## Journals

### Fractional Floquet theory. (arXiv:2302.02340v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

A fractional generalization of the Floquet theorem is suggested for fractional Schr\"odinger equations (FTSE)s with the time-dependent periodic Hamiltonians. The obtained result, called the fractional Floquet theorem (fFT), is formulated in the form of the Mittag-Leffler function, which is considered as the eigenfunction of the Caputo fractional derivative. The suggested formula makes it possible to reduce the FTSE to the standard quantum mechanics with the time-dependent Hamiltonian, where the standard Floquet theorem is valid. Two examples related to quantum resonances are considered as well to support the obtained result.

Categories: Journals, Physics

### Sufficient statistic and recoverability via Quantum Fisher Information metrics. (arXiv:2302.02341v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We prove that for a large class of quantum Fisher information, a quantum channel is sufficient for a family of quantum states, i.e., the input states can be recovered from the output, if and only if the quantum Fisher information is preserved under the quantum channel. This class, for instance, includes Winger-Yanase-Dyson skew information. On the other hand, interestingly, the SLD quantum Fisher information, as the most popular example of quantum analog of Fisher information, does not satisfy this property.

Our recoverability result is obtained by studying Riemannian monotone metrics on the quantum state space, i.e. metrics monotone decreasing under the action of quantum channels, a property often called data processing inequality. For two quantum states, the monotone metric gives the corresponding quantum $\chi^2$ divergence. We obtain an approximate recovery result in the sense that, if the quantum $\chi^2$ divergence is approximately preserved by a quantum channel, then two states can be approximately recovered by the Petz recovery map. We also obtain a universal recovery bound for the $\chi_{\frac{1}{2}}$ divergence.

Finally, we discuss applications in the context of quantum thermodynamics and the resource theory of asymmetry.

Categories: Journals, Physics

### Self-induced Transparency in a Semiconductor Quantum Dot medium at ultra-cold temperatures. (arXiv:2302.02377v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We investigate the feasibility of lossless and shape-preserving pulse propagation in an inhomogeneously broadened semiconductor quantum dot medium. We demonstrate that a short pulse can propagate inside the medium with minimal absorption and broadening in pulse shape. The stable pulse area becomes slightly higher than the prediction of the pulse area theorem and is also dependent on the environment temperature. The change in the final pulse shape is explained very well by numerically solving the propagation equation supported by the susceptibility of the medium. Our system also exhibits the pulse breakup phenomena for higher input pulse areas. Therefore, the considered scheme can have important applications in quantum communication, quantum information, and mode-locking with the advantage of scalability and controllability.

Categories: Journals, Physics

### A study of the violation of the Bell-CHSH inequality. (arXiv:2302.02385v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We analyze the violation of the Bell-CHSH inequality for bipartite systems described by pure states. Applications to the so called $N00N$ states as well as to coherent states are presented.

Categories: Journals, Physics

### High-Rate Point-to-Multipoint Quantum Key Distribution using Coherent States. (arXiv:2302.02391v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Quantum key distribution (QKD) which enables information-theoretically security is now heading towards quantum secure networks. It requires high-performance and cost-effective protocols while increasing the number of users. Unfortunately, qubit-implemented protocols only allow one receiver to respond to the prepared signal at a time, thus cannot support multiple users natively and well satisfy the network demands. Here, we show a 'protocol solution' using continuous-variable quantum information. A coherent-state point-to-multipoint protocol is proposed to simultaneously support multiple independent QKD links between a single transmitter and massive receivers. Every prepared coherent state is measured by all receivers to generate raw keys, then processed with a secure and high-efficient key distillation method to remove the correlations between different QKD links. It can achieve remarkably high key rates even with a hundred of access points and shows the potential improvement of two orders of magnitude. This scheme is a promising step towards a high-rate multi-user solution in a scalable quantum secure network.

Categories: Journals, Physics

### Toward random tensor networks and holographic codes in CFT. (arXiv:2302.02446v1 [hep-th])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

In holographic CFTs satisfying eigenstate thermalization, there is a regime where the operator product expansion can be approximated by a random tensor network. The geometry of the tensor network corresponds to a spatial slice in the holographic dual, with the tensors discretizing the radial direction. In spherically symmetric states in any dimension and more general states in 2d CFT, this leads to a holographic error-correcting code, defined in terms of OPE data, that can be systematically corrected beyond the random tensor approximation. The code is shown to be isometric for light operators outside the horizon, and non-isometric inside, as expected from general arguments about bulk reconstruction. The transition at the horizon occurs due to a subtle breakdown of the Virasoro identity block approximation in states with a complex interior.

