Quantum Physics (quant-ph)

Abstract: We report on the dynamics of the Rabi oscillation and the Larmor precession of $^{15}$N nuclear spin using nonselective short microwave pulses for initialization of $^{15}$N nuclear spins. We observe the Larmor precession of $^{15}$N nuclear spin depending on the angle between the applied magnetic field and the axis of the nitrogen vacancy center. We propose to utilize the change of the Larmor frequency of the nuclear spins to detect static magnetic fields at high sensitivity. Our results may contribute to enhancing the sensitivity of dc magnetic fields and devising novel protocols using $^{15}$N nuclear spin in nitrogen vacancy centers in diamonds.

Abstract: How much information a fermionic state contains? To address this fundamental question, we define the complexity of a particle-conserving many-fermion state as the entropy of its Fock space probability distribution, minimized over all Fock representations. The complexity characterizes the minimum computational and physical resources required to represent the state and store the information obtained from it by measurements. Alternatively, the complexity can be regarded a Fock space entanglement measure describing the intrinsic many-particle entanglement in the state. We establish universal lower bound for the complexity in terms of the single-particle correlation matrix eigenvalues and formulate a finite-size complexity scaling hypothesis. Remarkably, numerical studies on interacting lattice models suggest a general model-independent complexity hierarchy: ground states are exponentially less complex than average excited states which, in turn, are exponentially less complex than generic states in the Fock space. Our work has fundamental implications on how much information is encoded in fermionic states.

Abstract: Vortex pattern formation in electron-positron pair creation from vacuum by a time-dependent electric field of linear polarization is analyzed. It is demonstrated that in such scenario the momentum distributions of created particles exhibit vortex-antivortex pairs. Their sensitivity to the laser field parameters such as the field frequency and intensity is also studied. Specifically, it is shown that with increasing field frequency accross the one-photon threshold additional vortex-antivortex pairs appear. Their location in the momentum space is consistent with a general threshold behavior of probability distributions of created electrons (positrons). Namely, while for small field frequencies the particles tend to be created along the field polarization direction, for large enough frequencies they are predominantly generated in the perpendicular direction. Such change in longitudinal and transverse momentum sharing of created particles occurs accross the one-photon threshold.

Abstract: Noisy intermediate-scale quantum (NISQ) devices seek to achieve quantum advantage over classical systems without the use of full quantum error correction. We propose a NISQ processor architecture using a qubit `pipeline' in which all run-time control is applied globally, reducing the required number and complexity of control and interconnect resources. This is achieved by progressing qubit states through a layered physical array of structures which realise single and two-qubit gates. Such an approach lends itself to NISQ applications such as variational quantum eigensolvers which require numerous repetitions of the same calculation, or small variations thereof. In exchange for simplifying run-time control, a larger number of physical structures is required for shuttling the qubits as the circuit depth now corresponds to an array of physical structures. However, qubit states can be `pipelined' densely through the arrays for repeated runs to make more efficient use of physical resources. We describe how the qubit pipeline can be implemented in a silicon spin-qubit platform, to which it is well suited to due to the high qubit density and scalability. In this implementation, we describe the physical realisation of single and two qubit gates which represent a universal gate set that can achieve fidelities of $\mathcal{F} \geq 0.9999$, even under typical qubit frequency variations.

Abstract: A geometric construction of the arrival time in conventional quantum mechanics is presented. It is based on a careful mathematical analysis of different quantization procedures for classical observables as functions of positions and momenta. A class of observables is selected which possess a unique (if any) quantized version. A simple criterion for existence of such a quantized version is formulated. These mathematical results are then applied to the classical "arrival time" observable.

Abstract: In the framework of Impagliazzo's five worlds, a distinction is often made between two worlds, one where public-key encryption exists (Cryptomania), and one in which only one-way functions exist (MiniCrypt). However, the boundaries between these worlds can change when quantum information is taken into account. Recent work has shown that quantum variants of oblivious transfer and multi-party computation, both primitives that are classically in Cryptomania, can be constructed from one-way functions, placing them in the realm of quantum MiniCrypt (the so-called MiniQCrypt). This naturally raises the following question: Is it possible to construct a quantum variant of public-key encryption, which is at the heart of Cryptomania, from one-way functions or potentially weaker assumptions? In this work, we initiate the formal study of the notion of quantum public-key encryption (qPKE), i.e., public-key encryption where keys are allowed to be quantum states. We propose new definitions of security and several constructions of qPKE based on the existence of one-way functions (OWF), or even weaker assumptions, such as pseudorandom function-like states (PRFS) and pseudorandom function-like states with proof of destruction (PRFSPD). Finally, to give a tight characterization of this primitive, we show that computational assumptions are necessary to build quantum public-key encryption. That is, we give a self-contained proof that no quantum public-key encryption scheme can provide information-theoretic security.

