arXiv subjects

D. D. Yavuz

Publications and source records attributed to D. D. Yavuz.

16 recordsLinked to original sources

arXiv2025

Quantum statistics of single-mode radiation emitted by superradiant Dicke states

We study the quantum statistics of single-mode radiation emitted by an atomic ensemble when the ensemble is initially prepared in a superradiant Dicke state. We show that while the radiation is well approximated by the Glauber coherent state at early times in the evolution, the emission can be truly quantum at later times. In particular, one can observe a large amount of photon-number squeezing in the emission under certain conditions; even a Fock state can be produced. We discuss the quantum statistics of the emission for various parameters, including different initial conditions for the atomic ensemble. To obtain these results, we have developed a formalism where we are able to calculate the quantum statistics of the emission over long time-scales even when the number of atoms in the ensemble is quite large.

quant-ph↗

arXiv2025

Experimental observation of subabsorption

We predict and experimentally demonstrate a new type of collective (cooperative) coupling effect where a disordered atomic ensemble absorbs light with a rise-time longer (i. e., at a rate slower) than what is dictated by single-atom physics. This effect, which we name subabsorption, can be viewed as the absorptive analog of subradiance. The experiment is performed using a dilute ensemble of ultracold $^{87}$Rb atoms with a low optical depth, and time-resolving the absorption of a weak (tens of photons per pulse) resonant laser beam. In this dilute regime, the collective interaction relies on establishing dipole-dipole correlations over many atoms; i.e., the interaction is not dominated by the nearest neighbors. As a result, subabsorption is highly susceptible to motional dephasing: even a temperature increase of 60 $μ$K is enough to completely extinguish the subabsorption signal. We also present a theoretical model whose results are in reasonable agreement with the experimental observations. The model uses density-dependent dephasing rate of the long-range dipole-dipole correlations as a single adjustable parameter. Experiment-theory comparison indicates a dephasing coefficient of $β/2 π= 4.9 \times 10^{-5}$ Hz~cm$^3$, which is more than two orders of magnitude larger than the known dipole-dipole line broadening coefficient in $^{87}$Rb.

quant-ph↗

arXiv2023

Simulation of quantum algorithms using classical probabilistic bits and circuits

We discuss a new approach to simulate quantum algorithms using classical probabilistic bits and circuits. Each qubit (a two-level quantum system) is initially mapped to a vector in an eight dimensional probability space (equivalently, to a classical random variable with eight probabilistic outcomes). The key idea in this mapping is to store both the amplitude and phase information of the complex coefficients that describe the qubit state in the probabilities. Due to the identical tensor product structure of combining multiple quantum systems as well as multiple probability spaces, $n$ qubits are then mapped to a tensor product of $n$ 8-dimensional probabilistic vectors (i.e., the Hilbert space of dimension $2^n$ is mapped to a probability space of dimension $8^n$). After this initial mapping, we show how to implement the analogs of single-qubit and two-qubit gates in the probability space using correlation-inducing operations on these classical random variables. The key defining feature of both the mapping to the probability space and the transformations in this space (i.e., operations on the random variables) is that they are not linear, but instead affine. Using this architecture, the evolution of the $2^n$ complex coefficients of the quantum system can be tracked in the joint fully-correlated probabilities of the polynomial number of random variables. We then give specific procedures for implementing (1) the Deutsch-Jozsa algorithm, and (2) the Quantum Fourier Transform in the probability space. Identical to the Quantum case, simulating the Quantum Fourier Transform in the probability space requires $O(n)$ probabilistic bits and $O(n^2)$ (i.e., quadratic in the number of quantum bits) operations.

