The neighbor atom, as shown in red, is used to calculate the measurements of the blue atom. (Image Credit: RIKEN)


Researchers from the RIKEN Center for Emergent Matter Science in Japan have successfully taken repetitive measurements of the spin of an electron in a silicon quantum dot (QD), without altering the spin. This process of “non-demolition” measurement is crucial for building quantum computer systems that are fault-tolerant. The team published their findings in the Journal nature communications on March 2, 2020.


Quantum computers have the ability to perform certain sets of calculations a bit easier, such as many-body problems more efficiently, which are very challenging and time-consuming for traditional computers. Mainly, this involves quantifying a quantum value that doesn’t exist in a single state, like a conventional transistor, but instead, it exists as a “superimposed state,” similar to the Schrodinger’s cat concept. By using these systems, calculations can be performed with a qubit that is a superimposition of two values, and the correct result can be statistically determined. Quantum computers using single electron spins in silicon QDs are often seen as attractive devices mainly because of their promising scalability and silicon is commonly used in a wide range of electronic devices.


However, one of the main challenges when developing quantum computers is that they are extremely susceptible to external noise, which makes error fixes important. Nowadays, researchers have successfully built single electron spins in silicon QDs with very precise quantum operation along with a lengthy information retention time. However, quantum non-demolition measurement, a necessity for effective error correction, is rather complex. Traditional techniques to read out single electron spins in silicon involve converting the spins into charges that can be quickly recognized, but the detection process affects the electron spin.


The researchers were able to carry-out the non-demolition measurement by applying the Ising type interaction model, which is a model of ferromagnetism that observes how electron spins of adjacent atoms become aligned, resulting in ferromagnetism being formed in the entire lattice.  This allowed them to transfer the spin data from one electron in a QD to another in the adjacent QD by using the Ising type interaction in a magnetic field. The team can then measure the electron spin of the neighboring atoms using the traditional method, leaving the original spin intact, which also allowed them to rapidly and repetitively measure the adjacent atoms.


“Through this, we were able to achieve a non-demolition fidelity rate of 99%, and by using repeated measurements would get a readout accuracy of 95%. We have also shown that theoretically, this could be increased to out 99.6%, and plan to continue work toward reaching that level,” says Seigo Tarucha, who led the research group.


He continued, “This is very exciting, because if we can combine our work with high-fidelity single- and two-qubit gates, which are currently being developed, we could potentially build a variety of fault-tolerant quantum information processing systems using a silicon quantum-dot platform.”


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