Cosmology: 5 Needed Breakthroughs -- Robert Brandenberger

[Today's guest in our ongoing feature '5 Breakthroughs' is Robert Brandenberger, Canada Research Chair and Professor of Physics of McGill University, where he taught and conducted research since 2004. Before that he was a professor at Brown University for about 18 years.

In his long career spanning about a quarter of a century (He received his PhD from Harvard University in 1983; Thesis "Topics in Quantum Field Theory and Cosmology'), Prof. Brandenberger made crucial contributions in various important subfields of cosmology (link to a list of publications).

His current research interests cover a wide spectrum of topics in cosmology and related fields and include
(A) Conceptual problems in inflationary universe cosmology, in particular, trans-Planckian problem for cosmology,
(B) Theory of cosmological perturbations, in particular, back reaction problems, evolution of perturbations in nonsingular cosmologies, and parametric amplification of fluctuations during reheating,
(C) Superstring cosmology, in particular, string gas cosmology and structure formation, mechanisms for obtaining inflation from string theory, resolution of cosmological singularities in string theory, dualities and brane gases in the early universe,
(D) Topological defects in cosmology, in particular, topological defects and Baryogenesis, topological defects and direct signatures, stabilization of embedded defects by plasma effects,
(E) Nonequilibrium processes, in particular, parametric resonance during reheating in inflationary cosmology, nonequilibrium production of topological defects,
(F) Particle-Astrophysics, in particular, constraining physics beyond the Standard Model using cosmology, new mechanisms for CP violation and Baryogenesis,
(G) Large-scale structure, in particular, use of topological statistics to analyze large-scale redshift surveys, studies of weak gravitational lensing maps using new statistics.
(H) Formation of structure in the early universe, in particular, coupling of adiabatic and entropy fluctuations in multi-field, and cosmological models.

In March, 2008 issue of 'Physics Today', Prof. Brandenberger presented an excellent account of current status of inflationary cosmology in his article 'Alternatives to cosmological inflation' (article link here).

Prof. Brandenberger is a Fellow of the American Physical Society. He was an Alfred P. Sloan Research Fellow in years 1988-1992 and received the Outstanding Junior Investigator award of Department of Energy in years 1988-1991.

It gives us great pleasure to present this list of 5 most important breakthroughs that Prof. Brandenberger would like to see in Cosmology.
-- 2Physics.com ]

Breakthrough 1:
Solution of the (old) cosmological constant problem: why is the cosmological constant not given by the cut off scale of relativistic quantum field theory?

Breakthrough 2:
Solution of the new cosmological constant problem: why is there an apparent cosmological constant which is beginning to dominate the evolution of the universe at the current cosmological epoch?

Breakthrough 3:
Resolution of the cosmological singularity: without resolving the cosmological singularity a cosmological model will always be incomplete. Standard Big Bang cosmology had to be replaced by a new early universe cosmology because of this problem. The current paradigm, scalar field-driven inflationary cosmology still suffers from this problem and is therefore incomplete.

Breakthrough 4:
Non-perturbative understanding of superstring theory: will lead to a new cosmological model of the very early universe which will either yield a convincing realization of inflationary cosmology or else to an alternative model.

Breakthrough 5:
An observational discovery of a cosmic superstring: this will cement the link between string theory and cosmology and will also lead to a new theory of the very early universe.

Collaboration Helps Make JILA Strontium Atomic Clock Surpass Accuracy of NIST-F1 Fountain Clock

Jun Ye

A next-generation atomic clock that tops previous records for accuracy in clocks based on neutral atoms has been demonstrated by physicists at JILA, a joint institute of the Commerce Department's National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder. The new clock, based on thousands of strontium atoms trapped in grids of laser light, surpasses the accuracy of the current U.S. time standard based on a “fountain” of cesium atoms.

JILA’s experimental strontium clock, described in the Feb. 14 issue of Science Express and the Mar. 28 issue of Science, is now the world’s most accurate atomic clock based on neutral atoms, more than twice as accurate as the NIST-F1 standard cesium clock located just down the road at the NIST campus in Boulder.

The JILA strontium clock would neither gain nor lose a second in more than 200 million years, compared to NIST F-1’s current accuracy of over 80 million years.

