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Fast, Low-power All-optical Switch

Only 140 photons are needed to switch energy in a solid-state device | May 3, 2012

all-opticalswitch_fig1Figure 1
Setup of a waveguide made from a photonic crystal. A quantum dot (QD) is placed inside a tiny zone (cavity) clear of holes. Light is sent into and out of the waveguide via endcaps (the semi-circular structure at both ends, indicated by green arrows). If properly timed (the synchronicity time, tau, being less than about 100 ps), a pump (control) laser pulse will allow an accompanying probe pulse to exit out the side. If the probe and pump beams are not aligned, the probe beam will exit out the far end of the waveguide. (Figure from Ranojoy Bose.)
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An optical switch developed at the Joint Quantum Institute (JQI) spurs the prospective integration of photonics and electronics. What, isn’t electronics good enough? Well, nothing travels faster than light, and in the effort to speed up the processing and transmission of information, the combined use of light parcels (photons) along with electricity parcels (electrons) is desirable for developing a workable opto-electronic protocol.

The JQI (*) switch can steer a beam of light from one direction to another in only 120 picoseconds (120 trillionths of a second), requiring very little power, only about 90 attojoules (90 x 10-18 joules). At the wavelength used, in the near infrared (921 nm), this amounts to about 140 photons. These new results are being published in an upcoming issue of the journal Physical Review Letters (**).

The centerpiece of most electronic gear is the transistor, a solid-state component in which a gate signal is applied to a nearby tiny conducting pathway, thus switching on and off the passage of an information signal. The analogous process in photonics would be a solid-state component which acts as a gate, enabling or disabling the passage of light through a nearby waveguide, or as a router, for switching beams in different directions.

In the JQI experiment, prepared and conducted at the University of Maryland and at the National Institute for Standards and Technology (NIST) by Edo Waks and his colleagues, an all-optical switch has been created using a quantum dot (the equivalent of a gate) placed inside a resonant cavity. The dot, consisting of a nm-sized sandwich of the elements indium and arsenic, is so tiny that electrons moving inside can emit light at only discrete wavelengths, as if the dot were an atom. The quantum dot sits inside a photonic crystal, a material that has been bored with many tiny holes. The holes preclude the passage of light through the crystal except for a narrow wavelength range.

Actually, the dot sits inside a small hole-free arcade which acts like a resonant cavity. When light travels down the nearby waveguide some of it makes its way into the cavity, where it interacts with the quantum dot. And it is this interaction which can transform the waveguide’s transmission properties. Although 140 photons are needed in the waveguide to produce switching action, only about 6 photons actually are needed to bring about modulation of the QD, thus throwing the switch.

all-opticalswitch_fig2Figure 2
The switch in action. When the cavity is ON---when the quantum dot is resonant with the probe beam---the beam will exit the waveguide through a side port. When the cavity if OFF, the dot is not in resonance with the probe beam and it will exit out the end port. (Figure from Ranojoy Bose.)
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Previous optical switches have been able to work only by using bulky nonlinear-crystals and high input power. The JQI switch, by contrast, achieves high-nonlinear interactions using a single quantum dot and very low power input. Switching required only 90 aJ of power, some five times less than the best previous reported device made at labs in Japan (***), which itself used 100 times less power than other all-optical switches. The Japanese switch, however, has the advantage of operating at room temperature, while the JQI switch requires a temperature of around 40 K.

Continuing our analogy with electronics: light traveling down the waveguide (the equivalent of the conducting pathway in a transistor) in the form of an information-carrying (probe) beam can be switched from one direction to another using the presence of a second pulse, a control (pump) beam. To steer the probe beam out the side of the device, the slightly detuned pump beam needs to arrive simultaneously with the probe beam, which is on resonance with the dot. The dot lies just off the center track of the waveguide, inside the cavity. The temperature of the quantum dot is tuned to be resonant with the cavity, resulting in strong coupling. If the pump beam does not arrive at the same time as the probe, the probe beam will exit in another direction.

So, is this quantum-dot switch an “optical transistor”? Not quite, says JQI scientist Ranojoy Bose. “Our waveguide-dot setup can’t yet be used to modulate a beam of light using only a weak control pulse of light---what we would call a low-photon-number pulse.

But Bose says he expects an improvement (reduction) in the number of photons needed to switch the resonant cavity on and off. In the meantime, the JQI switch represents a great start toward creating a usable ultrafast, low-energy on-chip signal router. “Our paper shows that switching can be achieved physically by using only 6 photons of energy, which is completely unprecedented. This is the achievement of fundamental physical milestones—sub-100-aJ switching and switching near the single photon level,” Bose says.

(*)The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

(**) “Low photon number optical switching with a single quantum dot coupled to a photonic crystal cavity,” Deepak Sridharan, Ranojoy Bose, Hyochul Kim, Glenn S. Solomon, and Edo Waks. Physical Review Letters, in press.

