Atomic circuitry and quantum computing

Conventional supercomputers have limitations: they are logical and fast, certainly, can be run in parallel grids across the globe, but when it comes down to solving problems with no logical answer, such as cracking sophisticated encryption, working out the travelling sales-rep problem of logistics and deliveries, or modelling the climate, they have serious limitations.

A quantum computer, on the other hand, could find all the answers almost instantaneously and pluck out the most appropriate based on probabilities and quantum mechanics. Building such a quantum computer is not proving simple. Now, US researchers have demonstrated that they can exert delicate control over a pair of atoms within a mere seven-millionths-of-a-second window that suggests the necessary atomic circuitry for a quantum computer might one day be possible.

“At some point in time you get to the limit where a single transistor that makes up an electronic circuit is one atom, and then you can no longer predict how the transistor will work with classical methods,” explains physicist Mark Saffman of the University of Wisconsin-Madison. “You then have to use the physics that describes atoms – quantum mechanics.” In the quantum realm, new possibilities for processing information emerge that mean certain types of problems could be solved exponentially faster on a quantum computer than on any foreseeable classical computer.

Mark Saffman
Mark Saffman

Working with colleague Thad Walker, Saffman and co-workers have successfully used atoms to create a controlled-NOT (CNOT) gate, a basic type of circuit that will be an essential element of any quantum computer. They describe details of the work in the journal Physical Review Letters and explain that this is the first demonstration of a quantum gate formed between two uncharged atoms.

The use of neutral rubidium atoms chilled to a fraction of a degree above absolute zero, rather than charged ions or other materials, distinguishes this achievement from previous work. “The current gold standard in experimental quantum computing has been set by trapped ions … People can run small programs now with up to eight ions in traps,” explains Saffman. However, to be useful for computing applications, systems must contain enough quantum bits, or qubits, to be capable of running long programs and handling more complex calculations. An ion-based system presents challenges for scaling up because ions are highly reactive, which makes them difficult to control.

Thad Walker
Thad Walker

“Neutral atoms have the advantage that in their ground state they don’t talk to each other, so you can put more of them in a small region without having them interact with each other and cause problems,” Saffman says. “This is a step forward toward creating larger systems.” The team is now working towards arrays of up to 50 atoms to test the feasibility of scaling up the system.


Phys. Rev. Lett. 2010, 104, 010503

Mark Saffman

Thad Walker

Tiny acid drops

The world’s smallest droplet of acid has been produced by scientists working at very low temperatures. The droplet of hydrochloric acid comprises just a single hydrogen chloride (HCl) molecule and four water molecules. This seemingly esoteric piece of ultracold chemistry could have significant implications for our understanding of how chlorine-containing compounds such as chlorofluorocarbons (CFCs) damage the ozone layer in the upper atmosphere.

Writing in the 19th June issue of the journal Science, the team based at Ruhr-Universität Bochum, in Germany, explain that how acids dissociate into charged fragments, and are then surrounded by water molecules, or are solvated, at ultracold temperatures in nanoenvironments is very different from their behaviour in bulk volumes of water at room temperature. Such behaviour occurs outside our everyday experience in the upper atmosphere and in interstellar chemistry and is so far poorly understood, the researchers say.

Chemists Anna Gutberlet, Gerhard Schwaab, Özgür Birer, Marco Masia, Anna Kaczmarek, Harald Forbert, Martina Havenith, and Dominik Marx, formed their acid drop in superfluid helium at a fraction of a degree above absolute zero, 0.37 Kelvin.

In order to study this minimalist acid, the team used the powerful analytical technique of high-resolution mass-selective infrared laser spectroscopy. This tool revealed how the successive accumulation of water molecules around the central HCl molecule to form HCl(H2O)n, quickly leads to the formation of the hydronium ion (H3O+) when n = 4. Hydronium is essentially a water molecule with an added hydrogen ion, or proton, and it is the fundamental acidic species in solutions of hydrogen chloride and other acidic compounds.

To augment their studies, the team carried out a simulation of the process from first principles. This ab initio simulation uses the structures of the molecules and the various energies involved and predicts the same step-by-step assembly of undissociated clusters that the team observed. It also points to electrostatic “steering” of the water molecules around the HCl up to n = 3 which form an intermediate ring-shaped system.

The simulation explains how the addition of the fourth water molecule to this undissociated ring HCl(H2O) 3 spontaneously causes the structure to dissociate into an ion pair H3O+ with (H2O)3Cl–. Somehow, the aggregation mechanism bypasses the deep local energy minima that exists when four water molecules attempt to surround the HCl. “This offers a general paradigm for reactivity at ultracold temperatures,” the researchers explain in their Science paper.

Tiniest droplets of hydrochloric acid (Credit: Image courtesy of Ruhr-Universitaet-Bochum)

Science, 2009, 324, 1545-1548

Background reading wins physics Nobel

This year’s Nobel Prize for Physics cuts right to the chase, to one of the biggest questions that has vexed humanity ever since we first pondered the world around us – Where did we come from? In 1989, NASA launched a satellite to try and provide an answer. The COBE satellite would measure the background radiation, the microwave relics of the Big Bang that started it all and give us a glimpse at our very beginnings.

As part of a 1500 strong research team, the new Nobel physicists, John Mather of the NASA Goddard Space Flight Center, in Greenbelt, Maryland, and George Smoot of the University of California, Berkeley, used the readings from COBE to look back at the infancy of the Universe to learn how stars and galaxies were first formed.

Dr John C Mather (credit NASA)

Dr John C Mather (credit NASA)

A mere 380,000 years after the Big Bang, not even a blink of the eye given that the Universe is almost 14 billion years old, it had cooled sufficiently for photons to separate, or decouple, from matter. At this time the Universe was much hotter than it is today, but these photons travelling through ever-expanding space read just 2.7 degrees above absolute zero on COBE’s cosmic thermometer.

COBE also observed smallest variations in this temperature in different directions throughout the Universe. These differences of just a hundredth of a degree in the cosmic background radiation offer an important clue as to how the first galaxies formed and are reflected in the iconic image pasted on the covers of dozens of newspapers and magazines the world over when Smoot and his colleagues broke the news that they had a picture from the dawn of spacetime. These temperature variations reveal how matter began to aggregate, a process that ultimately led to stars and planets, the galaxies and eventually sentient life on Earth that would billions of years later ponder its own existence.

Professor George Smoot (credit Berkeley)

Professor George Smoot (credit Berkeley)

The success of COBE was the outcome of prodigious teamwork coordinated by Mather. Smoot’s main responsibility was to measure those tiny variations in temperature across the cosmos. The pair will share the 10m Swedish Krona (£0.72m, $1.4m) prize money.

Cosmic background

Cosmic background

Further reading

Advanced information on the 2006 physics Nobel prize

Dr. John C. Mather

Professor George Smoot

Suggested searches

cosmic microwave background
Big Bang
star formation