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Sougato Bose

Publications and source records attributed to Sougato Bose.

At least 91 records · Page 5Linked to original sources
arXiv2023

Micron-size spatial superpositions for the QGEM-protocol via screening and trapping

The quantum gravity-induced entanglement of masses (QGEM) protocol for testing quantum gravity using entanglement witnessing utilizes the creation of spatial quantum superpositions of two neutral, massive matter-wave interferometers kept adjacent to each other, separated by a distance d. The mass and the spatial superposition should be such that the two quantum systems can entangle solely via the quantum nature of gravity. Despite being charge-neutral, there are many electromagnetic backgrounds that can also entangle the systems, such as the dipole-dipole interaction, and the Casimir-Polder interaction. To minimize electromagnetic-induced interactions between the masses it is pertinent to isolate the two superpositions by a conducting plate. However, the conducting plate will also exert forces on the masses and hence the trajectories of the two superpositions would be affected. To minimize this effect, we propose to trap the two interferometers such that the trapping potential dominates over the attraction between the conducting plate and the matter-wave interferometers. The superpositions can still be created via the Stern-Gerlach effect in the direction parallel to the plate, where the trapping potential is negligible. The combination of trapping and shielding provides a better parameter space for the parallel configuration of the experiment, where the requirement on the size of the spatial superposition, to witness the entanglement between the two masses purely due to their quantum nature of gravity, decreases by at least two orders of magnitude as compared to the original protocol paper.

quant-ph↗
arXiv2023

Large Spin Stern-Gerlach Interferometry for Gravitational Entanglement

Recently, there has been a proposal to test the quantum nature of gravity in the laboratory by witnessing the growth of entanglement between two masses in spatial quantum superpositions. The required superpositions can be created via Stern-Gerlach interferometers, which couple an embedded spin qubit quantum state to the spatial dynamics of each mass. The masses would entangle only if gravity is quantum in nature. Here, we generalise the experiment to an arbitrary spin $j$ or equivalently to an ensemble of uniformly coupled spins. We first exemplify how to create a generalized Stern-Gerlach interferometer, which splits the mass into $2j+1$ trajectories. This shows that a controlled protocol can be formulated to encode the amplitudes of any spin state to a spatial superposition. Secondly, two masses in spatial superpositions of the above form are left to interact via gravity, and the entanglement is computed. Different families of initial spin states are varied to find the optimal spin state that maximizes the entanglement. We conclude that larger spins can offer a modest advantage in enhancing gravity-induced entanglement.

quant-ph↗
arXiv2023

Gravito-diamagnetic forces for mass independent large spatial superpositions

Creating a massive spatial quantum superposition, such as the Schrödinger cat state, where the mass and the superposition size within the range $10^{-19}-10^{-14}$ kg and $Δx \sim 10~{\rm nm}-100~μ{\rm m}$, is a challenging task. The methods employed so far rely either on wavepacket expansion or on a quantum ancilla, e.g. single spin dependent forces, which scale inversely with mass. In this paper, we present a novel approach that combines gravitational acceleration and diamagnetic repulsion to generate a large spatial superposition in a relatively short time. After first creating a modest initial spatial superposition of $1~μ{\rm m}$, achieved through techniques such as the Stern-Gerlach (SG) apparatus, we will show that we can achieve an $\sim 10^{2}-10^{3}$ fold improvement to the spatial superposition size ($1~{\rm μm}\rightarrow 980~μ{\rm m}$) between the wave packets in less than $0.02$~s by using the Earth's gravitational acceleration and then the diamagnetic repulsive scattering of the nanocrystal, neither of which depend on the object mass. Finally, the wave packet trajectories can be closed so that spatial interference fringes can be observed. Our findings highlight the potential of combining gravitational acceleration and diamagnetic repulsion to create and manipulate large spatial superpositions, offering new insights into creating macroscopic quantum superpositions.

quant-ph↗
arXiv2023

Relaxation of experimental parameters in a Quantum-Gravity Induced Entanglement of Masses Protocol using electromagnetic screening

To test the quantum nature of gravity in a lab requires witnessing the entanglement between the two test masses (nano-crystals) solely due to the gravitational interaction kept at a distance in a spatial superposition. The protocol is known as the quantum gravity-induced entanglement of masses (QGEM). One of the main backgrounds in the QGEM experiment is electromagnetic (EM) induced entanglement and decoherence. The EM interactions can entangle the two neutral masses via dipole-dipole vacuum-induced interactions, such as the Casimir-Polder interaction. To mitigate the EM-induced interactions between the two nano-crystals, we enclose the two interferometers in a Faraday cage and separate them by a conducting plate. However, any imperfection on the surface of a nano-crystal, such as a permanent dipole moment will also create an EM background interacting with the conducting plate in the experimental box. These interactions will further generate EM-induced dephasing which we wish to mitigate. In this paper, we will consider a parallel configuration of the QGEM experiment, where we will estimate the EM-induced dephasing rate, run-by-run systematic errors which will induce dephasing, and also provide constraints on the size of the superposition in a model-independent way of creating the spatial superposition.

