Nat. Phys. The first step toward achieving mechanical quantum bits
Quantum information (QI) processing could be the next game changer in technological development: it offers unprecedented computational power, security, and detection sensitivity. Quantum bits are the fundamental hardware element of quantum information and are the cornerstone of quantum computers and quantum information processing; however, there is still much debate as to which type of quantum bit is actually the best.
Research and development in this area is growing at an alarming rate to see which system/platform can outperform the others. Just to name a few, platforms like superconducting Josephson junctions, trapped ions, topological quantum bits, ultracold neutral atoms, and even diamond vacancies offer the possibility of making quantum bits.
However, so far, only a few quantum bit platforms have proven to have the potential to enable quantum computing - a hallmark of high-fidelity controllable gates, easy-to-implement quantum bit-quantum bit coupling, and good isolation from the environment (which means long enough to be coherent).
Example of a one-dimensional NEMS resonator
Examples of two-dimensional NEMS resonators.
Nanomechanical resonators (nano-mechanical resonators) may be part of these few platforms. They are oscillators (like springs and guitar strings) that, when driven, produce harmonic or non-harmonic sounds depending on the strength of the drive. But what happens when we cool a nano-resonator to an absolute zero temperature?
The energy level of the oscillator becomes quantized and the resonator vibrates with its characteristic zero-point motion; the zero-point motion arises from the Heisenberg uncertainty principle. In other words, the resonator maintains motion even when it is in its ground state. If the quantized energy levels of the resonator are not uniformly spaced, then the realization of mechanical quantum bits is possible.
The challenge is to maintain a large enough nonlinear effect in a quantum system in which the zero point shift of the oscillator is trivial. If this is achieved, then the system can be used as a quantum bit by manipulating it between the two lowest quantum energy levels (rather than driving it at a higher energy state).
For many years, there has been interest in implementing quantum bit systems with mechanical nanoresonators.2021, Fabio Pistolesi (CNRS, University of Bordeaux), Andrew N. Cleland (University of Chicago), and Professor Adrian Bachtold of ICFO have developed a nanotube resonator based on the ultra-strong coupling mechanism with double quantum dot coupling, a solid theoretical concept of mechanical quantum bits has been established.
These theoretical results demonstrate that these nanomechanical resonators can indeed be ideal candidates for quantum bits. Why? Because they are shown to have a long coherence time - a clear "must" for quantum computing.
Given that a theoretical framework is already in place, the challenge now is to actually make a quantum bit from a mechanical resonator and find the right conditions and parameters to control the nonlinearities in the system.
After several years of endless work, scientists have now finally achieved this challenge experimentally. In a recent study published in Nature Physics, ICFO researchers Chandan Samanta, Sergio Lucio de Bonis, Christoffer Moller, Roger Tormo-Queralt, W. Yang, Carles Urgell, under the leadership of ICFO Professor Adrian Bachtold, in collaboration with B. Stamenic and B. Thibeault of the University of California, Santa Barbara, Y. Jin of the Université de Paris-Saint-Georges, D.A. Czaplewski of the Argonne National Laboratory, and F. Pistolesi of the University of Bordeaux, to improve the performance of mechanical oscillators by demonstrating a new mechanism for quantum systems with anharmonicity, a first step towards the future realization of mechanical quantum bits.
The research results were published on June 8 in the journal Nature Physics under the title "Nonlinear nanomechanical resonators approaching the quantum ground state".
An array platform consisting of 36 mechanical resonator devices.
Specifically, the team fabricated a levitating nanotube device with a length of about 1.4 microns, the ends of which were hooked to the edges of two electrodes. They defined a quantum dot on the vibrating nanotube by electrostatically creating a "tunnel junction" at each end of the suspended nanotube - a two-level electron system.
Then, by adjusting the voltage on the gate electrode, they allow only one electron to flow onto the nanotube at a time. The mechanical motion of the nanotube is then coupled to the individual electrons in the system. This electromechanical coupling brings anharmonicity to the mechanical system.
They then lowered the temperature to mK (Milvin, almost absolute zero) and entered an ultra-strong coupling regime in which each additional electron on the nanotube shifted the equilibrium position of the nanotube away from its zero-point amplitude. Since the amplitude is only 13 times larger than the zero-point motion, they were able to notice these nonlinear vibrations.
The nonlinearity of the mechanical vibrations was enhanced at low temperatures.
These results are striking: since the vibrations present in other resonators cooled to the quantum ground state are shown to be nonlinear only when the amplitude is about 106 times larger than its zero-point motion.
This new mechanism shows remarkable physics; for contrary to what has been observed so far in all other mechanical resonators, the anharmonicity increases as the vibration is cooled to near ground state. As first author Chandan Samanta says, "When researchers first began studying nanomechanical resonators, a recurring question was whether it was possible to achieve nonlinearity in vibrations that were in a quantum ground state."
"Some researchers in the field believed that this would be a challenging feat due to technical limitations, and this view has remained the accepted paradigm until now. In this context, our work represents a major conceptual advance: because we demonstrate that nonlinear vibrations in quantum systems are indeed achievable."
Nonlinear mechanical vibrations
"We believe that nonlinear effects can be further enhanced by getting closer to the quantum ground state, but we are constrained by the temperature limitations of current cryostats. Our work provides a roadmap for implementing nonlinear vibrations in quantum systems."
-- The results of this research lay the first building blocks for the future development of mechanical quantum bits or even quantum simulators.
As Adrian Bachtold says: "It is remarkable that we have entered the superstrong coupling regime and observed strong anharmonicity in the resonator. But due to the coupling of the resonator to a quantum dot, the damping rate becomes large at low temperatures."
"In future experiments targeting cat states and mechanical quantum bits, coupling nanotube vibrations to a double quantum dot is advantageous because it allows strong nonlinearity to emerge along with long-lived mechanical states. The damping produced by the electrons in the double quantum dot is exponentially suppressed at low temperatures, so it should be possible to achieve damping rates of 10 Hz measured in nanotubes at low temperatures."
Reference link:
[1] https://arxiv.org/ftp/arxiv/papers/2208/2208.07164.pdf
[2]https://www.nature.com/articles/s41567-023-02065-9
[3] https://phys.org/news/2023-06-mechanical-qubits.html
