WEBVTT

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Before we look at how to build
a superconducting qubit,

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let's briefly discuss the requirements
we have on the system.

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These are the so-called DiVincenzo criteria,

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and they serve as a good guideline of what we need
to incorporate in our quantum processor.

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First, we need the qubit itself,

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acting as a two-level system
in which we can encode the two states, 0 and 1.

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Next, we need to be able to initiate the state
of the qubit in the beginning of the computation.

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For superconducting qubits, this is done
by operating the qubits at 10 mK.

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The cryogenic environment
will put the qubits in their ground state.

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As we saw in Module 1, it is important
that the qubits have long coherence times,

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which will allow us to implement
longer quantum algorithms

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containing a large number of quantum gates.

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Therefore, the coherence times need to be long

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compared to the time it takes
to perform qubit gate operations.

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This brings us to the next point, which is access
to a universal set of quantum gates.

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What we mean by a universal set of gates is that
the linear combination of the gates in the set

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can be used to generate any possible operation
on the qubit register.

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One example of such a universal set of gates
is shown within the brackets to the left here.

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We will return to discussing what these gates mean
and how to implement them,

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but for now I just want to point out
that the first four are single qubit gates

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whereas the last one, the CNOT gate,

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is a 2-qubit entangling gate.

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Finally, we need to be able to read out the state
of each individual qubit after the computation.

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As for the qubit gate operations,
it is important that the readout is also performed

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on a short time scale
compared to the qubit coherence time.

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For superconducting qubits today,
the coherence time is around 50 to 100 μs,

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whereas the readout duration is below 1 μs.

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Preferably, the readout should be performed
in a single shot,

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which means that we will not need
to re-run the protocol several times

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to take the average result
in order to know what the qubit state is.