quant-ph↗

arXiv2023

Mapping of Quantum Systems to the Probability Simplex

We start with the simplest quantum system (a two-level system, i.e., a qubit) and discuss a one-to-one mapping of the quantum state in a two-dimensional Hilbert space to a vector in an eight dimensional probability space (probability simplex). We then show how the usual transformations of the quantum state, specifically the Hadamard gate and the single-qubit phase gate, can be accomplished with appropriate transformations of the mapped vector in the probability simplex. One key defining feature of both the mapping to the simplex and the transformations in the simplex is that they are not linear. These results show that both the initial state and the time evolution of a qubit can be fully captured in an eight dimensional probability simplex (or equivalently using three classical probabilistic bits). We then discuss multi-partite quantum systems and their mapping to the probability simplex. Here, the key tool is the identical tensor product structure of combining multiple quantum systems as well as multiple probability spaces. Specifically, we explicitly show how to implement an analog of the two-qubit controlled-not (CNOT) gate in the simplex. We leave it an open problem how much the quantum dynamics of $N$ qubits can be captured in a probability simplex with $3N$ classical probabilistic bits. Finally, we also discuss the equivalent of the Schrodinger's equation for the wavefunction (in a Hilbert space of arbitrary dimension), which dictates the time evolution of the vectors in the simplex.

quant-ph↗

arXiv2023

Nanoscale addressing and manipulation of neutral atoms using electromagnetically induced transparency

We propose to integrate dark-state based localization techniques into a neutral atom quantum computing architecture and numerically investigate two specific schemes. The first scheme implements state-selective projective measurement by scattering photons from a specific qubit with very little cross talk on the other atoms in the ensemble. The second scheme performs a single-qubit phase gate on the target atom with an incoherent spontaneous emission probability as low as 0.01. Our numerical simulations in rubidium (Rb) atoms show that for both of these schemes a spatial resolution at the level of tens of nanometers using near-infrared light can be achieved with experimentally realistic parameters.

quant-ph↗

arXiv2022

Enhancing two-photon spontaneous emission in rare earths using graphene and graphene nanoribbons

The enhancement of two-photon spontaneous emission (2PSE) from trivalent and divalent rare earth ions in proximity to graphene and graphene nanoribbons is calculated for achievable experimental conditions using a combination of finite difference time domain simulations and direct computation of transition rates between energy levels in rare earths. For Er$^{3+}$, we find that the 2PSE rate is initially 8 orders lower than the single-photon spontaneous emission rate but that, with enhancement, 2PSE can reach 2.5% of the overall decay. When graphene nanoribbons are used, we also show that the emission of free-space photon pairs from Er$^{3+}$ at 3 - 3.2 $μ$m via 2PSE can be increased by $\sim 400$. Our calculations show significantly less relative graphene-enhanced 2PSE than previous works, and we attribute this variation to differences in emitter size and assumed graphene mobility. We also show that the internal energy structure of the ion can have an impact on degree of 2PSE enhancement achievable and find that divalent rare earths are more favorable.

cond-mat.mes-hall↗

arXiv2022

Generation and detection of axions using guided structures

We propose a new experimental technique to generate and detect axions in the lab with a good experimental sensitivity over a broad axion mass range. The scheme relies on using laser-based four-wave mixing, which is mediated by the hypothetical axion field. Intense pump and Stokes laser beams that are confined to a waveguide (i.e., for example, an optical fiber) with appropriately chosen frequencies resonantly drive axion generation. Under such a geometry, we predict the existence of guided axion waves, which we refer to as "axitons". These are solutions of the axion Klein-Gordon field equation that are spatially guided by the profiles of the driving pump and Stokes laser beams. These guided axitons can then couple to a nearby fiber and mix with another laser, affecting the propagation of a probe laser beam. A key advantage of the scheme is that the mass range of the hypothetical axion can be scanned by varying the frequencies of the pump and the Stokes laser beams. We predict that, using reasonable parameters, the technique will be able to detect axions in the mass range $10^{-6} $eV $< m<$ $10^{-2} $eV with a sensitivity at the level of $10^{-12}$ GeV$^{-1}$ for the axion-photon coupling constant.

hep-ph↗

arXiv2021

Spatial Coherence of Light in Collective Spontaneous Emission

When a quantum system is put into an excited state, it will decay back to the ground state through a process termed spontaneous emission. It is generally assumed that spontaneous emission between different individual emitters would not be coherent with each other; to produce coherent light one would need population inversion and stimulated emission. In this work, we show that an optically-thin ensemble of 11,000 radiating atoms spontaneously organize to produce spatially coherent light. The reason for this coherence is collective-coupling of the individual emitters via Dicke superradiance and subradiance (as opposed to amplification through stimulated emission).

