Nuclear clock

Last updated
Nuclear Clock
Nuclear clock concept.png
Concept of a thorium-229 based nuclear optical clock.

A nuclear clock or nuclear optical clock is a notional clock that would use the frequency of a nuclear transition as its reference frequency, [1] in the same manner as an atomic clock uses the frequency of an electronic transition in an atom's shell. Such a clock is expected to be more accurate than the best current atomic clocks by a factor of about 10, with an achievable accuracy approaching the 10−19 level. [2] The only nuclear state suitable for the development of a nuclear clock using existing technology is thorium-229m, a nuclear isomer of thorium-229 and the lowest-energy nuclear isomer known. With an energy of about 8 eV, [3] [4] [5] [6] [7] the corresponding ground-state transition is expected to be in the vacuum ultraviolet wavelength region around 150 nm, which would make it accessible to laser excitation. A comprehensive review can be found in reference. [8]

Contents

Principle of operation

Modern optical atomic clocks are by today the most accurate time-keeping devices. Their underlying principle of operation is based on the fact that the energy of an atomic transition (the energy difference between two atomic states) is independent of space and time. The atomic transition energy corresponds to a particular frequency of a light wave, which is required to drive the transition. Therefore, an atomic transition can be excited with the help of laser light, if the laser frequency is exactly matching the frequency corresponding to the energy of the atomic transition. Thus, in turn, the laser frequency can be stabilized to match the corresponding atomic transition energy by continuous verification of a successful laser excitation of the atomic transition. In case of successful stabilization to an atomic transition, the frequency of the laser light will always be the same (independent of space and time).

It is technologically possible to measure the frequency of laser light to extraordinary high accuracy by counting the oscillations of the light wave with the help of a frequency comb. This allows time to be measured simply by counting the number of oscillations of the laser light, that has been stabilized to a particular atomic transition. Such a device is known as optical atomic clock . [9] One prominent example for an optical atomic clock is the ytterbium (Yb) lattice clock, where a particular transition in the ytterbium-171 isotope is used for laser stabilization. [10] In this case, one second has elapsed after exactly 518295836590864 oscillations of the laser light stabilized to the corresponding transition. Other examples for optical atomic clocks of the highest accuracy are the ytterbium(Yb)-171 single-ion clock, [11] the strontium(Sr)-87 optical lattice clock [12] and the aluminum(Al)-27 single-ion clock. [13] The achieved accuracies of these clocks vary around 10−18, corresponding to about 1 second of inaccuracy in 30 billion years, significantly longer than the age of the universe.

For a nuclear optical clock the principle of operation remains unchanged, however, with the important difference that a nuclear transition instead of an atomic shell transition is used for laser stabilization. [1] The expected advantage of a nuclear clock compared to an atomic clock is that, figuratively speaking, the atomic nucleus is smaller than the atomic shell by up to five orders of magnitude and therefore (due to small magnetic dipole and electric quadrupole moments) significantly less affected by external influences like, e.g., electric and magnetic fields. Such external perturbations are the limiting factor for the achieved accuracies of atomic-shell based clocks. Due to this conceptual advantage, a nuclear optical clock is expected to achieve a time accuracy approaching 10−19, a ten-fold improvement over atomic-shell based clocks. [2]

Different nuclear clock concepts

Two different concepts for nuclear optical clocks have been discussed in the literature: trap-based nuclear clocks and solid-state nuclear clocks.

Trap-based nuclear clocks

For a trap-based nuclear clock either a single 229Th ion is trapped in a Paul trap, known as the single-ion nuclear clock, [1] [2] or a chain of multiple ions is trapped, considered as the multiple-ion nuclear clock. [8] Such clocks are expected to achieve the highest time accuracy, as the ions are to a large extent isolated from its environment. A multiple-ion nuclear clock could have a significant advantage over the single-ion nuclear clock in terms of stability performance.

