Physical Review A 50^th Anniversary Milestones

Surface codes: Towards practical large-scale quantum computation

It is well known that lowering the error thresholds for fault-tolerant quantum operations is an essential task needed to build a fault-tolerant quantum computer. The surface code approach based on a large number of identical physical qubits connected in a rectangular grid is less stringent than that of other quantum computational approaches. In this paper, the authors introduce and provide a detailed description of the concept. Further they offer a preliminary cost estimation for surface code quantum computation.

Theory of resonance fluorescence

In this paper, Kimble and Mandel treat the interaction of a two-level atom with a near-resonant quantized electromagnetic field without any of the common perturbative or semiclassical approximations. They present the general solution for the time evolution of the fluorescent light signal for an arbitrary initial state of the atom and an arbitrary initial coherent state of the field. As a result, they established new experimental approaches, particularly exploring two-time correlation functions, to investigate the quantum nature of the electromagnetic field.

Three qubits can be entangled in two inequivalent ways

Apart from being of fundamental interest in itself, entanglement is a crucial resource for quantum information processing. In this paper, Duer and collaborators consider genuinely multipartite entangled states of three qubits, i.e., that cannot be factorized across any bipartition. They show that these states fall into one of exactly two classes; states in the same class can be converted into each other by means of local operations and classical communication, but states belonging to different classes can not. The well-known GHZ and W states are shown to belong to different classes, and hence to be genuinely inequivalent.

Optimized effective atomic central potential

In this work, Talman and Shadwick introduce the optimized effective potential method to solve the Hartree-Fock equation. Since its introduction, the method has found unexpected success in the development of density functional theory. It has become a cornerstone of orbital-dependent functionals, one of the most promising avenues in modern density-functional theory.

Contextuality for preparations, transformations, and unsharp measurements

Since the introduction of the Bell-Kochen-Specker theorem, the study of contextuality has attracted significant attention in quantum foundations. In this paper, Robert W. Spekkens introduces an operational definition of contextuality of an experimental context and develops three no-go theorems for ontological models. These contributions could one day allow experimental tests of contextuality and make an impact on quantum information processing protocols.

Quantum Defect Theory of $l$ Uncoupling in ${\mathrm{H}}_2$ as an Example of Channel-Interaction Treatment

In this paper, Ugo Fano develops a multichannel quantum defect theory and applies it to the analysis of hydrogen molecular spectra. Since then, this important framework has been extended to analyze resonance structures, atomic collisions and recombinations, and other atomic systems.

Quantum memory for photons: Dark-state polaritons

Many quantum technologies rely on photons to carry quantum information from one place to another and on atoms to store and process the information, creating the need to transfer the information from one medium to the other in a way that fully preserves coherence. In this paper and an accompanying Physical Review Letter, Fleischhauer and Lukin showed how photons can be stopped and stored in an atomic vapor using the phenomenon of electromagnetically induced transparency. Since then, the storage and retrieval of light has been achieved in a variety of systems using the method described in this paper.

See also the PRA behind the research webinar series: Video Recording.

Laser cooling of atoms

The burgeoning field of ultracold atom research started from the technique of laser cooling of neutral atoms. The two papers by Wineland and Itano, and Gordon and Ashkin, belong to the early literature that proposed and analyzed Doppler cooling. This groundbreaking technique allows cooling neutral atoms to sub milliKelvin for the first time and laid the foundation for the later development of other cooling techniques.

The first paper was cited as an important contribution that leads to the 2012 Nobel Prize in Physics while the second paper contributed to Ashkin’s 2018 Nobel Prize in Physics.

Elementary gates for quantum computation

Nowadays quantum gates exist on the cloud, but back in 1995 scientists were discussing what set of gates enabled universal quantum computation. Barenco and collaborators showed that the combination of classical two-bit gate with quantum one-bit gates are universal, and derived bounds on the number of elementary gates required to construct several two- and three-bit quantum gates.

Probing many-body states of ultracold atoms via noise correlations

The authors propose the use of density-density correlations to probe many-body states of trapped ultracold atoms. With the help of recent development of quantum gas microscopes, this idea has been successfully employed to detect antiferromagnetic states in Fermi gases, Kosterlitz-Thouless transition in Bose gases, and many more intriguing phenomena in strongly correlated systems.

Squeezed spin states

For a long time, squeezing was the quintessential property of a quantum system, but the concept was mainly restricted to radiation fields. Kitagawa and Ueda generalized the formalism to the case of spin systems and provided a recipe for experimental manipulation, making squeezing available into the realm of atomic systems and opening the way for applications in metrology.