quant-ph↗
arXiv2023

Effects of XX-catalysts on quantum annealing spectra with perturbative crossings

In adiabatic quantum annealing the required run-time to reach a given ground-state fidelity is dictated by the size of the minimum gap that appears between the ground and first excited state in the annealing spectrum. In general the presence of avoided level crossings demands an exponential increase in the annealing time with the system size which has consequences both for the efficiency of the algorithm and the required qubit coherence times. One promising avenue being explored to produce more favourable gap scaling is the introduction of non-stoquastic XX-couplings in the form of a catalyst - of particular interest are catalysts which utilise accessible information about the optimisation problem in their construction. Here we show extreme sensitivity of the effect of an XX-catalyst to subtle changes in the encoding of the optimisation problem. In particular, we observe that a targeted catalyst containing a single coupling at constant strength can significantly reduce the gap closing with system size at an avoided level crossing. For slightly different encodings of the same problems however, these same catalysts result in closing gaps in the annealing spectrum. To understand the origin of these closing gaps, we study how the evolution of the ground-state vector is altered by the presence of the catalyst and find that the negative components of the ground-state vector are key to understanding the response of the gap spectrum. We also consider how and when these closing gaps could be utilised in diabatic quantum annealing protocols - a promising alternative to adiabatic quantum annealing in which transitions to higher energy levels are exploited to reduce the run time of the algorithm.

quant-ph↗
arXiv2023

Optimal Superpositions for Particle Detection via Quantum Phase

Exploiting quantum mechanics for sensing offers unprecedented possibilities. State of the art proposals for novel quantum sensors often rely on the creation of large superpositions and generally detect a field. However, what is the optimal superposition size for detecting an incident particle (or an incident stream of particles) from a specific direction? This question is nontrivial as, in general, this incident particle will scatter off with varied momenta, imparting varied recoils to the sensor, resulting in decoherence rather than a well defined measurable phase. By considering scattering interactions of directional particulate environments with a system in a quantum superposition, we find that there is an "optimal superposition" size for measuring incoming particles via a relative phase. As a consequence of the anisotropy of the environment, we observe a novel feature in the limiting behaviour of the real and imaginary parts of the system's density matrix, linking the optimality of the superposition size to the wavelength of the scatterer.

quant-ph↗
arXiv2023

Gravitational Optomechanics: Photon-Matter Entanglement via Graviton Exchange

The deflection of light in the gravitational field of the Sun is one of the most fundamental consequences for general relativity as well as one of its classical tests first performed by Eddington a century ago. However, despite its center stage role in modern physics, no experiment has tested it in an ostensibly quantum regime where both matter and light exhibit non-classical features. This paper shows that the interaction which gives rise to the light-bending also induces photon-matter entanglement as long as gravity and matter are treated at par with quantum mechanics. The quantum light-bending interaction within the framework of perturbative quantum gravity highlights this point by showing that the entangled states can be generated already with coherent states of light and matter exploiting the non-linear coupling induced by graviton exchange. Furthermore, the quantum light-bending interaction is capable of discerning between the spin-2 and spin-0 gravitons thus also providing a test for alternative theories of gravity at short distances and at the quantum level. We will conclude by estimating the order of magnitude of the entanglement generated by employing the linear entropy. In particular, we find that a half-ring cavity of radius $0.25$ m placed around a $10$ kg mechanical oscillator operating at $150$ Hz, could be used to generate linear entropy of order unity using a petawatt laser source at optical wavelengths. While the proposed scheme is beyond the current experimental realities it nonetheless initiates the discussion about testing the spin of the gravitational interaction at the quantum level.

gr-qc↗
arXiv2023

Decoherence of a matter-wave interferometer due to dipole-dipole interactions

Matter-wave interferometry with nanoparticles will enable the development of quantum sensors capable of probing ultraweak fields with unprecedented applications for fundamental physics. The high sensitivity of such devices however makes them susceptible to a number of noise and decoherence sources and as such can only operate when sufficient isolation from the environment is achieved. It is thus imperative to model and characterize the interaction of nanoparticles with the environment and to estimate its deleterious effects. The aim of this paper will be to study the decoherence of the matter-wave interferometer due to dipole-dipole interactions which is one of the unavoidable channels for decoherence even for a neutral micro-crystal. We will start the analysis from QED and show that it reduces to the scattering model characterized by the differential cross-section. We will then obtain simple expressions for the decoherence rate in the short and long wavelength limits that can be readily applied to estimate the available coherence time. We will conclude by applying the obtained formulae to estimate the dipole-dipole decoherence rate for the Quantum Gravity-induced Entanglement of Masses (QGEM) protocol and discuss if the effects should be mitigated.