Solid-state nuclear clocks

As the nucleus is largely unaffected by the atomic shell, it is also intriguing to embed many nuclei into a crystal lattice environment. This concept is known as the crystal-lattice nuclear clock. [1] Due to the high density of embedded nuclei of up to 1018 per cm3, this concept would allow to irradiate a huge amount of nuclei in parallel, thereby drastically increasing the achievable signal-to-noise ratio, [14] however, on the cost of potentially higher external perturbations. [15] It was also proposed to irradiate a metallic 229Th surface and to probe the isomer’s excitation in the internal conversion channel, which is known as the internal-conversion nuclear clock. [16] Both types of solid-state nuclear clocks were shown to offer the potential for comparable performance.

Transition requirements

From the principle of operation of a nuclear optical clock it is evident, that direct laser excitation of a nuclear state is a central requirement for the development of a nuclear clock. Until today no direct nuclear laser excitation has been achieved. The central reason is that the typical energy range of nuclear transitions (keV to MeV) is orders of magnitude above the maximum energy which is accessible with significant intensity by today's narrow-bandwidth laser technology (a few eV). There are only two nuclear excited states known, which possess an extraordinary low excitation energy (below 100 eV). These are 229mTh, a metastable nuclear excited state of the isotope thorium-229 with an excitation energy of only about 8 eV [5] [7] and 235mU, a metastable excited state of Uranium-235 with an energy of 76.7 eV. [17] For nuclear structure reasons, only 229mTh offers a realistic chance for direct nuclear laser excitation.

Further requirements for the development of a nuclear clock are, that the lifetime of the nuclear excited state is relatively long, thereby leading to a resonance of narrow bandwidth (a high quality factor) and that the ground-state nucleus is easily available and sufficiently long-lived to allow to work with moderate quantities of the material. Fortunately, with a radiative lifetime of 103 to 104 seconds of 229mTh [18] [19] and a lifetime of about 7917 years of a 229Th nucleus in its ground state, [20] both conditions are fulfilled for 229mTh, making it an ideal candidate for the development of a nuclear clock.

History

The history of the nuclear clock

A nuclear optical clock based on 229mTh was first proposed in 2003 by E. Peik and C. Tamm, who developed an idea of U. Sterr. [1] The paper contains both concepts, the single-ion nuclear clock, as well as the solid-state nuclear clock.

In their pioneering work, Peik and Tamm proposed to use individual laser-cooled 229Th3+ ions in a Paul trap to perform nuclear laser spectroscopy. [1] Here the 3+ charge state is advantageous, as it possesses a shell structure suitable for direct laser cooling. It was further proposed to excite an electronic shell state, to achieve 'good' quantum numbers of the total system of the shell plus nucleus that will lead to a reduction of the influence induced by external perturbing fields. A central idea is to probe the successful laser excitation of the nuclear state via the hyperfine-structure shift induced into the electronic shell due to the different nuclear spins of ground- and excited state. This method is known as the double-resonance method.

The expected performance of a single-ion nuclear clock was further investigated in 2012 by C. Campbell et al. with the result that a systematic frequency uncertainty (accuracy) of the clock of 1.5·10−19 could be achieved, which would be by about an order of magnitude better than the accuracy achieved by the best optical atomic clocks today. [2] The nuclear clock approach proposed by Campbell et al. slightly differs from the original one proposed by Peik and Tamm. Instead of exciting an electronic shell state in order to obtain the highest insensitivity against external perturbing fields, the nuclear clock proposed by Campbell et al. uses a stretched pair of nuclear hyperfine states in the electronic ground-state configuration, which appears to be advantageous in terms of the achievable quality factor and an improved suppression of the quadratic Zeeman shift.

The solid-state nuclear clock approach was further developed in 2010 by W.G. Rellergert et al. [15] with the result of an expected long-term accuracy of about 2·10−16. Although expected to be less accurate than the single-ion nuclear clock approach due to line-broadening effects and temperature shifts in the crystal lattice environment, this approach may have advantages in terms of compactness, robustness and power consumption. The expected stability performance was investigated by G. Kazakov et al. in 2012. [14] In 2020, the development of an internal conversion nuclear clock was proposed. [16]

