We discuss a possible realization of a quantum register with controllable decoherence in terms of /0> and /1> photon number states of a micromaser field. It is shown how to create in the Jaynes-Cummings model a superposition state of /0> and /1> photon number states inside a closed micromaser cavity. The loss of phase coherence between these two states can subsequently be measured by a second probe atom monitoring the decoherence of the field. A technique is proposed for forming the superposition of number states /0> and /1> using the time structure of the Rabi oscillation. The proposed method avoids problems with stray fields at the cavity holes, which disturb the coherence of the atomic superposition, and offers a way to study how the coupling strength to the environment influences the decoherence rate, displaying the robustness of physical qubits and the fidelity of quantum computations.
We discuss a possible realization of a quantum register with controllable decoherence in terms of /0> and /1> photon number states of a micromaser field. It is shown how to create in the Jaynes-Cummings model a superposition state of /0> and /1> photon number states inside a closed micromaser cavity. The loss of phase coherence between these two states can subsequently be measured by a second probe atom monitoring the decoherence of the field. A technique is proposed for forming the superposition of number states /0> and /1> using the time structure of the Rabi oscillation. The proposed method avoids problems with stray fields at the cavity holes, which disturb the coherence of the atomic superposition, and offers a way to study how the coupling strength to the environment influences the decoherence rate, displaying the robustness of physical qubits and the fidelity of quantum computations.
In a micromaser where a classical field strongly drives the atoms while they cross the cavity, remarkable atom–atom correlations show up at steady state, which vanish much faster than dissipative decay. Hence we consider atom pair correlation measurements in which the detection of the first probe atom prepares a mesoscopic superposition state of the cavity field, that entangles with a second probe atom. The conditional probabilities for the latter atomic detection provide a description of the decoherence of the superposition state, occurring in an open system in the presence of pumping, driving, dissipative, and thermal effects. The decoherence rate scales as the squared interaction time, that sets the separation in phase space between the superposition components, whereas the quantum coherence is unaffected by the atomic pumping. Hence we further investigate the system when the cavity is not pumped. Starting the correlation measurements from a thermal state, we can describe the effect of temperature on decoherence. Starting from a vacuum state, the superposition states are maximally separated Schrodinger cat states, whose decoherence can thus be monitored.
The microscopically pumped maser, or micromaser is a cavity QED experiment intended to be a physical realisation of the Jaynes-Cummings model of a single two-level atom interacting with a single mode electromagnetic field. This is the simplest model that describes the interaction between light and matter, yet it predicts behaviour unexpected from semiclassical models, such as the revival of Rabi oscillations of an atom interacting with an initially coherent field and non-monotonic linewidth as a function of pumping. The micromaser at the University of Leeds consists of a high quality superconducting microwave cavity designed to be resonant with the transition between two specific Rydberg states of rubidium. These two states behave like an ideal two level atom, and couple strongly to the cavity field due to a large dipole moment. These Rydberg atoms are passed through the cavity in a rarified beam such that in most instances when there is an atom in the cavity, there will only be one, closely approximating the Jaynes-Cummings model. I present experimental work on the build phase of the micromaser. Specifically I routed all of the wiring and microwave lines in the cryostat that contains the micromaser, and designed mounts for various components. I also designed several testing methods for probing high quality microwave cavity resonances and quality factors which are presented. Using the Jaynes-Cummings model as a prototype, I demonstrate how extensions to the model can be used to construct universal quantum logic gates that operate on photonic qubits in a multi-mode cavity. This could be realised in a micromaser with a multi-mode cavity. Conversely, I demonstrate that by using atoms as qubits, detuned cavities can be used to generate entangled resources such as the Greenberger-Horne-Zeilinger state, the W state, and graph states of atoms. I show that single qubit rotations on Rydberg atom qubits have already been experimentally demonstrated so that in combination these entangled resources are useful for quantum metrology, quantum computation and even tests of quantum gravity.
