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Interfering with Quantum Dots

April 6, 2010

Scientists at the Joint Quantum Institute have devised a new method that could be used to generate multiple pairs of “indistinguishable” photons – near-identical individual quanta of light – by fine-tuning the output from two separate quantum dots.

What good are two photons if you can’t tell them apart? As it happens, they can be worth a great deal in the burgeoning field of quantum information science. Manipulating single photons will almost certainly play a role in any eventual quantum communications network or quantum computer, and characterizing the behavior of single-photon sources is a subject of intense interest worldwide. Moreover, many planned applications call for multiple single photons that are virtually identical, with exactly the same frequency, spatial and temporal extent, direction and polarization. So researchers at JQI and elsewhere are engaged in an attempt to produce, study and control them.

The diagnostic hallmark of indistinguishable photons is a peculiar, counterintuitive property: When they are aimed at different sides of a 50-50 beamsplitter (an optical device in which a photon has an equal chance of passing straight through or being reflected at a 90-degree angle), both photons invariably emerge in the same direction. That is, they “coalesce” into a two-photon collective state. [See Figure 1]

This may not seem to make sense. If light has an equal chance of being reflected (R) or transmitted (T) by the beamsplitter, and two beams enter the splitter, then there are four possible and equally likely pathways for the two light beams as they travel through the device: both reflected (RR), both transmitted (TT), and one each way -- RT and TR.

Because the light input to the beamsplitter is made of discrete photons, they obey quantum mechanical rules of interference, whereby each possible outcome has a different “probability amplitude.” [See Figure 2.]

Image removed.And because the amplitudes for RR and TT are equal in all respects but opposite in sign, they cancel one another out in a process called destructive interference – analogous to the familiar process by which waves that are completely out of phase suppress each other. So the only two available pathways are RT and TR, both of which require that the photons emerge from the same side of the beamsplitter. This phenomenon, predicted and first observed 23 years ago, has been studied intensively since then by investigators including JQI Fellow Glenn Solomon who headed the research group that reports its new findings inPhysical Review Letters.*

There are several ways to produce photons with varying degrees of indistinguishability, and all are difficult to control. One method is to use a technique called parametric down conversion, in which one photon enters a nonlinear crystal, but two photons emerge. The combined momenta and energy of the two output photons are exactly equal to the momentum and energy content of the single input photon. This is a well-understood procedure, but it is impossible to guarantee that it will produce only one pair at a time (the desired goal) unless the input intensity is significantly reduced, and that in turn decreases the rate of photon pair production.

Alternatively, researchers have used pairs of atoms or ions to create indistinguishable photons, for instance by JQI Fellow Chris Monroe’s research group in 2007. And in 2002, a group at Stanford University that included Solomon showed that a single quantum dot in a resonant cavity can be used to emit a series of highly indistinguishable photons. In the latest work, Solomon’s group at the National Institute of Standards and Technology set out to see if indistinguishable photons could be produced by two completely separate quantum dots.

“Many proposed applications in quantum information science require indistinguishable photons, and for these applications to be practical, one wants many separate sources of mutually indistinguishable photons,” says Solomon. “Atoms or ions could do, but are difficult to manipulate, and thus hard to scale up to large numbers. Quantum dots have the advantage of being easily incorporated into standard semiconductor processing techniques, but the disadvantage of being randomly distributed in emission wavelength. The next step to large scale, solid-state, indistinguishable single-photon sources was to tune two quantum dots into resonance and show that their photons can interfere.”

Quantum dots, typically 10 to 30 billionths of a meter in diameter, are tiny lumps of semiconductor (indium arsenide in the JQI experiment) embedded in a layered construct of similar semiconductor materials such as gallium arsenide and aluminum arsenide. The dots behave very much like individual atoms or molecules, in that their dimensions permit only a discrete set of allowed optical excitations and emissions. As a result, the likelihood of finding two quantum dots that are identical in every respect is essentially zero. So it might seem to be impossible to find two dots that would emit indistinguishable photons.

To confront that problem, the group treated each dot differently. One dot (QD1) was in an open cavity formed between a mirror inside the semiconductor sample, below the location of the QD, and a mirror on the end an optical fiber above the sample. The sample was glued to a piezoelectric transducer, a device that changes shape when an electric voltage is applied. (See photo.) Sending a voltage to the transducer strains the semiconductor sample – slightly altering the spacing of atoms in QD1 and, thereby, the wavelength of the photons it can emit. The optical fiber carries excitation pulses in and emitted photons out, and the cavity causes the QD to preferentially emit into the fiber. 

The second dot, QD2, was enclosed in a microcavity within its sample, and also was fitted with an input-output fiber strand. Both samples were cooled to a temperature of 8 kelvin and simultaneously excited with pulses of near-infrared laser light spaced 13 nanoseconds apart. The researchers then adjusted the strain on QD1 to change the wavelength of the dot’s output photons until it matched that of QD2 as closely as possible.

To determine the extent to which the photons were indistinguishable, the group routed the output from each dot onto a different face of the beamsplitter and then measured the way that the photons emerged by comparing counts on two different photon detectors, one monitoring each output face of the beamsplitter. If there were no quantum coalescence effects at all in the system, then one photon would arrive at each detector simultaneously about 50 percent of the time. Conversely, in a perfect quantum system in which all the photon pairs coalesced and exited together, there should never be a photon in both detectors at the same time. The JQI team found substantial interference: The photons had a total coalescence rate of 18.1 percent.

That does not mean that “we have shown photons from separate QDs which are indistinguishable, just that they show a degree of indistinguishability,” first author Edward Flagg says.”Indistinguishability is not a binary value, yes or no. It's a continuous value. It turns out our photons are 18.1% indistinguishable. And showing partial indistinguishability of photons from two separate QDs is the complementary achievement to showing highly indistinguishable photons from a single QD. 

“The next step would be to combine those two traits to have highly indistinguishable photons, as from a [high-quality] cavity, with energy tuning to bring multiple QDs into resonance so their photons will all be mutually indistinguishable.”

* See reference publication