Important steps on the road towards a nuclear clock were a precision gamma-ray spectroscopy experiment which allowed to determine the isomeric energy to 7.8±0.5 eV, [3] [4] the successful direct laser cooling of 229Th3+ ions in a Paul trap achieved in 2011, [21] the direct detection of the 229mTh decay in 2016 [22] and a first detection of the isomer-induced hyperfine-structure shift, enabling the double-resonance method to probe a successful nuclear excitation in 2018. [23] In 2019, the isomer’s energy was measured via the detection of internal conversion electrons emitted in its direct ground-state decay to 8.28±0.17 eV. [5] Also a first successful excitation of the 29 keV nuclear excited state of 229Th via synchrotron radiation was reported. [24] In 2020, an energy of 8.10±0.17 eV was obtained from precision gamma-ray spectroscopy. [7]

The history of 229mTh

Since 1976, the 229Th nucleus has been known to possess a low energy excited state, [25] which was constrained to be of below 10 eV excitation energy in 1990 [26] and for which an energy value of 3.5±1.0 eV was determined in 1994. [27] As early as 1996 it was proposed to use the nuclear excitation as a highly stable source of light for metrology by E.V. Tkalya. [28]

At the time of the nuclear clock proposal in 2003 the parameters of 229mTh, in particular its energy, were not known to sufficient precision to allow for nuclear laser spectroscopy of individual thorium ions and thus the development of a nuclear clock. This fact triggered a multitude of experimental efforts to pin down the excited state's parameters like energy and half-life. The detection of light emitted in the direct decay of 229mTh would significantly help to determine its energy to higher precision, however until today all efforts failed to observe a secure signal of light emitted in the decay of 229mTh. [8] The failure of early experiments to observe any direct 229mTh decay signal can partly be explained by a correction of the energy value to 7.6±0.5 eV in 2007 [3] (slightly shifted to 7.8±05 eV in 2009 [4] ). However, also all recent experiments failed to observe any signal of light emitted in the direct decay, potentially pointing towards a strong non-radiative decay channel. [29] [30] [31] [32] In 2012 [33] and again in 2018 [34] the detection of light emitted in the decay of 229mTh was reported, but the observed signals are subject to controversial discussions within the community. [35]

A direct detection of electrons as being emitted in the isomer's internal conversion decay channel was achieved in 2016. [22] This detection laid the foundation for the determination of the 229mTh half-life in neutral, surface-bound atoms in 2017 [36] and a first laser-spectroscopic characterization in 2018. [23] In 2019 an improved energy value based on internal-conversion-electron spectroscopy could be determined. [5] Also, a secure excitation of the isomer via population of the 29 keV state with synchrotron radiation was achieved. [24] More recently, two additional papers about the isomeric energy were published. [6] [7]

Applications

When operational, a nuclear optical clock is expected to be applicable in various fields. Potential applications may arise in the field where already today's atomic clocks are in operation, like e.g., satellite-based navigation or data transfer. However, also potentially new applications may arise in the fields of relativistic geodesy, the search for topological dark matter, [37] or the determination of time-variations of fundamental constants. [38]

Especially a high sensitivity of a nuclear clock for potential time variations of fundamental constants, e.g., the fine-structure constant, has been highlighted. [39] The central idea is that a nuclear transition couples differently to the fine-structure constant than an atomic shell transition does. For this reason a comparison of the frequency of a nuclear clock with an atomic clock could lead to an extraordinary high sensitivity for potential time variations of the fine structure constant. The achievable factor of sensitivity, however, remains subject to speculation. A recent measurement is consistent with enhancement factors between 1 (no enhancement) and 104. [23]

Related Research Articles

<span class="mw-page-title-main">Ionization</span> Process by which atoms or molecules acquire charge by gaining or losing electrons

Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.

<span class="mw-page-title-main">Laser cooling</span> Class of methods for cooling atoms to very low temperatures

Laser cooling includes a number of techniques where atoms, molecules, and small mechanical systems are cooled with laser light. The directed energy of lasers is often associated with heating materials, e.g. laser cutting, so it can be counterintuitive that laser cooling often results in sample temperatures approaching absolute zero. Laser cooling relies on the change in momentum when an object, such as an atom, absorbs and re-emits a photon. For an ensemble of particles, their thermodynamic temperature is proportional to the variance in their velocity. That is, more homogeneous velocities among particles corresponds to a lower temperature. Laser cooling techniques combine atomic spectroscopy with the aforementioned mechanical effect of light to compress the velocity distribution of an ensemble of particles, thereby cooling the particles.

