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.
A quantum system driven by a weak deterministic force while under strong continuous energy measurement exhibits quantum jumps between its energy levels. This celebrated phenomenon isemblematic of the special nature of randomness in quantum physics. The times at which the jumps occur are reputed to be fundamentally unpredictable. However, certain classical phenomena, like tsunamis, while unpredictable in the long term, may possess a degree of predictability in the short term, and in some cases it may be possible to prevent a disaster by detecting an advance warning signal. Can there be, despite the indeterminism of quantum physics, a possibility to know if a quantum jump is about to occur or not? In this paper, we answer this question affirmatively by experimentally demonstrating that the completed jump from the ground to an excited state of a superconducting artificial atom can be tracked, as it follows its predictable „flight,“ by monitoring the population of an auxiliary level coupled to the ground state. Furthermore, we show that the completed jump is continuous, deterministic, and coherent. Exploiting this coherence, we catch and reverse a quantum jump mid-flight, thus preventing its completion. This real-time intervention is based on a particular lull period in the population of the auxiliary level, which serves as our advance warning signal. Our results, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as early detection of error syndromes.
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.
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.
Quantum jumps of a qubit are usually observed between its energy eigenstates, also known as its longitudinal pseudo-spin component. Is it possible, instead, to observe quantum jumpsbetween the transverse superpositions of these eigenstates? We answer positively by presenting the first continuous quantum nondemolition measurement of the transverse component of an individual qubit. In a circuit QED system irradiated by two pump tones, we engineer an effective Hamiltonian whose eigenstates are the transverse qubit states, and a dispersive measurement of the corresponding operator. Such transverse component measurements are a useful tool in the driven-dissipative operation engineering toolbox, which is central to quantum simulation and quantum error correction.
Entangling two remote quantum systems which never interact directly is an essential primitive in quantum information science. In quantum optics, remote entanglement experiments providesone approach for loophole-free tests of quantum non-locality and form the basis for the modular architecture of quantum computing. In these experiments, the two qubits, Alice and Bob, are each first entangled with a traveling photon. Subsequently, the two photons paths interfere on a beam-splitter before being directed to single-photon detectors. Such concurrent remote entanglement protocols using discrete Fock states can be made robust to photon losses, unlike schemes that rely on continuous variable states. This robustness arises from heralding the entanglement on the detection of events which can be selected for their unambiguity. However, efficiently detecting single photons is challenging in the domain of superconducting quantum circuits because of the low energy of microwave quanta. Here, we report the realization of a novel microwave photon detector implemented in the circuit quantum electrodynamics (cQED) framework of superconducting quantum information, and the demonstration, with this detector, of a robust form of concurrent remote entanglement. Our experiment opens the way for the implementation of the modular architecture of quantum computation with superconducting qubits.