Quantum feedback for rapid state preparation in the presence of control imperfections

Quantum feedback control protocols can improve the operation of quantum devices. Here we examine the performance of a purification protocol when there are imperfections in the controls. The ideal feedback protocol produces an $x$ eigenstate from a mixed state in the minimum time, and is known as rapid state preparation. The imperfections we examine include time delays in the feedback loop, finite strength feedback, calibration errors, and inefficient detection. We analyse these imperfections using the Wiseman-Milburn feedback master equation and related formalism. We find that the protocol is most sensitive to time delays in the feedback loop. For systems with slow dynamics, however, our analysis suggests that inefficient detection would be the bigger problem. We also show how system imperfections, such as dephasing and damping, can be included in model via the feedback master equation.Comment: 15 pages, 6 figures and 2 tables. V2 the published version, fig. 1 corrected and some minor changes to the tex

Maximum information gain in weak or continuous measurements of qudits: complementarity is not enough

To maximize average information gain for a classical measurement, all outcomes of an observation must be equally likely. The condition of equally likely outcomes may be enforced in quantum theory by ensuring that one's state $\rho$ is maximally different, or complementary, to the measured observable. This requires the ability to perform unitary operations on the state, conditioned on the results of prior measurements. We consider the case of measurement of a component of angular momentum for a qudit (a $D$-dimensional system, with $D=2J+1$). For weak or continuous-in-time (i.e. repeated weak) measurements, we show that the complementarity condition ensures an average improvement, in the rate of purification, of only 2. However, we show that by choosing the optimal control protocol of this type, one can attain the best possible scaling, $O(D^{2})$, for the average improvement. For this protocol the acquisition of information is nearly deterministic. Finally we contrast these results with those for complementarity-based protocols in a register of qbits.Comment: 21 pages, 21 figures. V2 published versio

Prospects for Higgs Searches with the Tri-bottom Channel in Unified SUSY Models

We investigate the prospects for the discovery of a neutral Higgs boson produced in association with a $b$ quark, followed by the Higgs decay into a pair of bottom quarks, $pp \to b\phi^0 \to b b\bar{b} +X$, at the CERN Large Hadron Collider (LHC) within the framework of unified supersymmetric models. The Higgs boson $\phi^0$ can be a heavy scalar $H^0$ or a pseudoscalar $A^0$. Furthermore, this direct discovery channel is compared with the indirect Higgs searches in the rare decay $B_s \to \mu^+\mu^-$ at hadron colliders. Promising results are found for the minimal supergravity (mSUGRA) model, the anomaly mediated supersymmetry breaking (AMSB) model, and the gauge mediated supersymmetry breaking (GMSB) model. We find that the indirect search for $B(B_s \to \mu^+\mu^-) \ge 5\times 10^{-9}$ is complementary to the direct search for $b\phi^0 \to bb\bar{b}$ with $\sqrt{s} = 14$ TeV and an integrated luminosity ($L$) of 300 fb$^{-1}$. In the AMSB and GMSB models, $b\phi^0 \to bb\bar{b}$ with $L = 300$ fb$^{-1}$ covers a larger area in the parameter space than $B(B_s \to \mu^+\mu^-) \ge 5\times 10^{-9}$. In addition, we present constraints from $b \to s\gamma$ and muon anomalous dipole moment ($\Delta a_\mu$) on the parameter space.Comment: REVTEX, 23 pages, 4 figure

Walking a tightrope into 2008

Inflation (Finance) ; Housing ; Consumer behavior

Collective force generation by groups of migrating bacteria

From biofilm and colony formation in bacteria to wound healing and embryonic development in multicellular organisms, groups of living cells must often move collectively. While considerable study has probed the biophysical mechanisms of how eukaryotic cells generate forces during migration, little such study has been devoted to bacteria, in particular with regard to the question of how bacteria generate and coordinate forces during collective motion. This question is addressed here for the first time using traction force microscopy. We study two distinct motility mechanisms of Myxococcus xanthus, namely twitching and gliding. For twitching, powered by type-IV pilus retraction, we find that individual cells exert local traction in small hotspots with forces on the order of 50 pN. Twitching of bacterial groups also produces traction hotspots, however with amplified forces around 100 pN. Although twitching groups migrate slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding, the second motility mechanism, is driven by lateral transport of substrate adhesions. When cells are isolated, gliding produces low average traction on the order of 1 Pa. However, traction is amplified in groups by a factor of ~5. Since advancing protrusions of gliding cells push on average in the direction of motion, we infer a long-range compressive load sharing among sub-leading cells. Together, these results show that the forces generated during twitching and gliding have complementary characters and both forces are collectively amplified in groups

Spin-Orbit Alignment for the Circumbinary Planet Host Kepler-16 A

Kepler-16 is an eccentric low-mass eclipsing binary with a circumbinary transiting planet. Here, we investigate the angular momentum of the primary star, based on Kepler photometry and Keck spectroscopy. The primary star’s rotation period is 35.1 ± 1.0 days, and its projected obliquity with respect to the stellar binary orbit is 1°.6 ± 2°.4. Therefore, the three largest sources of angular momentum—the stellar orbit, the planetary orbit, and the primary’s rotation—are all closely aligned. This finding supports a formation scenario involving accretion from a single disk. Alternatively, tides may have realigned the stars despite their relatively wide separation (0.2 AU), a hypothesis that is supported by the agreement between the measured rotation period and the “pseudosynchronous” period of tidal evolution theory. The rotation period, chromospheric activity level, and fractional light variations suggest a main-sequence age of 2–4 Gyr. Evolutionary models of low-mass stars can match the observed masses and radii of the primary and secondary stars to within about 3%

"Music for a National Defense": Making Martial Music during the Anti-Japanese War

This article examines the popularization of “mass songs” among Chinese Communist troops during the Anti-Japanese War by highlighting the urban origins of the National Salvation Song Movement and the key role it played in bringing songs to the war front. The diffusion of a new genre of march songs pioneered by Nie Er was facilitated by compositional devices that reinforced the ideological message of the lyrics, and by the National Salvation Song Movement. By the mid-1930s, this grassroots movement, led by Liu Liangmo, converged with the tail end of the proletarian arts movement that sought to popularize mass art and create a “music for national defense.” Once the war broke out, both Nationalists and Communists provided organizational support for the song movement by sponsoring war zone service corps and mobile theatrical troupes that served as conduits for musicians to propagate their art in the hinterland. By the late 1930s, as the United Front unraveled, a majority of musicians involved in the National Salvation Song Movement moved to the Communist base areas. Their work for the New Fourth Route and Eighth Route Armies, along with Communist propaganda organizations, enabled their songs to spread throughout the ranks.  Keywords: Anti-Japanese War, Li Jinhui, Liu Liangmo, Lü Ji, Mai Xin, mass song, National Salvation Song Movement, New Fourth Army, Nie Er, United Front, Xian Xinghai Related audio-visual links: “Original film performance of “March of the Volunteers”“Jiuguo junge”“Youjiduige”“Zai Taihangshan shang”“People’s Liberation Army March” “New Fourth Army Anthem