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
Artist's impression of entangled vibrating drums. Image credit: Juha Juvonen.
Gravitational coupling within a quantum system
In this project, the goal is to touch a hundred-year-old mystery of physics: Despite its success at describing phenomena in the low-energy limit, quantum mechanics is incompatible with general relativity that describes gravity and huge energies. The interface between these two has remained experimentally elusive, because only the most violent events in the universe have been considered to produce measurable effects due to the plausible quantum behavior of gravity. We aim at detecting gravitational forces for the first time within a quantum system. We use thin membrane oscillators loaded by milligram masses and bring two such gravitationally interacting oscillators into nonclassical motional states. Initially, we will measure the gravitational force between gold particles weighing a milligram, representing a new mass scale showing gravitational forces within a system. This work is part of the ERC Advanced Grant project “GUANTUM: Probing the limits of quantum mechanics and gravity with micromechanical oscillators”.
- Gravitational Forces Between Nonclassical Mechanical Oscillators. Phys. Rev. Applied 15, 034004 (2021).
In the experiment, gold spheres of 1 milligram mass rest on a very thin membrane so that the spheres are close to each other but free to vibrate, and the same time, interact through gravity.
Mode profile of the fundamental drum mode of around 2 kHz frequency. The 1 mg gold sphere vibrates up and down.
Gold spheres of 0.5 mm diameter have appreciable gravitational interaction at center-of-mass distances in mm range.
Quantum Backaction Evading Measurements
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. Some measurement techniques, however, rely on coupling a probe to the system in such a way that the measured observable is an invariant of the Hamiltonian evolution, which allows to preserve the state of at least this observable. These are often called quantum nondemolition measurements, or quantum backaction evading (BAE) measurements. An example is the measurement of only a single quadrature of the oscillator, which can evade the backaction and thus can be carried out with arbitrary precision.
We have extended the scope of BAE measurements to a new class of systems with a high degree of coherence and therefore immediately adapted to force or metric sensing. As the mechanical oscillator, we use a large 0.5 mm diameter silicon nitride (SiN) membrane oscillator with 707 kHz frequency, embedded in a microwave cavity. High-stress SiN has emerged as the material to realize the highest mechanical quality factors for usage in quantum optomechanics. The measurement shows that quantum backaction noise can be evaded in the quadrature measurement of the motion of a large object.
- Quantum backaction evading measurements of a silicon nitride membrane resonator. New J. Phys. 24 083043 (2022).
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).
Microwave signal processing
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.
A device patterned lithographically on a quartz chip. The structure, made of thin film of superconducting aluminum, supports two microwave resonance modes, and includes two drum oscillators marked with the dashed line.
Silicon nitride drum resonators with quality factors up to 108 can be operated by embedding in 3-dimensional microwave cavity resonator, which allows for accessing and manipulating the motion of the membrane down to the quantum level.
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.
- Coupling high-overtone bulk acoustic wave resonators via superconducting qubits. Appl. Phys. Lett. 123, 134004 (2023).
- 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 a 5-micron-long and 4-micron-wide bridge-type mechanical resonator (dashed box).
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 an analog to cavity optomechanics, with magnons replacing the electromagnetic cavity.
- Magnomechanics in suspended magnetic beams. Phys. Rev. B 104, 214416 (2021).