Bang! Bang! Let's kick these qubits!
Is this a sports story or a scientific step closer towards quantum supercomputers? You'll be the judge. But researchers from Oxford University have found a way to maintain a quantum bit (qubit) in a stable state by locking it up inside a buckyball. Then they kicked it repeatedly "with a strong pulse of microwaves which reverses the way in which it interacts with the environment," creating a bang-bang effect. This technique is pretty expensive and costs £7 million pounds per gram (US $12 million or 10 million Euro), so you will not see a quantum supercomputer before several years.
First, what is quantum computing?
The idea behind quantum computing is based on quantum mechanics, which allow an entity, such as an atom, to exist in multiple states simultaneously. Quantum computing is seen as the holy grail of computing because each individual piece of information, or ‘bit’, would exist in more than one state at once, making processing billions of times faster and thus dramatically widening the scope of what computers can do.
There’s just one problem: no-one knows how to build a quantum computer yet. The biggest hurdle is that the quantum state is only maintained as long as the quantum entity does not interact with anything.
And with an idea coming from -- I guess -- their love of soccer game, scientists from the Materials Science Department, led by John Morton and Simon Benjamin, decided to lock a qubit in a buckyball. But this was not enough to keep the qubit isolated from its environment.
The next step the researchers took was to apply the so-called 'bang-bang' method: the qubit is repeatedly hit with a strong pulse of microwaves which reverses the way in which it interacts with the environment. Dr John Morton said: 'The loss of information is like a child at a party game running with a blindfold on. We keep regularly turning the child around. If we do this quickly enough, the information remains intact (i.e. the child never gets very far).'
Below is an illustration showing some of the experiments. On the left part, you can see how "the natural evolution between two nuclear spin states of the nitrogen atom can be disrupted by the application of decoupling pulses (top: with microwave pulses applied on the electron spin at regular intervals; bottom: with a phase shift less than Π). On the right part, arbitrary nuclear phase gates have been implemented by driving two electron spin transitions simultaneously (top: only one transition; bottom: with both transitions detuned by equal and opposite amounts) (Credit for image and caption: Oxford University).
This research work has been published by Nature Physics under the title "Bang-bang control of fullerene qubits using ultrafast phase gates" (January 2006, Volume 2, No 1, Pages 40-43).
Here are some quotes from the abstract.
Quantum mechanics permits an entity, such as an atom, to exist in a superposition of multiple states simultaneously. Quantum information processing (QIP) harnesses this profound phenomenon to manipulate information in radically new ways. A fundamental challenge in all QIP technologies is the corruption of superposition in a quantum bit (qubit) through interaction with its environment.
Quantum bang–bang control provides a solution by repeatedly applying 'kicks' to a qubit, thus disrupting an environmental interaction. However, the speed and precision required for the kick operations has presented an obstacle to experimental realization. Here we demonstrate a phase gate of unprecedented speed3, 4 on a nuclear spin qubit in a fullerene molecule, and use it to bang–bang decouple the qubit from a strong environmental interaction. We can thus trap the qubit in closed cycles on the Bloch sphere, or lock it in a given state for an arbitrary period.
For more information, here is a link to the full paper (PDF format, 5 pages, 287 KB). [Please note that the Oxford server is currently unavailable.] The above illustration has been extracted from this document -- and redimensioned for layout purpose.
A final question remains -- at least for me: will this technique really have a strong impact on this research field and lead to quantum supercomputers? Please send me your thoughts.
Sources: University of Oxford news release, January 4, 2006; and various web sites
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