The group is part of the national Centre of Excellence – Quantum Technology Finland (QTF).
Aalto researchers awarded Physics World Breakthrough of the Year for macroscopic quantum entanglementAalto University Professor Mika A. Sillanpää, his team and collaborators at the University of New South Wales in Canberra, Australia, have won the Physics World 2021 Breakthrough of the Year. The prize was awarded for establishing quantum entanglement between a pair of macroscopic drumheads – two mechanical resonators that were tiny but still much larger than the subatomic particles that are usually entangled. The award has previously been given for the first direct observation of a black hole and for the detection of gravitational waves, which also received a Nobel Prize.
Prof. Mika Sillanpää
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).
Besides studying fundamental quantum concepts such as entanglement and backaction evasion, microwave optomechanics can be utilized for signal processing.
We have demonstrated that microwave 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.
Additionally, we have investigated nonreciprocal (i.e., directional) transport and amplification of electromagnetic or mechanical signals.
- Nonreciprocal transport based on cavity Floquet modes in optomechanics, Physical Review Letters 125 (2), 023603 (2020).
- 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).
Example of a drumhead resonator, imaged with a scanning electron microscope
Schematic of a sample with two microwave LC cavities coupled to a single mechanical drumhead resonator.
Nanobeam mechanical resonator coupled to a superconducting microwave cavity.
Micro acoustics coupled to superconducting qubits
Quantum systems with different types of degrees of freedom can intertwine, forming hybrid states with intriguing properties. We have explored setups for coupling transmon qubits to either low-frequency flexural resonators, or GHz-regime micro acoustic overtone (HBAR) resonances.
In a HBAR system, the modes mostly reside in the substrate chip and hence feature diluted strain and low acoustic losses. The system exhibits a dense spectrum of acoustic modes that interact near resonance with the qubit, suggesting a possibility to manipulate the many-mode system through the qubit. We have shown a qubit-HBAR system by controlling the qubit with longitudinal fields, allowing individually access a large number of acoustic modes.
- Sideband control of a multimode quantum bulk acoustic system, Phys. Rev. Applied 14, 054023 (2020).
- Landau-Zener-Stückelberg interference in a multimode electromechanical system in the quantum regime, Phys. Rev. Lett. 123, 240401 (2019).
- Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator, Nature 494, 211 (2013).
Assembly of a high-overtone bulk acoustic wave resonator (blue) on top of the Xmon qubit. The acoustic medium is a sapphire crystal that is first covered by a thin layer of molybdenum (60 nm), on top of which there is an approximately 1-micron-thick layer of polycrystalline aluminum nitride. The piezoelectric AlN layer acts as a transducer between the electric field of the qubit and the acoustic modes.
Right: Scanning electron micrograph showing the 5-micron-long and 4-micron-wide bridge-type mechanical resonator (dashed box).
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.