arXiv subjects

Joakim Bergli

Publications and source records attributed to Joakim Bergli.

At least 19 recordsLinked to original sources

arXiv2026

Universal Quantum Computation with Multi-Mode Schrödinger Cat States Stabilized by Non-Local Dissipation Engineering

Schrödinger cat states provide a hardware-efficient platform for bosonic quantum error correction by encoding logical information in protected manifolds of harmonic oscillators. While previous work has demonstrated the dissipative stabilization of multi-mode Schrödinger cat states as robust quantum memories, a framework for universal quantum computation has remained unavailable. Here we extend this approach by introducing a universal gate set for dissipatively stabilized multi-mode cat qubits. Using a chain of Kerr non-linear oscillators coupled through engineered non-local dissipation and an effective low-dimensional description, we show how arbitrary single-qubit control can be achieved through arbitrary rotation around the $X$-axis and $π/2$-rotation around the $Z$-axis. We further show how coupling two such stabilized arrays through just one oscillator on each respective array enables coherent entangling operations through implementation of the $XX(π/2)$ gate. Numerical simulations demonstrate high-fidelity gate dynamics and entanglement generation under realistic parameters. Finally, we analyze the effects of induced and intrinsic photon loss, disorder, and the validity regime of the effective low-dimensional theory. Our results establish dissipatively stabilized multi-mode Schrödinger cat states as a potential architecture for universal bosonic quantum computation.

quant-ph↗

arXiv2026

Symmetry-Protected Quantum Synchronization in Squeezed-Bath-Engineered Superradiance

A squeezed dissipative bath converts the coupling phase of a bipartite unconventional Dicke model into a control parameter that suppresses both static superradiant thresholds, opening a window where only a Hopf instability survives and the two spin ensembles synchronize completely via the shared cavity mode. The squeezed bath preserves a $\mathbb{Z}_2$ parity symmetry, so conventional broken-symmetry diagnostics vanish identically. We certify the synchronized state instead through parity-even, information-theoretic witnesses: a 30\% photon-number suppression, a Husimi-$Q$ lobe-count change, a 64\% suppression of spin--spin mutual information, and a robust discord-to-mutual-information ratio $D/I = 0.50 \pm 0.05$, confirmed by full quantum master-equation simulations. These results establish parity-even witnesses as a general, entanglement-free route to certifying quantum synchronization in symmetry-protected driven-dissipative systems.

quant-ph↗

arXiv2025

Energy landscape interpretation of universal linearly increasing absorption with frequency

Absorption of elastic waves in complex media is commonly found to increase linearly with frequency, for both longitudinal and shear waves. This ubiquitous property is observed in media such as rocks, unconsolidated sediments, and human tissue. Absorption is due to relaxation processes at the level of atomic scales and up to the sub-micron scale of biological materials. The effect of these processes is usually expressed as an integral over relaxation frequencies or relaxation times. Here we argue that these processes are thermally activated. Unusual for ultrasonics and seismics, we can therefore transform the expression for absorption from the frequency or time domains to an integral over an activation energy landscape weighted by an energy distribution. The universal power-law property surprisingly corresponds to a flat activation energy landscape. This is the solution which maximizes entropy or randomness. Therefore the linearly increasing absorption corresponds to the energy landscape with the fewest possible constraints.

cond-mat.soft↗

arXiv2021

Hanbury Brown and Twiss effect demonstrated for sound waves from a waterfall; an experimental, numerical and analytical study

