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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Saturday, July 11, 2009

Controlling Individual Qubits in Quantum Computers

Nathan Lundblad

In a paper published in 'Nature Physics', a team of physicists from the National Institute of Standards and Technology (NIST) reports a viable way of manipulating a single “bit” in a quantum processor without disturbing the information stored in its neighbors -- thus overcoming another hurdle in the development of quantum computer. The approach, which makes novel use of polarized light to create “effective” magnetic fields, could bring the long-sought computers a step closer to reality.

A great challenge in creating a working quantum computer is maintaining control over the carriers of information, the “switches” in a quantum processor while isolating them from the environment. These quantum bits, or “qubits,” have the uncanny ability to exist in both “on” and “off” positions simultaneously, giving quantum computers the power to solve problems conventional computers find intractable – such as breaking complex cryptographic codes.

One approach to quantum computer development aims to use a single isolated rubidium atom as a qubit. Each such rubidium atom can take on any of eight different energy states, so the design goal is to choose two of these energy states to represent the on and off positions. Ideally, these two states should be completely insensitive to stray magnetic fields that can destroy the qubit’s ability to be simultaneously on and off, ruining calculations. However, choosing such “field-insensitive” states also makes the qubits less sensitive to those magnetic fields used intentionally to select and manipulate them. “It’s a bit of a catch-22,” says NIST’s Nathan Lundblad. “The more sensitive to individual control you make the qubits, the more difficult it becomes to make them work properly.”

Optical lattices use lasers to separate rubidium atoms (red) for use as information “bits” in neutral-atom quantum processors -- prototype devices which designers are trying to develop into full-fledged quantum computers. The National Institute of Standards and Technology (NIST) physicists have managed to isolate and control pairs of the rubidium atoms with polarized light, an advance that may bring quantum computing a step closer to reality [image credit: NIST]

To solve the problem of using magnetic fields to control the individual atoms while keeping stray fields at bay, the NIST team used two pairs of energy states within the same atom. Each pair is best suited to a different task: One pair is used as a “memory” qubit for storing information, while the second “working” pair comprises a qubit to be used for computation. While each pair of states is field- insensitive, transitions between the memory and working states are sensitive, and amenable to field control. When a memory qubit needs to perform a computation, a magnetic field can make it change hats. And it can do this without disturbing nearby memory qubits.

The NIST team demonstrated this approach in an array of atoms grouped into pairs, using the technique to address one member of each pair individually. Grouping the atoms into pairs, Lundblad says, allows the team to simplify the problem from selecting one qubit out of many to selecting one out of two – which, as they show in their paper, can be done by creating an effective magnetic field, not with electric current as is ordinarily done, but with a beam of polarized light.

The polarized-light technique, which the NIST team developed, can be extended to select specific qubits out of a large group, making it useful for addressing individual qubits in a quantum processor without affecting those nearby. “If a working quantum computer is ever to be built,” Lundblad says, “these problems need to be addressed, and we think we’ve made a good case for how to do it.”

But, Lundblad adds, the long-term challenge to quantum computing remains: integrating all of the required ingredients into a single apparatus with many qubits.

Reference
"Field-sensitive addressing and control of field-insensitive neutral-atom qubits"
N. Lundblad, J.M. Obrecht, I.B. Spielman, and J.V. Porto,
Nature Physics, July 5, 2009 (doi:10.1038/nphys1330).
Abstract.

[We thank NIST for materials used in this posting]

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1 Comments:

At 3:58 PM, Anonymous Anonymous said...

This is a most enlightening article.
Can you tell me if much work is ongoing to have a base360 method of calculation and representation in the next generation of quantum computers?
Scott, Glasgow, UK

 

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