December 19, 2011

Quantum Computing Improves Magnetic Imaging Precision by ten times

Pitt physicists have been able to obtain higher-precision measurements with “single-electrons in-diamond” approach. The paper documents important progress towards realizing a nanoscale magnetic imager comprising single electrons encased in a diamond crystal.

They have used quantum computing methods to circumvent hardware limitations to view the entire magnetic field. By extending the field, the Pitt researchers have improved the ratio between maximum detectable field strength and field precision by a factor of 10 compared to the standard technique used previously. This puts them one step closer toward a future nanoscale MRI instrument that could study properties of molecules, materials, and cells in a noninvasive way, displaying where atoms are located without destroying them; current methods employed for this kind of study inevitably destroy the samples.

Top: lattice structure of an NV centre in diamond. Bottom: energy-level diagram of the NV centre showing the working microwave (MW) spin transition that is sensitive to the external field Bex

Nature Nanotechnology - High-dynamic-range magnetometry with a single electronic spin in diamond

Magnetic sensors capable of detecting nanoscale volumes of spins allow for non-invasive, element-specific probing. The error in such measurements is usually reduced by increasing the measurement time, and noise averaging the signal. However, achieving the best precision requires restricting the maximum possible field strength to much less than the spectral linewidth of the sensor. Quantum entanglement and squeezing can then be used to improve precision (although they are difficult to implement in solid-state environments). When the field strength is comparable to or greater than the spectral linewidth, an undesirable trade-off between field strength and signal precision occurs1. Here, we implement novel phase estimation algorithms on a single electronic spin associated with the nitrogen-vacancy defect centre in diamond to achieve an ~8.5-fold improvement in the ratio of the maximum field strength to precision, for field magnitudes that are large (~0.3 mT) compared to the spectral linewidth of the sensor (~4.5 µT). The field uncertainty in our approach scales as 1/T0.88, compared to 1/T0.5 in the standard measurement approach, where T is the measurement time. Quantum phase estimation algorithms have also recently been implemented using a single nuclear spin in a nitrogen-vacancy centre. Besides their direct impact on applications in magnetic sensing and imaging at the nanoscale, these results may prove useful in improving a variety of high-precision spectroscopy techniques.

9 pages of supplemental information.

2. Nature Nanotechnology - High-dynamic-range magnetometry with a single nuclear spin in diamond
Sensors based on the nitrogen-vacancy defect in diamond are being developed to measure weak magnetic and electric fields at the nanoscale. However, such sensors rely on measurements of a shift in the Lamor frequency of the defect, so an accumulation of quantum phase causes the measurement signal to exhibit a periodic modulation. This means that the measurement time is either restricted to half of one oscillation period, which limits accuracy, or that the magnetic field range must be known in advance. Moreover, the precision increases only slowly (as T−0.5) with measurement time T. Here, we implement a quantum phase estimation algorithm on a single nuclear spin in diamond to combine both high sensitivity and high dynamic range. By achieving a scaling of the precision with time to T−0.85, we improve the sensitivity by a factor of 7.4 for an accessible field range of 16 mT, or, alternatively, we improve the dynamic range by a factor of 130 for a sensitivity of 2.5 µT Hz−1/2. Quantum phase estimation algorithms have also recently been implemented using a single electron spin in a nitrogen-vacancy centre9. These methods are applicable to a variety of field detection schemes, and do not require quantum entanglement.

3 pages of supplemental material

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