The advance was made possible by Boulder’s critical mass of state-of-the-art timekeeping equipment and expertise. The JILA strontium clock was evaluated by remotely comparing it to a third NIST atomic clock, an experimental model based on neutral calcium atoms. The best clocks can be precisely evaluated by comparing them to other nearby clocks with similar performance; very long-distance signal transfer, such as by satellite, is too unstable for practical, reliable comparisons of the new generation of clocks. In the latest experiment, signals from the two clocks were compared via a 3.5-kilometer underground fiber-optic cable.

The strontium and calcium clocks rely on the use of optical light, which has higher frequencies than the microwaves used in NIST-F1. Because the frequencies are higher, the clocks divide time into smaller units, offering record precision. Laboratories around the world are developing optical clocks based on a variety of different designs and atoms; it is not yet clear which design will emerge as the best and be chosen as the next international standard. The work reported in Science is the first optical atomic clock comparison over kilometer-scale urban distances, an important step for worldwide development of future standards.

“This is our first comparison to another optical atomic clock,” says NIST/JILA Fellow Jun Ye (Link to Jun Ye Group), who leads the strontium project. “As of now, Boulder is in a very unique position. We have all the ingredients, including multiple optical clocks and the fiber-optic link, working so well. Without a single one of these components, these measurements would not be possible. It’s all coming together at this moment in time.”

NIST and JILA are home to optical clocks based on a variety of atoms, including strontium, calcium, mercury, aluminum, and ytterbium, each offering different advantages. Ye now plans to compare strontium to the world’s most accurate clock, NIST’s experimental design based on a single mercury ion (charged atom). The mercury ion clock (See our past posting). was accurate to about 1 second in 400 million years in 2006 and performs even better today, according to Jim Bergquist, the NIST physicist who built the clock. The “best” status in atomic clocks is a moving target.

The development and testing of a new generation of optical atomic clocks is important because highly precise clocks are used to synchronize telecom networks and deep-space communications, as well as for navigation and positioning. The race to build even better clocks is expected to lead to new types of gravity sensors, as well as new tests of fundamental physical laws to increase understanding of the universe. Because Ye’s group is able to measure and control interactions among so many atoms with such exquisite precision, the JILA work also is expected to lead to new scientific tools for quantum simulations that will help scientists better understand how matter and light behave under the strange rules governing the nanoworld.

In the JILA clock, a few thousand atoms of the alkaline-earth metal strontium are held in a column of about 100 pancake-shaped traps called an “optical lattice.” The lattice is formed by standing waves of intense near-infrared laser light. Forming a sort of artificial crystal of light, the lattice constrains atom motion and reduces systematic errors that occur in clocks that use moving balls of atoms, such as NIST-F1. Using thousands of atoms at once also produces stronger signals and eventually may yield more precise results than clocks relying on a single ion, such as mercury. JILA scientists detect strontium’s “ticks” (430 trillion per second) by bathing the atoms in very stable red laser light at the exact frequency that prompts jumps between two electronic energy levels. The JILA team recently improved the clock by achieving much better control of the atoms. For example, they can now cancel out the atoms’ internal sensitivity to external magnetic fields, which otherwise degrade clock accuracy. They also characterized more precisely the effects of confining atoms in the lattice.

Image caption: JILA Strontium Optical Atomic Clock: Keeping time with neutral atoms at the lowest uncertainty. After laser cooled to microKelvin temperatures, an optical lattice formed by a standing wave infrared laser beam traps the ultracold atoms in pancake-shaped traps. A highly coherent clock laser probes the atomic resonance and the result of the atom-light interaction is read out by state-sensitive strong fluorescence signals. This information is used to control the clock laser frequency, and a phase-coherent optical frequency comb distributes the clock signal to the radio frequency domain.

The NIST calcium clock, which was used to evaluate the performance of the new strontium clock, relies on the ticking of clouds of millions of calcium atoms. This clock offers high stability for short times, relatively compact size and simplicity of operation. NIST scientists believe it could be made portable and perhaps transported to other institutions for evaluations of other optical atomic clocks. JILA scientists were able to take advantage of the calcium clock's good short-term stability by making fast measurements of one property in the strontium clock and then quickly switching to a different property to start the comparison over again.

The JILA-NIST collaborations benefit both institutions by enabling scientists not only to compare and measure clock performance, but also to share tools and expertise. Another key element to the latest comparison was the use of two custom-made frequency combs, the most accurate tool for measuring optical frequencies, which helped to maintain stability during signal transfer between the two institutions. (For background, visit NIST frequency combs page).