(***) Nozaki et al., Nature Photonics, 2 May 2010.


Media Contact: Ranojoy Bose, rbose@umd.edu, 301-405-0030
A copy of the PRL paper can be obtained from Phillip F. Schewe, pschewe@umd.edu, 301-405-0989.

 

Inequality and Investment Bubbles

A clearer link is established | April 19, 2012

 

inequalityinvestment-fig1Figure 1
Fraction of tax returns above various adjusted gross incomes; the two curves fitting the data correspond to an exponential-type behavior (Boltzmann-Gibbs) and power-law behavior (Pareto). Courtesy Victor Yakovenko.
"Money, it’s a gas," says the sixties rock group Pink Floyd in their song “Money.” Indeed, physics professor Victor Yakovenko is an expert in statistical physics and studies how the flow of money and the distribution of incomes in American society resemble the flow of energy between molecules in a gas. In his lectures to be delivered on April 19 at New York University and April 20 at the New School for Social Research, Yakovenko will bring his physics-of-incomes study up to date, including a report on the correlation between levels of income inequality and the appearance of financial downturns, such as the dot-com bubble of 2000 and the more recent housing bubble of 2008.

That the rich really are different is a common opinion. It turns out that the rich even have their own physics. Yakovenko, who is a professor at the University of Maryland and also a fellow of the Joint Quantum Institute*, produces a plot of the cumulative percentage of the population versus income. The graph shows that the actual income distribution (the data coming from the IRS) for the poorer 97% of reported returns follows a type of curve---the Boltzmann-Gibbs curve---that applies to the energy distribution of molecules in a gas. The curve is named for 19th century physicists Ludwig Boltzmann and J. Williard Gibbs, pioneers in statistical physics.

By contrast, the upper 3 percent or so of incomes, starting at a tax-return level of about $140,000, lie along a different curve, one named for Vilfredo Pareto, an economist who studied income distributions in the 19th century. This distinction in income curves is generally attributed to the fact that the most affluent segment of society makes more of its income from investments, which are taxed at a lower rate, rather than income from labor.

“A mathematical analysis of the empirical data clearly demonstrates the two-class structure of a society,” Yakovenko says. The lower-97% curve is an example of exponential behavior, while the upper-3% curve is an example of a power-law behavior. The power-law curve is conspicuously different from the exponential curve in having a long tail, as shown in Figure 1.

Then, Yakovenko plots the percentage of total income lying in that tail on through the years. He finds that the periods of greatest inequality are also periods of bursting investment bubbles. Most recently the inequality peaks lined up very closely with the housing bubble of 2008, the dot.com bubble of 2000, and the savings-and-loan crisis of the late 1980s, as shown in Figure 2.

inequalityinvestment-fig2Figure 2
"A mathematical analysis of the empirical data clearly demonstrates the two-class structure of a society," Yakovenko says. The lower-97% curve is an example of exponential behavior, while the upper-3% curve is an example of a power-law behavior. The power-law curve is conspicuously different from the exponential curve in having a long tail, as shown in Figure 1. Courtesy Victor Yakovenko.
Yakovenko successfully models income distribution pretty well using basic statistical physics. In the case of a gas, molecules come to have a great inequality in energies, all through their random collisions with each other. People are not inanimate molecules and yet through their economic and social “collisions” they too come to have a very similar, and unequal, distribution of incomes. Previously the upper income bracket (the upper 3%) curve had been pretty well studied, but Yakovenko was one of the first, perhaps the first, to demonstrate that the lower bracket (the lower 97%) was described by the venerable Boltzmann-Gibbs curve developed to represent the spread of energies of molecules in a gas.

Yakovenko’s pioneering study of the 97% was summarized in a review paper in the journal Review of Modern Physics in 2009 (**) written in collaboration with the distinguished economist J. Barkley Rosser, Jr. Yakovenko got started in econophysics in the year 2000, at a time when statistical mechanics wasn’t used much to study economics. He has prepared an updated study of income distributions, for his participation in a celebration (April 20-21) of the career of economist Duncan Foley at the New School for Social Research in New York. Foley was a pioneer in marrying economics and statistical mechanics.


(*) The Joint Quantum Institute is operated by the University of Maryland in College Park and the National Institute of Standards and Technology in Gaithersburg, MD.

(**) Yakovenko V.M., and Rosser, J.B. Jr., “Colloquium: Statistical mechanics of money, wealth, and income," Reviews of Modern Physics 81, 1703 (2009).

Victor Yakovenko: 301-405-6151, yakovenk at umd.edu, http://physics.umd.edu/~yakovenk/econophysics/

Copies of Yakovenko’s 2012 report are available from the Joint Quantum Institute:
Phillip F. Schewe, 301-405-0989, pschewe at umd.edu

   

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