QUANTUM COMPUTING: Atomic storage of photons may hold key to quantum cryptography

Working independently, research teams at the Georgia Institute of Technology (Atlanta, GA), Harvard University (Cambridge, MA), and the California Institute of Technology (Caltech; Pasadena, CA) have demonstrated elementary operations for quantum communications.

The Georgia Tech researchers observed that a single photon maintained its quantum and particle-like properties through a process of retrieval and transport from one atomic quantum-memory node to a different quantum-memory node where the photon was also stored and retrieved. Quantum information can be encoded in the polarization states of photons (light) or in the spin states of atomic nuclei (matter), so the ability to transmit, store, and retrieve quantum bits (qubits) of information requires an ability to map quantum states both from matter to light and from light to matter.^{1, 2}

Storage, retrieval, and transmission of quantum bits are essential operations for eventual construction of a quantum-communications network, rendered ultrasecure by a quantum-cryptography key that cannot be observed by a third party without alerting both sender and receiver. The key must be sent from sender to receiver to enable the receiver to decode the message. And over long distances, as in classical fiberoptic communications, a repeater is required to periodically correct for naturally occurring attenuation and distortion of the traveling optical signal. To develop repeater technology suitable for quantum communication using a collection of atoms to store one qubit, researchers at Georgia Tech and Harvard used the so-called DLCZ quantum information protocols, proposed in 2001, based on Raman scattering of a photon between two atomic ground states.

The Georgia Tech researchers excited a cloud of rubidium atoms stored in a magneto-optical trap at temperatures approaching absolute zero to release a photon carrying information about the excitation state of the atoms from which it was generated. The photon traveled along about 100 m of optical fiber to a second supercooled cloud of trapped rubidium atoms (see figure). To control the velocity of the photon, the researchers used a laser beam that was switched off once the photon was inside the cloud, allowing the photon to come to a halt inside the dense ensemble of atoms. It was then converted into a single collective atomic excitation using a dark-state polariton approach.

“The information from the photon is stored in the state of excitation of many atoms of the second ensemble,” explained Stewart Jenkins, a graduate student on the Georgia Tech research team. “Each atom in the ensemble is slightly flipped, so the atomic ensemble is sharing this information-which is really information about spin.”

Restarting the photon

After allowing the photon to be stored in the atomic cloud for time periods that exceeded 10 µs (two orders of magnitude longer than required to achieve conversion between photonic and atomic quanta), the control beam was turned back on, allowing the photon to re-emerge from the atomic cloud. The researchers then compared the quantum information carried on the photon to verify that it matched the information carried into the cloud.

“When the single photon is generated, the first atomic ensemble is in an excited state,” explained Thierry Chaneliere, a postdoctoral fellow in the Georgia Tech lab of Alex Kuzmich, who led the research effort. “When we read the information from the second ensemble and find a coincidence between its excitation and the excitation of the first ensemble, we have demonstrated storage of the photon.”

The Harvard group reported a similar approach to manipulation of quantum pulses of light in optically dense atomic ensembles, using electromagnetically induced transparency for the generation, transmission, and storage of single photons with tunable frequency, timing, and bandwidth. Electromagnetically induced transparency is widely used for controlling the propagation of classical multiphoton light pulses in applications such as efficient nonlinear optics. The Harvard team studied the interaction of single photons produced in a source ensemble of rubidium 87 atoms at room temperature with another target ensemble.³

Researchers at Caltech and Georgia Tech have also demonstrated the use of a single photon to achieve quantum entanglement between two separate massive quantum objects.^{4, 5}

REFERENCE

1. T. Chanelière et al., Kuzmich A, Nature438(7069) 833 (Dec. 8, 2005).

2. P. Grangier, Nature 438(7069) 749 (Dec. 8, 2005).

3. M. D. Eisaman et al., Nature438(7069) 837 (Dec. 8, 2005).

4. C.W. Chou et al., Nature 438(7069) 828 (Dec. 8, 2005).

5. D.N. Matsukevich et al., Physical Rev. Lett., in press.