 Single-quantum coherence

Single-quantum coherence is a component of NMR coherence with coherence order $p=+1$ or $p=-1$. It connects basis states whose total spin projections differ by one unit. Under the convention $U(\varphi )=e^{-i\varphi F_z}$, an order-$p$ component acquires $e^{-ip\varphi }$.^[ref]

For one spin one-half nucleus, $I_{+}$ and $I_{-}$ are the positive- and negative-order components. Transverse magnetization is represented by their combinations,A Hermitian density matrix contains conjugate off-diagonal entries, so it can encode two real transverse components while containing both coherence-order signs.

An original state example usesIts eigenvalues are0.8 and0.2 for every $\varphi$, and its populations remain one half. Taking the traces against $I_x,I_y$ givesAt $\varphi =\pi /2$, the transverse expectation has moved from $+x$ to $+y$ without changing the populations. This is a constructed quantum-state comparison, not a measured thermal polarization.

Conventional transverse NMR detection can access the appropriate single-quantum contribution. The complex receiver signal chooses a phase convention for combining its two measured components. This is why a statement about a detected phase sign should specify its convention, while the operator transformation above remains explicit.^[ref]

Multispin single-quantum terms need not be merely isolated-spin magnetization: correlations can accompany the projection change. A coherence order identifies the transformation under a chosen common rotation, rather than uniquely specifying every spin correlation or the pulse sequence that generated it.