There are only fistful of systems in physics that can be described by an exact theory from first principles and, at the same time, can be investigated under experimental conditions approaching the idealized theoretical ones. The one-atom-maser or micromaser provides such an example, making a detailed study of the fundamental properties of the atom-field interaction possible. The situation realized in a micromaser is very close to the ideal case of a single two-level atom interacting with a single quantized mode of a cavity field. In the micromaser the atoms have a dual purpose of both pumping the field and also probing it via measurements of the outgoing atoms. Any other means of measuring the field inside the resonator has the detrimental effect of lowering its Q-factor and thereby the photon storage time. The long photon storage time of the resonator allows for the decay of the field to be negligible during the passage of an atom and its interaction with the field. The behavior of atoms in a cavity is governed by the oscillatory exchange of energy between the atoms and the field, which is called Rabi oscillation. For cavity fields in the vacuum and few photon number states (Fock states), Rabi oscillations have been measured in the past. To prevent thermal effects from polluting the pure number states these experiments were performed at low temperatures (below 1 K). However, the observed resolution of the Rabi oscillations measurements was quite disappointing (only about 2% of the predicted theoretical value) and precluded more ambitious experiments like atom-atom correlations or time resolved investigations of the maser field dynamics. The observed contrast of Rabi oscillations can be considered as a figure of merit for the experimental resolution of quantum features of a maser field. Therefore, a high resolution in the measurements of Rabi oscillations is crucial. This thesis describes the methods and improvements applied to enhance the resolution of Rabi oscillations by a factor of 8. The following upgrades and developments proved to be crucial in this work: 1) The base temperature of the cryogenic system was lowered to 0.3 K, i.e. one third of the original value. At the same time, the time during which the system remains at the base temperature was improved by almost of an order of magnitude (to more than 12 h). 2) For the promotion of the atoms to the Rydberg regime new three-step diode laser setup was constructed. The excitation efficiency compared to the old dye laser was increased by two orders. Using top-of-fringe stabilization with new spectroscopic methods, specially designed error detection and feedback schemes as well as state-of-the-art PID regulators the continuous locking of lasers is about 8 h (compared to about 20 min with the old dye laser system). 3) The flexibility of using the three step excitation allows to excite different maser states and for the first time to investigate pure maser transition. 4) Various improvements on the atomic oven and the channeltron detection unit led to the reliable and stable production and detection of the atomic beam. Observed distribution of the atomic beam statistics reaches to within 6% the Poisson limit which is expected in the absence of experimental imperfections. 5) New developed techniques in magnetic field compensation allow to perform such measurements with two orders of magnitude better accuracy compared to the previous experiments. To demonstrate the new capabilities of the apparatus and to apply its improved resolution of quantum fields we performed a previously impossible in-depth analysis of mean photon number and the atomic inversion produced by a micromaser for various detunings between the cavity- and atomic resonance frequency.
Coherent superpositions of quantum states are very sensitive to coupling to the environment. The fluctuations associated with this coupling tend to wash out quantum phases, so that in practice only quantum mixtures are observed at the macroscopic level. This problem has been the object of recent interest in connection with measurement theory (Schrodinger cat). Theoretical predictions indicate that the decay of quantum coherences in the so-called pointer basis is amplified by a factor proportional to a measure of the distance in Hilbert space between the states. Micromasers offer a possible experimental tool to investigate the decay of such coherences. Theoretical studies of a lossless version of the micromaster have shown the existence of trapping states | n q 〉 of the cavity mode. These number states are such that atoms injected in the upper state into the cavity experience a 2qπ interaction with the field. Cavity states of the form | ψ 〉 = Σ q C q | n q 〉 are easily shown to be dynamic steady states of the system. Number states are not a pointer basis for the micromaser, and our theoretical results will evidence the qualitative difference in the nature of coherence decay between this and previously studied cases. Because the decay times of micromaser cavities are typically of the order of hundreds of milliseconds, our results should be amenable to experimental verification.
We discuss a scheme to relate the phase diffusion dynamics of the micromaser field to the measured atomic population statistics. This can allow us to measure the linewidth of the micromaser spectrum and to solve a relevant decoherence problem in cavity quantum electrodynamics. The main steps are (i) a suitable preparation of the cavity field state to generate coherences, (ii) the transfer of information on the dynamics of field coherences to probe atoms by the action of an external resonant coherent field and (iii) the derivation of the phase diffusion rate, hence the micromaser linewidth, from the measured population statistics of the probe atoms. The method can be applied even in the presence of trapping states, where peculiar linewidth oscillations are expected for increasing pump rate, due to the quantum nature of the micromaser field.
We present a setup of two coupled micromaser cavities for studying decoherence. The system is prepared in a state where, initially, one photon is contained in one of the two cavities. The coupling of the two resonators allows the photon to tunnel into the other cavity with a certain probability. The system thus oscillates between the two states $\ensuremath{\mid}1{⟩}_{L}\ensuremath{\mid}0{⟩}_{R}$ and $\ensuremath{\mid}0{⟩}_{L}\ensuremath{\mid}1{⟩}_{R}$. It is shown how to map these states onto the states $\ensuremath{\mid}g⟩$ and $\ensuremath{\mid}e⟩$ of probe atoms used to investigate the status of the cavity system. It is an important result that the symmetry of our special setup allows decoherence to be monitored by measuring just the diagonal elements of the atomic density matrix. This property relies on the fact that only $\ensuremath{\mid}0⟩$ and $\ensuremath{\mid}1⟩$ states are relevant. To our knowledge, this is the first proposal that tries to exploit this particular property.