Researchers at the University of Copenhagen in Denmark have found a way to boost the sensitivity of a routine sensing technique known as magnetic induction tomography beyond the standard quantum limit. The improved method could find application in bio- and medical sensing.
In magnetic induction tomography, a magnetic field generated by a current-carrying coil produces minute eddy currents in the sample being analysed. These currents, in turn, alter the magnetic field, which is detected using the collective spin (or magnetization) of an atomic magnetometer. The properties of the detected field yield information about the electrical conductivity and magnetic permeability of the sample.
The technique is used in geophysical surveys, to non-destructively test metallic objects, as well as in medical imaging. But its sensitivity is constrained by the so-called quantum limit, or quantum fluctuations (uncertainty) of the sensor’s collective spin.
“Indeed, quantum mechanics and the uncertainty principle dictate that the spin direction cannot be determined with arbitrary precision,” explains Eugene Polzik, who led this new study. “Roughly speaking, in a sensor that contain N atomic spins, the direction of the collective spin cannot be determined with an angular certainty better than 1/√N, and it is this that we call the standard quantum limit (SQL).”
Reducing uncertainty
Polzik and colleagues showed that this uncertainty can be reduced by using an atomic magnetometer containing atoms whose spins are entangled to generate a so-called spin squeezed state. The angular uncertainty of one of the projections of this state is below the SQL. The researchers arranged the magnetic induction tomography protocol such that the useful signal is contained exactly in the projection with the reduced uncertainty. This approach results in a SQL sensitivity that is almost twice that of conventional atomic magnetometers.
“Conventional magnetic induction tomography techniques use a coil to detect the signal,” explains Polzik. “Such coils have intrinsic thermal noise, as well as picked-up environmental noise, which limits sensitivity. We have used an atomic sensor made of spins whose noise is only limited by intrinsic quantum fluctuations. This allowed us to substantially improve the sensitivity compared to conventional approaches.”
Atomic magnetometer measures cardiac conductivity
The researchers say they now plan to use their method in bio- and medical sensing, and in particular hope to develop it further for imaging internal organs, including the heart and even the brain.
“We also plan to continue working on this quantum-enhanced magnetic induction tomography with the goal of further improving its sensitivity and spatial resolution,” Polzik tells Physics World.
The research is detailed in Physical Review Letters.
