科学美国人60秒:Rainbow Photons Pack More Computing Power

Computers transmit information in bits: 0’s and 1’s. Off or on. But in quantum computers, that off/on logic becomes more flexible. Because the bits in quantum computing, called quantum bits or qubits, can actually be 0 AND 1 at the same time.

Think of it like this: a coin flipping through the air can be considered to be both heads and tails before it lands. In that way, it's like a qubit. Until you measure it, the qubit is 0 and 1, just as until the coin lands, it’s both heads and tails.   

But the quantum weirdness goes beyond simple qubits. Because physicists have also created pairs of photons, particles of light, called qudits, with a D, for multiple dimensions. And those dimensions are different colors of light. So instead of being just 0 and 1, like a regular quantum bit, these photon qudits are, for example, simultaneously pink and purple and red and orange. 

In fact, they can come in as many as 10 different colors…which mathematically means 10 different dimensions…compared to the two dimensions of a qubit. Again, if that sounds crazy, just think of a 10-sided die spinning through the air, instead of a two-sided coin. It's all 10 sides at once. The study is in the journal Nature. [Michael Kues et al., On-chip generation of high-dimensional entangled quantum states and their coherent control]

The big advantage of quantum bits, and the point of developing them, is they contain more information than a classical bit—meaning quantum computers can use them, in theory, to solve super complicated equations much faster than a normal computer could. And qudits? They can hold even more information.

study author Roberto Morandotti at the National Scientific Research Institute in Montreal, says the equipment they used to work with the qudits is mostly cheap, off-the-shelf telecoms gear. Which suggests quantum computing could someday become accessible to many of us. But for the time being, the world’s only commercially available quantum computers exist in just one state: expensive.

—Christopher Intagliata