Editors Note: Do you ever feel like a fish out of water? Try being a tech novice and talking to an engineer at a place like Google. Ask a Techspert is a new series on the Keyword asking Googler experts to explain complicated technology for the rest of us. This isnt meant to be comprehensive, but just enough to make you sound smart at a dinner party.
Quantum computing sounds like something out of a sci-fi movie. But its real, and scientists and engineers are working to make it a practical reality. Google engineers are creating chips the size of a quarter that could revolutionize the computers of tomorrow. But what is quantum computing, exactly?
The Keywords very first Techspert is Marissa Giustina, a research scientist and quantum electronics engineer in our Santa Barbara office. We asked her to explain how this emerging technology actually works.
What do we need to know about conventional computers when we think about quantum computers?
At a first glance, information seems like an abstract concept. Sure, information can be stored by writing and drawinghumans figured that out a long time ago. Still, there doesnt seem to be anything physically tangible about the process of thinking.
Enter the personal computer. Its a machinea purely physical objectthat manipulates information. So how does it do that, if its a physical machine and information is abstract? Well, information is actually physical. Computers store and process rich, detailed information by breaking it down. At a low level, a computer represents information as a series of bits. Each bit can take a value of either  or , and physically, these bits are tiny electrical switches that can be either open  or closed . Emails, photos and videos on YouTube are all represented by long sequences of bitslong rows of tiny electrical switches inside a computer.
The computer computes by manipulating those bits, like changing between  and  (opening or closing a switch), or checking whether two bits have equal or opposite values and setting another bit accordingly. These bit-level manipulations are the basis of even the fanciest computer programs.
Ones and zeros, like “The Matrix.” Got it. So then what is a quantum computer?
A quantum computer is a machine that stores and manipulates information as quantum bits, or qubits, instead of the classical bits we were talking about before. Quantum bits are good at storing and manipulating a different kind of information than classical bits, since they are governed by rules of quantum mechanicsthe same rules that govern the behavior of atoms and molecules.
Whats the difference between a bit and a qubit?
This is where it gets more complicated. Remember that a classical bit is just a switch: it has only two possible configurations: [open] or [closed]. A qubits configuration has a lot more possibilities. Physicists often think of a qubit like a little globe, with  at the north pole and  at the south pole, and the qubits configuration is represented by a point on the globe. In manipulating the qubit, we can send any point on the globe to any other point on the globe.
At first, it sounds like a qubit can hold way more information than a regular bit. But theres a catch: the rules of quantum mechanics restrict what kinds of information we can get out of a qubit. If we want to know the configuration of a classical bit, we just look at it, and we see that the switch is either open  or closed . If we want to know the configuration of a qubit, we measure it, but the only possible measurement outcomes are  (north pole) or  (south pole). A qubit that was situated on the equator will measure as  50 percent of the time and  the other 50 percent of the time. That means we have to repeat measurements many times in order to learn about a qubits actual configuration.
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Ask a Techspert: What is quantum computing? – blog.google
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