<span class="mw-page-title-main">Nuclear isomer</span> Metastable excited state of a nuclide

A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state (higher energy) levels. "Metastable" describes nuclei whose excited states have half-lives 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10−12 seconds). The term "metastable" is usually restricted to isomers with half-lives of 10−9 seconds or longer. Some references recommend 5 × 10−9 seconds to distinguish the metastable half life from the normal "prompt" gamma-emission half-life. Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the 180m
73Ta
 nuclear isomer survives so long (at least 1015 years) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by 180m
73Ta
 as well as 192m2
77Ir
, 210m
83Bi
, 242m
95Am
 and multiple holmium isomers.

<span class="mw-page-title-main">Polariton</span> Quasiparticles arising from EM wave coupling

In physics, polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation. They are an expression of the common quantum phenomenon known as level repulsion, also known as the avoided crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance. To this extent polaritons can also be thought of as the new normal modes of a given material or structure arising from the strong coupling of the bare modes, which are the photon and the dipolar oscillation. The polariton is a bosonic quasiparticle, and should not be confused with the polaron, which is an electron plus an attached phonon cloud.

<span class="mw-page-title-main">Electromagnetically induced transparency</span>

Electromagnetically induced transparency (EIT) is a coherent optical nonlinearity which renders a medium transparent within a narrow spectral range around an absorption line. Extreme dispersion is also created within this transparency "window" which leads to "slow light", described below. It is in essence a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium.

<span class="mw-page-title-main">Rydberg atom</span> Excited atomic quantum state with high principal quantum number (n)

A Rydberg atom is an excited atom with one or more electrons that have a very high principal quantum number, n. The higher the value of n, the farther the electron is from the nucleus, on average. Rydberg atoms have a number of peculiar properties including an exaggerated response to electric and magnetic fields, long decay periods and electron wavefunctions that approximate, under some conditions, classical orbits of electrons about the nuclei. The core electrons shield the outer electron from the electric field of the nucleus such that, from a distance, the electric potential looks identical to that experienced by the electron in a hydrogen atom.

An atom interferometer is an interferometer which uses the wave character of atoms. Similar to optical interferometers, atom interferometers measure the difference in phase between atomic matter waves along different paths. Today, atomic interference is typically controlled with laser beams. Atom interferometers have many uses in fundamental physics including measurements of the gravitational constant, the fine-structure constant, the universality of free fall, and have been proposed as a method to detect gravitational waves. They also have applied uses as accelerometers, rotation sensors, and gravity gradiometers.

Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is relatively stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of 230Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Darmstadtium (110Ds) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 269Ds in 1994. There are 11 known radioisotopes from 267Ds to 281Ds and 2 or 3 known isomers. The longest-lived isotope is 281Ds with a half-life of 14 seconds.

Livermorium (116Lv) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was 293Lv in 2000. There are five known radioisotopes, with mass numbers 288 and 290–293, as well as a few suggestive indications of a possible heavier isotope 294Lv. The longest-lived known isotope is 293Lv with a half-life of 70 ms.

This page deals with the electron affinity as a property of isolated atoms or molecules. Solid state electron affinities are not listed here.

<span class="mw-page-title-main">Nitrogen-vacancy center</span> Point defect in diamonds

The nitrogen-vacancy center is one of numerous photoluminescent point defects in diamond. Its most explored and useful properties include its spin-dependent photoluminescence, and its relatively long (millisecond) spin coherence at room temperature. The NV center energy levels are modified by magnetic fields, electric fields, temperature, and strain, which allow it to serve as a sensor of a variety of physical phenomena. Its atomic size and spin properties can form the basis for useful quantum sensors. It has also been explored for applications in quantum computing and spintronics.

