World’s Heaviest Schrödinger’s Cat States Achieved Thanks to Advanced Quantum Control
According to the Schrödinger equation, a physical system can be in any linear combination of its possible states. While the logic of this principle is routinely validated for microscopic systems, it is still unclear why we do not observe macroscopic objects – like cats – to be in superpositions. That’s where cat states start to play a key role. In this blog post, we delve into the fascinating world of Schrödinger’s cat states and share the groundbreaking achievement by Prof. Yiwen Chu and her team at ETH Zurich, who have shattered the world record for the heaviest object observed in a superposition of states thanks to their innovative use of advanced quantum control techniques, including Quantum Machines’ OPX+.
Letting Schrödinger’s Cat States Out of the Bag
A cat state is a quantum state composed of two diametrically opposed conditions at the same time, such as the possibility that a cat is simultaneously alive and dead. The term ‘cat state’ refers to any other quantum superposition involving two macroscopically distinct states. Such cat states have been experimentally realized in multiple ways and at various scales . Why is it that we can successfully create a microscopic cat state but encounter challenges when attempting to achieve macroscopic cat states where the cats don’t exhibit the desired behavior?
Factors such as increased interactions with the surrounding environment, thermal effects, and coupling with unwanted degrees of freedom contribute to the loss of coherence and the breakdown of the quantum superposition required for the cat states to persist. Hence, maintaining quantum coherence becomes increasingly challenging as the system’s size and mass grow, hindering the successful realization of quantum cat states in heavier systems. Scaling up the size of these states can be helpful to shed light on the boundary between the quantum and the classical worlds. That’s why we are thrilled to witness the groundbreaking achievement of a new world record being shattered by Marius Bild and Prof. Yiwen Chu’s group at ETH Zurich  with the help of Quantum Machines’ OPX+.
Herding Cats: The Experiment in Practice
In a remarkable milestone, Prof. Yiwen Chu and her team at ETH Zürich have accomplished an unprecedented feat, establishing a new benchmark for the heaviest object observed in a superposition of locations. Their groundbreaking work involved the manipulation of a mechanical resonator weighing an impressive 16.2 micrograms, little more than a grain of sand. This resonator, ingeniously prepared in Schrödinger cat states of motion, is several billion times heavier than an atom or molecule. With such a cat state, you could quite literally hold a quantum system in your palm.
These states encompassed the remarkable phenomenon of the constituent atoms oscillating in a superposition with two opposing phases. The team demonstrated precise control over the magnitude and phase of this superposition and delved into the intricate dynamics of decoherence that these states exhibited.
“The QUA interface of the OPX+ made our phonon measurement protocol particularly straightforward, enabling us to improve our phonon state tomography.”
– Marius Bild
Transferring Superpositions is Key
The solid-state mechanical resonator used by Chu’s group is a high-overtone bulk acoustic-wave resonator (HBAR) , which they coupled to a superconducting transmon qubit. The acoustic free spectral range used spans around 12 MHz. By tuning the qubits using this frequency range, they were able to effectively target multiple longitudinal phononic modes . These acoustic lattice displacements (Figure 1) represent the superpositions, which capture the idea of Schrödinger’s cat. The atoms of the sapphire crystal jiggle in two directions at the same time, up-down and down-up, as shown on the left of the figure below. These two directions represent the “alive” or “dead” states of the cat. But how does the superconducting qubit on the bottom chip communicate with the crystal on top?
The team at ETH Zurich developed an innovative solution by introducing a layer of piezoelectric material on the lower side of the HBAR. The piezoelectric layer generates an electric field as the crystal undergoes oscillation-induced shape changes. This electric field can effectively interface with the electric field of the qubit, allowing the transfer of the qubit’s superposition state to the crystal. The protocol for tomographic phonon measurements can be enhanced by averaging measurements across multiple phases of the qubit drive within a single repetition of the entire sequence. Quantum Machines’ OPX+, an all-in-one quantum controller with unmatched real-time processing capabilities, demonstrates its potential here, enabling the programming of such sequences. It also simplifies the process significantly thanks to the QUA interface, an intuitive quantum pulse-level programming language, contributing to the improvement of phonon state tomography.
Controlling Your Cat’s Size
Achieving precise control over the phase and amplitude of the cat state is crucial for various applications, including qubit state encoding. By manipulating the amplitude A of the phonon displacement drive, it is possible to gain the ability to govern both the initial coherent state’s amplitude and the overall size of the resultant cat state. Bigger displacement amplitudes mean bigger cat size (Figure 2(b)). The preparation of the initial state is performed with the OPX+, which applies a drive pulse with variable phase and amplitude to the qubits, playing a key role in manipulating the cat’s size.
“Using the OPX has been simplifying our work in many respects due to the intuitive implementation of sequences within QUA. The support by QM also helped us debug issues with swift responses and an easy way of getting in touch.”
– Prof. Yiwen Chu
Bridging the Quantum-Classical Gap to Push Quantum Breakthroughs
In addition to clarifying the interface between the quantum and classical domains, the findings of Chu et al. hold significant relevance for quantum technologies. They serve as a crucial stepping stone toward the realization of continuous-variable quantum information processing and quantum metrology utilizing mechanical resonators. Plus, the ability to use higher-mass objects rather than single atoms or ions may also have applications in future quantum technologies.
At Quantum Machines, we take great satisfaction in witnessing demonstrations like this one, as they affirm our commitment to pushing the limits of quantum research via advanced quantum control systems. By facilitating the faster exploration of quantum computing’s potential, we are delighted to support researchers in their groundbreaking endeavors.
To read more about this great milestone, check out the publication page.
A. Omran et al., Generation and manipulation of Schrödinger cat states in Rydberg atom arrays. Science 365, 570-574 (2019). DOI:10.1126/science.aax9743
 Marius Bild et al., Schrödinger cat states of a 16-microgram mechanical oscillator. Science 380, 274-278 (2023). DOI: 10.1126/science.adf7553
 Schrinski, Björn et al. “Macroscopic Quantum Test with Bulk Acoustic Wave Resonators.” Physical review letters vol. 130,13 (2023): 133604. doi:10.1103/PhysRevLett.130.133604
 Chu, Y. et al. Creation and control of multi-phonon Fock states in a bulk acoustic wave resonator. Nature 563, 666–670 (2018). DOI: 10.1038/s41586-018-0717-7