Luke Burkhart

Controlled release of multiphoton quantum states from a microwave cavity memory

The ability to transfer quantum information from a memory to a flying qubit is important for building quantum networks. The very fast release of a multiphoton state in a microwave cavity memory into propagating modes is demonstrated.

Faithful conversion of propagating quantum information to mechanical motion

Combining micrometre-sized mechanical resonators with superconducting quantum circuits, quantum information encoded with photons now can be converted to the motion of a macroscopic object.

On-demand quantum state transfer and entanglement between remote microwave cavity memories

Coupling isolated quantum systems through propagating photons is a central theme in quantum science1,2, with the potential for groundbreaking applications such as distributed, fault-tolerant quantum computing3–5. To date, photons have been used widely to realize high-fidelity remote entanglement6–12 and state transfer13–15 by compensating for inefficiency with conditioning, a fundamentally probabilistic strategy that places limits on the rate of communication. In contrast, here we experimentally realize a long-standing proposal for deterministic, direct quantum state transfer16. Using efficient, parametrically controlled emission and absorption of microwave photons, we show on-demand, high-fidelity state transfer and entanglement between two isolated superconducting cavity quantum memories. The transfer rate is faster than the rate of photon loss in either memory, an essential requirement for complex networks. By transferring states in a multiphoton encoding, we further show that the use of cavity memories and state-independent transfer creates the striking opportunity to deterministically mitigate transmission loss with quantum error correction. Our results establish a compelling approach for deterministic quantum communication across networks, and will enable modular scaling of superconducting quantum circuits.Sending quantum states as shaped microwave photonic wavepackets realizes on-demand, high-fidelity quantum state transfer and entanglement between two superconducting cavity quantum memories.

Normal-metal quasiparticle traps for superconducting qubits

Superconducting qubits are among the most promising elements for the implementation of the concept of quantum computing. Quasiparticles are an intrinsic sources of qubit decoherence, and are more generally detrimental to the operation of superconducting devices, e.g., Cooper pair pumps. Experiments reveal that quasiparticles fail to equilibrate and their density remains high even at low temperatures. Planting normal-metal traps on a superconducting device offers a way to reduce the quasiparticle density: once a quasiparticle tunnels into the normal metal and relaxes to subgap energy via inelastic processes, it cannot return to the superconductor. This paper presents a theoretical model for the time-resolved dynamics of quasiparticles injected into a qubit, and experiments with transmon qubits validating the model. The authors show that, contrary to expectations, the effective trapping rate depends on temperature, which is a consequence of the strong energy dependence of the quasiparticle density of states in the superconductor. At low temperatures, the relaxation process in the normal metal is the bottleneck limiting the effectiveness of traps. The authors also show that the trapping rate saturates for larger traps. At saturation, the rate is limited by the inverse of the time it takes for quasiparticles to diffuse across the device.