Point two laser beams so that they cross each other, and each goes through as if the other one did not exist. Light rays cannot interact with other light rays—or can they? With the help of a single atom, physicists have devised a system in which one light beam can turn another on or off. Such a light switch could serve as the basic component of futuristic optical quantum computers and may help open the way to a quantum version of the Internet, which would offer unbreakable data security.
The device makes use of a phenomenon called electromagnetically induced transparency, in which a laser beam can render opaque clouds of atoms temporarily transparent to a narrow wavelength of light. The cloud can then act as a switch for a second beam, either letting it through or blocking it. The result is similar to what happens with transistors in electronic circuits, where a voltage applied at one electrode controls whether current can flow between two other electrodes.
Applications such as quantum computing demand the control of beams down to single photons, the elementary particles of light. For that purpose, single atoms are better than clouds of them, says physicist Martin Mücke of the Max Planck Institute for Quantum Optics in Garching, Germany. He and his collaborators trapped a rubidium atom and aimed two different laser beams at it: one for probing, or transmitting, and the other one for switching. Ordinarily the atom acts as a barrier to photons from the probe beam because it would first absorb them—going from its “ground” state to an “excited” state—and then shoot them back, that is, reflect them. This condition would constitute the “off” state of the device.
But turning on the switching beam changed the atom’s possible states, so that it now had two different ground states. The probe beam then had two different ways of exciting the electron, each starting from a different ground state, but in the mathematics describing the atom’s quantum-mechanical nature, the two possibilities cancel out, so that no excitation was possible. Thus, the probe beam photons, rather than being absorbed, could get through, marking the “on” state.
Making single photons interact can be useful because a photon can carry the units of quantum information, called qubits. They can exist in two states simultaneously and thereby represent both the 0 and 1 of binary code at the same time. Thanks to this feature, quantum computers could perform certain operations in parallel. In principle, they could quickly perform calculations that a typical computer could not do, at least not before the sun swells up and bakes the earth five billion years from now.
Max Planck’s Gerhard Rempe, the senior researcher on the team, points out that a single-atom device could do more than mere switching. For example, it could store photons and release them at will without damaging their delicate quantum states—an application known as quantum random-access memory, which could be crucial for data routers of a quantum Internet. In such a network, privacy is guaranteed by the law of quantum physics [see “Privacy and the Quantum Internet,” by Seth Lloyd; Scientific American, October 2009].
The new device still needs improvement: in the off position, the atom still lets through 80 percent of photons from the second beam. But the researchers say that straightforward improvements, such as keeping the atom colder, could bring that number down to 10 percent, if not to 0. (A more substantial limitation is that handling single atoms requires a fairly sophisticated physics laboratory.) The team published its results in the June 10 Nature. (Scientific American is part of Nature Publishing Group.)
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10 Comments
Add CommentIs there some physical property of photons that offers inherent advantages over electrons in producing quantum logic circuits? It be disappointing to finally make a photonic computer only to find that it offers no real advantage in miniaturization or resulting speed over electronic computers...
Reply | Report Abuse | Link to thisPhotons travel much faster than electrons, among others.
Reply | Report Abuse | Link to thisDimitris - As I understand, contrary to popular conception, the propagation of an electron through a circuit can approach the 'speed of light' in a vacuum. Also, the speed of light traversing some unspecified optical circuit in an optical device could not attain the 'speed of light' in a vacuum, unless the device utilized a single light source and the entire photon path were within a single vacuum. I think remains to be seen whether a photonic computer would in practice be faster than an electronic computer.
Reply | Report Abuse | Link to thisWhat about this part though:
Reply | Report Abuse | Link to this"quantum computers could perform certain operations in parallel. In principle, they could quickly perform calculations that a typical computer could not do, at least not before the sun swells up and bakes the earth five billion years from now"
So while the photons might not in the end travel faster, it is possible their other characteristics could make the overall performance faster, or more secure, etc.
this article is mixing apples and oranges....
Reply | Report Abuse | Link to thistwo or more entangle photons could represent a qubit...as near as i can tell they are not talking about entanglement so no qubit....
they are just using one beam(photon) to control another....all by itself a great discovery...although at 20 percent they need several orders of magnitude to make it useable...
tharriss - Good catch; I wasn't really paying that close attention.
Reply | Report Abuse | Link to thisWayne Williamson - Mixing apples, oranges and grapes.
I don't think that optical computing provides any benefit relative to electronic computing regarding parallel computer architectures.
I agree there seems to be no entanglement going on here and don't understand the qubit reference.
I particularly like the prospects for really advanced computing:
"They can exist in two states simultaneously and thereby represent both the 0 and 1 of binary code at the same time."
I always wished I could use parallel processing to increase the uncertainty in my programs, but I never dreamed of parallel entangled quantum uncertain computing...
"which would offer unbreakable data security" From what I understand quantum algorithms do not create unbreakable security but make it so one knows if their message has been intercepted or tampered with. If this is incorrect I would be interested to know precisely how "quantum security" works.
Reply | Report Abuse | Link to thisOne of the values of photon transmission as opposed to electron is surge. Photons simple ramp up faster and terminate much faster. Making the switching effect very fast.
Reply | Report Abuse | Link to thisElectrons, however, can be "stored" much easier. We already know how to make capacitors.
With light, not so much.
On the other hand, we can "see" light, electrons are much tougher to see. So, for display interfaces it's light. For storage it's electrons.
But all bets are off once we find the Higgs.
Quinn the Eskimo - Excellent points.
Reply | Report Abuse | Link to thisI don't quite see how a device utilizing photon's switching speed advantage for a single photon emission could produce a device whose switch complexity is comparable to current electronic computers, without electronic components. I probably just lack the vision of the optical visionaries.
If the Higgs is ever found, I'll concede all my objections...
Ok, it's been a long time but since I just read this article others might as well so I thought I would add a few answers.
Reply | Report Abuse | Link to thisElectrons moving around produce electromagnetic noise in their environment which poses many very real difficulties for continued miniaturization. Nearby activity increasingly causes false signals/noise as you shrink the electronic components.
Photons do not interfere with one another directly (see first sentence of the article: "Point two laser beams so that they cross each other, and each goes through as if the other one did not exist"), so that makes them ideal for moving information around in very tiny spaces. Furthermore, it's not known how you would gate electric flow with a single atom, which is being demonstrated here.
As to the question of 'storage', I quote from the article "For example, it could store photons and release them at will".
Electrons do not generally move anywhere near the speed of light but the group velocity is much higher than the individual speed of an electron. When you turn on a switch the light comes on almost instantly but the individual electrons are just barely moving.
What you need to understand is the charge carrier density (you can google it). For a 100 volt, 1 amp DC circuit with 2 mm copper wire an electron would zip around at about 2 meters per DAY. With AC the electrons just jiggle back-and-forth slightly. The tricky electric company is selling you the same few electrons over and over again :) -- technically they are selling you the force to jiggle them back and forth.
Thought Experiment: Why would it take TREMENDOUS energies and very specialized equipment to accelerate electrons to near light speed in a particle accelerator if you could do it by just turning on the toaster?
If electrons were being accelerated with sufficient force to get them anywhere near the speed of light at the flick of your light switch we would be awash in cherenkov radiation.