The research takes the first steps towards the realization of mechanical qubits

The research takes the first steps towards the realization of mechanical qubits

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A platform for an array of 36 mechanical resonator devices. A nano-resonator is fabricated and electrically connected in one of the sections of this array. The entire piece is then placed inside the cryostat for measurements. Credit: ICFO

Quantum information processing (QI) could be the next tipping point in the evolution of technology, providing unprecedented computational power, security and detection sensitivity. Qubits, the basic piece of hardware for quantum information, are the building block of quantum computers and quantum information processing, but there is still much debate about which types of qubits are actually best.

Research and development in this field is growing at an astonishing rate to see which system or platform outperforms the other. Platforms as diverse as superconducting Josephson junctions, trapped ions, topological qubits, ultracold neutral atoms, or even vacancies in diamonds, to name a few, form a zoo of possibilities for creating qubits.

So far, only a handful of qubit platforms have been shown to have the potential for quantum computing, ticking the checklist of controlled gates with high fidelity, easy qubit-qubit coupling, and good isolation from the environment, meaning sufficiently long-lived consistency.

Nanomechanical resonators can be part of the handful of platforms. They are oscillators, like springs and strings (e.g. guitars) that when pushed create harmonic or anharmonic sounds depending on the strength of the drive. But what happens when we cool a nano resonator down to the temperature of absolute zero?

The energy levels of the oscillator are quantized and the resonator vibrates with its characteristic zero point movement. The zero-point motion comes from Heisenberg’s uncertainty principle. In other words, a resonator maintains motion even when in its ground state. The realization of a mechanical qubit is possible if the quantized energy levels of a resonator are not equally spaced.

The challenge is to keep the nonlinear effects large enough in the quantum regime, where the zero-point shift of the oscillator is tiny. If this is achieved, the system can be used as a qubit by manipulating it between the two lowest quantum levels without driving it into higher energy states.

For many years there has been much interest in building a qubit system with a mechanical nano resonator. In 2021, Fabio Pistolesi (Univ. Bordeaux-CNRS), Andrew N. Cleland (Univ. Chicago) and ICFO Prof. Adrian Bachtold, established a solid theoretical concept of a mechanical qubit, based on a nanotube resonator coupled to a double-quantum dot under an ultrastrong coupling regime.

These theoretical results demonstrated that these nanomechanical resonators could indeed become ideal candidates for qubits. Why? Because they have been shown to have long coherence times, a definite “must” for quantum computing.

Given that there was a theoretical framework to work with, the challenge now was to actually create a qubit from a mechanical resonator and find the appropriate conditions and parameters to control nonlinearities in the system.

After several years of endless work on these systems, the challenges of making it experimentally gave it its first very welcome green light. In a recent study published in Physics of nature, ICFO researchers Chandan Samanta, Sergio Lucio de Bonis, Christoffer Moller, Roger Tormo-Queralt, W. Yang, Carles Urgell, led by ICFO Prof. Adrian Bachtold, in collaboration with researchers B. Stamenic and B.Thibeault of the University of California Santa Barbara, Y. Jin of the University Paris-Saclay-CNRS, DA Czaplewski of the Argonne National Laboratory and F. Pistolesi of the Univ. Bordeaux-CNRS, achieved the first pre-experimental steps for the future realization of a mechanical qubit by demonstrating a new mechanism to enhance the anharmonicity of a mechanical oscillator in its quantum regime.

A platform for an array of 36 mechanical resonator devices. Credit: ICFO

The experiment: engineering anharmonicity close to the ground state

The team of researchers fabricated a suspended nanotube device about 1.4 micrometres long, with its ends hooked to the edges of two electrodes. They defined a quantum dot which is a two-level electronic system on the vibrating nanotube by electrostatically creating tunnel junctions at both ends of the suspended nanotube.

Then, by regulating the voltage across the gate electrode, they allowed only one electron to flow down the nanotube at a time. The mechanical motion of the nanotube was then coupled to the single electron in the single electron tunneling regime. This electromechanical coupling created a harmony with the mechanical system.

Next, they reduced the temperature down to mK (milikelvin, near absolute zero) and entered an ultrastrong coupling regime where each additional electron on the nanotube shifted the nanotube’s equilibrium position away from its zero-point amplitude. With an amplitude of only a factor of 13 of the zero point movement, they were able to notice these non-linear vibrations.

The results are surprising because the vibrations present in other resonators, cooled to the quantum ground state, have been shown to be nonlinear only at amplitudes of about 106 times greater than its zero-point motion.

This new mechanism exhibits remarkable physics because, contrary to what was expected, anharmonicity increases as the vibrations cool closer to the ground state. This is exactly the opposite of what has been observed in all other mechanical resonators so far. As first author Chandan Samanta says, “When researchers first started studying nanomechanical resonators, a recurring question was whether it was possible to achieve nonlinearities in the vibrations found in the quantum ground state.”

“Some prominent researchers in the field have argued that this would be a challenging undertaking due to technological limitations, and this view has remained the accepted paradigm until now. Against this background, our work represents a significant conceptual advance because we demonstrate that Nonlinear vibrations in the quantum regime are indeed achievable.”

