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Molecular spintronics for could help make quantum computing a reality

Spintronics is an emerging field of nanoscale electronics  whose central theme is the active manipulation of spin degrees of freedom of charge carriers in solid-state systems, in addition to their electronic charge used in traditional semiconductor electronics. Spintronics or spin electronics in contrast to conventional electronics uses the spin of electrons for sensing, information storage, transport, and processing.


Molecular spintronics in particular proposes to use molecules to perform spintronic functions in conventional applications and beyond. Molecular spintronics aims for the ultimate step towards miniaturization of spintronics by striving to actively control the spin states of individual molecules.


Quantum information science is the emerging field of science and engineering which seeks to leverage the quantum mechanics to perform computations, communication, and measurements that classical systems cannot. In quantum information, the primary building block is the qubit, a quantum system that can exist in a coherent superposition of two distinguishable states. A critical requirement for any physical qubit is preparation of a pure initial state. In addition, the preparation of two-qubit entangled states is necessary to execute fundamental quantum gate operations.


Qubits have ben implemented across many physical systems, including trapped ions, superconductors and diamonds, But, unfortunately, these require a near-perfect vacuum, extremely low temperatures and no disturbances to operate. They are also hard to scale up.


Spintronics also enable quantum computer based on electron  spins in tiny particles of semiconductors. Quantum computers use “qubits” – which can take any value between zero and one – giving them huge processing power. While normal electronic devices use electric charge to represent information as zeros and ones, quantum computers often use electron “spin” states to represent qubits. According to quantum mechanics, each electron in a material spins in a combination (superposition) of these states – a certain bit up and a certain bit down.


Electron and nuclear spins are two examples of potential qubits, however unless very high magnetic and extremely low temperatures are used, they have weak initial polarization. Photoexcitation of a covalent organic donor-acceptor molecule having a well-defined structure can result in sub-nanosecond electron transfer to produce a spin-entangled radical ion pair having an initial pure singlet spin configuration even at room temperature.


Now, scientists are realising that spintronics can also be done in organic molecules containing rings of carbon atoms. And that connects it with a whole other research field called molecular electronics, which aims to build electronic devices from single molecules and films of molecules.


The combination has proven useful. By carefully controlling and manipulating an electron’s spin within a molecule, it turns out we can actually do quantum computations. The preparation and readout of the electron’s spin state on molecules is made by zapping them with electric or magnetic fields.


Carbon-based organic molecules and polymer semiconductors also address the criteria of being easy to scale up. They do this through an ability to form molecular frameworks, within which molecular qubits sit in close proximity with each other. The tiny size of a single molecule automatically favours packing large numbers of them together on a small chip.


In addition, organic materials disturb quantum spins less than other electronic materials do. That’s because they are composed of relatively light elements such as carbon and hydrogen, resulting in weaker interactions with the spinning electrons. This avoids its spins from easily flipping state, causing them to be preserved for long periods of up to several microseconds. In one propeller-shaped molecule, this duration can even be up to a millisecond. These relatively long times are sufficient for operations to be performed – another great advantage.


But we still have much left to learn. In addition to understanding what causes extended spin lifetimes on organic molecules, a grasp on how far these spins can travel within organic circuits is necessary for building efficient spin-based electronic circuits.


There are also major challenges in getting such devices to work efficiently. The charged electrons that carry spins in an organic material constantly hop from molecule to molecule as they move. This hopping activity is unfortunately a source of electrical noise, making it difficult to electrically measure small spin current signatures using conventional architectures. That said, a relatively new technique known as spin pumping might prove suitable for generating spin currents with low noise in organic materials.


Another problem when trying to make organic molecules serious candidates within future quantum technologies is the ability to coherently control and measure spins on single molecules, or on a small number of molecules. This grand challenge is currently seeing tremendous progress. For example, a simple program for a quantum computer known as “Grover’s search algorithm” was recently implemented on a single magnetic molecule. This algorithm is known to significantly reduce the time necessary to perform a search on an unsorted database.


Wolfgang Wernsdorfer (Karlsruhe Institute of Technology, Germany) reported a prominent example of all-electrical nuclear-spin control in a terbium-based SMM entrapped in a metallic nanojunction. Though much less magnetic than electrons, atomic nuclei also carry a spin that is weakly coupled to its electronic counterpart. Relying on such a coupling, scientists were able to coherently drive transitions between the four nuclear spin states of terbium by perturbing the electron cloud with an oscillating electric field. These results bridge nanospintronics to quantum technologies, which have the potential to drive disruptive innovations in computation methods, secure communication and sensing


In another report, an ensemble of molecules were successfully integrated into a hybrid superconducting device. It provided a proof-of-concept in combining molecular spin qubits with existing quantum architectures. It provided a proof-of-concept in combining molecular spin qubits with existing quantum architectures.



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