How nanostrings and quantum bits go ‘out of tune’ (publication in Nature Communications by dr. Gary Steele (Kavli/QN) and his group)


(by: Webredactie Communication/TU Delft)

The key to quantum computing is preserving quantum information. A classic computer bit has two states: 1 or 0. A quantum bit however has the special property that it can be not just 1 or 0, but also 1 and 0 at the same time, and, in fact, everything in between. The biggest challenge for quantum researchers is that a qubit in these ‘in-between’ states lose their quantum information very quickly due to a process called ‘decoherence’ arising from disturbances of the qubit from its surroundings. Researchers from Delft University of Technology studied how decoherence could be measured in mechanical resonators, basically tiny vibrating strings made from carbon nanotubes, and found that the processes of decoherence in a vibrating nanotube can be thought of in a very similar way as the decoherence of a quantum bit. Using this similarity, you can visualize the loss of quantum information by thinking about a vibrating guitar string. Their work was reported in Nature Communications on Friday, December 19th.

Avoiding decoherence is the key to building a working quantum computer. As this decoherence is caused by random movements of matter surrounding qubits, qubits are usually operated at extremely low temperatures to keep their environment as quiet as possible. Potentially, quantum information can also be stored in nanostrings: miniscule vibrating carbon nanotubes. Because the nanotubes are so small and light, they are a sensitive tool for learning about mechanical decoherence. An important question is if mechanical quantum resonators could preserve quantum information for a much longer period than qubits. If they could, then they could be used as a ‘quantum memory’ in future quantum computers.

The nanostring is visible as a thin white line on this elecron microscope image

Playing the ‘quantum guitar’
‘One way to explain quantum decoherence is using an analogy with playing guitar. A vibrating nanotube is effectively a very, very small guitar string of just 500 nanometer long, about 1/160st of a human hair, and 1 nanometer wide’., Gary Steele of the Kavli Institute of Nanoscience explains. When you pluck a guitar string, it starts to vibrate, quantum information can be seen as the movement of that string. To be able to do useful quantum computations, it’s vital to know exactly how many times the string has vibrated.

‘In theory, this is easy’, Steele says. ‘Just multiply the frequency by the time. However, in the real world, two phenomena are messing up with your string: first, there is damping, the dissipation of energy. After you pluck a guitar string, the amplitude of the vibration reduces and the sound dies out. Once this happens, quantum information is lost. The same thing also happens in a qubit: after some time it will always fall back to the state with the lowest energy.’ This type of decoherence in current qubits, such as spin-qubits, can already be delayed for up to seconds, which is a very long time for a computer.

Messing with the knobs
However, there is an even trickier problem: at the same time, nature is ‘messing with the guitar string knobs’. Due to influences from the qubit’s surroundings, the frequency of the string is fluctuating. While the string is vibrating, ‘nature’ is randomly turning the tuning-knobs of the guitar string, causing the ‘tone’ to change unpredictably in time. As a result, the information about how many times the string has moved up and down is lost very quickly, and therefore also the quantum information. This ‘dephasing’ is much harder to control. For example, decoherence due to dephasing in spin qubits can already destroy quantum information after only 20 nanoseconds.

Illustration of decoherence by dissipation and dephasing using a guitar string

Using a nanotube with a new fast detection technique, the group of Gary Steele was able to study both decoherence processes at the same time in a nanoscale mechanical object. They found that decoherence of the phase information in the nanomechanical resonator was not only subject to dissipation, as has frequently been shown by others, but for the first time they also observed dephasing: nature toying with the knobs. This shows that the decoherence principles in mechanical objects are similar to those in qubits.

The advantage of using extremely small mechanical resonators to study decoherence is that they can behave as quantum objects. However, as they are much bigger then qubits based on one atom, they could be far less vulnerable to dephasing. Therefore, mechanical resonators might be able to store quantum information much longer than qubits, making them interesting candidates for a future quantum memory.

The study of decoherence of mechanical resonators was so far done in the classical domain. In future experiments, the group of Gary Steele is planning to execute similar experiments with resonators in the quantum regime.

For more information, see the article here in Nature Communications and also the news item on the Kavli/QN/MED website