Reference
"Sr lattice clock at 1x10^-16 fractional uncertainty by remote optical evaluation with a Ca clock"
A.D. Ludlow, T. Zelevinsky, G.K. Campbell, S. Blatt, M.M. Boyd, M.H.G. de Miranda, M.J. Martin, J. W. Thomsen, S.M. Foreman, J. Ye, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, Y. Le Coq, Z.W. Barber, N. Poli, N.D. Lemke, K.M. Beck, C. W. Oates.
Science Vol. 319, p. 1805-1808, (28 March 2008). Abstract Link

This report is based on a press release put together by Laura Ost in the public affairs office at NIST. The JILA research is supported by the Office of Naval Research, National Institute of Standards and Technology, National Science Foundation and Defense Advanced Research Projects Agency. As a non-regulatory agency of the Commerce Department, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

Ion Interferometers, the Bane of Chubby Photons?

Dallin S. Durfee poses with an elusive "fat photon" during the 2007 meeting of the APS Division of Atomic, Molecular, and Optical Physics (DAMOP).

[This is an invited article based on recent work of the author. -- 2Physics.com]

Author:
Dallin S. Durfee

Affiliation: Department of Physics, Brigham Young University

Our current model of electromagnetism has held up to 2.5 centuries of scrutiny. But like nearly every other theory that science has embraced, it will probably eventually be shown to be incomplete. In a recent article in Physical Review Letters, researchers at Brigham Young University examined the potential of using ion interferometry to search for Coulomb’s-law violating electric fields inside of a conducting cavity. If Coulomb’s law is correct, the absolute voltage of the cavity should not affect the fields inside the cavity. But if it is violated, changing the voltage should alter the fields in the cavity.

The proposed experiment was recently funded by a NIST Precision Measurement Grant and is currently under construction. In this experiment laser beams will be used to split the quantum wave functions of Strontium ions in two. The two waves will then recoil away from each other before being deflected back together and recombined by two additional laser beams. The last laser beam will cause the two waves to interfere, such that the final state of an ion will depend on the relative quantum phase of the two halves of its wave function.

The presence of electric fields inside the conducting shell would cause the two waves to travel through different potentials and acquire different quantum phase shifts. This would change the overall phase of the interference in a predictable way, making it possible to determine the magnitude of the electric field from the final state of the ions. By monitoring the state of ions exiting the apparatus as a changing voltage is applied to the conducting shell, a very sensitive test of Coulomb’s law can be conducted.

The theory of massive photons provides a useful way to compare experimental searches for Coulomb’s-law violations. This theory assumes that photons have a small, but non-zero rest mass, resulting in a limited range for Coulomb interactions. Although it is widely believed that the photon has zero rest mass, in today’s image conscious world it is just possible that photons aren’t telling us their true weight (after all, the neutrino maintained its massless image for decades). Based on calculations in their paper, the researchers predict that the experiment will be able to detect a rest mass of a few times 10^-50 grams, about 100 times smaller than previous laboratory measurements.

Reference
"Testing Nonclassical Theories of Electromagnetism with Ion Interferometry"
by B. Neyenhuis, D. Christensen, and D. S. Durfee
Phys. Rev. Lett. 99, 200401 (2007), Abstract Link

Squeezed Light – the first real application starts now

Fig. 1: Roman Schnabel at the squeezed light experiment.

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Roman Schnabel and Henning Vahlbruch

Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover and Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)

Not all light is the same. When looking at a light bulb, the light particles (photons) arrive at our eyes not in a well-organized stream, but in a chaotic fashion. Even in ultra-stable, single-color coherent laser beams, the photons are randomly distributed. This is demanded by the statistical nature of quantum physics. Similar to a rain shower, where many or just a few rain drops may hit the ground, sometimes a bunch of photons arrive, and sometimes just one. This fluctuation of the intensity, known as photon shot-noise, perturbs especially sensitive measurements.

However, quantum physics also allows for light with less (squeezed) photon noise. The very special property of such squeezed light is that the photons are not independent from each other but show quantum correlations. One way of interpreting squeezed light is the following. The occurrence of a photon, say on a photo-electric detector, is still unpredictable, because it obeys the statistical nature of quantum physics, but whenever a first photon has shown up, a second photon is guaranteed to follow a given time later. No wonder that the peculiar properties of squeezed light have been used to demonstrate quantum entanglement and teleportation [1].

