Super-fast quantum search achieved with individual atoms

December 2, 2005
An abstraction of two cadmium ions entangled

Researchers at the University of Michigan have been able to use a small quantum computer consisting of two atoms to do a super-fast data base search. This same system could someday be scaled to a much larger quantum computer that could outperform any conventional computer for certain applications.

Image: An abstraction of two cadmium ions entangled. The bright yellow balls are the ions (painted in cadmium yellow), and all the fun stuff in between depicts the "spooky action-at-a-distance" that is responsible for the power behind quantum computing. Painting by former U-M post doc Boris Blinov.

The super-fast search is called Grover's Quantum Search Algorithm, and it can be used to search unsorted databases for specific information. If you wanted to find a name belonging to a phone number in the phonebook, Grover's algorithm could be used to search for the corresponding name much faster than using a normal computer. For example, for a phone book with 1 million names, it would only take 1,000 "looks" to find the right match—the square root of 1 million—instead of an exhaustive search over all 1 million entries in the phone book.

The search was implemented using two atoms, each of which stores a single bit of information, for a total of four possible states. It's a system that increases exponentially, so by adding one atom the memory doubles, said Christopher Monroe, professor of physics and co-author of a paper on the topic, "Implementation of Grover's Quantum Search Algorithm in a Scalable System," appearing in the November issue of Physical Review.

"You don't have to add too many atoms before you have a huge system," he said. The research was led by graduate student Kathy-Anne Brickman in Monroe's research group at the U-M Department of Physics and the FOCUS Ultrafast Optics Center.

In this case, using the hypothetical phone book analogy, researchers used four numbers and tried to find the corresponding name. After looking only once, the algorithm was successful in finding the correct answer 60 percent of the time, better than the maximum possible success rate of 50 percent using a normal computer.

To understand how it works, think of the four states as a single wave, Monroe said. Researchers can manipulate the wave to mark any one of the four states and "look" at the system by zapping it with a specially tuned laser, which makes the atoms interact in certain ways. This involves the "entanglement" of the two atom bits, or a special linking that is only allowed in quantum systems. Einstein called entanglement "spooky action at a distance," and it is this feature of quantum physics that allows the fast search.

To test the algorithm, researchers marked one of the four states by adjusting the part of the wave corresponding to that particular state. Then, by manipulating the laser and entangling the atoms, researchers were able to make the incorrect values cancel out one another through quantum interference, leaving only the marked state.

"When we look at this four-state system, we can look at it in a way that you can't do in a regular phone book," Monroe said. "We don't want to exhaustively look at all possibilities before uncovering which one was marked. While this is obviously a very small quantum computer, the main point is that this exact system can be efficiently scaled to much larger memories."

For more information on Monroe's group, visit:

Source: University of Michigan

Explore further: A life: Hawking defied ALS to become pre-eminent physicist

Related Stories

Neutrons reveal the wild Weyl world of semimetals

February 20, 2018

The observation of an abnormal state of matter in a two-dimensional magnetic material is the latest development in the race to harness novel electronic properties for more robust and efficient next-generation devices.

Genetic targets for autism spectrum disorder identified

February 20, 2018

Autism is a spectrum of closely related symptoms involving behavioral, social and cognitive deficits. Early detection of autism in children is key to producing the best outcomes; however, searching for the genetic causes ...

Recommended for you

World's biggest battery in Australia to trump Musk's

March 16, 2018

British billionaire businessman Sanjeev Gupta will built the world's biggest battery in South Australia, officials said Friday, overtaking US star entrepreneur Elon Musk's project in the same state last year.

Plasmons triggered in nanotube quantum wells

March 16, 2018

A novel quantum effect observed in a carbon nanotube film could lead to the development of unique lasers and other optoelectronic devices, according to scientists at Rice University and Tokyo Metropolitan University.

Researchers measure gene activity in single cells

March 16, 2018

For biologists, a single cell is a world of its own: It can form a harmonious part of a tissue, or go rogue and take on a diseased state, like cancer. But biologists have long struggled to identify and track the many different ...


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.