quant-ph↗

arXiv2020

Subradiance and superradiance-to-subradiance transition in dilute atomic clouds

We experimentally study subradiance in a dilute cloud of ultracold rubidium (Rb) atoms where $n λ_a^3 \approx 10^{-2}$ ($n$: atomic density, $λ_a$ excitation wavelength) and the on-resonance optical depth of the cloud is of order unity. We show that in the strong excitation regime, the subradiant time-scales depend on the excitation fraction of the cloud; i.e., to the intensity of the excitation pulse. In this regime, the decay dynamics are highly complicated and there is not a single decay time-constant. Instead, the decay time constant varies during the dynamics. Specifically, we were able to observe signatures of superradiant-to-subradiant transition; i. e., initially the decay rate is faster than independent decay (superradiant emission), while at later times it transitions to slower (subradiant emission). We also discuss a theoretical model whose numerical results are in good agreement with the experiments.

quant-ph↗

arXiv2015

Large Sample Superradiance and Fault-Tolerant Quantum Computation

We quantitatively analyze superradiance (collective emission) in a three-dimensional array of qubits without imposing any restrictions on the size of the sample. We show that even when the spacing between the qubits become arbitrarily large, superradiance produces an error rate on each qubit that scales with the total number of qubits. This is because the sum of the norms of the effective Hamiltonians that decoheres each qubit scales with the total number of qubits and is, therefore, unbounded. In three spatial dimensions, the sum of the norms scales as $N^{2/3}$ where $N$ is the total number of qubits in the computer. Because the sum of the Hamiltonian norms are unbounded, the introduced errors are outside the applicability of the threshold theorem.

quant-ph↗

arXiv2010

Observation of atomic localization using Electromagnetically Induced Transparency

We present a proof-of-principle experiment in which the population of an atomic level is spatially localized using the technique of electromagnetically-induced transparency (EIT). The key idea is to utilize the sensitive dependence of the dark state of EIT on the intensity of the coupling laser beam. By using a sinusoidal intensity variation (standing-wave), we demonstrate that the population of a specific hyperfine level can be localized much tighter than the spatial period.

physics.atom-ph↗

arXiv2009

Far-off resonance conditional phase-shifter using the ac-Stark shift

We propose a simple technique that achieves a conditional phase shift of pi radians between two weak lasers with energies at the 1000-photon level. The key idea is to set up a V-system with two far-off resonant lasers by coupling the ground state to two excited electronic states. The lasers interact through the ac Stark shift of the ground state and thereby acquire a large conditional phase shift.

quant-ph↗

arXiv2008

Refractive Index Enhancement with Vanishing Absorption in an Atomic Vapor

We report a proof-of-principle experiment where the refractive index of an atomic vapor is enhanced while maintaining vanishing absorption of the beam. The key idea is to drive alkali atoms in a vapor with appropriate control lasers and induce a gain resonance and an absorption resonance for a probe beam in a two-photon Raman configuration. The strength and the position of these two resonances can be manipulated by changing the parameters of the control lasers. By using the interference between these two resonances, we obtain an enhanced refractive index without an increase in the absorption.

quant-ph↗

arXiv2008

Observation of Rydberg blockade between two atoms

We demonstrate experimentally that a single Rb atom excited to the $79d_{5/2}$ level blocks the subsequent excitation of a second atom located more than $10 μ\rm m$ away. The observed probability of double excitation of $\sim 30%$ is consistent with a theoretical model based on calculations of the long range dipole-dipole interaction between atoms.

quant-ph↗

arXiv2007

Rabi flopping between ground and Rydberg states with dipole-dipole atomic interactions

We demonstrate Rabi flopping of small numbers of $\rm{^{87}Rb}$ atoms between ground and Rydberg states with $n\le 43$. Coherent population oscillations are observed for single atom flopping, while the presence of two or more atoms decoheres the oscillations. We show that these observations are consistent with van der Waals interactions of Rydberg atoms.

quant-ph↗

arXiv2005

Fast Ground State Manipulation of Neutral Atoms in Microscopic Optical Traps

We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral $^{87}$Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with crosstalk on neighboring sites separated by $8 μ\rm m$ at the level of $10^{-3}$. Ramsey spectroscopy is used to measure a dephasing time of $870 μ\rm s$ which is $\approx$ 5000 times longer than the time for a $π/2$ pulse.

quant-ph↗