Phase-controlled atom-photon entanglement in a three-level V-type atomic system via spontaneously generated coherence

We investigate the dynamical behavior of the atom-photon entanglement in a V-type three-level quantum system using the atomic reduced entropy. It is shown that an atom and photons are entangled at the steady-state; however disentanglement can also be achieved in an especial condition. It is demonstrated that in the presence of quantum interference induced by spontaneous emission, the reduced entropy and the atom-photon entanglement are phase-dependent. Non-stationary solution is also obtained when the quantum interference due to the spontaneous emission is completely included.Comment: 16 pages, 7 figure

Coherence and entanglement dynamics of vibrating qubits

We investigate the dynamics of coherence and entanglement of vibrating qubits. Firstly, we consider a single trapped ion qubit inside a perfect cavity and successively we use it to construct a bipartite system made of two of such subsystems, taken identical and noninteracting. As a general result, we find that qubit vibration can lead to prolonging initial coherence in both single-qubit and two-qubit system. However, despite of this coherence preservation, we show that the decay of the entanglement between the two qubits is sped up by the vibrational motion of the qubits. Furthermore, we highlight how the dynamics of photon-phonon correlations between cavity mode and vibrational mode, which may serve as a further useful resource stored in the single-qubit system, is strongly affected by the initial state of the qubit. These results provide new insights about the ability of systems made of moving qubits in maintaining quantum resources compared to systems of stationary qubits.Comment: 7 pages, 5 figures. Prepared for the Virtual Special Issue (VSI) on Quantum Correlations, in the journal Optics Communications

Validating and controlling quantum enhancement against noise by the motion of a qubit

Experimental validation and control of quantum traits for an open quantum system are important for any quantum information purpose. We consider a traveling atom qubit as a quantum memory with adjustable velocity inside a leaky cavity, adopting a quantum witness as a figure of merit for quantumness assessment. We show that this model constitutes an inherent physical instance where the quantum witness does not work properly if not suitably optimized. We then supply the optimal intermediate blind measurements which make the quantum witness a faithful tester of quantum coherence. We thus find that larger velocities protect quantumness against noise, leading to a lifetime extension of hybrid qubit-photon entanglement and to higher phase estimation precision. Control of qubit motion thus reveals itself as a quantum enhancer

Protecting entanglement by adjusting the velocities of moving qubits inside non-Markovian environments

Efficient entanglement preservation in open quantum systems is a crucial scope towards a reliable exploitation of quantum resources. We address this issue by studying how two-qubit entanglement dynamically behaves when two atom qubits move inside two separated identical cavities. The moving qubits independently interact with their respective cavity. As a main general result, we find that under resonant qubit-cavity interaction the initial entanglement between two moving qubits remains closer to its initial value as time passes compared to the case of stationary qubits. In particular, we show that the initial entanglement can be strongly protected from decay by suitably adjusting the velocities of the qubits according to the non-Markovian features of the cavities. Our results supply a further way of preserving quantum correlations against noise with a natural implementation in cavity-QED scenarios and are straightforwardly extendable to many qubits for scalability.Comment: To be published in Laser Physics Letter

Low power data converters for specific applications

Due to increasing importance of portable equipment and reduction of the supply voltage due to technology scaling, recent efforts in the design of mixed-signal circuits have focused on developing new techniques to reduce the power dissipation and supply voltage. This requires research into new architectures and circuit techniques that enable both integration and programmability. Programmability allows each component to be used for different applications, reducing the total number of components, and increased integration by eliminating external components will reduce cost and power;Since data converters are used in many different applications, in this thesis new low voltage and low power data converter techniques at both the architecture and circuit design levels are investigated to minimize power dissipation and supply voltage. To demonstrate the proposed techniques, test the performance of the proposed architectures, and verify their effectiveness in terms of power savings, five prototype chips are fabricated and tested;First, a re-configurable data converter (RDC) architecture is presented that can be programmed as analog-to-digital converter (ADC), digital-to-analog converter (DAC), or both. The reconfigurability of the RDC to different numbers of ADCs and DACs having different speeds and resolutions makes it an ideal choice for analog test bus, mixed-mode boundary scan, and built-in self test applications. It combines the advantages of both analog test buses and boundary scan techniques while the area overhead of the proposed techniques is very low compared to the mixed-mode boundary scan techniques. RDC can save power by allowing the designer to program it as the right converter for desired application. This architecture can be potentially implemented inside a field programmable gate array (FPGA) to allow the FPGA communicate with the analog world. It can also be used as a stand-alone product to give flexibility to the user to choose ADC/DAC combinations for the desired application;Next, a new method for designing low power and small area ROMless direct digital frequency synthesizers (DDFSs) is presented. In this method, a non-linear digital-to-analog converter is used to replace the phase-to-sine amplitude ROM look-up table and the linear DAC in conventional DDFS. Since the non-linear DAC converts the phase information directly into analog sine wave, no phase-to-amplitude ROM look-up table is required;Finally, a new low voltage technique based on biased inverting opamp that can have almost rail-to-rail swing with continuously valid output is discussed. Based on this biasing technique, a 10-bit segmented R-2R DAC and an 8-bit successive approximation ADC are designed and presented

Quantumness and memory of one qubit in a dissipative cavity under classical control

Hybrid quantum–classical systems constitute a promising architecture for useful control strategies of quantum systems by means of a classical device. Here we provide a comprehensive study of the dynamics of various manifestations of quantumness with memory effects, identified by non-Markovianity, for a qubit controlled by a classical field and embedded in a leaky cavity. We consider both Leggett–Garg inequality and quantum witness as experimentally-friendly indicators of quantumness, also studying the geometric phase of the evolved (noisy) quantum state. We show that, under resonant qubit-classical field interaction, a stronger coupling to the classical control leads to enhancement of quantumness despite a disappearance of non-Markovianity. Differently, increasing the qubit-field detuning (out-of-resonance) reduces the nonclassical behavior of the qubit while recovering non-Markovian features. We then find that the qubit geometric phase can be remarkably preserved irrespective of the cavity spectral width via strong coupling to the classical field. The controllable interaction with the classical field inhibits the effective time-dependent decay rate of the open qubit. These results supply practical insights towards a classical harnessing of quantum properties in a quantum information scenari

Non-Markovianity and coherence of a moving qubit inside a leaky cavity

Non-Markovian features of a system evolution, stemming from memory effects, may be utilized to transfer, storage, and revive basic quantum properties of the system states. It is well known that an atom qubit undergoes non-Markovian dynamics in high quality cavities. We here consider the qubit-cavity interaction in the case when the qubit is in motion inside a leaky cavity. We show that, owing to the inhibition of the decay rate, the coherence of the traveling qubit remains closer to its initial value as time goes by compared to that of a qubit at rest. We also demonstrate that quantum coherence is preserved more efficiently for larger qubit velocities. This is true independently of the evolution being Markovian or non-Markovian, albeit the latter condition is more effective at a given value of velocity. We however find that the degree of non-Markovianity is eventually weakened as the qubit velocity increases, despite a better coherence maintenance.Comment: 16 pages and 5 figures. Written for the upcoming special volume "40 years of the GKLS equation", to be published in the journal Open Systems and Information Dynamics. A co-author and some references adde