Abstract: This paper deals with the broad class of magnetic systems having a single or collective spin $S$ with an energy barrier, such as Rare-Earth elements and their compounds, Single Molecule Magnets with uniaxial anisotropy and more generally any other quantum system made of single or multiple objects with discrete energy levels. Till now, the reversal of the magnetization of such systems at zero Kelvin required to make use of quantum tunneling with a significant transverse field or transverse anisotropy term, at resonance. Here, we show that another very simple method exists. It simply consists in the application of a particular sequence of electromagnetic radiations in the ranges of optical or microwave frequencies, depending on the characteristics of the system (spin and anisotropy values for magnetic systems). This produces oscillations of the Rabi type that pass above the barrier, thus extending these oscillations between the two energy wells. with mixtures of all the 2S+1 states. In addition to its basic character, this approach opens up new directions of research in quantum information with possible breakthroughs in the current use of multiple quantum bits.

Abstract: The violation of Mermin's inequalities is analyzed by making use of two different Bell setups built with pseudospin operators. Employing entangled states defined by means of squeezed and coherent states, the expectation value of Mermin's polynomials $M_n$ is evaluated for $n=3$ and $n=4$. In each case, we analyze the correlator $\langle M_n \rangle$ and identify the set of parameters leading to the violation of Mermin's inequalities and to the saturation of the bound predicted by Quantum Mechanics.

Abstract: The Helmholtz free energy of oscillators in thermal equilibrium with electromagnetic radiation is obtained from the Pauli-Hellmann-Feynman theorem and applied to some aspects of Lamb shifts and van der Waals interactions.

Abstract: It has been known that all bipartite pure quantum states can be self-tested, i.e., any such state can be certified completely by initially measuring both subsystems of this state by proper local quantum measurements and subsequently verifying that the correlation between the measurement choices and the outcomes satisfies a specific condition. In such a protocol, a key feature is that the conclusion can still be reliable even if involved quantum measurements are untrusted, where quantum nonlocality is crucial and plays a central role, and this means that each party has to conduct at least two different quantum measurements to produce a desirable correlation. Here, we prove that when the underlying Hilbert space dimension is known beforehand, an arbitrary $d\times d$ bipartite pure state can be certified completely (up to local unitary transformations) by a certain correlation generated by a single measurement setting on each party, where each measurement yields only $3d$ outcomes. Notably, our protocols do not involve any quantum nonlocality. We believe that our result may provide us a remarkable convenience when certifying bipartite pure quantum states in quantum labs.

Abstract: We use the reduced state of the field formalism [Entropy 21, 705 (2019)] to derive conditions under which a Bogoliubov transformation can be considered semi-classical. We apply this result to the dynamical Casimir effect in a moving medium [Phys. Rev. A 78, 042109 (2008)], discussing its classical and quantum features.

Abstract: The purpose of this paper is to study the coherent feedback control dynamics based on the network that an $N$-level atom is coupled with a cavity and the cavity is coupled with a single or multiple parallel waveguides through two semitransparent mirrors. When initially the atom is excited at the highest energy level, it can emit multiple photons into the cavity via the spontaneous emission, and the photons in the cavity can be transmitted into the waveguide and then re-interact with the cavity quantum electrodynamics (cavity-QED) system through the feedback channel. When the cavity is coupled with a single waveguide, the generation of multi-photon states in the waveguide can be characterized by the exponential stability of the linear control system with feedback delays determined by the feedback loop length. By tuning the feedback loop length, there can be zero or multiple photons in the waveguide. Besides, when the cavity-QED system is coupled with multiple parallel waveguides, the emitted photons oscillate among different waveguides and this process is influenced by the feedback loop length and coupling strengths among waveguides.

Abstract: Everett's many-worlds or multiverse theory is an attempt to find an alternative to the standard Copenhagen interpretation of quantum mechanics. Everett's theory is often claimed to be local in the Bell sense. Here, we show that this is not the case and debunk the contradictions by analyzing in detail the Greenberger--Horne--Zeilinger (GHZ) nonlocality theorem. We discuss and compare different notions of locality often mixed in the Everettian literature and try to explain the nature of the confusion. We conclude with a discussion of probability and statistics in the many-worlds theory and stress that the strong symmetry existing between branches in the theory prohibits the definition of probability and that the theory cannot recover statistics. The only way out from this contradiction is to modify the theory by adding hidden variables \`a la Bohm and, as a consequence, the new theory is explicitly Bell-nonlocal.

