MIT Technology Review -
Mike Grinolds et al at Harvard University have cracked the problem of increasing qubits using MRI (magnetic resonance imaging, previously stuck at about 7 qubits). They have shrunk the business end of a magnetic resonance machine to the size of a pinhead. They've done it by placing a powerful magnet at the scanning tip of an atomic force microscope. In this way, they can create a powerful magnetic field gradient in a volume of space just a few nanometres across. That allows them to stimulate and control the magnetic resonance of single electrons. They've tested their device on so-called nitrogen vacancies in diamond. These are created by burying single atoms of nitrogen in thin sheets of diamond. Quantum physicists are fascinated with these vacancies because they are well protected from the outside world and so stable, and are easy to see by the photons they emit.
Quantum control of individual spins in condensed matter systems is an emerging field with wide-ranging applications in spintronics, quantum computation, and sensitive magnetometry. Recent experiments have demonstrated the ability to address and manipulate single electron spins through either optical or electrical techniques. However, it is a challenge to extend individual spin control to nanoscale multi-electron systems, as individual spins are often irresolvable with existing methods. Here we demonstrate that coherent individual spin control can be achieved with few-nm resolution for proximal electron spins by performing single-spin magnetic resonance imaging (MRI), which is realized via a scanning magnetic field gradient that is both strong enough to achieve nanometric spatial resolution and sufficiently stable for coherent spin manipulations. We apply this scanning field-gradient MRI technique to electronic spins in nitrogen-vacancy (NV) centers in diamond and achieve nanometric resolution in imaging, characterization, and manipulation of individual spins. For NV centers, our results in individual spin control demonstrate an improvement of nearly two orders of magnitude in spatial resolution compared to conventional optical diffraction- limited techniques. This scanning-fi eld-gradient microscope enables a wide range of applications including materials characterization, spin entanglement, and nanoscale magnetometry.
The control and manipulation of individual spins using magnetic fi eld gradients is independent of the method used for spin readout. For optically addressable spins, such as NV spins, integrating far- field, sub-diff raction schemes - such as stimulated emission depletion (STED) and reversible saturable optical linear fluorescence (spin-RESOLFT) - with a scanning magnetic field gradient would allow for both robust individual spin control and readout with nanometric resolutions. Additionally, selective optical control of such systems is possible via the incorporation of an electric field gradient to the scanning tip, which would allow both spin and electronic degrees of freedom to be both addressed individually. Alternatively, using demonstrated single-shot electrical readout of individual spins would allow for MRI to be performed rapidly and efficiently, as acquisition times would not be limited by the readout integration time. Individual control of spins via the present technique is also extendable to nuclear spins, provided methods for reading out nuclear spins reach single-spin sensitivity. Nanoscale or atomic spatial resolution of nuclear spins is feasible as their long spin coherence times help to compensate for nuclear spins' small dipole moment. For any spin system, providing individual control of spins in dense ensembles, where mutual coupling is strong, allows for the creation of arbitrary entangled states. Such states have intriguing potential applications ranging from sensitive nanoscale magnetometers to scalable quantum information processors
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