Within quantum technology, a quantum sensor utilizes properties of quantum mechanics, such as quantum entanglement, quantum interference, and quantum state squeezing, which have optimized precision and beat current limits in sensor technology. The field of quantum sensing deals with the design and engineering of quantum sources and quantum measurements that are able to beat the performance of any classical strategy in a number of technological applications. This can be done with photonic systems or solid state systems.

<span class="mw-page-title-main">Atomic clock</span> Extremely accurate clock

An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, , the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

A trion is a localized excitation which consists of three charged particles. A negative trion consists of two electrons and one hole and a positive trion consists of two holes and one electron. The trion itself is a quasiparticle and is somewhat similar to an exciton, which is a complex of one electron and one hole. The trion has a ground singlet state (spin S = 1/2) and an excited triplet state (S = 3/2). Here singlet and triplet degeneracies originate not from the whole system but from the two identical particles in it. The half-integer spin value distinguishes trions from excitons in many phenomena; for example, energy states of trions, but not excitons, are split in an applied magnetic field. Trion states were predicted theoretically in 1958; they were observed experimentally in 1993 in CdTe/Cd1−xZnxTe quantum wells, and later in various other optically excited semiconductor structures. There are experimental proofs of their existence in nanotubes supported by theoretical studies. Despite numerous reports of experimental trion observations in different semiconductor heterostructures, there are serious concerns on the exact physical nature of the detected complexes. The originally foreseen 'true' trion particle has a delocalized wavefunction (at least at the scales of several Bohr radii) while recent studies reveal significant binding from charged impurities in real semiconductor quantum wells.

<span class="mw-page-title-main">Hughes–Drever experiment</span>

Hughes–Drever experiments are spectroscopic tests of the isotropy of mass and space. Although originally conceived of as a test of Mach's principle, they are now understood to be an important test of Lorentz invariance. As in Michelson–Morley experiments, the existence of a preferred frame of reference or other deviations from Lorentz invariance can be tested, which also affects the validity of the equivalence principle. Thus these experiments concern fundamental aspects of both special and general relativity. Unlike Michelson–Morley type experiments, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of protons, neutrons, and electrons. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity .

<span class="mw-page-title-main">Modern searches for Lorentz violation</span> Overview about the modern searches for Lorentz violation

Modern searches for Lorentz violation are scientific studies that look for deviations from Lorentz invariance or symmetry, a set of fundamental frameworks that underpin modern science and fundamental physics in particular. These studies try to determine whether violations or exceptions might exist for well-known physical laws such as special relativity and CPT symmetry, as predicted by some variations of quantum gravity, string theory, and some alternatives to general relativity.

<span class="mw-page-title-main">Time crystal</span> Structure that repeats in time; a novel type or phase of non-equilibrium matter

In condensed matter physics, a time crystal is a quantum system of particles whose lowest-energy state is one in which the particles are in repetitive motion. The system cannot lose energy to the environment and come to rest because it is already in its quantum ground state. Because of this, the motion of the particles does not really represent kinetic energy like other motion; it has "motion without energy". Time crystals were first proposed theoretically by Frank Wilczek in 2012 as a time-based analogue to common crystals – whereas the atoms in crystals are arranged periodically in space, the atoms in a time crystal are arranged periodically in both space and time. Several different groups have demonstrated matter with stable periodic evolution in systems that are periodically driven. In terms of practical use, time crystals may one day be used as quantum computer memory.

Double ionization is a process of formation of doubly charged ions when laser radiation is exerted on neutral atoms or molecules. Double ionization is usually less probable than single-electron ionization. Two types of double ionization are distinguished: sequential and non-sequential.

<span class="mw-page-title-main">Patrick Gill (scientist)</span> British physicist

Patrick Gill is a Senior NPL Fellow in Time & Frequency at the National Physical Laboratory (NPL) in the UK.

