Introduction
Out of all the different types of levitation that use physics, quantum levitation is by far the coolest! What else can beat a disk that can be placed in any orientation and orbit around a track all while giving up what looks like cold mist. If you don't believe me, watch the video below and find out for yourself.
The Physics
Superconductivity We know that a conductor, usually metal, is a material that allows for the flow of electric charge. For a regular conductor there is always going to be some resistance that will depend on factors such as temperature, the length of the conductor, and the material being used [1]. What makes a certain conductor super? No, not dressing it up in a cape! Superconductors have several characteristics but the one that usually defines them is zero resistivity when the material is cooled below some critical temperature. The property of interest for quantum levitation, the Meissner Effect, will be discussed soon.
There are two main types of superconductors; they are categorized depending on their behaviour in magnetic fields. They are referred to as type-I and type-II superconductors. In both types of superconductors, there is a limit to how strong the magnetic field can be until the material loses its superconducting nature, but for type-II there is a transitional stage where the mixed-state Meissner effect, discussed later, may occur [2]. Type-II superconductors are what are required for quantum levitation, specifically stabilized quantum locking.
Meissner Effect Even though quantum levitation videos have only recently become popular, the Meissner Effect, pictured below, has been around for years; since 1933 to be exact. It is described as the expulsion of magnetic fields from the inside of a superconducting material when it is cooled below its critical temperature [3]. You can think of it as someone standing stationary with a mob of people running at him or her without making contact. In this case each person acts as a magnetic field line and the stationary person is someone who was recently placed in a super cooled refrigerator such that they can behave like a superconductor. This phenomenon does allow for unstable levitation, but for stabilized levitation there are additional requirements.
Quantum locking Quantum locking, also known as flux pinning, is a result of a mixed-state Meissner Effect and it allows for stable levitation. For type-II superconductors, there exist a state in which if a large enough magnetic field is applied the material will stay superconducting even though it allows for discrete portions, or a quantized amount, of the external magnetic field to pass through tiny imperfections in the surface of the material [4]. This causes it to become trapped. You can think of these lines as acting as the supports that lift a bridge, but in this case they are lifting a superconducting material. These channels are known as flux tubes and are pictured below [5].
The expulsion of the magnetic field in the presence of flux tubes is what allows for the superconductor to lock in place. The existing forces can be overcome, for example by an individual's strength, allowing for the superconductor to be placed in a new spatial orientation. When this occurs the flux tubes re-establish themselves in a new arrangement to ensure a new locking position.
Summary
Even though quantum levitation is a highly intriguing visual phenomenon its current applications are minimal due to temperature restrictions of superconductors. With current research geared towards finding higher temperature superconductors, future applications of quantum levitation may be possible. With this in mind, future applications may include frictionless transportation, such as high-speed trains and advanced dolly systems for assistance in moving objects.
