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Shahid Qamar

Publications and source records attributed to Shahid Qamar.

14 recordsLinked to original sources

Quantum Filtering and Stabilization of Dissipative Quantum Systems via Augmented Neural Ordinary Differential Equations

Modeling open quantum dynamics without full knowledge of the system Hamiltonian or noise model is a key challenge in quantum control and quantum state estimation. We introduce an Augmented Quantum Neural Ordinary Differential Equation (AQNODE) framework that learns quantum trajectories and dissipation parameters directly from partial continuous measurement data. By embedding the system into a latent space evolved via neural ODEs, AQNODE captures both observable and hidden non-Markovian dynamics with temporal smoothness and physical consistency. Our approach integrates weak measurement data to reconstruct qubit states and time-dependent decoherence rates, enabling accurate state prediction and parameter inference without explicit physical equations. Furthermore, we incorporate AQNODE-based feedback control techniques, including proportional-derivative and time-varying linear-quadratic regulator (LQR) strategies, to steer the quantum system toward target states in real time. Extensive numerical simulations demonstrate AQNODE's ability to generalize across system configurations, achieve low prediction errors, and perform robust quantum filtering and control. These results establish AQNODE as a scalable, differentiable, and experimentally compatible framework for real-time modeling and control of dissipative quantum systems.

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Vacuum-induced atomic grating

Atom-field interactions, induced by the vacuum of the electromagnetic field, exhibit a variety of fundamental phenomena and effects. In this paper, we study the electromagnetically induced atomic grating due to the vacuum state of the radiation field. Using an ensemble of cold atoms, strongly coupled to an optical cavity, we show that a probe field, propagating through the atomic medium, diffracts to zeroth and first-order diffraction peaks with few photons and even by the electromagnetic vacuum field of the cavity mode. As the number of photons in the cavity increases, the intensity of the first-order diffraction peak initially rises and then exhibits a decreasing trend. Furthermore, we observe that the first-order peak intensity reaches its maximum at resonance for both the vacuum and single-photon cavity state. However, as the number of photons increases further, this peak at resonance transforms into a dip, accompanied by two side peaks at off-resonance positions. This transition from a peak to a dip may potentially be used to distinguish the quantum state of the cavity.

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Coherent control of photonic spin Hall effect in a Cavity

This paper theoretically investigates the manipulation of the Photonic Spin Hall Effect (photonic SHE) using a four-level closed coherent control coupling scheme in a cavity. The atomic system is configured to function as a combined Tripod and Lambda (CTL), Lambda $\Lambda$, and $N$ level model by properly adjusting the control field strengths and their relative phases. The system demonstrates multiple transparency windows in the CTL configuration, allowing the tunable photonic SHE to be used over a wider range of probe field detuning. At probe field resonance under the condition of electromagnetic induced transparency (EIT), the $\Lambda$-type system exhibits photonic SHE similar to the CTL system, showing a maximum upper limit of photonic SHE equal to half of the incident beam waist. This upper limit arises due to zero absorption and dispersion. Control field strengths and atomic density do not influence photonic SHE at resonance for both atomic configurations. Our findings reveal that atomic density and strength of control fields significantly influence photonic SHE in the $N$-type model at resonance, offering additional control parameters for tuning photonic SHE. Finally, the results are equally valid and applicable to conventional $\Lambda$-type and N-type atomic systems, making the findings broadly relevant in cavity atomic systems. The results of angular photonic SHE are also discussed.

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Magnomechanically controlled Goos-Hänchen shift in cavity QED

Phenomena involving interactions among magnons, phonons, and photons in cavity magnomechanical systems have attracted considerable attention recently, owing to their potential applications in the microwave frequency range. One such important effect is the response of a probe field to such tripartite interaction between photon-magnon-phonon. In this paper, we study Goos-Hänchen shift (GHS) of a reflected probe field in a cavity magnomechanical system. We consider a YIG sphere positioned within a microwave cavity. A microwave control field directly drives the magnon mode in YIG sphere, whereas the cavity is driven via a weak probe field. Our results show that the GHS can be coherently controlled through magnon-phonon coupling via the control field. For instance, GHS can be tuned from positive to negative by tuning the magnon-phonon coupling. Similarly, the effective cavity detuning is another important controlling parameter for GHS. Furthermore, we observe that the enhancement of GHS occurs when magnon-phonon coupling is weak at resonance, and when the magnon-photon coupling is approximately equal to the loss of microwave photons. Our findings may have potential significance in applications related to microwave switching and sensing.