Abstract: The quantum nature of light enables potentially revolutionary communication technologies. Key to advancing this area of research is a clear understanding of the concepts of states, modes, fields, and photons. The concept of field modes carries over from classical optics, while the concept of state has to be considered carefully when treating light quantum mechanically. The term 'photon' is an overloaded identifier in the sense that it is often used to refer to either a quantum particle or the state of a field. This overloading, often used without placing in context, has the potential to obfuscate the physical processes that describe the reality we measure. We review the uses and relationships between these concepts using modern quantum optics theory, including the concept of a photon wave function, the modern history of which was moved forward in a groundbreaking paper in this journal by Iwo Bia{\l}ynicki-Birula, to whom this article is dedicated.

Abstract: We study the statistical properties of a gas of interacting bosons trapped in a box potential in two and three dimensions. Our primary focus is the characteristic temperature $\tchar$, i.e. the temperature at which the fluctuations of the number of condensed atoms (or, in 2D, the number of motionless atoms) is maximal. Using the Fock State Sampling method, we show that $\tchar$ increases due to interaction. In 3D, this temperature converges to the critical temperature in the thermodynamic limit. In 2D we show the general applicability of the method by obtaining a generalized dependence of the characteristic temperature on the interaction strength. Finally, we discuss the experimental conditions necessary for the verification of our theoretical predictions.

Abstract: We start this short note by remembering the beginnings of the Warsaw School of Quantum Optics, evidently stimulated by Iwo Bialynicki-Birula at the Warsaw University, and then Centre for Theoretical Physics of Polish Academy of Sciences, and Adam Kujawski and Zofia Bialynicka-Birula at the Institute of Physics of Polish Academy of Sciences. In the theoretical approaches of the Warsaw School Quantum Field Theory was always present, and Quantum Optics was considered to be Applied Quantum Electrodynamics (QED). All of us who grew up in this fantastic community have carried and are still carrying the gospel to others. In particular, now QED began her run on the red carpet of Super Instense Laser Matter Interactions, Attosecond-physics, and Ultrafast Laser Physics, in general. We will elaborate on the recent progress in this direction, and on the open questions towards future investigations. This paper celebrates the 90th birthday of Prof. Iwo Bialynicki-Birula, our QED guru!

Abstract: Quantum memories are essential for quantum repeaters that will form the backbone of the future quantum internet. Such memory can capture a signal state for a controllable amount of time after which this state can be retrieved. In this work, we theoretically investigated how atomic material and engineering parameters affect the performance and bandwidth of a quantum memory. We have developed a theoretical model for quantum memory operation based on the Lindblad master equation and adiabatic quantum state manipulation. The material properties and their uncertainty are evaluated to determine the performance of Raman-type quantum memories based on defects in two-dimensional hexagonal boron nitride (hBN). We derived a scheme to calculate the signal bandwidth based on the material parameters as well as the maximum efficiency that can be realized. The bandwidth depends on four factors: the signal photon frequency, the dipole transition moments in the electronic structure, cavity volume, and the strength of the external control electric field. As our scheme is general, it can be applied to many other quantum materials with a suitable level structure. We therefore provided a promising route for designing and selecting materials for quantum memories. Our work is therefore an important step toward the realization of a large-scale quantum network.

Abstract: Quantum process tomography (QPT) methods aim at identifying a given quantum process. The present paper focuses on the estimation of a unitary process. This class is of particular interest because quantum mechanics postulates that the evolution of any closed quantum system is described by a unitary transformation. The standard approach of QTP is to measure copies of a particular set of predetermined (generally pure) states after they have been modified by the process to be identified. The main problem with this setup is that preparing an input state and setting it precisely to a predetermined value is challenging and thus yields errors. These errors can be decomposed into a sum of centred errors (i.e. whose average on all the copies is zero) and systematic errors that are the same on all the copies, the latter is often the main source of error in QPT. The algorithm we introduce in the current paper works for any input states that make QPT theoretically possible. The fact that we do not require the input states to be precisely set to predetermined values means that we can use a trick to remove the issue of systematic errors by considering that some states are unknown but measured before they go through the process to be identified. We achieve this by splitting the copies of each input state into several groups and measuring the copies of the $k$-th group after they have successively been transferred through $k$ instances of the process to be identified (each copy of each input state is only measured once). Using this trick we can compute estimates of the measured states before and after they go through the process without using the knowledge we might have on the initial states. We test our algorithm both on simulated data and experimentally to identify a CNOT gate on a trapped-ions qubit quantum computer.

Abstract: Phantom relaxation is relaxation with a rate that is not given by a finite spectral gap. Studying average purity dynamics in a staircase random circuit and the spectral decomposition of a matrix describing underlying Markovian evolution, we explain how that can arise out of an ordinary-looking spectrum. Crucial are alternating expansion coefficients that diverge in the thermodynamic limit due to the non-Hermitian skin effect. The mysterious phantom relaxation emerges out of localized generalized eigenvectors describing Jordan normal form kernel, and, independently, also out of localized true eigenvectors involving interesting trigonometric sums. All this shows that when dealing with non-Hermitian matrices it can happen that the spectrum is not the relevant object; rather, it is the eigenvectors, or, equivalently, the pseudospectrum.

