Search Results (8 total)

The Fermilab photoinjector produces electron bunches of 1–12 nC charge with an energy of 16–18 MeV. Detailed measurements and optimization of the transverse emittance have been carried out for a number of beam line optics conditions, and at a number of beam line locations. The length of the bunches has also been measured, first for an uncompressed beam (as a function of the charge) and then for a compressed beam of 8 nC charge (as a function of the 9-cell cavity phase). These measurements are presented and compared with the simulation codes HOMDYN and ASTRA.

Photon-number-resolving detectors are needed for a variety of applications including linear-optics quantum computing. Here we describe the use of time-multiplexing techniques that allow ordinary single-photon detectors, such as silicon avalanche photodiodes, to be used as photon-number-resolving detectors. The ability of such a detector to correctly measure the number of photons for an incident number state is analyzed. The predicted results for an incident coherent state are found to be in good agreement with the results of a proof-of-principle experimental demonstration.

We report a proof-of-principle demonstration of a probabilistic controlled-NOT gate for single photons. Single-photon control and target qubits were mixed with a single ancilla photon in a device constructed using only linear optical elements. The successful operation of the controlled-NOT gate relied on post-selected three-photon interference effects, which required the detection of the photons in the output modes.

Knill, Laflamme, and Milburn [Nature (London) 409, 46 (2001)] have shown that quantum logic operations can be performed using linear optical elements and additional ancilla photons. Their approach is probabilistic in the sense that the logic devices fail to produce an output with a failure rate that scales as 1/n, where n is the number of ancilla. Here we present an alternative approach in which the logic devices always produce an output with an intrinsic error rate that scales as 1/n², which may have several advantages in quantum computing applications.

Nonlocal dispersion cancellation is generalized to frequency-entangled states with large photon number N. We show that the same entangled states can simultaneously exhibit a factor of 1/sqrt[N] reduction in noise below the classical shot noise limit in precise timing applications, as was previously suggested by Giovannetti, Lloyd, and Maccone [Nature (London) 412, 417 (2001)]. The quantum-mechanical noise reduction can be destroyed by a relatively small amount of uncompensated dispersion and entangled states of this kind have larger timing uncertainties than the corresponding classical states in that case. Similar results were obtained for correlated states, anticorrelated states, and frequency-entangled coherent states, which shows that these effects are a fundamental result of entanglement.

When a relativistic electron bunch traverses a structure, strong electromagnetic fields are induced in its wake. For a 12 nC bunch of duration 4.2 ps FWHM, the peak field is measured >0.5 MV/m. Time resolution of ∼5 ps is achieved using electro-optic sampling with a lithium tantalate (LiTaO₃) crystal and a short-pulse infrared laser synchronized to the beam. We present measurements for both the longitudinal and radial components of the field and relate them to the wall impedance.

We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.

High-spin states were populated in ¹⁷²Hf with the ⁴⁸Ca+¹²⁸Te reaction at 200 and 214 MeV. All of the known rotational bands in ¹⁷²Hf have been extended and five new rotational bands have been observed using the Gammasphere spectrometer. This study establishes the highest known state in ¹⁷²Hf at spin 44ħ. One of the new bands is a K^π=(14⁺) strongly coupled rotational band. In conflict with the K-selection rule another newly established bandhead state, with spin I=K=12, is observed to γ decay directly to the yrast 12⁺ state. This large breakdown in the K-selection rule (ΔK≊12) may be due to a mixing between the low- and high-K states in ¹⁷²Hf.