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 work 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. The 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.

• 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. In recent work, we demonstrate 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).

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).

• Electrode configuration and electrical dissipation of mechanical energy in quartz crystal resonators, J. Micromech. Microeng. 28, 095014 (2018) 

Micromechanical resonators coupled to superconducting qubits

Quantum systems with different types of degrees of freedom can intertwine, forming hybrid entangled quantum states with intriguing properties. We have merged three quantum systems: a superconducting qubit (spheres) interacting with two different resonant cavities. A low frequency phonon cavity (vibrating string) was used as a storage of quantum information from the qubit, whereas an electrical microwave resonator (represented by the mirrors) acted as a means of communicating to the outside world. The idea could be used as a building block in the emerging field of quantum information and communication, as well as to enable creation of Schrödinger cat-like non-classical displacement states.

• Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator, Nature 494, 211 (2013).