Our work in the field of quantum science & engineering spans from theoretical investigations to experimental demonstrations of quantum effects in various light-matter interactions. Following are some of our key contributions to this field:
1) Single photon driven quantum phase transitions
2) Spin qubits and EM fluctuations
3) Spin qubits and photonics spin density
4) Vacuum fluctuations in matter
5) Quantum field theory for angular momentum of light
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MOVING PLATES CREATE NEGATIVE-FREQUENCY PHOTONIC RESONANCE
When in relative motion, plates of a Fabry-Perot interferometer generate a unique resonance with negative frequency if separated by a gap of critical size.
QUANTUM MODEL UNLOCKS NEW APPROACH TO SINGLE-PHOTON DETECTION
To become more pervasive in daily life, quantum technology needs to better detect single particles of light, called photons, carrying quantum information.
LIGHTING A FIRE WITH VACUUM FRICTION
One of the surprises of Quantum Mechanics is zero point fluctuations which pervade both vacuum and matter.
CONTROLLING FORCES BETWEEN ATOMS, PROMISING FOR ‘2-D HYPERBOLIC’ MATERIALS
A new approach to control forces and interactions between atoms and molecules, such as those employed by geckos to climb vertical surfaces, could bring advances in new materials for developing quantum light sources.
The temporal dynamics of large quantum systems perturbed weakly by a single excitation can give rise to unique phenomena at the quantum phase boundaries. Here, we develop a time-dependent model to study the temporal dynamics of a single photon interacting with a defect within a large system of interacting spin qubits (N > 100). Our model predicts a quantum resource, giant susceptibility, when the system of qubits is engineered to simulate a first-order quantum phase transition (QPT). We show that the absorption of a single-photon pulse by an engineered defect in the large qubit system can nucleate a single shot quantum measurement through spin noise read-out. This concept of a single-shot detection event (“click”) is different from parameter estimation, which requires repeated measurements. The crucial step of amplifying the weak quantum signal occurs by coupling the defect to a system of interacting qubits biased close to a QPT point. The macroscopic change in long-range order during the QPT generates amplified magnetic noise, which can be read out by a classical device. Our work paves the way for studying the temporal dynamics of large quantum systems interacting with a single-photon pulse.
Single-photon pulse induced giant response in N > 100 qubit system LP Yang, Z Jacob npj Quantum Information 6 (1), 76 (2020)
High-fidelity quantum gate operations are essential for achieving scalable quantum circuits. In spin qubit quantum computing systems, metallic gates and antennas that are necessary for qubit operation, initialization, and readout, also cause detrimental effects by enhancing fluctuations of electromagnetic fields. Therefore, evanescent wave Johnson noise (EWJN) caused by near-field thermal and vacuum fluctuations becomes an important unmitigated noise, which induces the decoherence of spin qubits and limits the quantum gate operation fidelity. Here, we first develop a macroscopic quantum electrodynamics theory of EWJN to account for the dynamics of two spin qubits interacting with metallic circuitry. Then we propose a numerical technique based on volume integral equations to quantify EWJN strength in the vicinity of nanofabricated metallic gates with arbitrary geometry. We study the limits to two-spin-qubit gate fidelity from EWJN-induced relaxation processes in two experimentally relevant quantum computing platforms: (a) the silicon quantum dot system and (b) nitrogen-vacancy centers in diamond. Finally, we introduce a Lindbladian engineering method to optimize the control pulse sequence design and show its enhanced performance over Hamiltonian engineering in mitigating the influence of thermal and vacuum fluctuations. Our work leverages advances in computational electromagnetics, fluctuational electrodynamics, and open quantum systems to suppress the effects of near-field thermal and vacuum fluctuations and reach the limits of two-spin-qubit gate fidelity.
Quantum sensing of photonic spin density using a single spin qubit
Nitrogen vacancy (NV) centers in diamond have emerged as promising room temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids, 2D magnetic materials, and magnons to hydrodynamic flow of current. Here, we propose and demonstrate that the nitrogen-vacancy center in a diamond can be used as a quantum sensor for detecting the photonic spin density. We exploit a single spin qubit on an atomic force microscope tip to probe the spinning field of an incident Gaussian light beam. The spinning field of light induces an effective static magnetic field in the single spin qubit probe. We perform room-temperature sensing using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-subwavelength volumes. We also put forth a rigorous theory of the experimentally measured phase change using the NV center Hamiltonian and perturbation theory involving only virtual photon transitions. The direct detection of the photonic spin density at the nanoscale using NV centers in diamond opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications in spin quantum electrodynamics (sQED).
