CyberSec Research

Donor-acceptor pairs (DAPs) in wide-bandgap semiconductors are promising platforms for the realization of quantum technologies, due to their optically controllable, long-range dipolar interactions. Specifically, Al-N DAPs in bulk silicon carbide (SiC) have been predicted to enable coherent coupling over distances exceeding 10 nm. However, their practical implementations require an understanding of the properties of these pairs near surfaces and interfaces. Here, using first principles calculations we investigate how the presence of surfaces influence the stability and optical properties of Al-N DAPs in SiC, and we show that they retain favorable optical properties comparable to their bulk counterparts, despite a slight increase in electron-phonon coupling. Furthermore, we introduce the concept of surface-defect pairs (SDPs), where an electron-hole pair is generated between a near-surface defect and an occupied surface state located in the bandgap of the material. We show that vanadium-based SDPs near OH-terminated 4H-SiC surfaces exhibit dipoles naturally aligned perpendicular to the surface, greatly enhancing dipole-dipole coupling between SDPs. Our results also reveal significant polarization-dependent modulation in the stimulated emission and photoionization cross sections of V-based SDPs, which are tunable by two orders of magnitude via the polarization angle of the incident laser light. The near-surface defects investigated here provide novel possibilities for the development of hybrid quantum-classical interfaces, as they can be used to mediate information transfer between quantum nodes and integrated photonic circuits.

We introduce "quantum barcodes," a theoretical framework that applies persistent homology to classify topological phases in quantum many-body systems. By mapping quantum states to classical data points through strategic observable measurements, we create a "quantum state cloud" analyzable via persistent homology techniques. Our framework establishes that quantum systems in the same topological phase exhibit consistent barcode representations with shared persistent homology groups over characteristic intervals. We prove that quantum phase transitions manifest as significant changes in these persistent homology features, detectable through discontinuities in the persistent Dirac operator spectrum. Using the SSH model as a demonstrative example, we show how our approach successfully identifies the topological phase transition and distinguishes between trivial and topological phases. While primarily developed for symmetry-protected topological phases, our framework provides a mathematical connection between persistent homology and quantum topology, offering new methods for phase classification that complement traditional invariant-based approaches.

We introduce the stellar decomposition, a novel method for characterizing non-Gaussian states produced by photon-counting measurements on Gaussian states. Given an (m+n)-mode Gaussian state G, we express it as an (m+n)-mode "Gaussian core state" G_core followed by a fixed m-mode Gaussian transformation T that only acts on the first m modes. The defining property of the Gaussian core state G_core is that measuring the last n of its modes in the photon-number basis leaves the first m modes on a finite Fock support, i.e. a core state. Since T is measurement-independent and G_core has an exact and finite Fock representation, this decomposition exactly describes all non-Gaussian states obtainable by projecting n modes of G onto the Fock basis. For pure states we prove that a physical pair (G_core, T) always exists with G_core pure and T unitary. For mixed states, we establish necessary and sufficient conditions for (G_core, T) to be a Gaussian mixed state and a Gaussian channel. Finally, we develop a semidefinite program to extract the "largest" possible Gaussian channel when these conditions fail. The stellar decomposition leads to practical bounds on achievable state quality in photonic circuits and for GKP state generation in particular. Our results are based on a new characterization of Gaussian completely positive maps in the Bargmann picture, which may be of independent interest. As a result, this work provides novel tools for improved simulations of quantum optical systems, and for understanding the generation of non-Gaussian resource states.

We analyze information transmission capacities of quantum channels acting on $d$-dimensional quantum systems that are highly Markovian divisible, i.e., channels of the form \begin{equation*} \Phi = \underbrace{\Psi\circ \Psi \circ \ldots \circ \Psi}_{l \,\operatorname{times}} \end{equation*} with $l \geq \gamma d^2 \log d$ for some constant $\gamma=\gamma(\Psi)$ that depends on the spectral gap of the dividing channel $\Psi$. We prove that capacities of such channels are approximately strongly additive and can be efficiently approximated in terms of the structure of their peripheral spaces. Furthermore, the quantum and private classical capacities of such channels approximately coincide and approximately satisfy the strong converse property. We show that these approximate results become exact for the corresponding zero-error capacities when $l \geq d^2$. To prove these results, we show that for any channel $\Psi$, the classical, private classical, and quantum capacities of $\Psi_\infty$, which is its so-called asymptotic part, satisfy the strong converse property and are strongly additive. In the zero-error case, we introduce the notion of the stabilized non-commutative confusability graph of a quantum channel and characterize its structure for any given channel.

