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Charge and magnetic flux/磁荷と磁束

Charge and magnetic flux/磁荷と磁束
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.
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?
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?
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.
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.







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