Assistant Professor Mohammad Mirhosseini (Caltech EE/APh) explains how his group built a mechanical quantum memory that stores microwave-photon quantum states far longer than typical superconducting qubits, and why that matters for hybrid quantum architectures. The discussion covers microwave photons, phonons, optomechanics, coherence versus lifetime (T2 vs. T1), current speed bottlenecks, and implications for quantum transduction and error mechanisms. The discussion centers on a paper from Mirhosseini's paper from December of 2024 titled, “A mechanical quantum memory for microwave photons,” detailing strong coupling between a transmon and a long‑lived nanomechanical oscillator for storage and retrieval of nonclassical states.

Guest

Mohammad Mirhosseini is an Assistant Professor of Electrical Engineering and Applied Physics at Caltech, where his group engineers hybrid superconducting–phononic–photonic systems at millikelvin temperatures for computing, communication, and sensing. He completed his PhD at the University of Rochester’s Institute of Optics and was a postdoc in Oscar Painter’s group at Caltech before starting his lab. His recent team effort demonstrates mechanical oscillators as compact, long‑lived quantum memories integrated with superconducting circuits.

Key topics

• What “microwave photons” are and how qubits emit/absorb single microwave photons in circuit QED analogously to atoms and optical photons.
• Why “memory” is missing in today’s quantum processors and how a dedicated long‑lived storage element can complement fast but dissipative superconducting qubits.
• Optomechanics 101: mapping quantum states between electrical and mechanical degrees of freedom, with phonons as the quantized vibrational excitations.
• T1 vs. T2: demonstrated order‑of‑magnitude gains in lifetime (T1) and more modest current gains in coherence (T2), plus paths to mitigate dephasing.
• Present bottleneck: state conversion between qubit and oscillator is about 100× slower than native superconducting operations, with clear engineering avenues to speed up.
• Quantum transduction: leveraging the same mechanical intermediary to bridge microwave and optical domains for interconnects and networking.
• Two‑level system (TLS) defects: shared decoherence mechanisms across mechanical oscillators and superconducting circuits and why comparing both can illuminate materials limits.

Why it matters

Hybrid architectures that pair fast processors with long‑lived memories are a natural route to scaling, and mechanical oscillators offer lifetimes far exceeding conventional superconducting storage elements while remaining chip‑integrable.. Demonstrating nonclassical state storage and retrieval with strong qubit–mechanics coupling validates mechanical oscillators as practical quantum memories and sets the stage for on‑chip transduction. Overcoming current speed limits and dephasing would lower the overhead for synchronization, buffering, and possibly future fault‑tolerant protocols in superconducting platforms.

Episode highlights

• A clear explanation of microwave photons and how circuit QED lets qubits create and absorb them one by one.
• Mechanical memory concept: store quantum states as phonons in a gigahertz‑frequency nanomechanical oscillator and read them back later.
• Performance today: roughly 10–30× longer T1 than typical superconducting qubits with current T2 gains of a few×, alongside concrete strategies to extend T2.
• Speed trade‑off: present qubit–mechanics state transfer is ~100× slower than native superconducting gates, but device design and coupling improvements are underway.
• Roadmap: tighter coupling for in‑oscillator gates, microwave‑to‑optical conversion via the same mechanics, and probing TLS defects to inform both mechanical and superconducting coherence.