‘Quantum Logic Clock’ Rivals Mercury Ion as World’s Most Accurate Clock A new limit on change in fine-structure constant

NIST physicist Till Rosenband adjusts the quantum logic clock, which derives its “ticks” from the natural vibrations of an aluminum ion. The aluminum ion is trapped together with one beryllium ion inside the copper-colored chamber in the foreground. [Photo credit and copyright: Geoffrey Wheeler]

In a paper published in today's issue of journal 'Science', a team of scientists from the National Institute of Standards and Technology (NIST) reports the development of an atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world and which now rivals the world's most accurate clock, based on a single mercury atom, previously designed and built by NIST scientists (See our past posting). Both clocks are at least 10 times more accurate than the current U.S. time standard.

An optical clock can be evaluated precisely only by comparison to another clock of similar accuracy serving as a “ruler.” NIST scientists used the quantum logic clock to measure the mercury clock, and vice versa. The measurements were made in a yearlong comparison of the two next-generation clocks with record precision, allowing scientists to record the relative frequencies of the two clocks to 17 digits—the most accurate measurement of this type ever made.

The aluminum and mercury clocks are both based on ions vibrating at optical frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1, the U.S. time standard based on neutral cesium atoms, and other similar time standards around the world. The aluminum and mercury clocks would neither gain nor lose one second in over 1 billion years—if they could run for such a long time—compared to about 80 million years for NIST-F1.

The NIST quantum logic clock uses two different kinds of ions, aluminum and beryllium, confined closely together in an electromagnetic trap and slowed by lasers to nearly “absolute zero” temperatures. Aluminum is a stable source of clock ticks, but its properties cannot be detected easily with lasers. The NIST scientists applied quantum computing methods to share information from the aluminum ion with the beryllium ion, a workhorse of their quantum computing research. The scientists can detect the aluminum clock’s ticks by observing light signals from the beryllium ion.

Highly accurate clocks are used to synchronize telecommunications networks and deep-space communications, and for satellite navigation and positioning. Next-generation clocks may also lead to new types of gravity sensors, which have potential applications in exploration for underground natural resources and fundamental studies of the Earth. NIST scientists have several other optical atomic clocks in development, including one based on thousands of neutral strontium atoms. The strontium clock recently achieved twice the accuracy of NIST-F1, but still trails the mercury and aluminum clocks.

Cosmology Connection: The comparison of these clocks produced the most precise results yet in the worldwide quest to determine whether some of the fundamental constants that describe the universe are changing slightly over time, a hot research question that may alter basic models of the cosmos.

Based on fluctuations in the frequencies of the two clocks relative to each other over time, NIST scientists were able to search for a possible change over time in a fundamental quantity called the fine-structure constant. This quantity measures the strength of electromagnetic interactions in many areas of physics, from studies of atoms and molecules to astronomy. Some evidence from astronomy has suggested the fine-structure constant may be changing very slowly over billions of years. If such changes are real, scientists would have to dramatically change their theories of the fundamental nature of the universe. [Readers may refer to the article "Changing Constants, Dark Energy and the Absorption of 21 cm Radiation" by Prof. Ben Wandelt of University of Illinois, 2Physics.com, July 25, 2007]

The NIST measurements indicate that the value of the fine-structure constant is not changing by more than 1.6 quadrillionths of 1 percent per year, with an uncertainty of 2.3 quadrillionths of 1 percent per year (a quadrillionth is a millionth of a billionth). The result is small enough to be “consistent with no change,” according to the paper. However, it is still possible that the fine-structure constant is changing at a rate smaller than anyone can yet detect. The new NIST limit is approximately 10 times smaller than the best previous measurement of the possible present-day rate of change in the fine-structure constant. The mercury clock is an especially useful tool for such tests because its frequency fluctuations are magnified by any changes in this constant.

Reference
"Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place"
T. Rosenband, D.B. Hume, P.O. Schmidt, C.W. Chou, A. Brusch, L. Lorini, W.H. Oskay, R.E. Drullinger, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, W.C. Swann, N.R. Newbury, W.M. Itano, D.J. Wineland, and J.C. Bergquist
Science, Vol. 319. no. 5871, pp. 1808 - 1812 (28 March 2008). Abstract Link

We thank Media Relations, NIST for materials used in this posting

Solitons Enable Semiconductor Microlasers

As water ripples on a pond, light waves usually spread out due to diffraction, if they are not confined by some means. Hence, typical lasers contain curved mirrors to refocus the beam and to create a “stable” cavity. It is also well known that nonlinearities can provide guiding. In that case the light field sustains its own waveguide leading to a soliton.

