Now, physicists at the Joint Quantum Institute (JQI), a collaborative organization of the National Institute of Standards and Technology (NIST) and the University of Maryland, have developed a promising new source of entangled photons using quantum dots tweaked with a laser.
The JQI technique may someday enable more compact and convenient sources of entangled photon pairs than presently available for quantum information applications such as the distribution of "quantum keys" for encrypting sensitive messages.
Quantum dots are nanometer-scale bits of semiconductor—so small that electrical charges in the dots are confined in all directions. They can be made to emit photons—fluoresce—by pumping in energy to create so-called "excitons," a pairing of an electron and the electron-less "hole." When the electron falls back into the hole, the excess energy is released as a photon. Quantum dots can also host the even more exotic "biexciton," composed of two electrons and two holes.
When a short-lived biexciton decomposes, it undergoes two drops in energy, analogous to descending two rungs of a ladder, and a photon is released at each stage. Physicists have long been trying to use this process to get pairs of entangled photons from quantum dots. What makes entanglement possible is that the biexciton could decay along one of two possible pathways, analogous to two different ladders that both get it to the ground. During its descent it releases a pair of photons with a different kind of polarization (electric field direction) depending on the ladder it descends. If the energy drop at each stage is exactly the same in both pathways, so that the ladders look identical, the pathways become indistinguishable—and as a result the biexciton releases photons with undetermined polarization values. Measuring a photon would both determine its polarization and instantly define its partners—a hallmark of entanglement.
But imperfections within the structure of the quantum dot create differences in the energy levels (rung heights) between the two pathways, making them distinguishable and creating photons with predetermined, clearly defined polarizations. Except in rare instances, this holds true even for the reliable, widely fabricated indium gallium arsenide (InGaAs) dots that JQI researcher Andreas Muller and his colleagues created at NIST. Muller and his coworkers solved this problem by beaming a laser at the quantum dot. The laser's electric field shifts the energy levels in one of the pathways so that the two pathways match up, resulting in the emission of entangled photons.
Entangled photons have come from individual quantum dots before, but they have been spotted by hunting for dots in large samples whose imperfections accidentally gave the two pathways identical energy structure. JQI group leader Glenn Solomon says that this entanglement technique could work for a wide variety of quantum dots. Though the dots must be cooled to cryogenic temperatures, he adds that quantum dots could offer advantages as entanglement sources over their conventional crystal counterparts as they are less bulky and can conveniently produce one pair of entangled photons at a time, instead of in bunches.
* A. Muller, W.F.Fang, J. Lawall and G.S. Solomon. Creating polarization-entangled photons from a quantum dot. Upcoming in Physical Review Letters.
Ben Stein | EurekAlert!
New NASA study improves search for habitable worlds
20.10.2017 | NASA/Goddard Space Flight Center
Physics boosts artificial intelligence methods
19.10.2017 | California Institute of Technology
University of Maryland researchers contribute to historic detection of gravitational waves and light created by event
On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...
Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....
Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).
When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...
Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.
How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...
Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...
17.10.2017 | Event News
10.10.2017 | Event News
10.10.2017 | Event News
20.10.2017 | Information Technology
20.10.2017 | Materials Sciences
20.10.2017 | Interdisciplinary Research