References

  1. 1 2 3 4 5 6 E. Peik; Chr. Tamm (2003). "Nuclear laser spectroscopy of the 3.5 eV transition in 229Th" (PDF). Europhysics Letters. 61 (2): 181–186. Bibcode:2003EL.....61..181P. doi:10.1209/epl/i2003-00210-x. S2CID   250818523. Archived from the original (PDF) on 2013-12-16. Retrieved 2019-03-17.
  2. 1 2 3 4 C. Campbell; et al. (2012). "A single ion nuclear clock for metrology at the 19th decimal place". Phys. Rev. Lett. 108 (12): 120802. arXiv: 1110.2490 . Bibcode:2012PhRvL.108l0802C. doi:10.1103/PhysRevLett.108.120802. PMID   22540568. S2CID   40863227.
  3. 1 2 3 B.R. Beck; et al. (2007). "Energy splitting in the ground state doublet in the nucleus 229Th". Phys. Rev. Lett. 98 (14): 142501. Bibcode:2007PhRvL..98n2501B. doi:10.1103/PhysRevLett.98.142501. PMID   17501268.
  4. 1 2 3 B.R. Beck; et al. (2009). Improved value for the energy splitting of the ground-state doublet in the nucleus 229Th (PDF). 12th Int. Conf. on Nuclear Reaction Mechanisms. Varenna, Italy. LLNL-PROC-415170. Archived from the original (PDF) on 2017-01-27. Retrieved 2019-03-17.
  5. 1 2 3 4 B. Seiferle; et al. (2019). "Energy of the 229Th nuclear clock transition". Nature . 573 (7773): 243–246. arXiv: 1905.06308 . Bibcode:2019Natur.573..243S. doi:10.1038/s41586-019-1533-4. PMID   31511684. S2CID   155090121.
  6. 1 2 A. Yamaguchi; et al. (2019). "Energy of the 229Th nuclear clock isomer determined by absolute gamma-ray energy difference". Phys. Rev. Lett. 123 (22): 222501. arXiv: 1912.05395 . Bibcode:2019PhRvL.123v2501Y. doi:10.1103/PhysRevLett.123.222501. PMID   31868403. S2CID   209202193.
  7. 1 2 3 4 T. Sikorsky; et al. (2020). "Measurement of the 229Th isomer energy with a magnetic micro-calorimeter". Phys. Rev. Lett. 125 (14): 142503. arXiv: 2005.13340 . Bibcode:2020PhRvL.125n2503S. doi:10.1103/PhysRevLett.125.142503. PMID   33064540. S2CID   218900580.
  8. 1 2 3 L. von der Wense; B. Seiferle (2020). "The 229Th isomer: prospects for a nuclear optical clock". Eur. Phys. J. A. 56 (11): 277. arXiv: 2009.13633 . Bibcode:2020EPJA...56..277V. doi:10.1140/epja/s10050-020-00263-0. S2CID   221995928.
  9. ↑ A.D. Ludlow; et al. (2015). "Optical atomic clocks". Rev. Mod. Phys. 87 (2): 637–699. arXiv: 1407.3493 . Bibcode:2015RvMP...87..637L. doi:10.1103/RevModPhys.87.637. S2CID   119116973.
  10. ↑ W.F. McGrew; et al. (2018). "Atomic clock performance enabling geodesy below the centimetre level". Nature. 564 (7734): 87–90. arXiv: 1807.11282 . Bibcode:2018Natur.564...87M. doi:10.1038/s41586-018-0738-2. PMID   30487601. S2CID   53803712.
  11. ↑ N. Huntemann; et al. (2016). "Single-ion atomic clock with 3·10−18 systematic uncertainty". Phys. Rev. Lett. 116 (6): 063001. arXiv: 1602.03908 . Bibcode:2016PhRvL.116f3001H. doi:10.1103/PhysRevLett.116.063001. PMID   26918984. S2CID   19870627.
  12. ↑ T.L. Nicholson; et al. (2015). "Systematic evaluation of an atomic clock at 2·10−18 total uncertainty". Nature Communications. 6: 6896. arXiv: 1412.8261 . doi:10.1038/ncomms7896. PMC   4411304 . PMID   25898253.