Suppose a black hole forms from a pure quantum state $\ket{\psi}$. The black hole information loss paradox arises from semiclassical arguments suggesting that, even in a closed system, the process of black hole formation and evaporation evolves a pure state into a mixed state. Resolution to the paradox typically demands violation of quantum mechanics or relativity in domains where they should hold. Instead, I propose that in a complete theory of quantum gravity, any region $\mathcal{U}$ that could collapse into a black hole should already be described by a mixed state, thus bypassing the paradox entirely. To that end, I present a model in which the universe is in a quantum error-corrected state, such that any local black hole appears mixed and encodes no information locally.

Cold atoms can adsorb to a surface with the emission of a single phonon when the binding energy is sufficiently small. The effects of phonon damping and adsorbent size on the adsorption rate in this quantum regime are studied using the multimode Rabi model. It is demonstrated that the adsorption rate can be either enhanced or suppressed relative to the Fermi golden rule rate, in analogy to cavity effects in the spontaneous emission rate in QED. A mesoscopic-sized adsorbent behaves as an acoustic cavity that enhances the adsorption rate when tuned to the adsorption transition frequency and suppresses the rate when detuned. This acoustic cavity effect occurs in the regime where the frequency spacing between vibrational modes exceeds the phonon linewidth.

Characterization of near-term quantum computing platforms requires the ability to capture and quantify dissipative effects. This is an inherently challenging task, as these effects are multifaceted, spanning a broad spectrum from Markovian to strongly non-Markovian dynamics. We introduce Quantum Liouvillian Tomography (QLT), a protocol to capture and quantify non-Markovian effects in time-continuous quantum dynamics. The protocol leverages gradient-based quantum process tomography to reconstruct dynamical maps and utilizes regression over the derivatives of Pauli string probability distributions to extract the Liouvillian governing the dynamics. We benchmark the protocol using synthetic data and quantify its accuracy in recovering Hamiltonians, jump operators, and dissipation rates for two-qubit systems. Finally, we apply QLT to analyze the evolution of an idling two-qubit system implemented on a superconducting quantum platform to extract characteristics of Hamiltonian and dissipative components and, as a result, detect inherently non-Markovian dynamics. Our work introduces the first protocol capable of retrieving generators of generic open quantum evolution from experimental data, thus enabling more precise characterization of many-body non-Markovian effects in near-term quantum computing platforms.

The study of holographic bulk-boundary dualities has led to the construction of novel quantum error correcting codes. Although these codes have shed new light on conceptual aspects of these dualities, they have widely been believed to lack a crucial feature of practical quantum error correction: The ability to support universal fault-tolerant quantum logic. In this work, we introduce a new class of holographic codes that realize this feature. These heterogeneous holographic codes are constructed by combining two seed codes in a tensor network on an alternating hyperbolic tiling. We show how this construction generalizes previous strategies for fault tolerance in tree-type concatenated codes, allowing one to implement non-Clifford gates fault-tolerantly on the holographic boundary. We also demonstrate that these codes allow for high erasure thresholds under a suitable heterogeneous combination of specific seed codes. Compared to previous concatenated codes, heterogeneous holographic codes achieve large overhead savings in physical qubits, e.g., a $21.8\%$ reduction for a two-layer Steane/quantum Reed-Muller combination. Unlike standard concatenated codes, we establish that the new codes can encode more than a single logical qubit per code block by applying ``black hole'' deformations with tunable rate and distance, while possessing fully addressable, universal fault-tolerant gate sets. Therefore, our work strengthens the case for the utility of holographic quantum codes for practical quantum computing.

