  The hyperpolarisation of nuclear spins within target molecules is a critical
and complex challenge in magnetic resonance imaging (MRI) and nuclear magnetic
resonance (NMR) spectroscopy. Hyperpolarisation offers enormous gains in signal
and spatial resolution which may ultimately lead to the development of
molecular MRI and NMR. At present, techniques used to polarise nuclear spins
generally require low temperatures and/or high magnetic fields, radio-frequency
control fields, or the introduction of catalysts or free-radical mediators. The
emergence of room temperature solid-state spin qubits has opened exciting new
pathways to circumvent these requirements to achieve direct nuclear spin
hyperpolarisation using quantum control. Employing a novel cross-relaxation
induced polarisation (CRIP) protocol using a single nitrogen-vacancy (NV)
centre in diamond, we demonstrate the first external nuclear spin
hyperpolarisation achieved by a quantum probe, in this case of $^1$H molecular
spins in poly(methyl methacrylate). In doing so, we show that a single qubit is
capable of increasing the thermal polarisation of $\sim 10^6$ nuclear spins by
six orders of magnitude, equivalent to an applied magnetic field of $10^5$\,T.
The technique can also be tuned to multiple spin species, which we demonstrate
using both \C{13} and $^1$H nuclear spin ensembles. Our results are analysed
and interpreted via a detailed theoretical treatment, which is also used to
describe how the system can be scaled up to a universal quantum
hyperpolarisation platform for the production of macroscopic quantities of
contrast agents at high polarisation levels for clinical applications. These
results represent a new paradigm for nuclear spin hyperpolarisation for
molecular imaging and spectroscopy, and beyond into areas such as materials
science and quantum information processing.
