Receiver front-end circuits and components for millimetre and submillimetre wavelengths

This dissertation focuses on the development of millimetre- and submillimetre-wave receiver front-end circuits and components. Seven scientific articles, written by the author, present this development work. A short introduction to the technology related to the designs of the thesis precedes the articles. The articles comprise several novel structures and techniques intended to further improve the performance of receivers or to provide new ways for receiver circuit implementation, summarised as follows. 1) Novel rectangular waveguide-to-CPW waveguide transition using a probe structure. The measured insertion and return loss of an X-band (8.2-12.4 GHz) back-to-back structure are less than 0.5 dB and more than 17 dB, respectively, over the entire frequency band (fractional bandwidth of > 40 %). The transition is used in a submm-wave mixer. 2) Novel rectangular waveguide-to-CPW transition using a fin-line taper. The measured insertion and return loss of an X-band (8.2-12.4 GHz) back-to-back structure are less than 0.4 dB and more than 16 dB, respectively, over the entire frequency band. 3) Novel tunable waveguide backshort based on a fixed waveguide short and movable dielectric slab. The measured return loss for a W-band backshort is less than 0.21 dB (VSWR > 82) over the entire frequency band of 75-110 GHz. 4) New coaxial bias T. The insertion loss is less than 0.5 dB at 3-16 GHz (fractional bandwidth of 137 %) and 0.1 dB at 5.2-14.1 GHz. In the latter range, the return loss is more than 30 dB. The RF isolation is greater than 30 dB at 1-17 GHz. 5) First millimetre-wave subharmonic waveguide mixer using European quasi-vertical Schottky diodes. The mixer utilises a single diode chip with quartz filters in a four-tuner waveguide housing. A single-sideband noise temperature of 3500 K and conversion loss of 9.2 dB (antenna loss included) have been measured at 215 GHz with an LO power of 3.5 mW. 6) Balanced-type fifth-harmonic submillimetre-wave mixer. It uses two planar Schottky diodes, quartz filters, and a tuner-less in-line waveguide housing with an integrated diagonal horn antenna and new LO transition structure. The designed RF range is 500-700 GHz enabling the use of an LO source at 100-140 GHz. A conversion loss of about 27 dB has been measured at 650 GHz with an LO power of 10 mW. The mixer has been in use in phase locking of a submm-wave signal source. 7) Characterisation procedure of planar Schottky diodes with extensive dc, capacitance, and wide-band (up to 220 GHz) S-parameter measurements and parameter extraction. Parameters of a simple diode equivalent circuit and results of extensive measurements are available for designers and diode manufacturers for further use.reviewe

Non-Abelian magnetic monopole in a Bose-Einstein condensate

Recently, an effective non-Abelian magnetic field with a topology of a monopole was shown to emerge from the adiabatic motion of multilevel atoms in spatially varying laser fields [J. Ruseckas et al., Phys. Rev. Lett. 95, 010404 (2005)]. We study this monopole in a Bose-Einstein condensate (BEC) of degenerate dressed states and find that the topological charge of the pseudospin cancels the monopole charge resulting in a vanishing gauge invariant charge. As a function of the laser wavelength, different stationary states are classified in terms of their effect to the monopole part of the magnetic field and a cross-over to vortex ground state is observed.Comment: 5 pages, 1 color figure; v2 modified according to referees' suggestions, some typos corrected; v3 minor modifications, published versio

Quantum circuits with uniformly controlled one-qubit gates

Uniformly controlled one-qubit gates are quantum gates which can be represented as direct sums of two-dimensional unitary operators acting on a single qubit. We present a quantum gate array which implements any n-qubit gate of this type using at most 2^{n-1} - 1 controlled-NOT gates, 2^{n-1} one-qubit gates and a single diagonal n-qubit gate. The circuit is based on the so-called quantum multiplexor, for which we provide a modified construction. We illustrate the versatility of these gates by applying them to the decomposition of a general n-qubit gate and a local state preparation procedure. Moreover, we study their implementation using only nearest-neighbor gates. We give upper bounds for the one-qubit and controlled-NOT gate counts for all the aforementioned applications. In all four cases, the proposed circuit topologies either improve on or achieve the previously reported upper bounds for the gate counts. Thus, they provide the most efficient method for general gate decompositions currently known.Comment: 8 pages, 10 figures. v2 has simpler notation and sharpens some result

Suppression of 1/f noise in one-qubit systems

We investigate the generation of quantum operations for one-qubit systems under classical noise with 1/f^\alpha power spectrum, where 2>\alpha > 0. We present an efficient way to approximate the noise with a discrete multi-state Markovian fluctuator. With this method, the average temporal evolution of the qubit density matrix under 1/f^\alpha noise can be feasibly determined from recently derived deterministic master equations. We obtain qubit operations such as quantum memory and the NOT}gate to high fidelity by a gradient based optimization algorithm. For the NOT gate, the computed fidelities are qualitatively similar to those obtained earlier for random telegraph noise. In the case of quantum memory however, we observe a nonmonotonic dependency of the fidelity on the operation time, yielding a natural access rate of the memory.Comment: 8 pages, 7 figure