each frequency, the re- and half comes from the linear amplifier itself. A convenient searchers pause for several minutes and listen for the signal— benchmark to keep in mind is that kBTSQL ≈ 50 mK at 1 GHz. At an excess of power over the noise floor—if the resonance con- high temperatures, kBT >> hν and TN ≈ T + TA. 2 dition is fulfilled, hν = mac . Here, ma is the axion mass, ν the The first microwave cavity experiments in the late 1980s at cavity frequency, and h Planck’s constant, with 1 GHz corre- Brookhaven National Laboratory and the University of Florida sponding to an axion mass of roughly 4 μeV. Think of the ex- used transistor-based amplifiers (heterojunction field-effect periment as a revved-up version of your car’s radio receiver. transistors that operated with system noise temperatures typ- The search strategy is dictated by the Dicke radiometer ically 5–20 K), some 200 times the standard quantum limit, TSQL, equation, over the 1–3 GHz frequency range. In the mid 1990s, the first S P t large-scale experiment, the Axion Dark Mater Experiment s (ADMX), began taking data. It used the best broadband ampli- N = kT · ν , BN Δ fiers of its time; based on high-electron-mobility transistors, those amplifiers steadily improved the noise temperature to where S/N is the signal-to-noise ratio, P is the signal power, k S B about 100 TSQL at subgigahertz frequencies. is Boltzmann’s constant, and TN is the system noise tempera- By virtue of its large volume cavity, ADMX had scanned a ture. The factors under the square root are the integration time significant range in mass at one of two representative axion– t at each step and the bandwidth of the axion signal Δν. photon couplings—corresponding to one variant of the KSVZ Although deceptively simple in concept, the experiment is (named for Jihn Eui Kim, Mikhail Shifman, Arkady Vainshtein, one of the most daunting endeavors in physics today. Three and Valentin Zakharov) family of models regarded by the axion factors complicate it. First, one must scan a range of axion community as a useful experimental goalpost. But delving much masses over at least three decades. Because the search is nar- deeper into the model space seemed out of reach. Furthermore, rowband, each decade must be covered by dint of many mil- the scanning rate was unacceptably slow. Unless much beter lions of tiny steps. Second, even for the most favorable axion– amplifiers were devised, the search for dark-mater axions photon couplings predicted, and in the largest such experiment, seemed headed for an abrupt dead end. the anticipated signal power is measured in units of yoctowats, a trillionth of a trillionth of a wat. Third—and herein lies the Closing in on the quantum real rub—unless one gets clever, a fundamental, irreducible The axion experiment unexpectedly gained a new lease on life noise floor set by quantum mechanics prevails for standard with a chance conversation during a 1994 workshop at the Uni- linear amplifiers. versity of California, Berkeley. Speaking to Leslie Rosenberg Known as the standard quantum limit (SQL), the noise floor and one of us (van Bibber), then ADMX spokespersons, John is expressed in terms of a temperature as kBTSQL = hν. More Clarke ventured that he could make gigahertz amplifiers based precisely, the system noise consists of a sum of two com - on superconducting quantum interference devices (SQUIDs).
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