Magnetic Quantum Dots
Optical control of a magnetic ion’s spin state in semiconductor quantum dots (QDs) is one of the exciting possible futures for magnetic data storage. These objects are particularly interesting as they may – in principle – be used as single bit cells far smaller than those currently used in Hard Disc Drives (HDD) in which miniaturization is limited by the magnetic interaction between adjacent bits.
“Like Raisins in a Cake”
A semiconductor quantum dot can be understood as a piece of semiconductor material, small enough to exhibit quantum confinement effects in all dimensions. They are often called artificial atoms, and have analogues with the quantum wire (1D – one dimension) or quantum well (2D – two dimensions). Quantum dots containing a single magnetic ion are produced using advanced epitaxy methods, such as molecular beam epitaxy, to create a suspension of quantum dots in a semiconductor matrix – like raisins in a cake.
Controlling With Light
Different approaches to control the spin-state of magnetic ions have emerged. Firstly, direct interaction of the ion and circularly polarized laser light can force a change in the electron configuration and consequently to change its spin state. That strategy has been extensively studied with nitrogen-vacancy (N-V) centers in diamond but still has to be developed in the field of quantum dots. Another idea is based upon the exchange interaction between ions and polarized excitons, which can be produced in a semiconductor using polarized laser light. An exchange interaction can take place because excitons have non-zero magnetic moment which can then force the ion’s magnetic moment to align along a specific direction. What is more, excitons are likely to reside in the quantum dots which leads to effective interaction between an ion in a quantum dot and an exciton. It is also easier to find and manipulate quantum dots with a single magnetic ion inside than just single magnetic ion.
Several systems with quantum dots have been studied so far . Successful and promising data write and read-out was demonstrated for example for a single Mn2+ ion in CdTe(dot)/ZnTe(barrier) [2,3], Mn2+ in InAs/GaAs , and Mn2+ in CdSe/ZnSe .
Difficulties To Overcome
Despite these successes, there are still many difficulties to overcome which make quantum dots an object of scientific investigations rather than currently applicable devices. The main challenges are:
- existence of non-radiative ways of exciton recombination – which can make write or read-out operations impossible;
- spin relaxation times – which limit the ‘memory’ of a device to a few hundred microseconds;
- low-temperature demands of the experiments. You’ll struggle to sell a hard drive that needs a bath of liquid helium to run in…
Having said that, let’s cross our fingers for magnetic ion quantum dots, whose impressive results reveal the beauty of quantum mechanics and offer a real opportunity for technological advance.
 Kobak J, Smolenski T, Goryca M, Papaj M, Gietka K, Bogucki A, Koperski M, Rousset JG, Suffczynski J, Janik E, Nawrocki M, Golnik A, Kossacki P, & Pacuski W (2014). Designing quantum dots for solotronics Nature communications (5) DOI: 10.1038/ncomms4191
 Goryca M, Kazimierczuk T, Nawrocki M, Golnik A, Gaj J, Kossacki P, Wojnar P & Karczewski G (2009). Optical Manipulation of a Single Mn Spin in a CdTe-Based Quantum Dot Physical Review Letters (103) DOI: 10.1103/PhysRevLett.103.087401
 Le Gall C, Besombes L, Boukari H, Kolodka R, Cibert J & Mariette H (2009). Optical Spin Orientation of a Single Manganese Atom in a Semiconductor Quantum Dot Using Quasiresonant Photoexcitation Physical Review Letters (102) DOI: 10.1103/PhysRevLett.102.127402
 Baudin E, Benjamin E, Lemaître A & Krebs O (2011). Optical Pumping and a Nondestructive Readout of a Single Magnetic Impurity Spin in an Quantum Dot Physical Review Letters (107) DOI: 10.1103/PhysRevLett.107.197402
 Smolenski T, Pacuski W, Goryca M, Nawrocki M, Golnik A & Kossacki P (2015). Optical spin orientation of an individual Mn2+ ion in a CdSe/ZnSe quantum dot Physical Review B (91) DOI: 1408.2928v1