Quantum Nanomechanics

Entangled mechanical oscillators

Entanglement is perhaps the most intriguing feature of quantum mechanics. It allows objects to affect each other across arbitrary distances without any direct interaction, defying both classical physics and our common-sense understanding of reality. Entanglement is now commonly observed in experiments with microscopic systems such as light or atoms, and is also the key resource for quantum technologies such as quantum computation, cryptography and measurement.

Quantum entanglement is, however, extremely fragile, and it will disappear if the entangled particles interact with their surroundings, through thermal disturbances, for example. For this reason, entanglement between the motion of macroscopic objects has long been an elusive goal.

In recent works we created and stabilised entanglement between the center-of-mass motion of two drumhead resonators. The drumheads, 15 micrometer in diameter, are capacitively coupled to a single microwave "cavity" formed by a superconducting circuit. By driving the system with suitable microwave fields, we cool the thermal disturbances and bring the drumheads to a steady state where they are entangled indefinitely. Our work qualitatively extends the range of entangled physical systems and has implications for quantum information processing, precision measurements and tests of the limits of quantum mechanics.

• Quantum mechanics–free subsystem with mechanical oscillators, Science 372, 625-629 (2021).
• Perspective: H.-K. Lau, A. Clerk, Macroscale entanglement and measurement, Science 372, 570-571 (2021).
• Stabilized entanglement of massive mechanical oscillators, Nature 556, 478 (2018)
• News & Views: Andrew Armour, "Entangled vibrations in mechanical oscillators", Nature 556, 478 (2018)
• Press release

Quantum Backaction Evading Measurement of Collective Mechanical Modes

The standard quantum limit constrains the precision of an oscillator position measurement. It arises from a balance between the imprecision and the quantum backaction of the measurement. However, a measurement of only a single quadrature of the oscillator can evade the backaction and be made with arbitrary precision. We have demonstrated quantum backaction evading measurements of a collective quadrature of two mechanical oscillators, both coupled to a common microwave cavity. The work allows for quantum state tomography of two mechanical oscillators, and provides a foundation for macroscopic mechanical entanglement and force sensing beyond conventional quantum limits.

• Quantum Backaction Evading Measurement of Collective Mechanical Modes, Phys. Rev. Lett. 117, 140401 (2016).

Magneto acoustics

In a new research effort, we integrate magnetic materials with nano- and micromechanical devices to advance fundamental science and to obtain new functionalities that can lead to disruptive technologies. We study strain-mediated interactions between magnons and phonons in magnetic mechanical oscillators. The hybrid potential is provided by magnetostriction, which couples mechanical strain to magnetization. This activity can be regarded as a strong analog to cavity optomechanics, with magnons replacing the electromagnetic cavity.

Optomechanics with macroscopic quartz resonators

We observe optomechanical physics in a truly macroscopic oscillator close to the quantum ground state. As the mechanical system, we use a mm-sized piezoelectric quartz disk oscillator. Its motion is coupled to a charge qubit which translates the piezo-induced charge into an effective radiation–pressure interaction between the disk and a microwave cavity. The work opens up opportunities for macroscopic quantum experiments.

• Optomechanical measurement of a millimeter-sized mechanical oscillator approaching the quantum ground state, New J. Phys. 19 103014 (2017).

• High-Precision Displacement Sensing of Monolithic Piezoelectric Disk Resonators Using a Single-Electron Transistor, J. Low Temp. Phys. 191, 316 (2018).