Categories: Journals, Physics

### On low-depth algorithms for quantum phase estimation. (arXiv:2302.02454v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Quantum phase estimation is one of the key building blocks of quantum computing. For early fault-tolerant quantum devices, it is desirable for a quantum phase estimation algorithm to (1) use a minimal number of ancilla qubits, (2) allow for inexact initial states with a significant mismatch, (3) achieve the Heisenberg limit for the total resource used, and (4) have a diminishing prefactor for the maximum circuit length when the overlap between the initial state and the target state approaches one. In this paper, we prove that an existing algorithm from quantum metrology can achieve the first three requirements. As a second contribution, we propose a modified version of the algorithm that also meets the fourth requirement, which makes it particularly attractive for early fault-tolerant quantum devices.

Categories: Journals, Physics

### Precision of quantum simulation of all-to-all coupling in a local architecture. (arXiv:2302.02458v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We present a simple 2d local circuit that implements all-to-all interactions via perturbative gadgets. We find an analytic relation between the values $J_{ij}$ of the desired interaction and the parameters of the 2d circuit, as well as the expression for the error in the quantum spectrum. For the relative error to be a constant $\epsilon$, one requires an energy scale growing as $n^6$ in the number of qubits, or equivalently a control precision up to $n^{-6}$. Our proof is based on the Schrieffer-Wolff transformation and generalizes to any hardware. In the architectures available today, $5$ digits of control precision are sufficient for $n=40,~ \epsilon =0.1$. Comparing our construction, known as paramagnetic trees, to ferromagnetic chains used in minor embedding, we find that at chain length $>3$ the performance of minor embedding degrades exponentially with the length of the chain, while our construction experiences only a polynomial decrease.

Categories: Journals, Physics

### A High Performance Compiler for Very Large Scale Surface Code Computations. (arXiv:2302.02459v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We present the first high performance compiler for very large scale quantum error correction: it translates an arbitrary quantum circuit to surface code operations based on lattice surgery. Our compiler offers an end to end error correction workflow implemented by a pluggable architecture centered around an intermediate representation of lattice surgery instructions. Moreover, the compiler supports customizable circuit layouts, can be used for quantum benchmarking and includes a quantum resource estimator. The compiler can process millions of gates using a streaming pipeline at a speed geared towards real-time operation of a physical device. We compiled within seconds 80 million logical surface code instructions, corresponding to a high precision Clifford+T implementation of the 128-qubit Quantum Fourier Transform (QFT). Our code is open-sourced at \url{https://github.com/latticesurgery-com}.

Categories: Journals, Physics

### Experimental full network nonlocality with independent sources and strict locality constraints. (arXiv:2302.02472v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Nonlocality arising in networks composed of several independent sources gives rise to phenomena radically different from that in standard Bell scenarios. Over the years, the phenomenon of network nonlocality in the entanglement-swapping scenario has been well investigated and demonstrated. However, it is known that violations of the so-called bilocality inequality used in previous experimental demonstrations cannot be used to certify the non-classicality of their sources. This has put forward a stronger concept for nonlocality in networks, called full network nonlocality. Here, we experimentally observe full network nonlocal correlations in a network where the source-independence, locality, and measurement-independence loopholes are closed. This is ensured by employing two independent sources, rapid setting generation, and space-like separations of relevant events. Our experiment violates known inequalities characterizing non-full network nonlocal correlations by over five standard deviations, certifying the absence of classical sources in the realization.

Categories: Journals, Physics

### Approximate reconstructability of quantum states and noisy quantum secret sharing schemes. (arXiv:2302.02509v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We introduce and analyse approximate quantum secret sharing in a formal cryptographic setting, wherein a dealer encodes and distributes a quantum secret to players such that authorized structures (sets of subsets of players) can approximately reconstruct the quantum secret and omnipotent adversarial agents controlling non-authorized subsets of players are approximately denied the quantum secret. In particular, viewing the map encoding the quantum secret to shares for players in an authorized structure as a quantum channel, we show that approximate reconstructability of the quantum secret by these players is possible if and only if the information leakage, given in terms of a certain entanglement-assisted capacity of the complementary quantum channel to the players outside the structure and the environment, is small.

Categories: Journals, Physics

### The principle of learning sign rules by neural networks in qubit lattice models. (arXiv:2302.02523v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

A neural network is a powerful tool for generalizing hidden laws beyond human intuition; however, it looks like a black box due to complicated nonlinear structures. Based on the Gutzwiller mean-field theory, we exhibit a principle of learning sign rules for the ordered states in qubit lattice models. Accordingly, we construct a shallow feed-forward neural network with a single hidden neuron and systematically make benchmarks in the generalized Ising, XY, frustrated Heisenberg chains, antiferromagnetic XY on the triangle lattice, and the Fermi-Hubbard chain at an arbitrary filling. All the leading-order or mean-field sign rule characters are visualized in classical forms, such as the gauge field gradient, pitch angles, etc. Besides, quantum fluctuations violate the sign rule and quantitatively yield an imperfect accuracy rate in the prediction.