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Entangled atomic ensemble and an yttrium-iron-garnet sphere in coupled microwave cavities

We present a scheme to generate distant bipartite and tripartite entanglement between an atomic ensemble and an yttrium iron garnet (YIG) sphere in coupled microwave cavities. We consider an atomic ensemble in a single-mode microwave cavity which is coupled with a second single-mode cavity having a YIG sphere. Our system, therefore, has five excitation modes namely cavity-1 photons, atomic ensemble, cavity-2 photons, a magnon and a phonon mode in the YIG sphere. We show that significant bipartite entanglement exists between indirectly coupled subsystems in the cavities, which is robust against temperature. Moreover, we present suitable parameters for a significant tripartite entanglement of ensemble, magnon, and phonon modes. We also demonstrate the existence of tripartite entanglement between magnon and phonon modes of the YIG sphere with indirectly coupled cavity photons. Interestingly, this distant tripartite entanglement is of the same order as previously found for a single-cavity system. We show that cavity-cavity coupling strength affects both the degree and transfer of quantum entanglement between various subsystems. Therefore, an appropriate cavity-cavity coupling optimizes the distant entanglement by increasing the entanglement strength and its robustness against temperature.

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Zero-photon catalysis based eight-state discrete modulated measurement-device-independent continuous-variable quantum key distribution

Zero-photon catalysis (ZPC) introduces noiseless attenuation and can be implemented by existing technologies in quantum key distribution (QKD) protocols. In this paper, we present a ZPC-based eight-state measurement-device-independent continuous-variable QKD (MDI-CV-QKD) combined with discrete modulation and reverse reconciliation. This ZPC-involved eight-state protocol shows better efficiency in terms of optimal modulation variances, secret key rates, transmission distances, tolerable excess noises, and reconciliation efficiency compared to the eight-state protocol without ZPC, the four-state protocol without ZPC, and the four-state protocol with ZPC, at a low signal-to-noise ratio (SNR).

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Multiphoton blockade and antibunching in an optical cavity coupled with dipole-dipole interacting $Λ$-type atoms

We study multiphoton blockade effects in a single-mode cavity interacting with two three-level atoms in $Λ$-configuration having position-dependent atom-field coupling. We consider the effects of dipole-dipole interaction (DDI) between the three-level atoms and show how the presence of DDI strongly influences the multiphoton blockade. For symmetric coupling of the atoms with the field, the DDI induces an asymmetry in the emission spectra as a function of pump field detuning. At positive detuning, the single-photon blockade gets stronger as a function of DDI strength, leading to photon antibunching. However, it becomes weaker at negative detuning and can also completely vanish. We show that this vanishing single-photon blockade is associated with a strong two-photon blockade, leading to two-photon bunching. Therefore, by just tuning the frequency of the pump field, we can achieve two very distinct features. We also study the effects of DDI when the atoms are asymmetrically coupled with the field and show that the proposed system exhibits two-photon bunching. We believe our results are important for the experimental realization of such systems where DDI may be present.

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Entanglement enhancement in cavity magnomechanics by an optical parametric amplifier

We propose a method to enhance bipartite and tripartite entanglement in cavity magnomechanics using an optical parametric amplifier (OPA). We analyze this system and identified parametric regimes where different types of entanglement are enhanced. We show that a proper choice of the phase of the parametric amplifier leads to the enhancement of the bipartite entanglements. Moreover, the tripartite entanglement is also significantly enhanced in the presence of OPA. The OPA not only enhances the strength of entanglement but also increases the domain of entanglement over a wider space of detunings as compared to the system when no OPA is present. Similarly, the robustness of entanglement against temperature is also enhanced. Another important consequence of OPA is the fact that it relaxes the requirement of strong magnon-phonon coupling to generate cavity-magnon entanglement which is necessary for the case when OPA is not present. We believe that the presented scheme is a step forward to realize robust quantum entanglement using current technology.