Buhrman, Patro, and Speelman presented a framework of conjectures that together form a quantum analogue of the strong exponential-time hypothesis and its variants. They called it the QSETH framework. In this paper, using a notion of quantum natural proofs (built from natural proofs introduced by Razborov and Rudich), we show how part of the QSETH conjecture that requires properties to be `compression oblivious' can in many cases be replaced by assuming the existence of quantum-secure pseudorandom functions, a standard hardness assumption. Combined with techniques from Fourier analysis of Boolean functions, we show that properties such as PARITY and MAJORITY are compression oblivious for certain circuit class $\Lambda$ if subexponentially secure quantum pseudorandom functions exist in $\Lambda$, answering an open question in [Buhrman-Patro-Speelman 2021].

Rydberg atoms in static electric fields possess permanent dipole moments. When the atoms are close to a surface producing an inhomogeneous electric field, such as by the adsorbates on an atom chip, depending on the sign of the dipole moment of the Rydberg-Stark eigenstate, the atoms may experience a force towards or away from the surface. We show that by applying a bias electric field and coupling a desired Rydberg state by a microwave field of proper frequency to another Rydberg state with opposite sign of the dipole moment, we can create a trapping potential for the atom at a prescribed distance from the surface. Perfectly overlapping trapping potentials for several Rydberg states can also be created by multi-component microwave fields. A pair of such trapped Rydberg states of an atom can represent a qubit. Finally, we discuss an optimal realization of the swap gate between pairs of such atomic Rydberg qubits separated by a large distance but interacting with a common mode of a planar microwave resonator at finite temperature.

In a recent article [Phys. Rev. A 111, 022204 (2025)], Genovese and Piacentini analyzed recent experiments measuring what they call "the entire Bell-CHSH parameter". They claimed those experiments may have implications for interpreting loophole-fee tests of the Bell-CHSH inequality. We explain that the Bell-CHSH inequality is not based on the entire Bell parameter, so these experiments are unrelated to their empirical tests and cannot close eventual loopholes that still might persist. We point out that the physical meaning of these new experiments measuring the entire Bell parameter could be interpreted differently.

Recent experiments demonstrate all-electric spinning of levitated nanodiamonds with embedded nitrogen-vacancy spins. Here, we argue that such gyroscopically stabilized spin rotors offer a promising platform for probing and exploiting quantum spin-rotation coupling of particles hosting a single spin degree of freedom. Specifically, we derive the effective Hamiltonian describing how an embedded spin affects the rotation of rapidly revolving quantum rotors due to the Einstein-de Haas and Barnett effects, which we use to devise experimental protocols for observing this coupling in state-of-the-art experiments. This will open the door for future exploitations of quantum spin rotors for superposition experiments with massive objects.

Cross-talk between qubits is one of the main challenges for scaling superconducting quantum processors. Here, we use the density-matrix renormalization-group to numerically analyze lattices of superconducting qubits from a perspective of many-body localization. Specifically, we compare different architectures that include tunable couplers designed to decouple qubits in the idle state, and calculate the residual ZZ interactions as well as the inverse participation ratio in the computational basis states. For transmon qubits outside of the straddling regime, the results confirm that tunable C-shunt flux couplers are significantly more efficient in mitigating the ZZ interactions than tunable transmons. A recently proposed fluxonium architecture with tunable transmon couplers is demonstrated to also maintain its strong suppression of the ZZ interactions in larger systems, while having a higher inverse participation ratio in the computational basis states than lattices of transmon qubits. Our results thus suggest that fluxonium architectures may feature lower cross talk than transmon lattices when designed to achieve similar gate speeds and fidelities.

