  Semiconductor qubits are appealing for building quantum processors as they
may be densely integrated due to small footprint. However, a high density
raises the issue of noise correlated across different qubits, which is of
practical concern for scalability and fault tolerance. Here, we analyse and
quantify in detail the degree of noise correlation in a pair of neighbouring
silicon spin qubits ~100 nm apart. We evaluate all a-priori independent auto-
and cross- power spectral densities of noise as a function of frequency. We
reveal strong inter-qubit noise correlation with a correlation strength as
large as ~0.7 at ~1 Hz (70% of the maximum in-phase correlation), even in the
regime where the spin-spin exchange interaction contributes negligibly. We
furthermore find that fluctuations of single-spin precession rates are strongly
correlated with exchange noise, giving away their electrical origin. Noise
cross-correlations have thus enabled us to pinpoint the most influential noise
in the present device among compelling mechanisms including nuclear spins. Our
work presents a powerful tool set to assess and identify the noise acting on
multiple qubits and highlights the importance of long-range electric noise in
densely packed silicon spin qubits.
