New way Improves Performance of Qubits, by Reducing Interference from the environment

A new strategy helps quantum bits stay on task. Credit: Image courtesy of Florida State University

Development of quantum computers may be expedited by collaboration between physicists and chemists. Quantum computer power will dwarf that of today’s machines, with huge implications for cryptography, computational chemistry and other fields. While qubits can take many different forms, the MagLab team worked with carefully designed tungsten oxide molecules that contained a single magnetic holmium ion. The magnetic electrons associated with each holmium ion circulate either clockwise or counterclockwise around the axis of the molecule. These spin states are analogous to the “0s” and “1s” of computer bits. Qubit aka quantum bits can be in both the 0 and 1 states at the same time ie quantum superposition. a mix of the 2 spin states, with a spectrum of almost infinite possibilities.

Magnetic qubits can also interact with each other over relatively large distances using their magnetic fields, a phenomenon known as entanglement. In a useful quantum computer, large numbers of entangled qubits would perform in perfect unison. Unfortunately, the real world is full of magnetic disturbances ie “noise” that entangle with the qubits, interfering with calculations. This breakdown = “decoherence.” The MagLab team describes a new way to significantly reduce this decoherence in magnetic molecules.

It turns out chemists can assemble molecules with special spin states that, when placed in a magnetic field, are immune to magnetic disturbances. This sweet spot that allows qubits to interact without interference is called an atomic clock transition, or ACT. Atomic clocks rely on the same quantum physics principle to remain accurate.

The MagLab team was able to keep its holmium qubit working coherently for 8.4 microseconds – long enough for it to potentially perform useful computational tasks. Now that the MagLab team has shown that ACTs can be used as a mechanism to make quantum computers work, it’s up to chemists to tweak more molecules so that they are capable, under the right conditions, of creating a coherence sweet spot for qubits.
Komijani said: “you can play around with your qubit by changing the magnetic field it’s in and moving from where the coherence is very low to the sweet spot, where it’s very high.”
http://news.fsu.edu/More-FSU-News/New-strategy-helps-quantam-bits-stay-on-task  http://www.nature.com/nature/journal/v531/n7594/full/nature16984.html