Scientists demonstrate quantum state exchange between light and matter

Quantum computers offer the promise of processing information much more efficiently than classical computers. But before quantum computers can be built, scientists must confront several challenges, one of which is quantum computers' vulnerability to their surroundings. Interaction with outside forces would immediately damage a quantum computer's information; this problem is known as "decoherence."

One method to coherently process quantum information involves cavity quantum electrodynamics (QED). In this method, scientists use a small cavity to achieve coherent dynamics between an atom and a photon by manipulating an atom's radiation properties with mirrors. Scientists from the California Institute of Technology are among the leaders in cavity QED, and have recently reported an important advance to enable a coherent distribution of quantum information across a network.

In their paper published in Physical Review Letters, physicist David Boozer and his colleagues have demonstrated the reversible state transfer of a coherent light pulse to and from the internal state of an atom trapped in an optical cavity. This observation is the first verification of atomic physicist Ignacio Cirac's proposal for the reversible mapping of quantum states between light and matter using cavity QED to provide strong coupling for the atom-photon interaction.

“The most significant result of this work is the demonstration of reversibility (i.e., coherence) for the light emission and absorption processes,” Boozer told PhysOrg.com. “The fact that this process is coherent means that it preserves superpositions of quantum states, hence it is a way of mapping quantum information between an atom and light.”

In quantum networks, qubits (the information states for quantum computers) can be represented by either atoms or photons. Atoms, which have long coherence times, serve as "stationary" qubits, or nodes of a network, where they are stored and locally manipulated. Photons, on the other hand, serve as "flying" qubits, or quantum channels that connect nodes over long distances. While many single-photon sources have been demonstrated in the past decade, none have been experimentally shown to be reversible until now.

“In principle, in a quantum computer there are several logic gates, each of which performs an elementary quantum operation on one or two stationary qubits,” Boozer explained. “The gates are connected together in a network, so that the output of one gate can be transported as a flying qubit to the input of the next gate. Hence, one needs a way to turn stationary qubits into flying qubits and vice-versa, which is what our recent work has demonstrated.”

In the Caltech scientists' experiment, a cesium atom is localized within the cavity by a far off-resonant optical trap, where it repeatedly undergoes a series of light absorption and reemission cycles, lasting a total of 360 ms. During each such cycle, the cavity is first illuminated by an incident pulse of coherent light. Whenever the atom-cavity system absorbs this pulse, the quantum state of the light is written onto the internal state of the atom.

After a delay of about 300 ns, the atomic state gets mapped back onto an emitted pulse of light, which is allowed to interfere with the source of the original coherent pulse. Observing the resulting interference fringe demonstrates the reversibility of the overall absorption-reemission process.

“Our optical cavity has a very small mode volume (the cavity length is only 42 microns), which ensures that the coherent interaction between the atom and light field occurs on a much faster time scale than the decoherence caused by atomic spontaneous emission or cavity leakage,” Boozer explained. “Thus the atom and cavity field can exchange quantum information coherently many times before an incoherent process occurs. This regime is known as strong-coupling in cavity QED.”

The scientists explain that the efficiency of the light-to-atom transfer is limited in this scenario by factors such as passive mirror losses, equal transmission coefficients of the cavity mirrors, and the coupling of the atom to both polarization modes of the cavity.

With the ability to reversibly transfer a qubit's state from "flying" to "stationary" and back again, the scientists have taken a step toward coherently transferring quantum information across a network, without disruption with the outside world. Still, Boozer and his colleagues look forward to future improvements.

“In the present work, the qubit is encoded in the photon-number states of light and in the hyperfine levels of the atom,” he said. “A more robust scheme which we may pursue in the future would be to instead use the polarization degree of freedom of the light, and the magnetic sublevels of the atom. Another future goal will be to increase the efficiency of the state transfer process, for instance by using cavity mirrors with unequal transmissivities and/or even higher reflectivities.”

Citation: Boozer, A. D., Boca, A., Miller, R., Northup, T. E., and Kimble, H. J. "Reversible State Transfer between Light and a Single Trapped Atom." Physical Review Letters 98, 193601 (2007).

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Related stories:

Terahertz laser source at room temperature
“There is a growing interest in utilizing terahertz radiation, or T-rays, for a variety of applications,” Mikhail Belkin, a scientist at Harvard University, tells PhysOrg.com. “The terahertz region is a part of the electromagnetic spectrum that lies between the radio waves and infrared/visible light.”
Precise Alignment to Quantum Dots
“Precise lithographic alignment to site-controlled quantum dots is of major importance for numerous nano-photonic, nano-electronic and nano-spintronic devices,” Sven Höfling tells PhysOrg.com.
Quantum computers take step toward practicality with demonstration of new device
Computers based on the powerful properties of quantum mechanics have the potential to revolutionize information technology and security, but for decades they have remained more theoretical than practical, and difficult to scale up. That is changing, however, as demonstrated in a report this week in the journal Science.
IBM researchers unveil green optical network technology prototype
IBM researchers today unveiled the fastest and most highly integrated optical data bus ever developed. The prototype technology could bring massive amounts of bandwidth in an energy-efficient way to all kinds of machines—from cell phones to supercomputers. This could revolutionize the way we access, use and share information across many different applications.
Scientists make single-photon sources brighter
Scientists have achieved a major advance in developing a single-photon light source, bringing quantum applications such as quantum computing and quantum cryptography closer to reality.
Researchers developed a quantum 'light switch'
Infinitely secure cryptography that renders any computer unhackable. Computers that can solve the structure of a complicated protein at the drop of a hat. Programs to decrypt complicated enemy secrets. Optical data connections up to 100 times faster than current technology allows.
NIST Debuts Superconducting Quantum Computing Cable
Physicists at the National Institute of Standards and Technology have transferred information between two “artificial atoms” by way of electronic vibrations on a microfabricated aluminum cable, demonstrating a new component for potential ultra-powerful quantum computers of the future. The setup resembles a miniature version of a cable-television transmission line, but with some powerful added features, including superconducting circuits with zero electrical resistance, and multi-tasking data bits that obey the unusual rules of quantum physics.
New optical microcavity could lead to more efficient quantum computing
Right now, there is no shortage of proposed architectures for quantum computers. Scientists are constantly looking for, and developing viable candidates for quantum information processing. And with the production of an open and scalable microcavity, the group of Ed Hinds at the Centre for Cold Matter at Imperial College, London thinks it might have found at least one possibility.

News discussion:

Multiferroics using UV Photon Induced Electric Field Poling in Physics news