Quantum computation with quantum dots

In this paper, Loss and DiVincenzo laid out a proposal for quantum computation based on quantum dots. A detailed implementation of a universal set of one- and two-quantum-bit gates using the spin states of coupled single-electron quantum dots is presented. Following the proposal, significant theoretical and experimental achievements have made quantum dots another candidate platform for quantum computation.

Quantum dynamics of an atomic Bose-Einstein condensate in a double-well potential

The authors study the quantum dynamics of a Bose-Einstein condensate in a double-well potential and predict the collapse and revival of the matter-wave coherence as a result of two-body interaction between atoms. This important feature is subsequently observed in several experiments and finds important applications in matter-wave interferometry.

Atomic Coherent States in Quantum Optics

Nowadays, atomic coherent states are part of the toolbox explained in any quantum optics textbook. Introduced by Arecchi and collaborators in 1972, such states describe the coherence properties of atoms in a laser cavity and have properties similar to optical coherent states.

Measuring the scrambling of quantum information

Out-of-time-order correlation functions have been suggested as powerful tools for characterizing the scrambling of information. Therefore, accessing them experimentally is a subject that spurred a wave of research across condensed-matter and high-energy fields. Brian Swingle and collaborators proposed one of the first practical measuring protocols and detailed an implementation based on a platform of cold atoms.

Stimulated Emission and Absorption near Resonance for Driven Systems

The Mollow triplet corresponds to the three peaks of the resonance fluorescence spectrum of a single two-level atom. It is considered the most fundamental spectral lines of quantum optics. Focusing on the absorption spectrum, this paper belongs to a collection of early work by B. R. Mollow explaining the mechanism of the Mollow triplet.

Ghost imaging with a single detector

The quantum nature of light has been the source of many twists and surprises. In the case of ghost imaging, it has surely tricked many scientists to believe that the effect had a quantum nature. After a decade of controversies, a beautiful experiment by Silberberg and collaborators settles the dispute: there is no need to invoke nonlocal quantum correlations.

Measurement-based quantum computation on cluster states

Raussendorf and collaborators explore entangled cluster states in a novel quantum computational model based on single qubit measurements. In the paradigm of one way quantum computation, the correlations arising in a sequence of measurements drive the computation.

Realistic Rashba and Dresselhaus spin-orbit coupling for neutral atoms

In this paper, the authors propose a class of generalized Raman schemes to realize spin-orbit-coupled Hamiltonians for ultracold neutral atoms in two dimensions. The idea has inspired several successful experimental realizations in both ultracold Bose and Fermi gases, paving the way for exploring exotic quantum phases in a new platform.

Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes

It started as a fundamental investigation on the properties of laser light: a Gaussian mode is proven to carry orbital angular momentum (OAM). In 1992, Allen and co-authors provided a recipe for creating higher-order Hermite modes by using standard cylindrical lenses. Since then, applications of light beams carrying OAM have grown exponentially.

Computable measure of entanglement

Witnessing and quantifying quantum correlations have been in the focus of intense research for several years. Vidal and Werner pinpointed the intricacies of mixed bipartite states, offering an entanglement measure that is both operationally simple and useful for assessing protocols such as distillation and teleportation.

Theory of high-harmonic generation by low-frequency laser fields

In this paper, the authors develop one of the most popular models for the exploration of high-harmonic generation in terms of a three-step picture involving ionization, propagation, and rescattering. The framework, now known as the Lewenstein model, provides the foundation for much of the current research in strong-field interaction and attosecond physics.

See also the PRA behind the research webinar series: Video Recording

Two-photon coherent states of the radiation field

Squeezed light is a special state of light where the noise caused by quantum effects has been reduced, or “squeezed out.” In this paper, Horace P. Yuen details the properties of a two-photon coherent state, now better known as a squeezed state of light, and lays the groundwork for its potential applications in precision measurements and optical communications that require a low signal-to-noise ratio.

Scheme for reducing decoherence in quantum computer memory

One of the biggest challenges in the current race to design quantum computers is how to deal with unavoidable errors. In this landmark paper, Peter Shor shows how to circumvent the problem by using a smart error-correcting code. Since its publication, quantum error correction has developed into a burgeoning field with several promising strategies, based on Shor’s original foresight.

See Physics article: Focus: Landmarks—Correcting Quantum Computer Errors

Density-functional exchange-energy approximation with correct asymptotic behavior

In this paper, Axel D. Becke introduces a gradient-corrected exchange-energy functional that reproduces the exact asymptotic behavior of finite many-electron systems. This functional marks an important development in density functional theory with far-reaching applications in atomic and condensed matter physics, chemistry, and materials science.