A Beautiful Memory
"Entanglement" might sound like a description of a difficult relationship, or that snarl of wires generated by your home theatre system, or even the current home mortgage meltdown; but, to a physicist, the word means something altogether different--a counter-intuitive state of affairs from the quantum realm that is essential for the advancement of quantum information science and technology.
Quantum networks, if they become a reality, could help information technology to escape the fast approaching limitations imposed Moore's Law. In 1965, Intel co-founder Gordon Moore predicted the number of transistors that could be placed on a computer chip would double every two years. His prediction held out as industry produced a succession of increasingly faster and smaller computers but at some point in the coming 10-20 years existing materials will be physically incapable of further miniaturization since component sizes will shrink to the limit of single atoms. Quantum science offers a radically different perspective for the processing and distribution of information--one that enables tasks to be accomplished in ways that are impossible with traditional hardware.
"Quantum Information Science is an emerging field with the potential to cause revolutionary advances in science and engineering fields involving computation, communication, precision measurement and fundamental quantum science," said NSF Program Manager Robert Dunford. "The exciting scientific opportunities offered by this field are attracting the interest of a growing community of scientists and technologists and promoting unprecedented interactions across traditional disciplinary boundaries. Advances in the field will become increasingly critical to our national competitiveness in information technology during the coming century."
In one such example, a group of researchers at the California Institute of Technology, with funding from the National Science Foundation and the Intelligence Advanced Research Projects Activity, is advancing the field of quantum information science by creating and manipulating entangled states of light and matter. Entanglement leads to correlations between the various components of a physical system--regardless of the distance separating them--that are "stronger" than those possible for classical systems. Einstein famously referred to these correlations as "spooky action at a distance." In the Caltech experiment, entanglement between two spatially distinct beams of light was transferred to and from separated quantum memories in a fashion that is applicable to quantum communication protocols. While entanglement has a long history of important experiments, the Caltech researchers are the first to "map" entanglement to and from distinct quantum memories in a deterministic fashion.
"Being able to control the coherent conversion of photonic entanglement into and out of separated quantum memories is an important building block for realizing functional quantum networks," said Caltech Valentine Professor of Physics H. Jeff Kimble. "My group and the experimental community in general are in the process of going from abstract ideas to exciting new realities. The basic science that we are pursuing is the harnessing of real physical processes in the service of quantum information, which includes the realization of quantum computers."
In the Caltech demonstration, a single photon is split along two distinct paths, generating an entangled state of light in which the photon simultaneously propagates along both paths. At this point it, helps to remembers Einstein's discovery that light has a combination of particle and wave-like behaviors. The photon may come out of the chute like a particle of light, but the splitter separates it in the fashion of a buoy separating a wave of water.
Afterward, the photon exists in an entangled state. Although it remains a single entity, it can occupy two places at once. (More about this later.) The researchers then transcribes, or "map," the entangled beams of light onto two distinct groups of roughly 100,000 laser-cooled cesium atoms, separated from each other by 1 millimeter. The groups of atoms serve as the quantum memories in this demonstration. At this point, the photon essentially "dissolves" itself into the internal quantum states of the atoms--think of a jaw breaker dissolving itself in water--causing the two groups of atoms to enter an entangled state. With prompting by a control laser, the entangled photon reconstitutes itself from the atoms and propagates on, once again, as two entangled beams of light. Ultimately, these beams could be directed to other locations to distribute entanglement across larger networks.
"Entanglement is fragile and to use quantum protocols over long distances, the channel has to be divided into many segments and entanglement generated and stored into material systems before connecting them all together," Dunford said. "The significant achievement of the Caltech group is that they have demonstrated an initial version of one of these segments." Papers describing their research appeared in the March 6, 2008, and June 19, 2008, issues of Nature.
One reason quantum science holds so much promise for computing and communication technologies is that in the quantum world a particle, like the entangled photon in the Caltech demonstration, can be in two states at once. It's called "superposition." For example, if you lived in a quantum world you could be working at your desk and driving your car at the exact same time. Think how much more you could accomplish! It's the same with quantum computers. Every quantum bit, or qubit, can store two numbers at the same time. Adding a single qubit to a quantum computer doubles its capacity. In a quantum network of 300 qubits, multiply the number "2" by itself 300 times and you get the idea of how powerful this thing could be.
"By funding this research, the National Science Foundation is laying a foundation for quantum information science, which could enable new technologies and devices," Kimble said. "The research at Caltech and other labs also advances our understanding of exotic states of matter, such as high-temperature superconductors. We know at given temperatures certain materials become superconducting--the particles all behave in concert in a way that allows them to travel along wires with little or no resistance--why and how this happens has defied our attempts to understand it."
The next step for the Caltech researchers is to explore the nature of entanglement in multicomponent systems. "We're trying to build exotic quantum systems one piece at a time." So far, they have successfully split a photon into four independent beams to create four-party entanglement. "The hardest and undoubtedly most important part of the research is understanding how to make measurements in a model independent fashion that unambiguously verify and characterize entanglement in complex quantum systems," Kimble noted.
At some point in the future, the successful pieces will combine to become networks of quantum nodes for processing and storing quantum states and channels for distributing quantum information for quantum computing, communication--one might even be so bold as to envision a Quantum Internet.
Physicists have now been generating entangled pairs of components for 30 years but four-party entanglement is still somewhat of an unknown. Kimble and his colleagues are investigating these multicomponent entangled states to better understand them. There are also variations on the theme known as "classes of entanglement" that can be defined. These are not understood in the case of the entanglement of an arbitrary number of components N, or so-called N-party states.
"What is the zoology of the kingdom of entanglement as we increase the number N of participating systems?" Kimble asked. "Ultimately, we need to understand a lot more about the nature of entanglement to exploit it in actual applications, such as quantum computers."