Recently, researchers from the University of Strathclyde (Yann Tanguy, Thorsten Ackemann, Willie J. Firth) and from Ulm Photonics (R. Jäger) married the curious properties of soliton waves with a major semiconductor laser technology to create a new type of semiconductor laser that can be switched on and off using light pulses. This cavity soliton laser can be applied in all-optical telecoms systems, in which data are switched and routed without the need to convert light pulses into electrical signals and back again.

It has also interesting fundamental aspects because the emerging microlasers have the freedom to choose frequency, phase and polarization in contrast to other optical solitons where these degrees of freedom are usually imprinted by the driving beam. The achievement is published in Physical Review Letters (PRL 100, 013907, 2008) . Fig. 1 shows some of the authors together with the system and some solitons. Yann Tanguy left the group by now but Neal Radwell is working on the further development of the scheme.

Fig. 1: Willie, Thorsten and Neal working on the advancement of the cavity soliton laser. The setup for the external cavity is in the foreground with the mount for the VCSEL on the right (on the heat sink). The monitor shows five solitonic mircrolasers present in the aperture.

The device is based on a vertical-cavity surface-emitting laser (VCSEL), which is used in a wide range of applications including optical datacoms. The VCSEL was built by R. Jäger from ULM Photonics in the framework of the European project FunFACS(Fundamentals, Functionalities and Applications of Cavity Solitons). These devices are magnificent pieces of engineering where nearly hundred of different semiconductor layers with widths in the nanometer to some hundreds of nanometers range (1 nm = 0.000000001 m) are grown using a process called molecular beam epitaxy (MBE).

The VCSEL is driven with a rather small current such that it is amplifying but not yet above threshold, if operating isolated. The device is coupled to an external cavity containing a diffraction grating as a frequency-selective element. The frequency experiencing optimal feedback is deliberately offset slightly from the frequency of the free-running VCSEL. Then a pulse from an external laser is fired at a small spot (12 µm diameter) within the 200 µm aperture of the VCSEL. This changes the index of refraction at that spot and aligns the resonance frequency of the VCSEL locally with the one of the external cavity modes causing a bright spot of light – the soliton - to form in that region.

This soliton represents a small microlaser. It can then be switched off by firing a second laser pulse at the region, which perturbs the refraction index profile in such a way to destroy the soliton. Hence, this microlaser is bistable similar to electronic flip-flops. Similarly, additional microlasers can be set and reset with the external control beam, (in principle) anywhere in the active area, leading to an “ensemble” of small microlasers. Due to the self-localization there is no need for micro-fabrication of individual emitters. The switching sequence is illustrated in Fig. 2.

Fig. 2: Intensity distribution within the active area of the VCSEL. The independent switch-on and switch-off of two microlasers with an external control beam (arrow) is demonstrated. The individual microlasers have a size of about 10 µm. There are also other solitons as well as background states which are attributed to the finite bandwidth of the feedback and inhomogeneities.

As a result, we have optically controllable microlasers, being an example of a major thrust of the field of photonics, the control of light by light.

In the first 2008 issue, a possible application of cavity solitons was starring on the title page of Applied Physics Letters. A collaboration of French, Italian, German and UK researchers from the FunFACS project were reporting on a cavity soliton device used to delay the propagation of light pulses (Appl. Phys. Lett. 92, 011101, 2008). All-optical delay lines and “slow light” are a hot topic in photonics because of fundamental issues as well as of the possible use as all-optical buffers in future high-speed photonic networks.

Currently, the Strathclyde group is working on the advancement of the device by miniaturizing it and achieving the delay functionality in a cavity soliton laser.

References:
"Realization of a Semiconductor-Based Cavity Soliton Laser"
Y. Tanguy, T. Ackemann, W. J. Firth, R. Jäger,
Phys. Rev. Lett. 100, 013907 (2008), Abstract Link.

