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Cubic silicon-carbide crystals (3C-SiC), known for their high thermal conductivity and in-plane stress, hold significant promise for the development of high-quality ( Q ) mechanical oscillators. We reveal degeneracy-breaking phenomena in 3C-phase crystalline silicon-carbide membrane and present high- Q mechanical modes in pairs or clusters. The 3C-SiC material demonstrates excellent microwave compatibility with superconducting circuits. Thus, we can establish a coherent electromechanical interface, enabling precise control over 21 high- Q mechanical modes from a single 3C-SiC square membrane. Benefiting from extremely high mechanical frequency stability, this interface enables tunable light slowing with group delays extending up to an impressive duration of an hour . Coherent energy transfer between distinct mechanical modes are also presented. In this work, the studied 3C-SiC membrane crystal with their significant properties of multiple acoustic modes and high-quality factors, provide unique opportunities for the encoding, storage, and transmission of quantum information via bosonic phonon channels. Coherent electrical manipulation of long-lived, multimode mechanical resonators is crucial for several applications. Here the authors report the integration of a high quality cubic SiC-membrane mechanical resonator with a 3D microwave cavity, enabling long-lived phononic memory and tunable light slowing.

Catching the quantum bus Microfabricated superconducting circuit elements can harness the power of quantum behaviour for information processing. Unlike classical information bits, quantum information bits (qubits) can form superpositions or mixture states of ON and OFF, offering a faster, natural form of parallel processing. Previously, direct qubit–qubit coupling has been achieved for up to four qubits, but now two independent groups demonstrate the next crucial step: communication and exchange of quantum information between two superconducting qubits via a quantum bus, in the form of a resonant cavity formed by a superconducting transmission line a few millimetres long. Using this microwave cavity it is possible to store, transfer and exchange quantum information between two quantum bits. It can also perform multiplexed qubit readout. This basic architecture lends itself to expansion, offering the possibility for the coherent interaction of many superconducting qubits. The cover illustrates a zig-zag-shaped resonant cavity or quantum bus linking two superconducting phase qubits. One of two papers that demonstrate the communication of individual quantum states between superconducting qubits via a quantum bus. This quantum bus is a resonant cavity formed by a superconducting transmission line of several millimetres. Quantum information, initially defined in one qubit on one end, can be stored in this quantum bus and at a later time retrieved by a second qubit at the other end. As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states 1 , 2 , and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits 3 , 4 . Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor 5 , 6 , 7 , 8 , 9 . Although these systems have successfully passed tests of coherent coupling for up to four qubits 10 , 11 , 12 , 13 , communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended superconducting transmission line of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved later by the second qubit connected to the opposite end of the cavity. Beyond simple state transfer, these results suggest that a high-quality-factor superconducting cavity could also function as a useful short-term memory element. The basic architecture presented here can be expanded, offering the possibility for the coherent interaction of a large number of superconducting qubits.

Dissipation and the accompanying fluctuations are often seen as detrimental for quantum systems since they are associated with fast relaxation and loss of phase coherence. However, it has been proposed that a pure state can be prepared if external noise induces suitable downwards transitions, while exciting transitions are blocked. We demonstrate such a refrigeration mechanism in a cavity optomechanical system, where we prepare a mechanical oscillator in its ground state by injecting strong electromagnetic noise at frequencies around the red mechanical sideband of the cavity. The optimum cooling is reached with a noise bandwidth smaller than but on the order of the cavity decay rate. At higher bandwidths, cooling is less efficient as suitable transitions are not effectively activated. In the opposite regime where the noise bandwidth becomes comparable to the mechanical damping rate, damping follows the noise amplitude adiabatically, and the cooling is also suppressed. Sideband cooling is a well-known technique exploiting coherent pumping for cooling a quantum system, and recent theoretical work suggested that even broadband noise might be used to the same effect. Here, the authors demonstrate this by cooling a mechanical oscillator in its ground state using synthetic noise.

Quantum backaction disturbs the measurement of the position of a mechanical oscillator by introducing additional fluctuations. In a quantum backaction measurement technique, the backaction can be evaded, although at the cost of losing part of the information. In this work, we carry out such a quantum backaction measurement using a large 0.5 mm diameter silicon nitride membrane oscillator with 707 kHz frequency, via a microwave cavity readout. The measurement shows that quantum backaction noise can be evaded in the quadrature measurement of the motion of a large object.

The coupling of distinct systems underlies nearly all physical phenomena. A basic instance is that of interacting harmonic oscillators, giving rise to, for example, the phonon eigenmodes in a lattice. Of particular importance are the interactions in hybrid quantum systems, which can combine the benefits of each part in quantum technologies. Here we investigate a hybrid optomechanical system having three degrees of freedom, consisting of a microwave cavity and two micromechanical beams with closely spaced frequencies around 32 MHz and no direct interaction. We record the first evidence of tripartite optomechanical mixing, implying that the eigenmodes are combinations of one photonic and two phononic modes. We identify an asymmetric dark mode having a long lifetime. Simultaneously, we operate the nearly macroscopic mechanical modes close to the motional quantum ground state, down to 1.8 thermal quanta, achieved by back-action cooling. These results constitute an important advance towards engineering of entangled motional states. Optomechanical systems allow for the exploration of macroscopic behaviour at or near the quantum limit. Massel et al . use micromechanical resonators to study the hybridisation of one photonic and two phononic modes with phonon numbers down to 1.8, showing a coupling between all three degrees of freedom.

We report a study of a cavity optomechanical system driven by narrowband electromagnetic fields, which are applied either in the form of uncorrelated noise, or as a more structured spectrum. The bandwidth of the driving spectra is smaller than the mechanical resonant frequency, and thus we can describe the resulting physics using concepts familiar from regular cavity optomechanics in the resolved-sideband limit. With a blue-detuned noise driving, the noise-induced interaction leads to anti-damping of the mechanical oscillator, and a self-oscillation threshold at an average noise power that is comparable to that of a coherent driving tone. This process can be seen as noise-induced dynamical amplification of mechanical motion. However, when the noise bandwidth is reduced down to the order of the mechanical damping, we discover a large shift of the power threshold of self-oscillation. This is due to the oscillator adiabatically following the instantaneous noise profile. In addition to blue-detuned noise driving, we investigate narrowband driving consisting of two coherent drive tones nearby in frequency. Also in these cases, we observe deviations from a naive optomechanical description relying only on the tones’ frequencies and powers.

Routers, switches and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmission.

The mathematical description of pure dephasing in a three-level system is not as straightforward as it is in a two-level system. Here we provide a detailed derivation of the Liouvillean for pure dephasing for a ladder three-level system, based on [J Li et al., Phys. Rev. B 84, 104527 (2011)]. Numerical calculations based on this model give good fittings to the spectral line and Autler-Townes splitting observed in a superconducting phase qubit.