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## Keio University

Skip to 0 minutes and 4 seconds I am here with Professor Yasunobu Nakamura at the University of Tokyo in his laboratory. Professor Nakamura built the first superconducting qubit and now he runs his research group here at the University of Tokyo. Professor Nakamura, welcome. Hello. I want to ask you what are you doing here? Okay, here, we are running experiments on superconducting qubit, so as you see here in our cryostat, we have superconducting qubit devices at the very bottom. So inside this can, there is a vacuum and at the very end the temperature is very low, like 10 millikelvin and superconducting qubit is there and we have all the control wiring to send and read out the signal.

Skip to 0 minutes and 51 seconds So we send microwave pulses to the device and then read out the signal coming back from the device through the electronics. So here we have all the control electronics to create microwave pulses and then read out microwave signals, so by doing that, we can control the superposition and entanglement of the superconducting qubit. I see, thank you Professor Nakamura. I want to ask to you how data is represented in superconducting system and what kind of state variable can we use in superconducting system?

Skip to 1 minute and 25 seconds As you know, when electrical circuit becomes superconducting, there is no resistance in the circuit, that is very important for maintaining quantum information in our circuits and if we properly design the circuit with a small Josephson junction, then we can create quantum bits in our system. There, we use either charge or flux degrees of freedom in our circuit. Charge means Cooper pair in superconducting system where two electrons are combined and flux is a quantum of magnetic field, which is confined in superconducting circuit. Okay, what is Josephson junction?

Skip to 2 minutes and 10 seconds Josephson junction is a small tunnel junction, which means there is a thin insulating barrier between two superconducting electrodes and there Cooper pairs – a pair of superconducting electrons can tunnel free across the junction and the junction works as a non-linear inductive element which is very important for making Josephson superconducting qubit. Okay, which type of state variable is more popular today? Actually, neither of them.

Skip to 2 minutes and 45 seconds So both kind of qubits using charge degrees of freedom or flux degrees of freedom do work properly, but each of them can cover charge noise or flux noise rather strongly, so actually the most popular design these days is kind of in the middle where we use quantum states which is rather insensitive to those noise in the environment. I see. Thank you professor Nakamura.

# Charge and magnetic flux

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 $$|1\rangle$$ state, and the absence of the pair as the $$|0\rangle$$ 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 $$|0\rangle$$ state and counterclockwise as our $$|1\rangle$$ (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.