Quantum heating of a nonlinear resonator probed by a superconducting qubit.

We measure the quantum fluctuations of a pumped nonlinear resonator using a superconducting artificial atom as an in situ probe. The qubit excitation spectrum gives access to the frequency and amount of excitation of the intracavity field fluctuations, from which we infer its effective temperature. These quantities are found to be in agreement with theoretical predictions; in particular, we experimentally observe the phenomenon of quantum heating.

Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise.

We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f~1 GHz and find agreement with values extrapolated from a 1/f(α) fit to the measured flux noise spectral density below 1 Hz.

Circuit QED with a nonlinear resonator: ac-Stark shift and dephasing.

We have performed spectroscopic measurements of a superconducting qubit dispersively coupled to a nonlinear resonator driven by a pump microwave field. Measurements of the qubit frequency shift provide a sensitive probe of the intracavity field, yielding a precise characterization of the resonator nonlinearity. The qubit linewidth has a complex dependence on the pump frequency and amplitude, which is correlated with the gain of the nonlinear resonator operated as a small-signal amplifier. The corresponding dephasing rate is found to be close to the quantum limit in the low-gain limit of the amplifier.

Control and tomography of a three level superconducting artificial atom.

A number of superconducting qubits, such as the transmon or the phase qubit, have an energy level structure with small anharmonicity. This allows for convenient access of higher excited states with similar frequencies. However, special care has to be taken to avoid unwanted higher-level populations when using short control pulses. Here we demonstrate the preparation of arbitrary three level superposition states using optimal control techniques in a transmon. Performing dispersive readout, we extract the populations of all three levels of the qutrit and study the coherence of its excited states. Finally we demonstrate full quantum state tomography of the prepared qutrit states and evaluate the fidelities of a set of states, finding on average 95%.

The enhancement by pervanadate of tyrosine phosphorylation on prostatic proteins occurs through the inhibition of membrane-associated tyrosine phosphatases.

The relapse of prostate cancer during endocrine therapies is attributed to the proliferation of growth factor (GF)-dependent epithelial cells. Such cells are present but in a quiescent state in the normal adult human and dog (experimental model) prostates. GF-signaling pathways involve the activation of protein tyrosine kinases (PTK) whose action is also modulated by phosphotyrosine protein phosphatases (PTPs). To that effect, we have previously reported that dividing canine prostatic epithelial cells exhibited high levels of phosphotyrosyl-(pY)-proteins which were greatly enhanced when incubated in the presence of vanadate. The aim of this study, performed with pervanadate (pV), was to determine whether pV acts either directly by stimulating prostatic PTKs or indirectly by inhibiting PTPs. Upon fractionation, most of the PTK activity was found in membranes of dividing cells and pV selectively increased its activity. This was due to an inhibition of intrinsic PTPs, as demonstrated by dephosphorylation of endogenous pY-proteins which was abolished by pV. This activity was very sensitive to pV (IC50: 150 nM) and was due to non-secreted forms of prostatic acid phosphatase (PAP), a pV inhibited-enzyme, as well as to PTP-1 B, as demonstrated by gel filtration, isoelectric focusing and probing with antibodies. These enzymes were also detected in membranes from human hyperplastic/neoplastic prostates but only PTP-1 B was present in those of prostatic carcinoma PC3 cells. These PTPs, bound to membranes of dividing cells (normal vs neoplastic) where activated PTKs are also located, may be of importance in the development and progression of prostatic proliferative diseases.

Transforming growth factor beta down-regulates Src family protein tyrosine kinase signaling pathways.

Transforming growth factor beta (TGF beta) inhibits the proliferation of a wide range of cell types through interaction with its cell surface receptor (R-TGF beta). R-TGF beta possesses serine/threonine kinase activity rather than the tyrosine kinase activity normally associated with peptide growth factor receptors; nevertheless, TGF beta triggers a signaling pathway that leads to the repression of transcription factors, which appear to mediate the action of receptor tyrosine kinases within the nucleus. Accumulating evidence has also shown that the nonreceptor protein tyrosine kinases of the Src family play essential roles in the signal transduction pathways that regulate cell proliferation, differentiation, and function. Here, we investigate whether signals initiated by R-TGF beta are transduced, at least in part, through members of the Src family of tyrosine kinases. Treatment of the responsive human prostate carcinoma cell line PC3 with TGF beta induces a rapid and specific decrease in cellular levels of pp60Src and pp53/56Lyn and a corresponding decrease in their protein kinase activity when the assays were performed in vitro using the exogenous substrate enolase. Consistent with suppression of pp60Src and pp53/56Lyn kinase activity, TGF beta also caused a substantial intracellular accumulation of the unphosphorylated form of SH2-containing protein (SHC), a substrate of the Src family kinases. This was paralleled by decreased formation of a complex between the adaptor protein known as growth factor receptor-bound protein 2 and SHC. These results suggest, for the first time, that TGF beta induces down-regulation of Src family kinases, leading to disruption of the SHC-growth factor receptor-bound protein 2 complex. These events may play a crucial role in the negative regulation of Ras, as well as in the control of downstream effector molecules involved in the regulation of cell growth.