quant-ph↗
arXiv2023

Massive Spatial Qubits for Testing Macroscopic Nonclassicality and Casimir Induced Entanglement

An open challenge in physics is to expand the frontiers of the validity of quantum mechanics by evidencing nonclassicality of the centre of mass state of a macroscopic object. Yet another equally important task is to evidence the essential nonclassicality of the interactions which act between macroscopic objects. Here we introduce a new tool to meet these challenges: massive spatial qubits. In particular, we show that if two distinct localized states of a mass are used as the $|0\rangle$ and $|1\rangle$ states of a qubit, then we can measure this encoded spatial qubit with a high fidelity in the $σ_x, σ_y$ and $σ_z$ bases simply by measuring its position after different durations of free evolution. We show how this technique can be used to reveal an irreducible nonclassicality through a Bell-inequality violation arising from the entanglement of the centre of mass of a nano-crystal with its spin in a Stern-Gerlach setup. Secondly, we show how our methodology, in conjuction with the Casimir interaction, offers a powerful method to create and certify non-Gaussian entanglement between two neutral nano-objects. Fundamentally, the generation of such an entanglement provides an empirical means for demonstrating an inherent quantumness of the Casimir interaction.

quant-ph↗
arXiv2023

Quantum Gravitational Sensor for Space Debris

Matter-wave interferometers have fundamental applications for gravity experiments such as testing the equivalence principle and the quantum nature of gravity. In addition, matter-wave interferometers can be used as quantum sensors to measure the local gravitational acceleration caused by external massive moving objects, thus lending itself for technological applications. In this paper, we will establish a three dimensional model to describe the gravity gradient signal from an external moving object, and theoretically investigate the achievable sensitivities using the matter-wave interferometer based on the Stern-Gerlach set-up. As an application we will consider the Mesoscopic Interference for Metric and Curvature (MIMAC) and Gravitational wave detection scheme [New J. Phys. 22, 083012 (2020)] and quantify its sensitivity to gravity gradients using frequency-space analysis. We will consider objects near Earth-based experiments and space debris in proximity of satellites and estimate the minimum detectable mass of the object as a function of their distance, velocity, and orientation.

gr-qc↗
arXiv2023

Entanglement of Magnetically Levitated Massive Schrödinger Cat States by Induced Dipole Interaction

Quantum entanglement provides a novel way to test short-distance quantum physics in a non-relativistic regime. We provide entanglement-based protocols to potentially test the magnetically induced dipole-dipole interaction and the Casimir-Polder potential between the two nano-crystals kept in a Schrodinger Cat state. Our scheme is based on the Stern-Gerlach (SG) apparatus, where we can witness the entanglement mediated by these interactions for the nano-crystal mass m~10^-19 kg with a spatial superposition size of order 0.1 micron in a trap relying on diamagnetic levitation. We show that it is possible to close the SG interferometer in position and momentum with a modest gradient in the magnetic field.

quant-ph↗
arXiv2023

Low-Dissipation Data Bus via Coherent Quantum Dynamics

The transfer of information between two physical locations is an essential component of both classical and quantum computing. In quantum computing the transfer of information must be coherent to preserve quantum states and hence the quantum information. We establish a simple protocol for transferring one- and two-electron encoded logical qubits in quantum dot arrays. The theoretical energetic cost of this protocol is calculated - in particular, the cost of freezing and unfreezing tunnelling between quantum dots. Our results are compared with the energetic cost of shuttling qubits in quantum dot arrays and transferring classical information using classical information buses. Only our protocol can manage constant dissipation for any chain length. This protocol could reduce the cooling requirements and constraints on scalable architectures for quantum dot quantum computers.

quant-ph↗
arXiv2023

Entanglement Witness for the Weak Equivalence Principle

The Einstein equivalence principle is based on the equality of gravitational mass and inertial mass, which has led to the universality of a free-fall concept. The principle has been extremely well tested so far and has been tested with a great precision. However, all these tests and the corresponding arguments are based on a classical setup where the notion of position and velocity of the mass is associated with a classical value as opposed to the quantum entities. Here, we will provide a simple protocol based on creating large spatial superposition states in a laboratory to test the fully quantum regime of the equivalence principle where both matter and gravity are treated at par as a quantum entity. We will argue that such a quantum protocol is unique with regard to testing especially the generalization of the weak equivalence principle via witnessing quantum entanglement.

