An interdisciplinary collaboration of physicists and chemists from the National High Magnetic Field Laboratory (MagLab) (CSA CSM) has demonstrated a way to improve the performance of the powerful but persnickety building blocks of quantum computers (called quantum bits or qubits) by reducing interference from the environment. The research, which may hasten the development of quantum computers, is available in the journal Nature.
Quantum computers are one of the holy grails of modern applied physics. Compared to computers that rely on transistors to process bits of information in the form of binary 0s or 1s, quantum computers hold the promise of performing certain computational tasks exponentially faster. Their power could potentially dwarf that of today’s machines, with huge implications for cryptography, computational chemistry and other fields.
That’s the paradoxical nut that a team of scientists, including physicists Dorsa Komijani and Stephen Hill, director of the MagLab’s Electron Magnetic Resonance Facility, have spent years attacking. And while they have not broken this nut open entirely, they have made an important crack.
Understanding the significance of this crack requires grasping a few basics about quantum mechanics.
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 so-called spin states are analogous to the 0s and 1s of modern computers, but in the quantum world there’s a bonus: the qubit can be in both the 0 and 1 states at the same time in what is termed a quantum superposition. In this case, the superposition involves a mix of the two spin states, with a spectrum of almost infinite possibilities between the fully clockwise and fully counterclockwise states. This is where the added computational power comes from.
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 (physicists call this “noise”) that can also become entangled with the qubits, interfering with the calculations. This breakdown is called “decoherence,” a state researchers say is not unlike being interrupted when you’re trying to do complex arithmetic in your head and having to start over.
In the Nature paper, the MagLab team describes a new way to significantly reduce this decoherence in magnetic molecules. It turns out that chemists can assemble molecules with special spin states that, when placed in a magnetic field, are immune to magnetic disturbances, similar to the way noise-canceling headphones allow you to listen to your favorite music in high fidelity. This sweet spot, called an atomic clock transition, allows qubits to interact without interference. 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. “I know 8.4 microseconds doesn’t seem like a big deal,” says Komijani. “But in molecular magnets, it is a big deal, because it’s very, very long. But the important point is not the long coherence time, it’s the approach that we used to get to this coherence time.”
Now that the MagLab team has shown 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. “That’s why this is important,” says Komijani. “We’re saying, ‘See, we found this capability in molecular magnets. Now you guys, you chemists, go ahead and make stuff that has this capability so we can find the atomic clock transitions.'”
The Nature paper is part of a larger research effort expected to yield additional publications. “We’re just contributing a tiny, tiny amount of research,” says Komijani. “But it’s important because it’s saying that 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.”