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Charge and magnetic flux

Let's take a look at the laboratory of Dr. Nakamura of the University of Tokyo which conducts the latest research.

In a superconductor, there is no electrical resistance. If you start an electrical current flowing, it will run forever. It can also maintain the state of individual quanta. In this visit with Professor Yasunobu Nakamura of the University of Tokyo, we learn about three ways of using this special capability to make the quantum states we use for qubits.

The first state variable was charge. In a superconductor, electrons form Cooper pairs, in which two electrons, which normally repel each other, become loosely bound to each other, and behave together. A charge qubit uses the presence of a Cooper pair in a small, isolated island as the (|1rangle) state, and the absence of the pair as the (|0rangle) state.

The second choice of state variable is magnetic flux. Electrical current flowing in a loop creates a magnetic field, the basis for all electromagnets. If we have a microscopic loop of superconductor, the current can flow either clockwise or counterclockwise around the loop, so we can use clockwise as our (|0rangle) state and counterclockwise as our (|1rangle) (or vice versa).

The third type of state variable is an intermediate between the two, known as a transmon.

The key to all three of these state variables is being able to control the presence or absence of Cooper pairs very precisely. A Josephson junction is a tiny gap (perhaps only a few atoms across) in the metal conductor. It might seem that such a gap would prevent current from flowing (unless the voltage is high enough to make spark across the gap, but the voltages and energies here are much, much too small for that). However, because of the way that the quantum probability amplitude waves work, there is a small probability that our Cooper pair will tunnel through this barrier. Used appropriately, at close to absolute zero, this gives us the ability to control very precisely the number of Cooper pairs, giving us the states we can use as our state variable. In an upcoming article, we will learn more about the hardware necessary.








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Understanding Quantum Computers

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