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C. Schlager, R. Albert, G. Kirchmair Fabrication and characterization of vacuum-gap microstrip resonators,
Appl. Phys. Lett. 127 74001 (2025-08-05),
http://dx.doi.org/10.1063/5.0269702 doi:10.1063/5.0269702
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M. Zanner, R. Albert, E. I. Rosenthal, S. Casulleras-Guardia, I. Yang, C. Schneider, O. Romero-Isart, G. Kirchmair Spatial Addressing of Qubits in a Dispersive Waveguide,
Phys. Rev. Applied 24 14051 (2025-07-28),
http://dx.doi.org/10.1103/np2t-48rm doi:10.1103/np2t-48rm
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A. Sharafiev, G. Kirchmair, M. L. Juan, M. Cattaneo Leveraging collective effects for thermometry in waveguide quantum electrodynamics,
Phys. Rev. Lett. 134 213602 (2025-05-28),
http://dx.doi.org/10.1103/PhysRevLett.134.213602 doi:10.1103/PhysRevLett.134.213602
We report a proof-of-principle experiment for a new method of temperature measurements in waveguide quantum electrodynamics (wQED) experiments, allowing one to differentiate between global and local baths. The method takes advantage of collective states of two transmon qubits located in the center of a waveguide. The Hilbert space of such a system forms two separate subspaces (bright and dark) which are coupled differently to external noise sources. Measuring transmission through the waveguide allows one to extract separately the temperatures of the baths responsible for global and local excitations in the system. Such a system would allow for building a new type of primary temperature sensor capable of distinguishing between local and global baths.
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N. Diaz-Naufal, L. Deeg, D. Zoepfl, C. Schneider, M. L. Juan, G. Kirchmair, A. Metelmann Kerr enhanced optomechanical cooling in the unresolved sideband regime,
Phys. Rev. A 111 53505 (2025-05-25),
http://dx.doi.org/10.1103/PhysRevA.111.053505 doi:10.1103/PhysRevA.111.053505
Dynamical backaction cooling has been demonstrated to be a successful method for achieving the motional quantum ground state of a mechanical oscillator in the resolved-sideband regime, where the mechanical frequency is significantly larger than the cavity decay rate. Nevertheless, as mechanical systems increase in size, their frequencies naturally decrease, thus bringing them into the unresolved-sideband regime, where the effectiveness of the sideband cooling approach decreases. Here we demonstrate, however, that this cooling technique in the unresolved-sideband regime can be significantly enhanced by utilizing a nonlinear cavity as shown in the experimental work of Zoepfl et al. [Phys. Rev. Lett. 130, 033601 (2023)]. The above arises due to the increased asymmetry between the cooling and heating processes, thereby improving the cooling efficiency. In addition, we show that injecting a squeezed vacuum into the nonlinear cavity paves the way to ground-state cooling of the mechanical mode. Notably, the required squeezing parameters are far less stringent than in the linear case, simplifying experimental implementation.
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D. Atanasova, I. Yang, T. Hönigl-Decrinis, D. Gusenkova, I. M. Pop, G. Kirchmair In-situ tunable interaction with an invertible sign between a fluxonium and a post cavity,
PRX Quantum 6 20318 (2025-04-25),
http://dx.doi.org/10.1103/PRXQuantum.6.020318 doi:10.1103/PRXQuantum.6.020318
Quantum computation with bosonic modes presents a powerful paradigm for harnessing the principles of quantum mechanics to perform complex information processing tasks. In constructing a bosonic qubit with superconducting circuits, nonlinearity is typically introduced to a cavity mode through an ancillary two-level qubit. However, the ancilla's spurious heating has impeded progress towards fully fault-tolerant bosonic qubits. The ability to in situ decouple the ancilla when not in use would be beneficial but has so far only been realized with tunable couplers or additional parametric drives. This work presents a novel architecture for quantum information processing, comprising a 3D post cavity coupled to a fluxonium ancilla via a readout resonator. This system's intricate energy level structure results in a complex landscape of interactions whose sign can be tuned in situ by the magnetic field threading the fluxonium loop without the need of additional elements. Our results could significantly advance the lifetime and controllability of bosonic qubits.
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J. Yang, T. E. Agrenius Gustafsson, V. Usova, O. Romero-Isart, G. Kirchmair Hot Schrödinger Cat States,
Sci. Adv. 11 (2025-04-04),
http://dx.doi.org/10.1126/sciadv.adr4492 doi:10.1126/sciadv.adr4492
The observation of quantum phenomena often necessitates sufficiently pure states, a requirement that can be challenging to achieve. In this study, our goal is to prepare a non-classical state originating from a mixed state, utilizing dynamics that preserve the initial low purity of the state. We generate a quantum superposition of displaced thermal states within a microwave cavity using only unitary interactions with a transmon qubit. We measure the Wigner functions of these ``hot'' Schrödinger cat states for an initial purity as low as 0.06. This corresponds to a cavity mode temperature of up to 1.8 Kelvin, sixty times hotter than the cavity's physical environment. Our realization of highly mixed quantum superposition states could be implemented with other continuous-variable systems e.g. nanomechanical oscillators, for which ground-state cooling remains challenging.
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L. Deeg, D. Zoepfl, N. Diaz-Naufal, M. L. Juan, A. Metelmann, G. Kirchmair Optomechanical Backaction in the Bistable Regime,
Phys. Rev. Applied 23 (2025-01-31),
http://dx.doi.org/10.1103/PhysRevApplied.23.014082 doi:10.1103/PhysRevApplied.23.014082
With a variety of realizations, optomechanics utilizes its light-matter interaction to test fundamental physics. By coupling the phonons of a mechanical resonator to the photons in a high-quality cavity, control of increasingly macroscopic objects has become feasible. In such systems, state manipulation of the mechanical mode is achieved by driving the cavity. To be able to achieve high drive powers the system is typically designed such that it remains in a linear response regime when driven. A nonlinear response, and especially bistability, in a driven cavity is often considered detrimental to cooling and state preparation in optomechanical systems and is avoided in experiments. Here we show that with an intrinsic nonlinear cavity backaction cooling of a mechanical resonator is feasible operating deep within the nonlinear regime of the cavity. With our theory taking the nonlinearity into account, precise predictions on backaction cooling can be achieved even with a cavity beyond the bifurcation point, where the cavity photon number spectrum starts to deviate from a typical Lorentzian shape.