“We are confident that the nonlinear effects could have been further enhanced by approaching the quantum ground state, but we were limited by the temperature of our current cryostat. Our work provides a roadmap for achieving nonlinear vibrations in the quantum regime.”

Contrary to what has been observed in other mechanical resonators so far, the research team has found a method to increase the anharmonicity of a mechanical oscillator close to its quantum regime. The results of this study set the first milestones for the future development of mechanical qubits or even quantum simulators.

As Adrian Bachtold notes, “It is remarkable that we entered a regime of ultra-strong coupling and observed strong anharmonicity in the resonator. But the damping rate becomes high at low temperature due to the coupling of the resonator to a quantum dot.”

“In future experiments targeting cat states and mechanical qubits, it will be advantageous to couple the nanotube vibrations to a quantum double-dot, as it allows for strong nonlinearities along with long-lived mechanical states. The electron-derived damping in the double-dot The quantum dot is suppressed exponentially at low temperatures, so it should be possible to achieve a damping rate of 10Hz measured in nanotubes at low temperatures.”

More information:
C. Samanta et al, nonlinear nanomechanical resonators approaching the quantum ground state, Physics of nature (2023). DOI: 10.1038/s41567-023-02065-9

About the magazine:
Physics of nature

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Six steps for retirees to help protect against financial losses due to dementia and cognitive decline

Six steps for retirees to help protect against financial losses due to dementia and cognitive decline

Surprisingly, one of the first signs of mild cognitive decline can be suffering a financial loss due to a mistake, such as not completing an investment transaction correctly or being the victim of fraud or exploitation. In many of these situations, retirees are still highly functioning and have no severe symptoms of dementia or Alzheimer’s disease.

While your first course of action is to take steps to delay, mitigate, or prevent cognitive decline, despite your best efforts, you may still experience some form of it in your later years. Once you accept this possibility, it simply makes sense to develop strategies to protect yourself from financial loss Before symptoms of cognitive decline occur.

MORE FROM FORBESSix winning strategies for retirees to help prevent or delay dementia and cognitive decline

With those thoughts in mind, let’s look at six steps you can take to protect your finances, assets, and retirement income. This way you can help prevent not only a financial loss, but also the heartache that comes with it for both you and your family.

Step 1. Simplify and set your income and expenses on autopilot. As much as possible, have your various sources of retirement income automatically deposited into your checking account. Plus, simplify your investments by combining accounts where it makes sense.

Likewise, eliminate all unnecessary regular expenses and reduce the number of credit cards you have. Also consider making payment for your regular expenses on autopay from your checking account or a credit card. If you do, be sure to monitor these accounts closely so you don’t overcharge your checking account or exceed your credit card limits.

Step 2. Blocking of investment accounts. Some financial institutions allow you to block your investment and savings accounts. The lockout feature offers extra security on your accounts by requiring you to go through stricter security procedures to make any changes to your investments or withdrawals. This feature helps prevent fraud and exploitation. Before setting it up, however, you’ll want to specify how you want your accounts invested and choose a regular form of automatic payment into your checking account to help pay for your living expenses.

Step 3. Purchase a low-cost annuity to provide a regular retirement income. You may not have considered this type of product for your retirement, as they are often criticized for their lack of liquidity and flexibility. They usually do not allow withdrawals other than the regular monthly check and give investors little or no control over investments or their ability to adjust the payout amount. However, these features can turn into advantages in your later years when you are less able to manage your money, thus providing protection against losing all your savings through mistakes or becoming a victim of fraud or exploitation.

Step 4. Select a financial lawyer to help you manage your money. One of the best forms of financial protection is to designate a trusted person to help you manage your money should you ever need help in the future. Often a spouse, adult child, relative, or close friend can serve this purpose. However, if you don’t have someone in your life you trust to help, you can also hire professional day money managers. This type of financial attorney can help you with tasks such as paying regular and periodic expenses, tracking your retirement income and investments, paying taxes, and filing insurance claims.

Step 5. Take an inventory of your finances. Helping you manage your finances can be an important job for a financial supporter. You will really help your attorney by taking a detailed inventory of all your living expenses, sources of retirement income and savings, and insurance records.

Step 6. Develop an early warning system. Unfortunately, you’re not like a car with warning lights that come on when something goes wrong. However, there are often signs that it may be time for your financial attorney to step in and start helping you with your finances. You’ll want to develop an early warning system that contains both red emergency signs, such as a doctor diagnosing dementia or a serious illness, and yellow warning lights, such as forgetting to pay some bills or having trouble managing financial assets that previously managed well .

If you want to implement these steps but need more information, there’s a solid, online, free source that provides details on all of these steps. It’s the Thinking Ahead Roadmap: A Guide to Keeping Your Money Safe as You Age. It was funded by the AARP and the Society of Actuaries and developed by a team of a social science researcher, a senior attorney, and a retired specialty actuary (full disclosure, yours truly).

Smart planning Now it can help you prevent disruptive pain in the future, when you’ll be vulnerable and less able to recover from significant losses. Take the right steps to protect yourself and your money.

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