By Jason Palmer
Science and technology reporter, BBC News
Researchers have created a “quantum state” in the largest object yet.
Such states, in which an object is effectively in two places at once, have until now only been accomplished with single particles, atoms and molecules.
In this experiment, published in the journal Nature, scientists produced a quantum state in an object billions of times larger than previous tests.
The team says the result could have significant implications in quantum computing.
One of the pillars of quantum mechanics is the idea that objects absorb and emit energy in tiny discrete packets known as quanta.
This can be seen in a piece of coloured glass, which absorbs a certain colour of light.
That light is made up of photons – packets of light energy – and the glass atoms absorb only photons with the quanta (or amount) of energy that corresponds to that colour.
What we see through the glass is the light that has not been absorbed.
At the atomic level, quantum mechanics predicts – and experiments demonstrate – a number of surprising effects beyond that, however.
If all the energy that an atom gets from the jostling atoms in its environment is removed by cooling it to phenomenally low temperatures, it can reach its “quantum ground state” – no more energy can be removed.
If just one quantum of energy is then carefully put back in a certain way, the atom can be said to be in two states at the same time: a superposition of states.
Although only one quantum of energy is put in, any measurements will show either zero or one quanta; strictly, the atom has both.
Down to ground
These superpositions of states have long been predicted to be useful for a pursuit known as quantum computing; if used in place of the zeroes and ones of digital computing, a quantum computer would be vastly more powerful.
However, creating these states in anything bigger than single atoms and molecules has proven difficult, because the larger an object is, the more tricky it becomes to isolate it from its environment and put it in its ground state.
“There is this question of where the dividing line is between the quantum world and the classical world we know,” said Andrew Cleland of the University of California, Santa Barbara.
“We know perfectly well that things are not in two places at the same time in our everyday experience, but this fundamental theory of physics says that they can be,” he told BBC News.
Now, Professor Cleland and his team have moved that dividing line, using an object just big enough to be seen with the naked eye.
They used a tiny piece of what is known as a piezoelectric material, which expands and contracts when an electrical current is run through it.
A current applied at a certain frequency causes it to expand and contract regularly and, just like a violin string, the material has a frequency at which it is inclined to vibrate.
They connected this resonator to an electric circuit that the team has been developing for three years. This can be tuned to put in just one quantum of electrical energy.
Markus Aspelmeyer University of Vienna
They cooled the whole apparatus down to a thousandth of a degree above absolute zero and confirmed that their resonator was in its quantum ground state.
The researchers designed the system so that they could “pump in” just one quantum of electrical energy at a time and see the oscillator begin to vibrate as it converted that quantum into one quantum of vibrational energy.
As it vibrated, the team showed that the resonator was in one of the slippery superpositions of states, with both one and zero quanta of energy.
Sensors and sensibility
The result is a huge push toward answering the question of whether quantum mechanical effects simply disappear in objects beyond a certain size.
“As far as mechanical objects are concerned, the dividing line was at around 60 atoms,” Professor Cleland said.
“With this experiment, we’ve shown that the dividing line can be pushed up all the way to about a trillion atoms.”
The ability to create these superpositions of states and to read them out using the same circuit that created them would make for a quantum-based memory storage system – the heart of a potential quantum computer.
Markus Aspelmeyer of the University of Vienna believes that the mechanical oscillator approach will, in time, prove its worth in the business of quantum computing.
“What they’ve shown here is a mechanical oscillator as a completely new quantum system, and I personally think it’s a really important one,” he told BBC News.
“It means that you can now utilise mechanical resonators in quantum experiments and that opens a completely new perspective, in particular for quantum information science.”
Although these tiny resonators could be made in huge arrays using techniques that are standard in the computer industry, Professor Cleland says that using different systems based on photons instead of vibrations would most likely perform better in any eventual computers.
But, he said, the devices might be used in reverse, to detect the tiniest of vibrations that are created when light interacts with matter or when chemical reactions take place.
In either case, these devices have added to the debate about quantum mechanics and whether its surprising and, as Albert Einstein famously put it, “spooky” effects play a role in the everyday objects around us.
“I don’t think there is a limit, that there will be a certain size where quantum mechanics starts to break down,” Dr Aspelmeyer said.
“The larger we go, it becomes increasingly difficult and we will bump into more and more practical limitations. So the only reason that things could break down is that we run out of money.”
Published: 2010/03/17 19:16:59 GMT
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