The Hanbury Brown and Twiss effect (HBT) is described by numerical and analytical modeling, as well as experimentally, using sound waves and easily available instrumentation. An interesting phenomenon that has often been considered too difficult to be included in standard physics studies at bachelor and master level, can now be introduced even for second year bachelor students and up. In the original Hanbury Brown and Twiss effect the angular size of the source (the star Sirius) was calculated by determining the distance between two detectors that lead to a drop in the cross-correlations in the signals from the detectors. We find that this principle works equally well by sound waves from a waterfall. This is remarkable, since we use a completely different kind of waves from the HBT case, the frequency of the waves differ by a factor $\sim 10^{12}$ and the wavelength as well as the angular extension of the source seen from the observer's position differ by a factor $\sim 10^{7}$. The original HBT papers were based on measurements of \emph{intensity} fluctuations recorded by two detectors and correlations between these signals. The starting point for the theory that explained the effect was therefore intensity fluctuations per see, and the theory is not easy to understand, at least not for an undergraduate physics student. Our starting point is descriptions of broadband waves at the amplitude level (not at intensity level) by numerical modeling. Important properties of broadband waves can easily be revealed and understood by numerical modeling, and time-resolved frequency analysis (TFA) based on Morlet wavelets turns out to be a very useful tool. In fact, we think it has been far too little attention to broadband waves in physics education hitherto, but the growth of use of numerical methods in basic physics courses opens up new possibilities.

physics.ed-ph↗

arXiv2020

Synchronization in two-level quantum systems

Recently, it was shown that dissipative quantum systems with three or more levels are able to synchronize to an external signal, but it was stated that it is not possible for two-level systems as they lack a stable limit cycle in the unperturbed dynamics. At the same time, several papers, demonstrate, under a different definition of what is synchronization, that the latter is possible in qubits, although in different models which also include other elements. We show how a quantum two-level system can be understood as containing a valid limit cycle as the starting point of synchronization, and that it can synchronize its dynamics to an external weak signal. This is demonstrated by analytically solving the Lindblad equation of a two-level system coupled to an environment, determining the steady state. This is a mixed state with contributions from many pure states, each of which provides a valid limit cycle. We show that this is sufficient to phase-lock the dynamics to a weak external signal, hence clarifying synchronization in two-level systems. We use the Husimi Q representation to analyze the synchronization region, defining a synchronization measure which characterizes the strength of the phase-locking. Also, we study the stability of the limit cycle and its deformation with the strength of the signal in terms of the components of the Bloch vector of the system. Finally, we generalize the model of the three-level system from in order to illustrate how the stationary fixed point of that model can be changed into a limit cycle similar to the one that we describe for the two-level system.

quant-ph↗

arXiv2019

Frequency and phase relations of entangled photons observed by a two-photon interference experiment

An entangled photon experiment has been performed with a large variation of the temperature of the non-linear crystal generating the entangled pair by spontaneous downconversion. The photon pairs are separated by a nonpolarizing beamsplitter, and the polarization modes are mixed by half wave plates. The correlation function of the coincidences is studied as a function of the temperature. In the presence of a narrow interference filter we observe that the correlation changes between -1 and +1 about seven times within a temperature interval of about 30 degrees C. We show that the common simplified single-mode pair representation of entangled photons is insufficient to describe the results, but that the biphoton description that includes frequency and phase details gives close to perfect fit with experimental data for two different choices of interference filters. We explain the main ideas of the underlying physics, and give an interpretation of the two-photon amplitude which provides an intuitive understanding of the effect of changing the temperature and inserting interference filters.

quant-ph↗

arXiv2019

Deep reinforcement learning for robust quantum optimization

Machine learning techniques based on artificial neural networks have been successfully applied to solve many problems in science. One of the most interesting domains of machine learning, reinforcement learning, has natural applicability for optimization problems in physics. In this work we use deep reinforcement learning and Chopped Random Basis optimization, to solve an optimization problem based on the insertion of an off-center barrier in a quantum Szilard engine. We show that using designed protocols for the time dependence of the barrier strength, we can achieve an equal splitting of the wave function (1/2 probability to find the particle on either side of the barrier) even for an asymmetric Szilard engine in such a way that no information is lost when measuring which side the particle is found. This implies that the asymmetric non-adiabatic Szilard engine can operate with the same efficiency as the traditional Szilard engine, with adiabatic insertion of a central barrier. We compare the two optimization methods, and demonstrate the advantage of reinforcement learning when it comes to constructing robust and noise-resistant protocols.

