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.
Illustration of the 15 micrometre wide vibrating drumheads coupled to a superconducting microwave cavity.
- 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. (2018).
Electrode configuration and electrical dissipation of mechanical energy in quartz crystal resonators, arXiv:1803.03706.
Millimeter-scale piezoelectric quartz resonators
An on-chip microwave resonator can be capacitively coupled to a nanomechanical resonator, similar to an optical cavity with a movable end mirror. We create nanobeam and drumhead mechanical resonators coupled to superconducting microwave circuits. The movement of the mechanical resonator modulates the frequency of the microwave resonance, giving rise to the optomechanical coupling.
In recent work we have demonstrated that such optomechanical systems can be used as ultra low-noise microwave amplifiers. In a phase-preserving mode the incoming microwave signal can be amplified while adding only half a quantum of noise, the minimum amount required by Heisenberg's uncertainty principle. When configured as a phase-senstive amplifier, the device amplifies a single quadrature of the incoming signal while adding almost no noise at all. The novel type of amplifiers may offer improved performance for information processing in certain applications.
- Noiseless Quantum Measurement and Squeezing of Microwave Fields Utilizing Mechanical Vibrations, Physical Review Letters 118, 103601 (2017). See also: press release.
- Low-Noise Amplification and Frequency Conversion with a Multiport Microwave Optomechanical Device, Physical Review X 6, 041024 (2016).
Microwave amplification with nanomechanical resonators, Nature 480, 351 (2011).
Left: Two microwave LC cavities coupled to a single mechanical drumhead resonator. Right: Example of a drumhead resonator.
Nanobeam mechanical resonator coupled to a superconducting microwave cavity.
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).
Right: Scanning electron micrograph showing the 5-mm-long and 4-mm-wide bridge-type mechanical resonator (dashed box).