Single superconducting qubits have been demonstrated by a number of groups. Only very

recently 2- and 3-qubit system have been implemented and shown to operate successfully.

Scaling-up to larger qubit numbers implies three well-identifiable stages: from 2 to 3 and

from 3 to ~ 10, and finally from 10 to “large”. The last step is well into the R&D stage,

requiring appropriate industrial methods for it to develop. Going from 3 to ~ 10 should be a

“straightforward” continuation of the preceding stage of going from 2 to 3 qubits.

Stepping from 2 to 3 qubits, two fundamental problems have to be addressed and resolved:

the controllable coupling/entanglement of all three individual 2-qubit pairs, and the

individual addressing and control (and readout) of each qubit. For 3 qubits (and with larger

numbers in mind) this requires proper topological and geometrical considerations, employing

symmetries in circuit layout and system operation. We believe that, once this 2-to-3-step is

realized in practice, the subsequent 3-to-10-step is within clear reach. The EuroSQIP Y4

objectives are to design, build, test and operate such integrated platforms containing 3 qubits.

With the additional readout and coupling elements the circuit will have a complexity of

typically 5-7 coupled quantum coherent components.

Fundamentally, there may be no clear distinction between quantum oscillators used for

processing, memory register, readout or communication. By "effective" qubits we will refer

to the number of coherent quantum components that can be coupled together and serve as

qubits. We intend to design flexible platforms that make efficient use of quantum resources. In

this way, a 3-qubit platform may operate with, say, 5-7 “effective” qubits for specific

purposes.

It goes without saying that some components and functions may be rudimentary or non-

existent at any given time due to the state of the art. During Y4, long-term storage memory

and communication interfaces will not be available but will have to be investigated and

designed. Long-term memory will involve hybrid circuits, coupling JJ-circuits to systems

with long coherence times, e.g. ion in microtraps, or cold molecular ensembles attached to

supeconducting microstrip resonators. Communication interfaces will couple JJ-circuits to

microwave transmission lines for both internal and external coherent communication and

entanglement via photons.