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arXiv:1401.5016 · Scientific Reports

Decoherence control by quantum decoherence itself

Katarzyna Roszak, Radim Filip, Tomáš Novotný

Published Jun 11, 2015• 26 citations
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Abstract

We propose a general approach of protecting a two-level system against decoherence via quantum engineering of non-classical multiple superpositions of coherent states in a non-Markovian reservoir. The scheme surprisingly only uses the system-environment interaction responsible for the decoherence and projective measurements of the two-level system. We demonstrate the method on the example of an excitonic qubit in self-assembled semiconductor quantum dots coupled to the super-Ohmic reservoir of acoustic phonons.

DOI
10.1038/srep09796

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•Jan 1, 2016•0 citations
We propose a general approach of protecting a two-level system against decoherence via quantum engineering of non-classical multiple superpositions of coherent states in a non-Markovian reservoir. The scheme surprisingly only uses the system-environment interaction responsible for the decoherence and projective measurements of the two-level system. We demonstrate the method on the example of an excitonic qubit in self-assembled semiconductor quantum dots coupled to the super-Ohmic reservoir of acoustic phonons.
We evaluate exactly the non-Markovian effect on the decoherence dynamics of a qubit interacting with a dissipative vacuum reservoir and find that the coherence of the qubit can be partially trapped in the steady state when the memory effect of the reservoir is considered. Our analysis shows that it is the formation of a bound state between the qubit and its reservoir that results in this residual coherence in the steady state under the non-Markovian dynamics. A physical condition for the decoherence suppression is given explicitly. Our results suggest a potential way to decoherence control by modifying the system–reservoir interaction and the spectrum of the reservoir to the non-Markovian regime in the scenario of reservoir engineering.
Controlling the non-Markovian dynamics of open quantum systems is essential in quantum information technology since it plays a crucial role in preserving quantum memory. Albeit in many realistic scenarios the quantum system can simultaneously interact with composite environments, this condition remains little understood, particularly regarding the effect of the coupling between environmental parts. We analyze the non-Markovian behavior of a qubit interacting at the same time with two coupled single-mode cavities which in turn dissipate into memoryless or memory-keeping reservoirs. We show that increasing the control parameter, that is the two-mode coupling, allows for triggering and enhancing a non-Markovian dynamics for the qubit starting from a Markovian one in the absence of coupling. Surprisingly, if the qubit dynamics is non-Markovian for the zero control parameter, increasing the latter enables multiple transitions from non-Markovian to Markovian regimes. These results hold independently on the nature of the reservoirs. This work highlights that suitably engineering the coupling between parts of a compound environment can efficiently harness the quantum memory, stored in a qubit, based on non-Markovianity.
Although coupling to a super-Ohmic bosonic reservoir leads only to partial dephasing on short time scales, exponential decay of coherence appears in the Markovian limit (for long times) if anharmonicity of the reservoir is taken into account. This effect not only qualitatively changes the decoherence scenario but also leads to localization processes in which superpositions of spatially separated states dephase with a rate that depends on the distance between the localized states. As an example of the latter process, we study the decay of coherence of an electron state delocalized over two semiconductor quantum dots due to anharmonicity of phonon modes.
We investigate two techniques for controlling decoherence, focusing on the crucial role played by the environmental spectrum. We show how environments with different spectra lead to very different dynamical behaviors. Our study clearly proves that such differences must be taken into account when designing decoherence control schemes. The two techniques we consider are reservoir engineering and quantum Zeno control. We focus on a quantum harmonic oscillator initially prepared in a nonclassical state and derive analytically its non-Markovian dynamics in the presence of different bosonic thermal environments. On the one hand, we show how, by modifying the spectrum of the environment, it is possible to prolong or reduce the life of a Schr\"odinger cat state. On the other hand, we study the effect of nonselective energy measurements on the degradation of quantumness of initial Fock states. In this latter case, we see that the crossover between quantum Zeno and anti-Zeno effects, discussed by Maniscalco et al. [Phys. Rev. Lett. 97, 130402 (2006)], is highly sensitive to the details of the spectrum. In particular, for certain types of spectra, even very small variations of the system frequency may cause a measurement-induced acceleration of decoherence rather than its inhibition.
Self-assembled semiconductor quantum dots are often referred to as artificial atoms. They are bright single photon sources implemented in an environment easier scalable then implementations with single atoms. These properties promote quantum dots as promising candidates for quantum information technology. A high fidelity state preparation reduces the error rate in quantum information protocols and is hence an essential requirement. \n \nIn this thesis the technique of rapid adiabatic passage is implemented with ultra-fast pulse parameters and is used to study single self-assembled quantum dots by detecting the resonance fluorescence signal following an optical excitation. Different experimental schemes provide insights into different aspects of a quantum dot and its environment. \n \nThe negative trion transition in a single quantum dot represents a good approach to creating an ideal two-level system. With this two-level system, the interaction of the electron trapped in the QD and phonons in the host material is studied with a single chirped pulse. Measuring the resonance fluorescence response of the quantum dot as a function of the pulse area for a set of chirp parameters reveals the electron-phonon interaction. Phonons in the semiconductor environment are a source of decoherence. The non-monotonic behaviour of the coupling to phonons is experimentally demonstrated. Furthermore, a decoupling regime where the electron oscillation is too fast for phonons to follow is reached. A high fidelity state preparation with a vanishing coupling to phonons is demonstrated. Experimental results are affirmed with an excellent agreement with simulations. \n \nIn another scheme, a three-level system consisting of the crystal ground state of the quantum dot, the neutral exciton and the biexciton is investigated. The goal in this experiment is a coherent, resonant and high fidelity preparation of a biexciton state robust against system parameter fluctuations. A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. An excitation with chirped pulses applying the technique of rapid adiabatic passage is the key for the biexciton preparation scheme. In contrast to other state of the art techniques, an interaction with phonons is here intentionally minimized reducing the dephasing and maintaining at the same time a robustness with respect to pulse area and detuning. A fidelity close to one is reached over a pulse area range of more than π. Also in this case, the interpretation of the experiment is confirmed by an excellent agreement with simulations which include a microscopic coupling to phonons. \n \n \nIn a third experiment, a sequence of two chirped pulses with different point spread functions is used to overcome the diffraction limit in spectroscopy. A universal technique - optical nanoscopy via quantum control - to perform diffraction-unlimited spectroscopy on two-level systems is presented. A model for simulating the system is developed and gives a prediction for the expected spatial resolution as a function of the pulse parameters. The concept is demonstrated and the prediction is fulfilled on self-assembled semiconductor quantum dots. A resolution down to 30 nm with an excitation wavelength of 950 nm is reached.