Fault tolerant quantum information processing requires specific nonlinear interactions acting within the Hilbert space of the physical system that implements a logical qubit. The requiredorder of nonlinearity is often not directly available in the natural interactions of the system. Here, we experimentally demonstrate a route to obtain higher-order nonlinearities by combining more easily available lower-order nonlinear processes, using a generalization of the Raman transitions. In particular, we demonstrate a Raman-assisted transformation of four photons of a high-Q superconducting cavity into two excitations of a superconducting transmon mode and vice versa. The resulting six-quanta process is obtained by cascading two fourth-order nonlinear processes through a virtual state. This process is a key step towards hardware efficient quantum error correction using Schrödinger cat-states.
We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state ofthe qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconducting qubits and implementations of bosonic quantum error-correcting codes.
We present a new process for fabricating vertical NbN-MgO-NbN Josephson junctions using self-aligned silicon nitride spacers. It allows for a wide range of junction areas from 0.02um^2 to several 100 um^2. At the same time, it is suited for the implementation of complex microwave circuits with transmission line impedances ranging from < 1 Ohm to > 1 kOhm. The constituent thin films and the finished junctions are characterized. The latter are shown to have high gap voltages (> 4 mV) and low sub-gap leakage currents.
The quantum Zeno effect (QZE) is the apparent freezing of a quantum system in one state under the influence of a continuous observation. It has been further generalized to the stabilizationof a manifold spanned by multiple quantum states. In that case, motion inside the manifold can subsist and can even be driven by the combination of a dissipative stabilization and an external force. A superconducting microwave cavity that exchanges pairs of photons with its environments constitutes an example of a system which displays a stabilized manifold spanned by Schr\“odinger cat states. For this driven-dissipative system, the quantum Zeno stabilization transforms a simple linear drive into photon number parity oscillations within the stable cat state manifold. Without this stabilization, the linear drive would trivially displace the oscillator state and push it outside of the manifold. However, the observation of this effect is experimentally challenging. On one hand, the adiabaticity condition requires the oscillations to be slow compared to the manifold stabilization rate. On the other hand, the oscillations have to be fast compared with the coherence timescales within the stabilized manifold. Here, we implement the stabilization of a manifold spanned by Schr\“odinger cat states at a rate that exceeds the main source of decoherence by two orders of magnitude, and we show Zeno-driven coherent oscillations within this manifold. While related driven manifold dynamics have been proposed and observed, the non-linear dissipation specific to our experiment adds a crucial element: any drift out of the cat state manifold is projected back into it. The coherent oscillations of parity observed in this work are analogous to the Rabi rotation of a qubit protected against phase-flips and are likely to become part of the toolbox in the construction of a fault-tolerant logical qubit.
Nature sets fundamental limits regarding how accurate the amplification of analog signals may be. For instance, a linear amplifier unavoidably adds some noise which amounts to halfa photon at best. While for most applications much higher noise levels are acceptable, the readout of microwave quantum systems, such as spin or superconducting qubits requires noise as close as possible to this ultimate limit. To date it is approached only by parametric amplifiers exploiting non-linearities in superconducting circuits and driven by a strong microwave pump tone. However, this microwave drive makes them much more difficult to implement and operate than conventional DC powered amplifiers, which, so far suffer from much higher noise. Here we present the first experimental proof that a simple DC-powered setup allows for amplification close to the quantum limit. Our amplification scheme is based on the stimulated microwave photon emission accompanying inelastic Cooper pair tunneling through a DC-biased Josephson junction, with the key to low noise lying in the separation of nonlinear and dissipative elements, in analogy to parametric amplifiers.
Stabilization of quantum manifolds is at the heart of error-protected quantum information storage and manipulation. Nonlinear driven-dissipative processes achieve such stabilizationin a hardware efficient manner. Josephson circuits with parametric pump drives implement these nonlinear interactions. In this article, we propose a scheme to engineer a four-photon drive and dissipation on a harmonic oscillator by cascading experimentally demonstrated two-photon processes. This would stabilize a four-dimensional degenerate manifold in a superconducting resonator. We analyze the performance of the scheme using numerical simulations of a realizable system with experimentally achievable parameters.
Niobium nitride (NbN) is widely used in high-frequency superconducting electronics circuits because it has one of the highest superconducting transition temperatures (Tc ∼ 16.5 K)and largest gap among conventional superconductors. In its thin-film form, the Tc of NbN is very sensitive to growth conditions and it still remains a challenge to grow NbN thin film (below 50 nm) with high Tc. Here, we report on the superconducting properties of NbN thin films grown by high-temperature chemical vapor deposition (HTCVD). Transport measurements reveal significantly lower disorder than previously reported, characterized by a Ioffe-Regel (kFℓ) parameter of ∼ 14. Accordingly we observe Tc ∼ 17.06 K (point of 50% of normal state resistance), the highest value reported so far for films of thickness below 50 nm, indicating that HTCVD could be particularly useful for growing high quality NbN thin films.