Washington
Quantum computing with light
Researchers at MIT and Harvard University have described an experiment that allows a single photon to control the quantum state of another photon.
The result could have wide-ranging consequences for quantum computing and quantum communication.
To date, the most promising approach to building quantum computers has been to use ions trapped in electric fields. Using photons — particles of light — instead would have many advantages, but it’s difficult to get photons to interact: Two photons that collide in a vacuum simply pass through each other.
Slowing light
Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT and associates have developed an optical switch that consists of a small cluster of cesium atoms suspended between two tiny mirrors in a vacuum cavity. “The only way to make two photons interact with one another is to use atoms as a mediator,” Vuletic says. “The [first] photon changes the state of the atom, and therefore it modifies the atom’s interaction with the other photon.”
When a photon enters the cavity, it begins bouncing back and forth between the mirrors, delaying its emission on the other side. If another photon has already struck the cesium atoms, then each pass through them delays this second photon even more. The delay induced by a single pass through the atoms would be imperceptible, but the mirror-lined cavity, Vuletic explains, “allows us to pass the photon many, many times through the atoms. In our case, it’s like passing the photon 40,000 times through the atoms.”
When it emerges from the cavity, the second photon thus has two possible states — delayed or extra-delayed — depending on whether another photon has preceded it. With these two states, it could, in principle, represent a bit of information. And if the first photon was in some weird quantum state, where it can’t be said to have struck the atoms or not, the second photon will be both extra-delayed and not extra-delayed at the same time. The cavity would thus serve as a quantum switch, the fundamental building block of a quantum computer.
Quantum Internet
Currently, the extra delay is not quite long enough that delayed and extra-delayed photons can be entirely distinguished, but if the researchers can increase its duration, the switch could have other uses as well. Many potential applications of quantum optics, such as quantum cryptography, quantum communication, and quantum-enhanced imaging, require photons that are emitted in definite numbers — usually one or two. But the most practical method of emitting small numbers of photons — a very weak laser — can promise only an average of one photon at a time: There might sometimes be two, or three, or none. The CUA researchers’ switch could be tailored to separate photons into groups of one, two or three and route them onto different paths.
Because the switch allows the state of one photon to determine that of another, it could also serve as an amplifier in a quantum Internet, increasing the strength of an optical signal without knocking the individual photons out of superposition. It could also serve as a probe that detects photons without knocking them out of superposition, improving the efficiency of quantum computation.
Read more: http://goo.gl/yy2jh
Researchers at MIT and Harvard University have described an experiment that allows a single photon to control the quantum state of another photon.
The result could have wide-ranging consequences for quantum computing and quantum communication.
To date, the most promising approach to building quantum computers has been to use ions trapped in electric fields. Using photons — particles of light — instead would have many advantages, but it’s difficult to get photons to interact: Two photons that collide in a vacuum simply pass through each other.
Slowing light
Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT and associates have developed an optical switch that consists of a small cluster of cesium atoms suspended between two tiny mirrors in a vacuum cavity. “The only way to make two photons interact with one another is to use atoms as a mediator,” Vuletic says. “The [first] photon changes the state of the atom, and therefore it modifies the atom’s interaction with the other photon.”
When a photon enters the cavity, it begins bouncing back and forth between the mirrors, delaying its emission on the other side. If another photon has already struck the cesium atoms, then each pass through them delays this second photon even more. The delay induced by a single pass through the atoms would be imperceptible, but the mirror-lined cavity, Vuletic explains, “allows us to pass the photon many, many times through the atoms. In our case, it’s like passing the photon 40,000 times through the atoms.”
When it emerges from the cavity, the second photon thus has two possible states — delayed or extra-delayed — depending on whether another photon has preceded it. With these two states, it could, in principle, represent a bit of information. And if the first photon was in some weird quantum state, where it can’t be said to have struck the atoms or not, the second photon will be both extra-delayed and not extra-delayed at the same time. The cavity would thus serve as a quantum switch, the fundamental building block of a quantum computer.
Quantum Internet
Currently, the extra delay is not quite long enough that delayed and extra-delayed photons can be entirely distinguished, but if the researchers can increase its duration, the switch could have other uses as well. Many potential applications of quantum optics, such as quantum cryptography, quantum communication, and quantum-enhanced imaging, require photons that are emitted in definite numbers — usually one or two. But the most practical method of emitting small numbers of photons — a very weak laser — can promise only an average of one photon at a time: There might sometimes be two, or three, or none. The CUA researchers’ switch could be tailored to separate photons into groups of one, two or three and route them onto different paths.
Because the switch allows the state of one photon to determine that of another, it could also serve as an amplifier in a quantum Internet, increasing the strength of an optical signal without knocking the individual photons out of superposition. It could also serve as a probe that detects photons without knocking them out of superposition, improving the efficiency of quantum computation.
Read more: http://goo.gl/yy2jh
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