Simulating Hamiltonian dynamics using many-qudit Hamiltonians and local unitary control

When can a quantum system of finite dimension be used to simulate another quantum system of finite dimension? What restricts the capacity of one system to simulate another? In this paper we complete the program of studying what simulations can be done with entangling many-qudit Hamiltonians and local unitary control. By entangling we mean that every qudit is coupled to every other qudit, at least indirectly. We demonstrate that the only class of finite-dimensional entangling Hamiltonians that aren't universal for simulation is the class of entangling Hamiltonians on qubits whose Pauli operator expansion contains only terms coupling an odd number of systems, as identified by Bremner et. al. [Phys. Rev. A, 69, 012313 (2004)]. We show that in all other cases entangling many-qudit Hamiltonians are universal for simulation

Movement of Adult Colorado Potato Beetles, \u3ci\u3eLeptinotarsa Decemlineata\u3c/i\u3e (Coleoptera: Chrysomelidae), in Response to Isolated Potato Plots

Mark recapture techniques were used to determine Colorado potato beetle movement in circular arenas with isolated plots of potatoes at each ordinal direction. Post-diapause beetles aggregated on one or a few of the plants in one of the plots for each release, but not on the same plants in different releases. Differences in plant attractiveness were therefore not likely responsible for the aggregatory behavior. Aggregations were probably a result of either coordinated movement from the release site to the plants or an aggregatory signal with a range of at least 15 m. Summer adults did not aggregate on plants. Correlations of summer beetle recapture distributions to wind direction showed that anemotactic behavior could not account for the major portion of variation in beetle orientation to the potato plots. Some positive attraction to the plots was indicated because more beetles were recaptured at the plots than would be expected from random motion. The number of beetles recaptured at plots covered by cheesecloth was not significantly different from recaptures on uncovered plots, indicating little reliance on visual cues specific to potatoes for location of the plants

Measurement of the fundamental modulation response of a semiconductor laser to millimeter wave frequencies by active-layer photomixing

The room-temperature modulation response of a GaAs/GaAlAs semiconductor laser (relaxation resonance frequency, vR=6.5 GHz) is measured to 37 GHz using the active-layer photomixing technique. The measured response function agrees with the theoretical ideal, and there is no indication of device parasitic effects. An ultrahigh-finesse Fabry–Perot interferometer is used to detect the optical modulation, which appears as sidebands in the laser field spectrum. With a moderately faster laser diode (i.e., vR~10 GHz), the modulation response should be measurable beyond 100 GHz

Low-temperature measurement of the fundamental frequency response of a semiconductor laser by active-layer photomixing

We use the active-layer photomixing technique to directly modulate the output of a GaAs semiconductor laser operating at temperatures as low as 4.2 K. The technique produces modulation with nearly perfect immunity to device parasitic effects, revealing the laser diode's intrinsic modulation response. At 4.2 K the parasitic corner frequency is estimated to be 410 MHz, yet the response appears ideal out to 15 GHz. We measure the dynamical parameters governing the response function, the relaxation resonance frequency, and the damping rate, and discuss their low-temperature behavior

Amplitude-phase decorrelation: a method for reducing intensity noise in semiconductor lasers

It has been shown that the method of amplitude-phase decorrelation can reduce the fundamental intensity noise floor of semiconductor laser light over a wide bandwidth by the ratio 1/(1+α^2), where α is the linewidth enhancement factor. The method uses a dispersive element to convert phase noise into intensity noise. This technique was recently demonstrated by reducing intensity noise from a DFB (distributed feedback) laser as much as 7 dB below its intrinsic level. In the present work, the authors extend these results by characterizing the frequency dependence of the noise reduction. Optimum reduction is achieved in the flat region of the spectrum and diminishes at higher frequencies approaching the relaxation resonance. The correlation properties of the fluctuations are also investigated