quant-ph↗

arXiv2018

Quantum particle in a split box: Excitations to the ground state

We discuss two different approaches for splitting the wavefunction of a single-particle-box (SPB) into two equal parts. Adiabatic insertion of a barrier in the center of a SPB in order to make two compartments which each have probability 1/2 to find the particle in it is one of the key steps for a Szilard engine. However, any asymmetry between the volume of the compartments due to an off-center insertion of the barrier results in a particle that is fully localized in the larger compartment, in the adiabatic limit. We show that rather than exactly splitting the eigenfunctions in half by a symmetric barrier, one can use a non-adiabatic insertion of an asymmetric barrier to induce excitations to the first excited state of the full box. As the barrier height goes to infinity the excited state of the full box becomes the ground state of one of the new boxes. Thus, we can achieve close to exact splitting of the probability between the two compartments using the more realistic non-adiabatic, not perfectly centered barrier, rather than the idealized adiabatic and central barrier normally assumed.

quant-ph↗

arXiv2018

A Monte Carlo method for solving the NEGF equations for electron transport

We derive, by introducing restrictions to the lesser self energy, a Monte Carlo scheme that solves the NEGF equations for electron transport. In doing so we formally prove that the Monte Carlo estimator has an expectation value equal to the lead current of the NEGF solution, and we provide a simple test of the Monte Carlo scheme by calculating conductivity in nanowires within Buttiker's approximation of scattering. Good agreement between the Monte Carlo simulations and the alternative approaches are obtained, and we also demonstrate the existence of a regime where the Monte Carlo method is the fastest method. In our tests this regime is to extreme to be of practical use. We discuss various ways in which to speed up our prototype and how it can be extended to include more physics. Although the full applicability range of our assumptions about the lesser self energy remains to be better understood, we argue that they should apply as long as the transport process can be considered stationary.

physics.comp-ph↗

arXiv2017

The influence of measurement error on Maxwell's demon

In any general cycle of measurement, feedback and erasure, the measurement will reduce the entropy of the system when information about the state is obtained, while erasure, according to Landauer's principle, is accompanied by a corresponding increase in entropy due to the compression of logical and physical phase space. The total process can in principle be fully reversible. A measurement error reduces the information obtained and the entropy decrease in the system. The erasure still gives the same increase in entropy and the total process is irreversible. Another consequence of measurement error is that a bad feedback is applied, which further increases the entropy production if the proper protocol adapted to the expected error rate is not applied. We consider the effect of measurement error on a realistic single-electron box Szilard engine. We find the optimal protocol for the cycle as a function of the desired power $P$ and error $ε$, as well as the existence of a maximal power $P^{\max}$.

cond-mat.stat-mech↗

arXiv2015

Cooling by Heating: Restoration of the Third Law of Thermodynamics

We have made a simple and natural modification of a recent quantum refrigerator model presented by Cleuren et al. in Phys. Rev, Lett.108, 120603 (2012). The original model consist of two metal leads acting as heat baths, and a set of quantum dots that allow for electron transport between the baths. It was shown to violate the dynamic third law of thermodynamics (the unattainability principle, which states that cooling to absolute zero in finite time is impossible), but by taking into consideration the finite energy level spacing in metals we restore the third law, while keeping all of the original model's thermodynamic properties intact.

cond-mat.stat-mech↗

arXiv2014

Universal scaling form of AC response in variable range hopping

We have studied the AC response of a hopping model in the variable range hopping regime by dynamical Monte Carlo simulations. We find that the conductivity as function of frequency follows a universal scaling law. We also compare the numerical results to various theoretical predictions. Finally, we study the form of the conducting network as function of frequency.

cond-mat.dis-nn↗

arXiv2013

Accuracy of energy measurement and reversible operation of a microcanonical Szilard engine