"All-optical delay line using semiconductor cavity solitons"
F. Pedaci, S. Barland, E. Caboche, P. Genevet, M. Giudici, J. R. Tredicce, T. Ackemann, A. J. Scroggie, W. J. Firth, G.-L. Oppo, G. Tissoni,
Appl. Phys. Lett. 92, 011101 (2008), Abstract Link.

Related Links:
-- FunFACS project
-- Nonlinear Photonics at Strathclyde
-- Computational Nonlinear Optics and Quantum Optics at Strathclyde
-- Ulm Photonics
-- Coverage at Physicsworld.com
-- Popular account of VCSELs

Entangled Memory

Jeff Kimble [Photo courtesy: Caltech]

In a paper published in today's issue of the journal Nature, Caltech's Valentine Professor of Physics H. Jeff Kimble and his colleagues have laid the groundwork for a crucial step in quantum information science. They demonstrate for the first time an important capability required for the control of quantum information and quantum networks, namely the coherent conversion of photonic entanglement into and out of separated quantum memories.

Entanglement lies at the heart of quantum physics, and is a state where parts of a composite system are more strongly correlated than is possible for any classical counterparts regardless of the distance separating them. Entanglement is a critical resource for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. Quantum networks rely on entanglement for the teleportation of quantum states from place to place.

Entanglement lies at the heart of quantum physics, and is a state where parts of a composite system are more strongly correlated than is possible for any classical counterparts regardless of the distance separating them. Entanglement is a critical resource for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. Quantum networks rely on entanglement for the teleportation of quantum states from place to place.

In a quest to turn these abstract ideas into real laboratory systems and to distribute entanglement to remote locations (even on a continental scale), Kimble explains that quantum physicists have studied ways to propagate photonic information into and out of quantum memory using a system called a quantum repeater, invented in 1998 by H. Briegel, J.I. Cirac, and P. Zoller at the University of Innsbruck. Until now, work in Kimble's group on the realization of a quantum repeater with atomic ensembles relied upon the probabilistic creation of entanglement. In this setting entanglement between two clouds of atoms was generated probabilistically but with an unambiguous heralding event.

While such systems hold the potential for scalable quantum networks, it has been difficult for Kimble's Quantum Optics Group to apply such schemes to certain protocols necessary for quantum networks, such as entanglement connection. Now, with the new protocol and future improvements, "We can push a button and generate entanglement," says physics graduate student Kyung Soo Choi, one of four authors of the Caltech experiment.

[Image Courtesy: Nature]

In the Caltech experiment, a single photon is first split, generating an entangled state of light with quantum amplitudes for the photon to propagate two distinct paths, taking both at once. The Caltech team in turn transcribed, or mapped, the entanglement onto distinct atomic ensembles separated by one millimeter. To create the interface between the light and matter, the team employed laser-cooled cesium atoms whose atomic states interact with a control laser to create destructive quantum interference, making the atomic ensembles either invisible or highly opaque to the input light. Called Electromagnetically Induced Transparency and pioneered by S. Harris at Stanford University, the mechanism manipulates the speed of the light for the incoming entangled photon and that kicks off the entire procedure.

In this experiment, the photonic entanglement was mapped into the atomic ensembles in a time ~ 20 nanoseconds and then stored in the atomic ensembles for one microsecond, with storage times extendable up to 10 microseconds. The photonic entanglements of the input and output of the quantum interface were explicitly quantified with a conversion efficiency of 20 percent. However, the researchers emphasize, real-world realization of a quantum network remains far out of reach even with these parameters and the state-of-the-art of quantum controls. Choi comments, "Further improvements in quantum control and storage capabilities in matter-light interfaces will lead to fruitful and exciting discoveries in Quantum Information Science, including for the realization of quantum networks."

In addition to Kimble and Choi, other authors are Hui Deng, a postdoctoral scholar at the Center for the Physics of Information; and Julien Laurat, a former Caltech physics postdoctoral scholar who is now an associate professor at Laboratoire Kastler Brossel (Universite P. et M. Curie, Ecole Normale Superieure and CNRS) in Paris, France.

Reference
"Mapping photonic entanglement into and out of a quantum memory"
K. S. Choi, H. Deng, J. Laurat & H. J. Kimble,
Nature 452, 67-71 (6 March 2008), Abstract Link

[We thank Caltech Media Relations for materials used in this posting]