  13. ↑ S.M. Brewer; et al. (2019). "An 27Al+ quantum-logic clock with systematic uncertainty below 10−18". Phys. Rev. Lett. 123 (3): 033201. arXiv: 1902.07694 . doi:10.1103/PhysRevLett.123.033201. PMID   31386450. S2CID   119075546.
  14. 1 2 G.A. Kazakov; et al. (2012). "Performance of a 229 Thorium solid-state nuclear clock". New Journal of Physics. 14 (8): 083019. arXiv: 1204.3268 . Bibcode:2012NJPh...14h3019K. doi:10.1088/1367-2630/14/8/083019. S2CID   118341064.
  15. 1 2 W.G. Rellergert; et al. (2010). "Constraining the evolution of the fundamental constants with a solid-state optical frequency reference based on the 229Th nucleus" (PDF). Phys. Rev. Lett. 104 (20): 200802. doi:10.1103/PhysRevLett.104.200802. PMID   20867019.
  16. 1 2 L. von der Wense; C. Zhang (2020). "Concepts for direct frequency-comb spectroscopy of 229mTh and an internal-conversion-based solid-state nuclear clock". Eur. Phys. J. D. 74 (7): 146. arXiv: 1905.08060 . Bibcode:2020EPJD...74..146V. doi:10.1140/epjd/e2020-100582-5. S2CID   209322360.
  17. ↑ F. Ponce; et al. (2018). "Accurate measurement of the first excited nuclear state in 235U". Phys. Rev. C. 97 (5): 054310. Bibcode:2018PhRvC..97e4310P. doi: 10.1103/PhysRevC.97.054310 .
  18. ↑ E.V. Tkalya; et al. (2015). "Radiative lifetime and energy of the low-energy isomeric level in 229Th". Phys. Rev. C. 92 (5): 054324. arXiv: 1509.09101 . Bibcode:2015PhRvC..92e4324T. doi:10.1103/PhysRevC.92.054324. S2CID   118374372.
  19. ↑ N. Minkov; A. Pálffy (2017). "Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in 229mTh". Phys. Rev. Lett. 118 (21): 212501. arXiv: 1704.07919 . Bibcode:2017PhRvL.118u2501M. doi:10.1103/PhysRevLett.118.212501. PMID   28598657. S2CID   40694257.
  20. ↑ Z. Varga; A. Nicholl; K. Mayer (2014). "Determination of the 229Th half-life". Phys. Rev. C . 89: 064310. doi: 10.1103/PhysRevC.89.064310 .
  21. ↑ C.J. Campbell; A.G. Radnaev; A. Kuzmich (2011). "Wigner crystals of 229Th for optical excitation of the nuclear isomer". Phys. Rev. Lett. 106 (22): 223001. arXiv: 1110.2339 . doi:10.1103/PhysRevLett.106.223001. PMID   21702597. S2CID   20801170.
  22. 1 2 L. von der Wense; et al. (2016). "Direct detection of the 229Th nuclear clock transition". Nature . 533 (7601): 47–51. arXiv: 1710.11398 . Bibcode:2016Natur.533...47V. doi:10.1038/nature17669. PMID   27147026. S2CID   205248786.
  23. 1 2 3 J. Thielking; et al. (2018). "Laser spectroscopic characterization of the nuclear-clock isomer 229mTh". Nature. 556 (7701): 321–325. arXiv: 1709.05325 . Bibcode:2018Natur.556..321T. doi:10.1038/s41586-018-0011-8. PMID   29670266. S2CID   4990345.
  24. 1 2 T. Masuda; et al. (2019). "X-ray pumping of the 229Th nuclear clock isomer". Nature . 573 (7773): 238–242. arXiv: 1902.04823 . Bibcode:2019Natur.573..238M. doi:10.1038/s41586-019-1542-3. PMID   31511686. S2CID   119083861.
  25. ↑ L.A. Kroger; C.W. Reich (1976). "Features of the low energy level scheme of 229Th as observed in the α decay of 233U". Nucl. Phys. A. 259 (1): 29–60. Bibcode:1976NuPhA.259...29K. doi:10.1016/0375-9474(76)90494-2.
  26. ↑ C.W. Reich; R.G. Helmer (1990). "Energy separation of the doublet of intrinsic states at the ground state of 229Th". Phys. Rev. Lett. American Physical Society. 64 (3): 271–273. Bibcode:1990PhRvL..64..271R. doi:10.1103/PhysRevLett.64.271. PMID   10041937.
