Valley Degree of Freedom: Harnessing Electron Valleys for Quantum Technologies

What is the Valley Degree of Freedom?

The valley degree of freedom is a quantum property of electrons in certain materials, where the energy bands have multiple minima or "valleys" in the momentum space. These valleys are regions of low energy where electrons can reside, and they can be treated as a discrete degree of freedom, similar to the spin of an electron. The valley degree of freedom offers a new platform for encoding, manipulating, and processing quantum information, opening up opportunities for valleytronic devices and quantum technologies.

Valley Physics in 2D Materials

The valley degree of freedom is particularly prominent in two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs) like MoS₂ and WSe₂. These materials have a hexagonal lattice structure, which gives rise to two inequivalent valleys (K and K') in the electronic band structure. The valley-dependent properties of these materials can be exploited for various applications:

Valleytronic Devices: The ability to control and manipulate the valley degree of freedom enables the development of valleytronic devices, such as valley filters, valley valves, and valley-based logic gates. These devices can process information using the valley index, complementing or even replacing traditional electronic devices.
Quantum Information Processing: The valley degree of freedom can be used as a qubit, the fundamental unit of quantum information. Valley qubits can be initialized, manipulated, and read out using optical and electrical methods, making them promising candidates for quantum computing and communication.
Optoelectronic Applications: The valley-dependent optical selection rules in 2D materials allow for the selective excitation and detection of valley-polarized carriers. This property can be harnessed for applications such as valley-based light-emitting diodes (LEDs), photodetectors, and solar cells.

Controlling the Valley Degree of Freedom

To harness the potential of the valley degree of freedom, researchers have developed various methods to control and manipulate valley states:

Optical Control

Circularly polarized light can be used to selectively excite electrons in a specific valley, thanks to the valley-dependent optical selection rules. By using left- or right-handed circularly polarized light, researchers can initialize and read out valley states, enabling valley-based quantum information processing.

Electrical Control

Electric fields can be used to control the valley degree of freedom through the valley Hall effect. By applying an in-plane electric field, electrons from different valleys can be separated and accumulated at opposite edges of the material, leading to valley-polarized currents. This effect can be used for valley-based electronic devices and sensors.

Magnetic Control

Magnetic fields can also influence the valley degree of freedom through the valley Zeeman effect. In the presence of a magnetic field, the energy levels of the valleys split, allowing for the selective population and manipulation of valley states. This effect can be exploited for valley-based spintronics and quantum sensing applications.

Challenges and Future Perspectives

Despite the exciting possibilities offered by the valley degree of freedom, there are still challenges to overcome for its widespread application. One of the main challenges is the preservation of valley coherence, which is essential for reliable quantum information processing. The interaction of valley states with their environment, such as lattice vibrations and impurities, can lead to decoherence and loss of information.

Future research will focus on developing techniques to enhance valley coherence and reduce decoherence effects. This may involve the use of high-quality, defect-free 2D materials, as well as the design of valley-based quantum error correction schemes. Additionally, the integration of valley-based devices with existing electronic and photonic technologies will be crucial for practical applications.

Another area of interest is the exploration of valley physics in novel material systems, such as twisted bilayer graphene and van der Waals heterostructures. These systems offer new opportunities for engineering valley-dependent properties and creating hybrid valley-spin-orbit devices.