Physicists Demonstrate Quantum Memory with Matter Qubits

Jul 03, 2009 By Lisa Zyga feature
Teleportation chambers used to trap, entangle, and teleport ions. Credit: Curt Suplee.

( -- For the first time, scientists have successfully operated a quantum gate between two remote particles of matter, marking an important step toward the development of a quantum computer. In previous experiments, researchers have used photons, which are difficult to store. Using matter qubits enables the researchers to store the obtained quantum information, opening up new possibilities for the generation of remote networks of entangled qubits.

Physicists Peter Maunz and coauthors from the University of Maryland Department of Physics and National Institute of Standards and Technology in College Park, Maryland, and the University of Michigan in Ann Arbor, Michigan, published their study in a recent issue of Physical Review Letters.

“Our work demonstrates a probabilistic remote entangling quantum gate,” Maunz told “Remote entangling gates are an essential building block for quantum repeaters which facilitate quantum communication over long distances. Furthermore, the remote link established by the entangling gate could be used to interconnect remote quantum processors and thus could be an important additional possibility to scale a future quantum computer.”

As the scientists explain, their quantum gate works by entangling two ytterbium ions, each confined in its own trap positioned one meter apart. The scientists suspended the ions into either a one or a zero state using . The use of ion traps prevents anything from interacting with the ytterbium. This allows the ions to hold states of both zero and one simultaneously so that the ions function as qubits.

Next, the scientists shined a laser on the ions, causing each ion to emit a single photon. The scientists collected a small amount of the emitted light from each ion and coupled the light into fibers. The outputs of the fibers were then sent to interfere at a beam splitter, after which single photons were detected. A simultaneous detection of two photons indicated the successful operation of the gate. Finally, the atomic states were identified to verify the claim.

Close-up image of trap electrodes. Credit: Curt Suplee.

As the physicists explain, this quantum gate could lead to the development of a “one-way” quantum computer. In this technique, a large collection of qubits is first prepared in a highly entangled state, and then single rotations and measurements are performed on the qubits to realize quantum computation. Since the measurements destroy the state of the qubits, the technique is called “one-way.” Because it has certain advantages, this technique may be easier to implement than the quantum circuit model, which is the conventional model of .

“The quantum circuit model and the one-way quantum computer a different but equivalent approaches to use quantum interactions and quantum entanglement for computation,” Maunz said. “The one-way quantum computer starts with a very sophisticated, highly entangled state of a collection of qubits. The computation is realized by measurements on single quantum bits and a classical feedforward of the results on rotations and measurements on the remaining qubits.”

In order to realize a useful one-way quantum computer, the physicists still face some challenges. For instance, they need to generate large collections of entangled qubits, since the current study demonstrates the technique with just one pair of qubits. The scientists also hope to improve the reliability and speed of their entanglement method.

“One focus of our research will be to increase the success probability of the remote gate,” Maunz said. “This can be done by increasing the fraction of scattered light that is collected and used for the gate. We hope to achieve an increase high enough to allow us to scale the entanglement to more than two ions. The remote entangling gate can also be combined with local gates which have been realized with trapped ions before. This should facilitate the development of deterministic remote quantum gates and the transmission of quantum information over larger distances.”

More information: P. Maunz, S. Olmschenk, D. Hayes, D.N. Matsukevich, L.-M. Duan, and C. Monroe. “Heralded Quantum Gate between Remote Quantum Memories.” Physical Review Letters 102, 250502 (2009).

Copyright 2009
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of

Explore further: Simon's algorithm run on quantum computer for the first time—faster than on standard computer

add to favorites email to friend print save as pdf

Related Stories

2 qubits in action, new step towards the quantum computer

Jun 14, 2007

Researchers at Delft University of Technology have succeeded in carrying out calculations with two quantum bits, the building blocks of a possible future quantum computer. The Delft researchers are publishing ...

Plugging the leaks in a quantum computer

Aug 12, 2005

New work by two researchers at HP Laboratories Bristol sets out to solve one of the major difficulties in quantum computer architectures that use directly interacting qubits.

A Quantum CPU: the Pentium Q?

May 23, 2006

A new design scheme for a quantum processor core makes potential quantum computers more technically feasible, more efficient, and in many cases faster by keeping all of the quantum bits active all the time, rather than switching ...

Recommended for you

Ultra-short X-ray pulses explore the nano world

2 hours ago

Ultra-short and extremely strong X-ray flashes, as produced by free-electron lasers, are opening the door to a hitherto unknown world. Scientists are using these flashes to take "snapshots" of the geometry ...

Physicist pursues superconductivity mysteries

6 hours ago

More than a quarter of a century after its discovery, high-temperature superconductivity still challenges condensed matter physicists. For Binghamton's Pegor Aynajian, the key to unlocking the mystery—which will ultimately ...

Why does coffee spill more often than beer? (w/ Video)

6 hours ago

Watch even the most careful waiter bring a cup of coffee to your table, and you'll realize that carrying liquid is not easy. When set in motion, the coffee starts sloshing, little waves appear, and spilling ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (1) Jul 03, 2009
We hope to achieve an increase high enough to allow us to scale the entanglement to more than two ions. The remote entangling gate can also be combined with local gates which have been realized with trapped ions before. This should facilitate the development of deterministic remote quantum gates and the transmission of quantum information over larger distances.

Well when you put it like that it just seems obvious.
1 / 5 (1) Jul 10, 2009

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.