Categories: Journals, Physics

### Virtual quantum error detection. (arXiv:2302.02626v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Quantum error correction and quantum error detection necessitate syndrome measurements to detect errors. Syndrome measurements need to be performed for each stabilizer generator with single-shot measurements, which can be a significant overhead, considering the fact that the readout fidelity is generally lower than gate fidelity in the current quantum hardware. Here, by generalizing a quantum error mitigation method called symmetry expansion, we propose a protocol that we call virtual quantum error detection (VQED). This method virtually allows for evaluating computation results corresponding to post-selected quantum states obtained through quantum error detection during circuit execution without syndrome measurements. Furthermore, while the conventional quantum error detection needs the implementation of Hadamard test circuits for each stabilizer generator, our VQED protocol can be performed with a constant depth shallow quantum circuit with an ancilla qubit, irrespective of the number of stabilizer generators. In addition, VQED is fully compatible with other error mitigation schemes for further improvement of computation accuracy, thus leading to high-fidelity quantum computing.

Categories: Journals, Physics

### Extending Matchgate Simulation Methods to Universal Quantum Circuits. (arXiv:2302.02654v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Matchgates are a family of parity-preserving two-qubit gates, nearest-neighbour circuits of which are known to be classically simulable in polynomial time. In this work, we present a simulation method to classically simulate an $\boldsymbol{n}$-qubit circuit containing $\boldsymbol{N}$ matchgates and $\boldsymbol{m}$ universality-enabling gates in the setting of single-qubit Pauli measurements and product state inputs. The universality-enabling gates we consider include the SWAP, CZ and CPhase gates. We find in the worst and average cases, the scaling when $\boldsymbol{m \, < \, \lfloor \frac{n}{2} \rfloor -1}$ is given by $\sim \mathcal{O}(\boldsymbol{N(\frac{n}{m+1})^{2m+2}})$ and $\sim \mathcal{O}( \boldsymbol{\frac{N}{m+1}(\frac{n}{m+1})^{2m+2}})$, respectively. For $\boldsymbol{m \, \geq \, \lfloor \frac{n}{2} \rfloor -1}$, we find the scaling is exponential in $\boldsymbol{n}$, but always outperforms a dense statevector simulator in the asymptotic limit.

Categories: Journals, Physics

### Symmetry Resolved Entanglement of Excited States in Quantum Field Theory III: Bosonic and Fermionic Negativity. (arXiv:2302.02666v1 [hep-th])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

In two recent works, we studied the symmetry resolved R\'enyi entropies of quasi-particle excited states in quantum field theory. We found that the entropies display many model-independent features which we discussed and analytically characterised. In this paper we extend this line of investigation by providing analytical and numerical evidence that a similar universal behavior arises for the symmetry resolved negativity. In particular, we compute the ratio of charged moments of the partially transposed reduced density matrix as an expectation value of twist operators. These are fused" versions of the more traditionally used branch point twist fields and were introduced in a previous work. The use of twist operators allows us to perform the computation in an arbitrary number of spacial dimensions. We show that, in the large-volume limit, only the commutation relations between the twist operators and local fields matter, and computations reduce to a purely combinatorial problem. We address some specific issues regarding fermionic excitations, whose treatment requires the notion of partial time-reversal transformation, and we discuss the differences and analogies with their bosonic counterpart. We find that although the operation of partial transposition requires a redefinition for fermionic theories, the ratio of the negativity moments between an excited state and the ground state is universal and identical for fermions and bosons as well as for a large variety of very different states, ranging from simple qubit states to the excited states of free quantum field theories. Our predictions are tested numerically on a 1D Fermi chain.

Categories: Journals, Physics

### The semiclassical limit of a quantum Zeno dynamics. (arXiv:2302.02673v1 [math-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Motivated by a quantum Zeno dynamics in a cavity quantum electrodynamics setting, we study the asymptotics of a family of symbols corresponding to a truncated momentum operator, in the semiclassical limit of vanishing Planck's constant $\hbar\to0$ and large quantum number $N\to\infty$, with $\hbar N$ kept fixed. In a certain topology, the limit is the discontinuous symbol $p\chi_D(x,p)$ where $\chi_D$ is the characteristic function of the classically permitted region $D$ in the phase space. A refined analysis shows that the symbol is asymptotically close to a smooth function $p\chi_D^{(N)}(x,p)$, where $\chi_D^{(N)}$ is a smooth version of $\chi_D$ related to the integrated Airy function. We also discuss the limit from the dynamical point of view.