The eigenstate thermalization hypothesis (ETH) underpins much of our modern understanding of the thermalization of closed quantum many-body systems. Here, we investigate the statistical properties of observables in the eigenbasis of the Lindbladian operator of a Markovian open quantum system. We demonstrate the validity of a Lindbladian ETH ansatz through extensive numerical simulations of several physical models. To highlight the robustness of Lindbladian ETH, we consider what we dub the dilute-click regime of the model, in which one postselects only quantum trajectories with a finite fraction of quantum jumps. The average dynamics are generated by a non-trace-preserving Liouvillian, and we show that the Lindbladian ETH ansatz still holds in this case. On the other hand, the no-click limit is a singular point at which the Lindbladian reduces to a doubled non-Hermitian Hamiltonian and Lindbladian ETH breaks down.

The presence of a dissipative environment disrupts the unitary spectrum of dynamical quantum maps. Nevertheless, key features of the underlying unitary dynamics -- such as their integrable or chaotic nature -- are not immediately erased by dissipation. To investigate this, we model dissipation as a convex combination of a unitary evolution and a random Kraus map, and study how signatures of integrability fade as dissipation strength increases. Our analysis shows that in the weakly dissipative regime, the complex eigenvalue spectrum organizes into well-defined, high-density clusters. We estimate the critical dissipation threshold beyond which these clusters disappear, rendering the dynamics indistinguishable from chaotic evolution. This threshold depends only on the number of spectral clusters and the rank of the random Kraus operator. To characterize this transition, we introduce the eigenvalue angular velocity as a diagnostic of integrability loss. We illustrate our findings through several integrable quantum circuits, including the dissipative quantum Fourier transform. Our results provide a quantitative picture of how noise gradually erases the footprints of integrability in open quantum systems.

Quantum simulation is a promising way toward practical quantum advantage, but noise in current quantum hardware poses a significant obstacle. We theoretically and numerically revealed that not only the physical error but also the algorithmic error in a single Trotter step decreases exponentially with the circuit depth. In particular, according to our results, we derive the optimal number of Trotter steps and the noise requirement to guarantee total simulation precision. At last, we demonstrate that our improved error analysis leads to significant resource-saving for fault-tolerant Trotter simulation. By addressing these aspects, this work systematically characterizes the robustness of Trotter simulation errors in noisy quantum devices and paves the way toward practical quantum advantage.

In this work, a novel quantum Fourier ordinary differential equation (ODE) solver is proposed to solve both linear and nonlinear partial differential equations (PDEs). Traditional quantum ODE solvers transform a PDE into an ODE system via spatial discretization and then integrate it, thereby converting the task of solving the PDE into computing the integral for the driving function $f(x)$. These solvers rely on the quantum amplitude estimation algorithm, which requires the driving function $f(x)$ to be within the range of [0, 1] and necessitates the construction of a quantum circuit for the oracle R that encodes $f(x)$. This construction can be highly complex, even for simple functions like $f(x) = x$. An important exception arises for the specific case of $f(x) = sin^2(mx+c)$, which can be encoded more efficiently using a set of $Ry$ rotation gates. To address these challenges, we expand the driving function $f(x)$ as a Fourier series and propose the Quantum Fourier ODE Solver. This approach not only simplifies the construction of the oracle R but also removes the restriction that $f(x)$ must lie within [0,1]. The proposed method was evaluated by solving several representative linear and nonlinear PDEs, including the Navier-Stokes (N-S) equations. The results show that the quantum Fourier ODE solver produces results that closely match both analytical and reference solutions.

In the Quantum Internet, multipartite entanglement enables a new form of network connectivity, referred to as artificial connectivity namely and able to augment the physical connectivity with artificial links between pairs of nodes, without any additional physical link deployment. In this paper, by engineering such an artificial connectivity, we theoretically determine upper and lower bounds for the number of EPR pairs and GHZ states that can be extracted among nodes that are not adjacent in the artificial network topology. The aforementioned analysis is crucial, since the extraction of EPR pairs and GHZ states among remote nodes constitutes the resource primitives for on-demand and end-to-end communications. Indeed, within the paper, we not only determine whether a certain number of remote EPR pairs and GHZ states can be extracted, but we also provide the locations, namely the identities, of the nodes interconnected by such entangled resources. Thus, our analysis is far from being purely theoretical, rather it is constructive, since we provide the sequence of operations required for performing such extractions.