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Laser Physics

Light-Induced Decoherence in the Driven Evolution of an Atom

Chr. Balzer, Th. Hannemann, Dirk Reiß, Werner Neuhauser, P. E. Toschek, C. Wunderlich

Published Jan 1, 2002• 4 citations
OpenAlex

Abstract

Atoms or ions prepared in traps for applications in quantum information processing (QIP) often become addressed by radiation within schemes of alternating microwave-optical double resonance. QIP requires thorough assessment of decoherence. A well-defined amount of decoherence may be applied to the system when spurious resonance light is admitted simultaneously with the driving radiation. This decoherence is quantified in terms of longitudinal and transversal relaxation. It may serve for calibrating observed decoherence as well as for testing error-correcting quantum codes.

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Atoms or ions prepared in traps for applications in quantum information processing (QIP) often become addressed by radiation within schemes of alternating microwave-optical double resonance. QIP requires thorough assessment of decoherence. A well-defined amount of decoherence may be applied to the system when spurious resonance light is admitted simultaneously with the driving radiation. This decoherence is quantified in terms of longitudinal and transversal relaxation. It may serve for calibrating observed decoherence as well as for testing error-correcting quantum codes.
Quantum information processing (QIP) requires thorough assessment of decoherence. Atoms or ions prepared for QIP often become addressed by radiation within schemes of alternating microwave-optical double resonance. A well-defined amount of decoherence may be applied to the system when spurious resonance light is admitted simultaneously with the driving radiation. This decoherence is quantified in terms of longitudinal and transversal relaxation. It may serve for calibrating observed decoherence as well as for testing error-correcting quantum codes.
Quantum Information Processing (QIP) is expected to bring revolutionary enhancement to various technological areas. However, today's QIP applications are far from being practical. The problem involves both hardware issues, i.e., quantum devices are imperfect, and software issues, i.e., the functionality of some QIP applications is not fully understood.Aiming to improve the practicality of QIP, in my PhD research I have studied various topics in quantum cryptography and ion trap quantum computation. In quantum cryptography, I first studied the security of position-based quantum cryptography (PBQC). I discovered a wrong assumption in the previous literature that the cheaters are not allowed to share entangled resources. I proposed entanglement attacks that could cheat all known PBQC protocols.I also studied the practicality of continuous-variable (CV) quantum secret sharing (QSS). While the security of CV QSS was considered by the literature only in the limit of infinite squeezing, I found that finitely squeezed CV resources could also provide finite secret sharing rate. Our work relaxes the stringent resources requirement of implementing QSS.In ion trap quantum computation, I studied the phase error of quantum information induced by dc Stark effect during ion transportation. I found an optimized ion trajectory for which the phase error is the minimum. I also defined a threshold speed, above which ion transportation would induce significant error.In addition, I proposed a new application for ion trap systems as universal bosonic simulators (UBS). I introduced two architectures, and discussed their respective strength and weakness. I illustrated the implementations of bosonic state initialization, transformation, and measurement by applying radiation fields or by varying the trap potential. When comparing with conducting optical experiments, the ion trap UBS is advantageous in higher state initialization efficiency and higher measurement accuracy.Finally, I proposed a new method to re-cool ion qubits during quantum computation. The idea is to transfer the motional excitation of a qubit to another ion that is prepared in the motional ground state. I showed that my method could be ten times faster than current laser cooling techniques, and thus could improve the speed of ion trap quantum computation.
Reiterated measurement blocks the quantum evolution of the measured object. An attempt at experimental demonstration of this postulate was reported a decade ago; it applied series of light pulses during the microwave-induced evolution of an ensemble of trapped ions, followed by a measurement of the ensemble’s state. The results agreed with quantum mechanical predictions. However, the blockade of the quantum evolution by acts of measurement cannot, in principle , be verified on an ensemble, since such a measurement yields, as the result, an expectation value, not an eigenvalue, and the micro-state of the system remains unknown. Moreover, each radiative intervention requires subsequent detection of the emerging state in order to qualify as a measurement. A recent experiment has used a single quantum entity (an ion) and has included repeated probing of the ion’s state alternating with the laser-driven evolution. Here, each measurement signals the eigen state of the system. The statistics of correlated results contains complete information on the evolution of the ion, in particular on the blocking effect of reiterated measurements. Recently, an extended relaxationless version has included coherently driving such an ion on the ground-state hyperfine transition by a microwave.
The micromotion of ion crystals confined in Paul traps is usually considered an inconvenient nuisance, and is thus typically minimized in high-precision experiments such as high-fidelity quantum gates for quantum information processing (QIP). In this work, we introduce a particular scheme where this behavior can be reversed, making micromotion beneficial for QIP. We show that using laser-driven micromotion sidebands, it is possible to engineer state-dependent dipole forces with a reduced effect of off-resonant couplings to the carrier transition. This allows one, in a certain parameter regime, to devise entangling gate schemes based on geometric phase gates with both a higher speed and a lower error, which is attractive in light of current efforts towards fault-tolerant QIP. We discuss the prospects of reaching the parameters required to observe this micromotion-enabled improvement in experiments with current and future trap designs.