First silicon-based, long-lasting nuclear spin qubit created by quantum researchers
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Researchers at the University of New South Wales in Australia have created the first quantum bit (qubit) based on the nuclear spin of an atom, within a silicon transistor. This breakthrough is significant for two reasons: The qubit produced by the researchers is highly stable — and it’s in silicon, meaning it can be wired up and controlled electronically, just like a conventional computer chip.
Late last year, the same research group at UNSW produced a qubit based on the electron spin of a phosphorous atom trapped inside a silicon transistor. For more information on how electron spin — spintronics — can be used in computing, and thus as the basis of futuristic, ultra-low-power computers, read our spintronics and straintronics explainer. Now the UNSW researchers have effectively done the same thing, but with the nuclear spin of a phosphorus atom trapped inside a silicon transistor.
Every atom consists of a nucleus, made out of protons and neutrons (except hydrogen, which has no neutrons), and electrons that orbit the nucleus. Protons have a positive charge, neutrons are neutral, and electrons have a negative charge. The number of protons in the nucleus dictates what element it is (one proton = hydrogen, six = carbon, eight = oxygen), and the number of neutrons defines the isotope (essentially whether it’s stable or radioactive). The main thing, though, is that the nucleus of an atom — except in extreme cases such as fusion and fission — is so small, dense, and stable as to be almost immutable. Electrons, on the other hand, will readily change their orbits, hop between atoms, or disassociate from their atoms entirely (thus turning the atom into an ion). According to UNSW, the nucleus of an atom is around one million times smaller than the atom itself, and 2,000 times less magnetic than the orbiting electrons.
Now, there are a few ways of building a qubit, but one of the most common uses the spin — the magnetism — of an electron orbiting an atom. By measuring or changing the spin/magnetism, you can read/write the qubit’s value. You can also change and measure the spin of the nucleus, however — which is exactly what the researchers at the University of New South Wales have done. The advantage of using the nucleus is that it’s almost immune to outside electromagnetic interference, and thus the qubit has very high coherence (the integrity of the data lasts a very long time); the disadvantage is that it’s incredibly hard to measure the magnetism of something that is 2,000 times less magnetic than an electron.
To set the value of their nuclear qubit, the researchers use nuclear magnetic resonance — the same phenomenon/technique used in magnetic resonance imaging (MRI) — to alter the spin of a single nucleus. To read the value is more difficult, because of the tiny magnetic field generated by the nucleus, but the researchers succeeded by exploiting a process known as spin-to-charge conversion. Basically, the location of an orbiting electron is altered by the magnetism of the nucleus, and this displacement is easy to detect with a nanoscale electrometer. Perhaps most excitingly, both the read and write processes are performed on-chip by standard CMOS components.
All told, this new qubit achieves between 99.8 and 99.99% read-out fidelity, and a coherence time of 60 milliseconds — unheard of for a solid-state qubit (as opposed to an electromagnetic trap inside a supercooled vacuum chamber). For comparison, IBM’s qubit is only coherent for 0.1 milliseconds.
Moving forward, nuclear spin qubits aren’t likely to replace electron spin qubits as the qubit of choice — but they could act as a kind of quantum memory, or assist with the implementation of two-electron-qubit logic gates, both of which are being actively explored by the UNSW research team.
Research paper: arXiv:1302.0047 - “High-fidelity readout and control of a nuclear spin qubit in silicon”