gr-qc↗
arXiv2023

Mass-independent scheme for enhancing spatial quantum superpositions

Placing a large mass in a large spatial superposition, such as a Schrödinger Cat state is a significant and important challenge. In particular, the large spatial superposition (${\cal O}(10-100)$ $μ$m) of mesoscopic masses ($m\sim {\cal O}(10^{-14} -10^{-15})$ kg) makes it possible to test the quantum nature of gravity via entanglement in the laboratory. To date, the proposed methods of achieving this spatial delocalization are to use wavepacket expansions or quantum ancilla (for example spin) dependent forces, all of whose efficacy reduces with mass. Thus increasing the spatial splitting independent of the mass is an important open challenge. In this paper, we present a method of achieving a mass-independent enhancement of superposition via diamagnetic repulsion from current-carrying wires. We analyse an example system which uses the Stern-Gerlach effect to creating a small initial splitting, and then apply the diamagnetic repulsion method to enhance the superposition size ${\cal O}(400-600)$ $μ$m from an initial modest split of the wavefunction. We provide an analytic and numeric analysis of our scheme.

quant-ph↗
arXiv2023

Floquet-induced localization in long-range many-body systems

The fate of many-body localization in long-range interacting systems is not fully settled. For instance, the phase boundary between ergodic and many-body localized regimes is still under debate. Here, we use Floquet dynamics which can induce many-body localization in a clean long-range interacting system through spatiotemporal disorder, which are realized by regular operation of random local rotations. The phase diagram has been determined for two types of uniform and nonuniform long-range couplings. Our Floquet mechanism shows more localizing power than conventional static disorder methods as it pushes the phase boundary in favor of the localized phase. Moreover, our comprehensive long-time simulations provide strong support for obtained results based on static analysis.

quant-ph↗
arXiv2022

Catapulting towards massive and large spatial quantum superposition

Large spatial quantum superposition of size ${\cal O}(1-10)~{\rm μ\text{m}}$ for mass $m \sim 10^{-17}-10^{-14}~\text{kg}$ is required to probe the foundations of quantum mechanics and testing classical and quantum nature of gravity via entanglement in a laboratory. In this paper, we will show that it is possible to accelerate the two spin states of a macroscopic nano-crystal sourced by the inhomogeneous nonlinear magnetic field in the Stern-Gerlach type setup. We will assume that the electronic spin can be embedded at the centre of the nano-crystal, such as the nitrogen-vacancy (NV) centre of diamond. Our analysis will be generic to any dopant or any material. We will show that we can create a desired superposition size within $1-2$ seconds by catapulting the trajectories of the two spin states with a modest magnetic field gradient and then recombine the trajectories for a coherent interference. We will show the demanding nature of the precision required in the magnetic field to recover $99\%$ spin coherence confidence level at the moment of interference.

quant-ph↗
arXiv2022

Sequential measurements for quantum-enhanced magnetometry in spin chain probes

Quantum sensors outperform their classical counterparts in their estimation precision, given the same amount of resources. So far, quantum-enhanced sensitivity has been achieved by exploiting the superposition principle. This enhancement has been obtained for particular forms of entangled states, adaptive measurement basis change, critical many-body systems, and steady-state of periodically driven systems. Here, we introduce a different approach to obtain quantum-enhanced sensitivity in a many-body probe through utilizing the nature of quantum measurement and its subsequent wave-function collapse without demanding prior entanglement. Our protocol consists of a sequence of local measurements, without re-initialization, performed regularly during the evolution of a many-body probe. As the number of sequences increases, the sensing precision is enhanced beyond the standard limit, reaching the Heisenberg bound asymptotically. The benefits of the protocol are multi-fold as it uses a product initial state and avoids complex initialization (e.g. prior entangled states or critical ground states) and allows for remote quantum sensing.

quant-ph↗
arXiv2022

Entanglement based tomography to probe new macroscopic forces

Quantum entanglement provides a novel way to test short distance physics in the non-relativistic regime. We will provide a protocol to {\it potentially} test new physics by bringing two charged massive particle interferometers adjacent to each other. Being charged, the two superpositions will be entangled via electromagnetic interactions mediated by the photons, including the Coulomb and the Casimir-Polder potential. We will bring a method of {\it entanglement based tomography} to seek time evolution of very small entanglement phases to probe new physical effects mediated by {\it hitherto unknown macroscopic force} which might be responsible for entangling the two charged superpositions modelled by the Yukawa type potential. We will be able to constrain the Yukawa couplings $α\geq 10^{-35}$ for $r\geq 10^{-6}$m for new physics occurring in the electromagnetic sector, and in the gravitational potential $α_g \geq 10^{-8}$ for $r \geq 10^{-6}$m. Furthermore, our protocol can also constrain the axion like particle mass and coupling, which is complimentary to the existing experimental bounds.

hep-ph↗