imredesiuk / Shutterstock
Quantum microscopy uses quantum principles to capture images and take measurements at the sub-atomic scale. Scanning tunnelling microscopes and photoionization microscopes are two common microscopy tools that apply quantum effects. Researchers are also currently investigating a ‘quantum electron microscope’ based on an idea called ‘interaction-free measurement’.
Scanning Tunnelling Microscopy
The scanning tunnelling microscope (STM) generates images by passing an extremely sharp metal tip over a conductive surface. By placing this tip extremely close to a surface and by applying an electrical charge, it is possible to image the sample at the atomic scale.
An STM is founded on multiple scientific principles, one of which is the quantum effect called tunnelling. This effect occurs when electrons proceed through a barrier that they shouldn’t be capable of going through.
In conventional terms, sufficient energy is required for matter to pass thorough a barrier at the atomic level. However, at the quantum level, electrons exhibit wavelike qualities. Electrons "waves" don’t stop suddenly at a barrier but decrease rapidly in strength.
If a barrier isn't sufficiently thick, the wave function may be able to pass through the barrier. Due to the small likelihood of an electron passing to the opposite side of the barrier, and provided there are enough electrons encountering this barrier, some amount of charge passes through to the other side of a sample being imaged by an STM. Due to the sharp decrease of the wave function through the barrier, the amount of charge that tunnels is based on the width of the barrier.
Based on the setup on an STM, the origin of a tunnelling electron can be either the tip or sample. Recording the tunnelling current allows for precise control of the distance between the tip of an STM and the sample.
Wavefunction is a main principle of quantum theory. Simply put, it can provide the most knowledge available on the condition of a quantum system. The square of a wavefunction indicates the likelihood of where a particle may be located at a set point in time. Even though it is quite prominent in quantum theory, directly gauging or seeing wavefunction is not easy, as direct observation disrupts the wavefunction before detection is possible.
By using photons to create ions, a process known as photoionization, scientists have been capable of evaluating various facets of an atom’s electron orbital structure. Atomic photoionization in an electric field offers a distinctive opportunity where quantum qualities of the wavefunction are extrapolated to macroscopic levels.
This imaging method starts with placing an atom in an electric field and exciting that atom with laser pulses. A resulting ionized electron breaks away from the atom in a trajectory until it strikes a sensor, which is at a 90-degree angle to the field. Given that there are numerous trajectories that can hit the same point on the sensor, interference patterns can be identified and then magnified. The interference pattern produced by this technique is representative of the electron structure of the sample atom.
To generate images, standard electron microscopes use radiation that can be harmful, even destructive, to biological samples. In recent years, researchers have been investigating a new kind of electron microscope based on quantum effects that could image biological samples with minimal damage.
The new kind of microscope is based on an idea known as interaction-free measurement (IFM). The technique has been investigated both in theory and experimentally using photons to probe samples, rather than electrons.
The principle behind IFM can best be explained on the single-particle level. After going through a beam splitter, a particle’s waveform is divided into two tracks that are still coherent with each other. If there is a sample in the first track, it will prevent the amplitude of the first waveform from being transferred to the second track. If there is no sample, the amplitude of the first track will completely transferred. This phenomenon, known as the quantum Zero effect, allows for an IFM system to detect a particle by inference, and without direct interaction.