Categories: Journals, Physics

### Electromagnetic vacuum stresses and energy fluxes induced by a cosmic string in de Sitter spacetime. (arXiv:2302.02685v1 [hep-th])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

For the electromagnetic field in (D+1)-dimensional locally de Sitter (dS) spacetime, we analyze the effects of a generalized cosmic string type defect on the vacuum expectation value of the energy-momentum tensor. For the Bunch-Davies vacuum state, the topological contributions are explicitly extracted in both the diagonal and off-diagonal components. The latter describes the presence of radially directed energy flux in the vacuum state. It vanishes for $D=3$ because of the conformal invariance of the electromagnetic field and is directed towards the cosmic string for $D\geq 4$ . The topological contributions in the vacuum stresses are anisotropic and, unlike to the geometry of a cosmic string in the Minkowski spacetime, for $D>3$ the stresses along the directions parallel to the string core differ from the energy density. Depending on the planar angle deficit and the distance from the cosmic string, the corresponding expectation values can be either positive or negative. Near the cosmic string the effect of the gravitational field on the diagonal components of the topological part is weak. The spacetime curvature essentially modifies the behavior of the topological terms at proper distances from the cosmic string larger than the dS curvature radius. In that region, the topological contributions in the diagonal components of the energy-momentum tensor decay in inverse proportion to the fourth power of the proper distance and the energy flux density behaves as inverse-fifth power for all values of the spatial dimension $D$. The exception is the energy density in the special case $D=4$.

Categories: Journals, Physics

### Heterogeneous integration of superconducting thin films and epitaxial semiconductor heterostructures with Lithium Niobate. (arXiv:2302.02712v1 [physics.app-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots on strong piezoelectic and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III-V compound semiconductor membranes on the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors $Q_i > 16000$. Superconducting operation of $\approx 400\,\mathrm{MHz}$ resonators is achieved to temperatures $T>7\mathrm{K}$ and electrical radio frequency powers $P_{\mathrm{rf}}>+9\,\mathrm{dBm}$. Position and frequency selective coupling of single quantum dots heterogeneously integrated within the resonant phononic field of the surface acoustic wave resonator is validated using time-integrated and time-resolved optical spectroscopy. These experiments confirm deformation coupling as the dominant coupling mechanism. Finally, acousto-electric charge state control is achieved in a modified device geometry in which the large piezoelectric fields strongly couple to the semiconductor. The implemented hybrid quantum dot - surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes to realize quantum phase modulation and transduction between photons and phonons via the quantum dot. Finally, the employed materials allow for the implementation of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.

Categories: Journals, Physics

### Low-loss interconnects for modular superconducting quantum processors. (arXiv:2302.02751v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

Scaling is now a key challenge in superconducting quantum computing. One solution is to build modular systems in which smaller-scale quantum modules are individually constructed and calibrated, and then assembled into a larger architecture. This, however, requires the development of suitable interconnects. Here, we report low-loss interconnects based on pure aluminium coaxial cables and on-chip impedance transformers featuring quality factors up to $8.1 \times 10^5$, which is comparable to the performance of our transmon qubits fabricated on single-crystal sapphire substrate. We use these interconnects to link five quantum modules with inter-module quantum state transfer and Bell state fidelities up to 99\%. To benchmark the overall performance of the processor, we create maximally-entangled, multi-qubit Greenberger-Horne-Zeilinger (GHZ) states. The generated inter-module four-qubit GHZ state exhibits 92.0\% fidelity. We also entangle up to 12 qubits in a GHZ state with $55.8 \pm 1.8\%$ fidelity, which is above the genuine multipartite entanglement threshold of 1/2. These results represent a viable modular approach for large-scale superconducting quantum processors.

Categories: Journals, Physics

### Accumulation of scale-free localized states induced by local non-Hermiticity. (arXiv:2302.02798v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2023-02-07 11:45

The bulk states of Hermitian systems are believed insensitive to local Hermitian impurities or perturbations except for a few impurity-induced bound states. Thus an important question is whether local non-Hermiticity can cause drastic changes to the original Hermitian systems. Here we answer this question affirmatively and present exact solutions for the double chain model with local non-Hermitian terms that has parity-time ($\mathcal{PT}$) symmetry. Induced by the non-Hermiticity, the system undergoes a sequence of $\mathcal{PT}$-symmetry breakings, after which the eigenenergies appear in complex conjugate pairs. The associated bulk states then become scale-free localized, and unidirectionally accumulated around the impurity. There exist mobility edges separating the residual extended states and the scale-free localized states. In a narrow intermediate region, all eigenstates become scale-free localized states. Further increasing the non-Hermitian term counter-intuitively brings the system to a $\mathcal{PT}$-restoration regime with fully real bulk spectra and a pair of imaginary bound states. We demonstrate that accumulation of local non-Hermiticity generated scale-free localized states is a general phenomenon and can even survive in the presence of quasiperiodical disorder. Our results indicate that the bulk properties of the original Hermitian system can be globally reshaped by local non-Hermiticity.

Categories: Journals, Physics