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Fast Quantum Calibration using Bayesian Optimization with State Parameter Estimator for Non-Markovian Environment

As quantum systems expand in size and complexity, manual qubit characterization and gate optimization will be a non-scalable and time-consuming venture. Physical qubits have to be carefully calibrated because quantum processors are very sensitive to the external environment, with control hardware parameters slowly drifting during operation, affecting gate fidelity. Currently, existing calibration techniques require complex and lengthy measurements to independently control the different parameters of each gate and are unscalable to large quantum systems. Therefore, fully automated protocols with the desired functionalities are required to speed up the calibration process. This paper aims to propose single-qubit calibration of superconducting qubits under continuous weak measurements from a real physical experimental settings point of view. We propose a real-time optimal estimator of qubit states, which utilizes weak measurements and Bayesian optimization to find the optimal control pulses for gate design. Our numerical results demonstrate a significant reduction in the calibration process, obtaining a high gate fidelity. Using the proposed estimator we estimated the qubit state with and without measurement noise and the estimation error between the qubit state and the estimator state is less than 0.02. With this setup, we drive an approximated pi pulse with final fidelity of 0.9928. This shows that our proposed strategy is robust against the presence of measurement and environmental noise and can also be applicable for the calibration of many other quantum computation technologies.

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Efficient Classical Computation of Quantum Mean Values for Shallow QAOA Circuits

The Quantum Approximate Optimization Algorithm (QAOA), which is a variational quantum algorithm, aims to give sub-optimal solutions of combinatorial optimization problems. It is widely believed that QAOA has the potential to demonstrate application-level quantum advantages in the noisy intermediate-scale quantum(NISQ) processors with shallow circuit depth. Since the core of QAOA is the computation of expectation values of the problem Hamiltonian, an important practical question is whether we can find an efficient classical algorithm to solve quantum mean value in the case of general shallow quantum circuits. Here, we present a novel graph decomposition based classical algorithm that scales linearly with the number of qubits for the shallow QAOA circuits in most optimization problems except for complete graph case. Numerical tests in Max-cut, graph coloring and Sherrington-Kirkpatrick model problems, compared to the state-of-the-art method, shows orders of magnitude performance improvement. Our results are not only important for the exploration of quantum advantages with QAOA, but also useful for the benchmarking of NISQ processors.

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Dynamics and multiqubit entanglement in distant resonators

We consider the dynamics of the photon states in distant resonators coupled to a common bus resonator at different positions. The frequencies of distant resonators from a common bus resonator are equally detuned. These frequency detunings are kept larger than the coupling strengths of each resonator to the common bus resonator to satisfy the dispersive interaction regime. In the dispersive regime, we show that the time dynamics of the system evolve to an arbitrary W-type state in a single step at various interaction times. Our results show that a one-step generation of arbitrary W-type states can be achieved with high fidelity in a system of superconducting resonators.

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Realization of quantum gates with multiple control qubits or multiple target qubits in a cavity

In this paper, we propose a scheme to realize three-qubit controlled phase gate and multiqubit controlled-NOT gate of one qubit simultaneously controlling n target qubit with four level quantum system in a cavity. Adjustment of level spacing during the gates implementation is not required. Implementation time for multiqubit controlled NOT gate is independent of number of qubit. Three-qubit phase gate is generalized to n-qubit phase gate with multiple control qubit and the number of steps (complexity) reduces linearly as compare to conventional gate decomposition method. Our scheme can be applied to various types of physical systems such as superconducting qubits coupled to resonator and trapped atoms in a cavity. Experimental possibility of our approach is also presented.

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Three qubit Grover's algorithm using superconducting quantum interference devices in cavity QED

We present a scheme for the implementation of three qubit Grover's algorithm using four-level superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. The scheme is based on resonant, off-resonant interaction of the cavity field with SQUIDs and the application of classical microwave pulses. We show that adjustment of SQUID level spacing during the gate operations, adiabatic passage, and second-order detuning are not required that leads to faster implementation. We also show that the marked state can be searched with high fidelity even in the presence of level decay and cavity dissipation.

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Multiqubit quantum phase gate using four-level superconducting quantum interference devices coupled to superconducting resonator

In this paper, we propose a scheme to realize three-qubit quantum phase gate of one qubit simultaneously controlling two target qubits using four-level superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. The two lowest levels |0> and |1> of each SQUID are used to represent logical states while the higher energy levels |2> and |3> are utilized for gate realization. Our scheme does not require adiabatic passage, second order detuning, and the adjustment of the level spacing during gate operation which reduce the gate time significantly. The scheme is generalized for an arbitrary n-qubit quantum phase gate. We also apply the scheme to implement three-qubit quantum Fourier transform. key words: quantum phase gate, superconducting quantum interference devices (SQUIDs), superconducting resonator, quantum Fourier transform

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