  27. ↑ R.G. Helmer; C.W. Reich (1994). "An Excited State of 229Th at 3.5 eV". Physical Review C . 49 (4): 1845–1858. Bibcode:1994PhRvC..49.1845H. doi:10.1103/PhysRevC.49.1845. PMID   9969412.
  28. ↑ E.V. Tkalya; et al. (1996). "Processes of the nuclear isomer 229mTh(3/2+, 3.5±1.0 eV) Resonant excitation by optical photons". Physica Scripta. 53 (3): 296–299. Bibcode:1996PhyS...53..296T. doi:10.1088/0031-8949/53/3/003. S2CID   250744766.
  29. ↑ J. Jeet; et al. (2015). "Results of a Direct Search Using Synchrotron Radiation for the Low-Energy". Physical Review Letters. 114 (25): 253001. arXiv: 1502.02189 . Bibcode:2015PhRvL.114y3001J. doi:10.1103/physrevlett.114.253001. PMID   26197124. S2CID   1322253.
  30. ↑ A. Yamaguchi; et al. (2015). "Experimental search for the low-energy nuclear transition in 229 Th with undulator radiation". New Journal of Physics. 17 (5): 053053. Bibcode:2015NJPh...17e3053Y. doi: 10.1088/1367-2630/17/5/053053 . ISSN   1367-2630.
  31. ↑ L. von der Wense (2018). On the direct detection of 229mTh (PDF). Springer Theses, Berlin. ISBN   978-3-319-70461-6.
  32. ↑ S. Stellmer; et al. (2018). "Attempt to optically excite the nuclear isomer in 229Th". Phys. Rev. A. 97 (6): 062506. arXiv: 1803.09294 . Bibcode:2018PhRvA..97f2506S. doi:10.1103/PhysRevA.97.062506. S2CID   4946329.
  33. ↑ X. Zhao; et al. (2012). "Observation of the Deexcitation of the 229mTh Nuclear Isomer". Physical Review Letters. 109 (16): 160801. Bibcode:2012PhRvL.109p0801Z. doi: 10.1103/PhysRevLett.109.160801 . ISSN   0031-9007. PMID   23215066.
  34. ↑ P.V. Borisyuk; et al. (2018). "Excitation of 229Th nuclei in laser plasma: the energy and half-life of the low-lying isomeric state". arXiv: 1804.00299 [nucl-th].
  35. ↑ E. Peik; K. Zimmermann (2013). "Comment on "Observation of the Deexcitation of the 229mTh Nuclear Isomer"". Physical Review Letters. 111 (1): 018901. Bibcode:2013PhRvL.111a8901P. doi:10.1103/PhysRevLett.111.018901. PMID   23863029.
  36. ↑ B. Seiferle; L. von der Wense; P.G. Thirolf (2017). "Lifetime measurement of the 229Th nuclear isomer". Phys. Rev. Lett. 118 (4): 042501. arXiv: 1801.05205 . Bibcode:2017PhRvL.118d2501S. doi:10.1103/PhysRevLett.118.042501. PMID   28186791. S2CID   37518294.
  37. ↑ A. Derevianko; M. Pospelov (2014). "Hunting for topological dark matter with atomic clocks". Nature Physics. 10 (12): 933–936. arXiv: 1311.1244 . Bibcode:2014NatPh..10..933D. doi:10.1038/nphys3137. S2CID   53630878.
  38. ↑ P.G. Thirolf; B. Seiferle; L. von der Wense (2019). "Improving Our Knowledge on the 229mThorium Isomer: Toward a Test Bench for Time Variations of Fundamental Constants". Annalen der Physik. 531 (5): 1800381. Bibcode:2019AnP...53100381T. doi:10.1002/andp.201800381.
  39. ↑ V.V. Flambaum (2006). "Enhanced Effect of Temporal Variation of the Fine Structure Constant and the Strong Interaction in 229mTh". Phys. Rev. Lett. 97 (9): 092502. arXiv: physics/0601034 . doi:10.1103/PhysRevLett.97.092502. PMID   17026357. S2CID   4109230.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.