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A quantum bit, or qubit, is the basic unit of information for a quantum computer, analogous to a bit in ordinary machines.

But unlike a bit, which can have the value 0 or 1, a qubit can take on an infinite number of values. Physicists call these the states of the qubit.

It turns out that specifying a qubit's state is a lot like specifying your position on Earth using latitude and longitude. The yellow arrow in the figure below points to a spot at a particular latitude and longitude. Notice how the arrow moves when you change the sliders.

Just as two numbers can pinpoint a spot on a globe, there are two numbers that determine a qubit state. In general, that state will be a combination of two special quantum states, which are called 0 and 1 to match the naming convention for ordinary bits. These special states are like the north and south pole on a globe.

Although there are an infinite number of possible qubit values, observing a qubit’s state—by making a quantum measurement—yields either 0 or 1. The result of a given measurement is probabilistic and depends on the details of this combination. In particular, the chance of observing the qubit in one of the special states depends on its distance from either pole. (The opacity of the colored discs on the right side of the animation below represents this chance, with more opaque colors indicating a higher probability for 0 or 1.) Qubits with values on the equator of the sphere are equally likely to be 0 or 1 when measured, but they are all subtly different. As the arrow traces traces a path along the equator, different colors represent phase differences—numbers responsible for interference effects when two qubits interact. 

Different physical systems can store a qubit, such as the polarization of a photon, the spin of an electron or the amount of magnetic field that passes through the middle of a loop of superconductor. Researchers at JQI freqently use atoms to store qubits. Creating a real qubit in a lab requires detailed knowledge of these platforms, but the abstraction of a qubit allows scientists to treat them on equal an equal footing when studying their properties.

Finally, just as bits undergo digital logic gates like AND, OR and NOT, qubits, too, have gates. Visually, these are rotations of a qubit's state to a new position on the surface of the sphere. Quantum computations consist of preparing many qubits in a certain state, rotating them and making measurements.

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Quantum physics began with revolutionary discoveries in the early twentieth century and continues to be central in today’s physics research. Learn about quantum physics, bit by bit. From definitions to the latest research, this is your portal. Subscribe to receive regular emails from the quantum world. Previous Issues...

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