Compressed ‘bits’ store tons of quantum data

"This work sheds light on some of the striking differences between information in the classical and quantum worlds," Aephraim Steinberg says. (Credit: Paul Hocksenar/Flickr)

Scientists recently demonstrated that it’s possible to compress quantum bits, or qubits, without losing information. The ability to compress quantum information—just as we do with digital data—could open up huge potential for more powerful computing.

They also showed that the compression would scale exponentially. So it would require only 10 qubits to store all of the information about 1,000 qubits, and only 20 qubits to store all of the information about a million.

Digital compression in the world of classical information theory is fairly straightforward. As a simple example, if you have a string of 1,000 zeros and ones and are only interested in how many zeros there are, you can simply count them and then write down the number.

In the quantum world it’s more complicated. A qubit can be in a “superposition” between both zero and one until you measure it, at which point it collapses to either a zero or a one.

Why it’s complicated

Not only that, but you can extract different values depending on how you make the measurement. Measured one way, a qubit might reveal a value of either zero or one. Measured another way it might show a value of either plus or minus.


So, you don’t want to collapse the quantum state of the qubit until you’re ready to. Once you’ve made a single measurement, any other information you might have wanted to extract from the qubit disappears.

You could just store the qubit until you know you’re ready to measure its value. But you might be dealing with thousands or millions of qubits.

“Our proposal gives you a way to hold onto a smaller quantum memory but still have the possibility of extracting as much information at a later date as if you’d held onto them all in the first place,” says Aephraim M. Steinberg of the University of Toronto and a senior fellow at the Canadian Institute for Advanced Research (CIFAR).

In the experiment, Lee Rozema, a researcher in Steinberg’s lab and lead author on the paper, prepared qubits in the form of photons, which carried information in the form of their spin and in their path.

The experiment showed that the information contained in three qubits could be compressed into only two qubits—and that it can be compressed exponentially.

One caveat is that the information has to be contained in qubits that have been prepared by an identical process. However, many experiments in quantum information make use of just such identically prepared qubits, making the technique potentially very useful.

“This work sheds light on some of the striking differences between information in the classical and quantum worlds. It also promises to provide an exponential reduction in the amount of quantum memory needed for certain tasks,” Steinberg says.

The paper will appear in an upcoming issue of Physical Review Letters.

The Natural Sciences and Engineering Research Council of Canada supported the project.

Source: University of Toronto via CIFAR