Kalhor, Farid, Li-Ping Yang, Leif Bauer, and Zubin Jacob. "Quantum sensing of photonic spin density." arXiv preprint arXiv:2102.11373 (2021).
Singular evanescent wave resonances in moving media
Resonators fold the path of light by reflections leading to a phase balance and thus constructive addition of propagating waves. However, amplitude decrease of these waves due to incomplete reflection or material absorption leads to a finite quality factor of all resonances. Here we report on our discovery that evanescent waves can lead to a perfect phase and amplitude balance causing an ideal Fabry-Perot resonance condition in spite of material absorption and non-ideal reflectivities. This counterintuitive resonance occurs if and only if the metallic Fabry-Perot plates are in relative motion to each other separated by a critical distance. We show that the energy needed to approach the resonance arises from the conversion of the mechanical energy of motion to electromagnetic energy. The phenomenon is similar to lasing where the losses in the cavity resonance are exactly compensated by optical gain media instead of mechanical motion. Nonlinearities and non-localities in material response will inevitably curtail any singularities however we show the giant enhancement in non-equilibrium phenomena due to such resonances in moving media.
Guo, Yu, and Zubin Jacob. "Singular evanescent wave resonances in moving media." Optics express 22, no. 21 (2014): 26193-26202.
PT-symmetric spectral singularity and negative-frequency resonance
Vacuum consists of a bath of balanced and symmetric positive- and negative-frequency fluctuations. Media in relative motion or accelerated observers can break this symmetry and preferentially amplify negative-frequency modes as in quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of a universal negative-frequency-momentum mirror symmetry in the relativistic Lorentzian transformation for electromagnetic waves. We show the connection of our discovered symmetry to parity-time (PT) symmetry in moving media and the resulting spectral singularity in vacuum fluctuation-related effects. We prove that this spectral singularity can occur in the case of two metallic plates in relative motion interacting through positive- and negative-frequency plasmonic fluctuations (negative-frequency resonance). Our work paves the way for understanding the role of PT-symmetric spectral singularities in amplifying fluctuations and motivates the search for PT-symmetry in novel photonic systems.
Pendharker, Sarang, Yu Guo, Farhad Khosravi, and Zubin Jacob. "PT-symmetric spectral singularity and negative-frequency resonance." Physical Review A 95, no. 3 (2017): 033817.
Giant non-equilibrium vacuum friction: role of singular evanescent wave resonances in moving media
We recently reported on the existence of a singular resonance in moving media which arises due to perfect amplitude and phase balance of evanescent waves. We show here that the nonequilibrium vacuum friction (lateral Casimir–Lifshitz force) between moving plates separated by a finite gap is fundamentally dominated by this resonance. Our result is robust to losses and dispersion as well as polarization mixing which occurs in the relativistic limit.
Guo, Yu, and Zubin Jacob. "Giant non-equilibrium vacuum friction: role of singular evanescent wave resonances in moving media." Journal of Optics 16, no. 11 (2014): 114023.
All elementary particles in nature can be classified as fermions with half-integer spin and bosons with integer spin. Within quantum electrodynamics (QED), even though the spin of the Dirac particle is well defined, there exist open questions on the quantized description of spin of the gauge field particle—the photon. Using quantum field theory, we discover the quantum operators for the spin angular momentum (SAM) SM=(1/c)∫d3xπ×A and orbital angular momentum (OAM) LM=−(1/c)∫d3xπμx×∇Aμ of the photon, where πμ is the conjugate canonical momentum of the gauge field Aμ. We also reveal a perfect symmetry between the angular momentum commutation relations for Dirac fields and Maxwell fields. We derive the well-known OAM and SAM of classical electromagnetic fields from the above-defined quantum operators. Our work shows that the spin and OAM operators commute, which is important for simultaneously observing and separating the SAM and OAM. The correct commutation relations of orbital and spin angular momentum of the photon has applications in quantum optics, topological photonics as well as nanophotonics and can be extended in the future for the spin structure of nucleons.