In a recent paper [Vaikuntanathan and Jarzynski, Phys. Rev. E {\bf 83}, 061120 (2011), arXiv:1105.1744] a model was introduced whereby work could be extracted from a thermal bath by measuring the energy of a particle that was thermalized by the bath and manipulating the potential of the particle in the appropriate way, depending on the measurement outcome. If the extracted work is $W_1$ and the work $W_{\text{er}}$ needed to be dissipated in order to erase the measured information in accordance with Landauer's principle, it was shown that $W_1\leq W_{\text{er}}$ in accordance with the second law of thermodynamics. Here we extend this work in two directions: First, we discuss how accurately the energy should be measured. By increasing the accuracy one can extract more work, but at the same time one obtains more information that has to be deleted. We discuss what are the appropriate ways of optimizing the balance between the two and find optimal solutions. Second, whenever $W_1$ is strictly less than $W_{\text{er}}$ it means that an irreversible step has been performed. We identify the irreversible step and propose a protocol that will achieve the same transition in a reversible way, increasing $W_1$ so that $W_1 = W_{\text{er}}$.

cond-mat.stat-mech↗

arXiv2013

Observation of bosonic coalescence and fermionic anti-coalescence with indistinguishable photons

The symmetrization postulate asserts that the state of particular species of particles can only be of one permutation symmetry type: symmetric for bosons and antisymmetric for fermions. We report some experimental results showing that pairs of photons indistinguishable by all degrees of freedom can exhibit not only a bosonic behavior, as expected for photons, but also a surprisingly sharp fermionic behavior under specific conditions.

quant-ph↗

arXiv2012

Bloch-sphere approach to correlated noise in coupled qubits

By use of a generalized Bloch vector construction, we study the decoherence of a system composed of two interacting qubits in a general noisy environment. In particular, we investigate the effects of correlations in the noise acting on distinct qubits. Our treatment of the two-qubit system by use of the generalized Bloch vector leads to tractable analytic equations for the dynamics of the 4-level Bloch vector and allows for the application of geometrical concepts from the well known 2-level Bloch sphere. We find that in the presence of correlated or anticorrelated noise, the rate of decoherence is very sensitive to the initial two-qubit state, as well as to the symmetry of the Hamiltonian. In the absence of symmetry in the Hamiltonian, correlations only weakly impact the decoherence rate.

cond-mat.mes-hall↗

arXiv2012

Decoherence of a qubit due to a quantum fluctuator or to a classical telegraph noise

We investigate the decoherence of a qubit coupled to either a quantum two-level system (TLS) again coupled to an environment, or a classical fluctuator modeled by random telegraph noise. In order to do this we construct a model for the quantum TLS where we can adjust the temperature of its environment, and the decoherence rate independently. The model has a well-defined classical limit at any temperature and this corresponds to the appropriate random telegraph process, which is symmetric at high temperatures and becomes asymmetric at low temperatures. We find that the difference in the qubit decoherence rates predicted by the two models depends on the ratio between the qubit-TLS coupling and the decoherence rate in the pointer basis of the TLS. This is then the relevant parameter which determines whether the TLS has to be treated quantum mechanically or can be replaced by a classical telegraph process. We also compare the mutual information between the qubit and the TLS in the classical and quantum cases.

cond-mat.mes-hall↗

arXiv2012

The importance of level statistics for the decoherence of a central spin due to a spin environment

We study the decoherence of a central spin-1/2 due to a closed environment composed of spin-1/2 particles. It is known that a frustrated spin environment, such as a spin glass, is much more efficient for decoherence of the central spin than a similar size environment without frustration. We construct a Hamiltonian where the degree of frustration is parametrized by a single parameter $κ$. By use of this model we find that the environment can be classified by two distinct regimes with respect to the strength of level repulsion. These regimes behave qualitatively different with respect to decoherence of the central spin and might explain the strong enhancement of decoherence observed for frustrated environments.

quant-ph↗

arXiv2011

Effects of external driving on the coherence time of a Josephson junction qubit in a bath of two level fluctuators

We study the effect of external driving on the two level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid state Josephson-junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS, the significance of this effect compared to thermal fluctuations depend on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS, the driving frequency and at beatings between these two frequencies. Finally we study the ensemble averaged noise spectrum for a typical distribution of TLSs known to give origin to $\propto 1/f$ noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time $T_2^Q$ of the qubit. However this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.

cond-mat.mes-hall↗