A new world record in the strength of squeezing has been achieved this year by the team made up of Henning Vahlbruch, Roman Schnabel and co-workers at the Albert-Einstein-Institute (AEI) in Hanover/Germany. In their experiment an ultra-stable, infra-red laser beam was further refined by squeezing its photon shot-noise by a factor of ten (or 10 dB in the Decibel scale) [2]. The first squeezed light sources were demonstrated in 1985. However, after 20 years of intensive research, doubts arose whether strong squeezing could ever be realized as required for imminent applications. The new experiment at the AEI showed that strong squeezing of light’s quantum noise is possible. If a 10 dB squeezed laser beam replaces a standard coherent laser beam in an optical measurement device, its noise variance would decrease by a factor of 10. Similarly, the new world-record technology can be used to improve entanglement and teleportation experiments.

At the heart of the AEI squeezing experiment was an artificially grown birefringent crystal, that was precisely cut and polished. Two laser beams were sent through the crystal. A green laser beam of wavelength 532 nm caused the electron clouds of the crystal's atoms to oscillate with the frequency of the green light. In this state, the crystal could redistribute photons from a second, infrared beam that had exactly twice the wavelength (1064 nm). When the infrared beam sent a large bunch of photons through the crystal, the crystal stored those photons, and only returned them to the infrared beam when the photon flux became less. In this way, a more regular photon distribution for the infrared laser beam was achieved, and the photon noise of the infra-red beam became squeezed.

Figure 2 shows a photograph of the original optical components of the squeezed light source: a magnesium oxide doped lithium niobate crystal pumped with green laser light. The crystal had a length of 6.5 mm. The additional mirror was used to form a resonator together with one dielectrically coated crystal surface. In the real experiment the crystal was inside a temperature controlled housing and not visible.







Figure 3 shows the result of the successful squeezing experiment. When the green pump light was switched on, the photon noise of the infrared laser dropped by 10 dB to the lower noise level (red). The experimental techniques used for this result are independent of the absolute laser power. In principle any laser beam of arbitrary power could be realized with the same quantum noise reduction of a factor of 10.

Squeezed light offers a lot of fantastic applications in quantum communication and optical quantum computation. However in gravitational wave detection, squeezed light will find its first real application. Gravitational wave detectors are sophisticated laser interferometers which use cutting-edge technology in order to observe tiny changes in space-time that originate from distant black hole binaries or neutron star mergers. The circulating laser powers in these interferometers reach up to several kilowatts in order to reach high measurement sensitivities. A further increase of laser power can in principle further improve the detectors. But there may be a better way.

Imagine that you had a fixed amount of money, say a billion Euros, to build the most sensitive gravitational wave detector. Squeezed light would certainly be one of your key technologies. In order to reduce thermal motions inside your detector, you would use cryogenic techniques to cool it to liquid Helium temperature (-269°C). At the same time you would want to use high laser powers to boost its sensitivity to gravitational waves. You would find that the high laser power inside the detector would heat it up, such that any operation at low temperatures would become technologically extremely challenging, if not impossible. The 10 dB squeezed light technology can solve this problem. It ensures the same high sensitivity to gravitational waves for only one tenth the laser power, and your ultra-sensitive gravitational wave detector could be cost-effectively realized.

All prototype experiments for the application of squeezed light in gravitational wave detectors have been rather successful [3], and the implementation in the gravitational wave detector GEO600 [4] is currently in preparation. The routine use of squeezed light in GEO600 is envisaged for 2009.

References
[1] "Experimental investigation of continuous variable quantum transportation"
A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble, and E. S. Polzik, Science 282, 706 (1998); W. P. Bowen, N. Treps, B. C. Buchler, R. Schnabel, T. C. Ralph, H.-A. Bachor, T. Symul, and P. K. Lam,
Phys. Rev. A 67, 032302 (2003), Abstract Link, arXiv:quant-ph/0207179.
[2] "Observation of squeezed light with 10dB quantum noise reduction"
H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R. Schnabel,
Phys. Rev. Lett. 100, 033602 (2008), Abstract Link, arXiv:0706.1431.
[3] "Coherent control of vacuum squeezing in the gravitational-wave detection band"
H. Vahlbruch, S. Chelkowski, B. Hage, A. Franzen, K. Danzmann, and R. Schnabel,
Phys. Rev. Lett. 97, 011101 (2006), Abstract Link, arXiv:0707.0164.
[4] http://geo600.aei.mpg.de/