Quantum control of a cat qubit with bit-flip times exceeding ten seconds.

Quantum bits (qubits) are prone to several types of error as the result of uncontrolled interactions with their environment. Common strategies to correct these errors are based on architectures of qubits involving daunting hardware overheads1. One possible solution is to build qubits that are inherently protected against certain types of error, so the overhead required to correct the remaining errors is greatly reduced2-7. However, this strategy relies on one condition: any quantum manipulations of the qubit must not break the protection that has been so carefully engineered5,8. A type of qubit known as a cat qubit is encoded in the manifold of metastable states of a quantum dynamical system, and thereby acquires continuous and autonomous protection against bit-flips. Here, in a superconducting-circuit experiment, we implemented a cat qubit with bit-flip times exceeding 10 s. This is an improvement of four orders of magnitude over previously published cat-qubit implementations. We prepared and imaged quantum superposition states, and measured phase-flip times greater than 490 ns. Most importantly, we controlled the phase of these quantum superpositions without breaking the bit-flip protection. This experiment demonstrates the compatibility of quantum control and inherent bit-flip protection at an unprecedented level, showing the viability of these dynamical qubits for future quantum technologies.

Benchmarking Maximum-Likelihood State Estimation with an Entangled Two-Cavity State.

The efficient quantum state reconstruction algorithm described by Six et al. [Phys. Rev. A 93, 012109 (2016)PLRAAN2469-992610.1103/PhysRevA.93.012109] is experimentally implemented on the nonlocal state of two microwave cavities entangled by a circular Rydberg atom. We use information provided by long sequences of measurements performed by resonant and dispersive probe atoms over timescales involving the system decoherence. Moreover, we benefit from the consolidation, in the same reconstruction, of different measurement protocols providing complementary information. Finally, we obtain realistic error bars for the matrix elements of the reconstructed density operator. These results demonstrate the pertinence and precision of the method, directly applicable to any complex quantum system.

Using Spontaneous Emission of a Qubit as a Resource for Feedback Control.

Persistent control of a transmon qubit is performed by a feedback protocol based on continuous heterodyne measurement of its fluorescence. By driving the qubit and cavity with microwave signals whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we show that it is possible to stabilize permanently the qubit in any targeted state. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η=35%, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output analog Markovian feedback in the quantum regime.

Stabilization of nonclassical states of the radiation field in a cavity by reservoir engineering.

We propose an engineered reservoir inducing the relaxation of a cavity field towards nonclassical states. It is made up of two-level atoms crossing the cavity one at a time. Each atom-cavity interaction is first dispersive, then resonant, then dispersive again. The reservoir pointer states are those produced by an effective Kerr Hamiltonian acting on a coherent field. We thereby stabilize squeezed states and quantum superpositions of multiple coherent components in a cavity having a finite damping time. This robust decoherence protection method could be implemented in state-of-the-art experiments.

Purification and characterization of a new potential in vivo inhibitor of cathepsin L from bovine skeletal muscle.

Four papain-inhibiting peaks, labeled F-I, F-II, F-III, and F-IV, were fractionated from a crude bovine muscle extract by gel filtration chromatography on Sephadex G100, and the F-III fraction was analyzed. From F-III, a cysteine proteinase inhibitor was purified by two successive anionic exchange chromatography steps on Q-Sepharose and Mono-Q columns. This inhibitor has a molecular weight of about 30 kDa. Regarding its specificity toward different proteinases, the purified 30 kDa inhibitor was inactive against serine (trypsin and chymotrypsin) and aspartyl (pepsin) families. In contrast, cathepsin L, H, B, and papain, four enzymes of the cysteine class were strongly inhibited suggesting that this inhibitor was specific to the cysteine proteinase group. However, no inhibitory activity was shown against calpains. Kinetic parameters, including inhibition constants (Ki), rate constant for association (kass) and time required for almost complete inhibition of proteinase in vivo were determined. The values are consistent with a possible physiological function for this inhibitor protein in controlling in vivo cathepsin L activity.

Purification and characterisation of a polymorphic low M(r) bovine muscle cysteine proteinase inhibitor: structural identity with fatty-acid-binding proteins.

Three low molecular mass cysteine proteinase inhibitors were purified from a bovine skeletal muscle crude extract using a three-step procedure. The crude extract was first subjected to gel filtration on a Sephadex G100 column which separated five active fractions (F-I to F-V). Three papain inhibitors, P1, P2 and P3, were fractionated from the F-V fraction by chromatofocalisation on a poly buffer exchanger column. Purification was completed by chromatography on a Mono Q column. After SDS-PAGE, the three inhibitors showed only one band with an M(r) of 14,300. P1, P2 and P3 appeared to be highly resistant to temperature (40-90 degrees C), pH (3-10), reducing agents (5-50 mM) and to be specific for cysteine proteinases since no activity was detected against either serine or aspartyl proteinases. Although to a varying extent, P1, P2 and P3 inhibited papain, cathepsin B and cathepsin L. Analysis of the peptide mixtures of these inhibitors by RP-HPLC after hydrolysis with CNBr or aspartly endoproteinase N together with their amino acid composition revealed that P1, P2 and P3 cysteine proteinase inhibitors are isoforms of the same protein. As their N-terminal ends were blocked, partial sequence of some of these peptides was determined. Computer search in protein identification resources did not reveal any homology of these sequences with proteinase inhibitors of known primary structure. In contrast, they matched well with different parts of the total sequence of a fatty acid binding protein isolated from bovine heart. This homology was supported by the ability of these inhibitors to bind long chain fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)