Abstract: Coulomb crystals of cold trapped ions are a leading platform for the realisation of quantum processors and quantum simulations and, in quantum metrology, for the construction of optical atomic clocks and for fundamental tests of the Standard Model. For these applications, it is not only essential to cool the ion crystal in all its degrees of freedom down to the quantum ground state, but also to be able to determine its temperature with a high accuracy. However, when a large ground-state cooled crystal is interrogated for thermometry, complex many-body interactions take place, making it challenging to accurately estimate the temperature with established techniques. In this work we present a new thermometry method tailored for ion crystals. The method is applicable to all normal modes of motion and does not suffer from a computational bottleneck when applied to large ion crystals. We test the temperature estimate with two experiments, namely with a 1D linear chain of 4 ions and a 2D crystal of 19 ions and verify the results, where possible, using other methods. The results show that the new method is an accurate and efficient tool for thermometry of ion crystals.

Abstract: In this article a study was made of the conditions under which a Hamiltonian which is an element of the complex $ \left\{ h (1) \oplus h(1) \right\} \uplus u(2) $ Lie algebra admits ladder operators which are also elements of this algebra. The algebra eigenstates of the lowering operator constructed in this way are computed and from them both the energy spectrum and the energy eigenstates of this Hamiltonian are generated in the usual way with the help of the corresponding raising operator. Thus, several families of generalized Hamiltonian systems are found, which, under a suitable similarity transformation, reduce to a basic set of systems, among which we find the 1:1, 2:1, 1:2, $su(2)$ and some other non-commensurate and commensurate anisotropic 2D quantum oscillator systems. Explicit expressions for the normalized eigenstates of the Hamiltonian and its associated lowering operator are given, which show the classical structure of two-mode separable and non-separable generalized coherent and squeezed states. Finally, based on all the above results, a proposal for new ladder operators for the $p:q$ coprime commensurate anisotropic quantum oscillator is made, which leads us to a class of Chen $SU(2)$ coherent states.

Abstract: Measurement incompatibility and the negativity of quasiprobability distribution functions are well-known non-classical aspects of quantum systems. Both of them are widely accepted resources in quantum information processing. We acquaint an approach to establish a connection between the negativity of the Wigner function, a well-known phase-space quasiprobability distribution, of finite-dimensional Hermitian operators and incompatibility among them. We calculate the negativity of the Wigner distribution function for noisy eigenprojectors of qubit Pauli operators as a function of the noise and observe that the amount of negativity increases with the decrease in noise vis-\`a-vis the increase in the incompatibility. It becomes maximum for the set of maximally unbiased operators. Our results, although qualitatively, provide a direct comparison between relative degrees of incompatibility among a set of operators for different amounts of noise. We generalize our treatment for higher dimensional qudits for specific finite-dimensional Gell-Mann operators to observe that with an increase in the dimension of the operators, the negativity of their Wigner distribution, and hence incompatibility, decreases.

Abstract: The propagation of general electronic quantum states provides information of the interaction of molecular systems with external driving fields. These can also offer understandings regarding non-adiabatic quantum phenomena. Well established methods focus mainly on propagating a quantum system that is initially described exclusively by the ground state wavefunction. In this work, we expand a previously developed formalism within coupled cluster theory, called second response theory, so it propagates quantum systems that are initially described by a general linear combination of different states, which can include the ground state, and show how with a special set of time-dependent cluster operators such propagations are performed. Our theory shows strong consistency with numerically exact results for the determination of quantum mechanical observables, probabilities, and coherences. We discuss unperturbed non-stationary states within second response theory and their ability to predict matrix elements that agree with those found in linear and quadratic response theories. This work also discusses an approximate regularized methodology to treat systems with potential instabilities in their ground-state cluster amplitudes, and compare such approximations with respect to reference results from standard unitary theory.

Abstract: We discuss the possibility of localizing an electron in a highly excited Rydberg state. The second-order correlation of emitted photons is the tool for the determination of electron position. This second-order correlation of emitted radiation and, therefore, the correlation of operators describing the acceleration of the electron allows for a partial localization of the electron in its orbit. The correlation function is found by approximating the transition matrix elements by their values in the classical limit. It is shown that the second-order correlation, depending on two times, is a function of the time difference and is a periodic function of this argument with the period equal to the period of the corresponding classical motion. The function has sharp maxima corresponding to large electron acceleration in the vicinity of the ``perihelion.'' This allows the localization of the electron in its consecutive approach to the perihelion point.