Collective Quantum States Research Articles

Exploring heavy fermions from macroscopic to microscopic length scales

Strongly correlated systems present fundamental challenges, especially in materials in which electronic correlations cause a strong increase of the effective mass of the charge carriers. Heavy fermion metals — intermetallic compounds of rare earth metals (such as Ce, Sm and Yb) and actinides (such as U, Np and Pu) — are prototype systems for complex and collective quantum states; they exhibit both a lattice Kondo effect and antiferromagnetic correlations. These materials show unexpected phenomena; for example, they display unconventional superconductivity (beyond Bardeen–Cooper–Schrieffer (BCS) theory) and unconventional quantum criticality (beyond the Landau framework). In this Review, we focus on systems in which Landau's Fermi-liquid theory does not apply. Heavy fermion metals and semiconductors are well suited for the study of strong electronic correlations, because the relevant energy scales (for charge carriers, magnetic excitations and lattice dynamics) are well separated from each other, allowing the exploration of concomitant physical phenomena almost independently. Thus, the study of these materials also provides valuable insight for the understanding — and tailoring — of other correlated systems. Heavy fermion systems are ideally suited to study strong electronic correlations. These fascinating materials are characterized by a clear separation of the relevant energy scales and may exhibit quantum critical points, non-Fermi-liquid behaviour and unconventional superconductivity coexisting or competing with magnetism.

Magnon BEC Versus Atomic BEC

The Bose–Einstein condensation (BEC) corresponds to the formation of a collective quantum state in which macroscopic number of particles is governed by a single wave function. The magnon BEC, the coherent state of a macroscopic number of magnons, manifests itself by coherent precession of magnetization even in an inhomogeneous magnetic field. The magnon BEC has many similarities with the atomic BEC. But it shows a great variety of properties in different magnetic materials. Here we will classify the different types of magnonic BECs.

Noise-induced collective quantum state preservation in spin qubit arrays

The hyperfine interaction with nuclear spins (or, Overhauser noise) has long been viewed as a leading source of decoherence in individual quantum dot spin qubits. Here we show that in a coupled multi-qubit system consisting of as few as four spins, interactions with nuclear spins can have the opposite effect where they instead preserve the collective quantum state of the system. This noise-induced state preservation can be realized in a linear spin qubit array using current technological capabilities. Our proposal requires no control over the Overhauser fields in the array; only experimental control over the average interqubit coupling between nearest neighbors is needed, and this is readily achieved by tuning gate voltages. Our results illustrate how the role of the environment can transform from harmful to helpful in the progression from single-qubit to multi-qubit quantum systems.

Manipulation of collective quantum states in Bose-Einstein condensates by continuous imaging

We develop a Gaussian state treatment that allows a transparent quantum description of the continuous, nondestructive imaging of and feedback on a Bose-Einstein condensate. We have previously demonstrated [Phys. Rev. Lett. \textbf{115}, 060401 (2015)] that the measurement back action of stroboscopic imaging leads to selective squeezing and entanglement of quantized density oscillations. Here, we investigate how the squeezing and entanglement are affected by the finite spatial resolution and geometry of the probe laser beam and of the detector and how they can be optimized.

Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.

Reducing collective quantum state rotation errors with reversible dephasing

We demonstrate that reversible dephasing via inhomogeneous broadening can greatly reduce collective quantum state rotation errors, and observe the suppression of rotation errors by more than 21 dB in the context of collective population measurements of the spin states of an ensemble of 2.1×105 laser cooled and trapped 87Rb atoms. The large reduction in rotation noise enables direct resolution of spin state populations 13(1) dB below the fundamental quantum projection noise limit. Further, the spin state measurement projects the system into an entangled state with 9.5(5) dB of directly observed spectroscopic enhancement (squeezing) relative to the standard quantum limit, whereas no enhancement would have been obtained without the suppression of rotation errors.

Bose-Einstein condensation in antiferromagnets at low temperatures

The Bose-Einstein condensation (BEC) was predicted by Einstein in 1925 and this effect is characterized by the formation of a collective quantum state, when macroscopic number of particles is governed by a single wave function. The BEC of magnons was discovered experimentally in superfluid phase of 3He. In the present work we report our progress on the BEC of magnons investigations in solid antiferromagnets at low temperatures by magnetic resonance methods. The duration of the FID signal in two samples of easy-plane antiferromagnets CsMnF3 has been studied. Obtained data confirm the formation of magnon BEC in antiferromagnet CsMnF3.

Collective excitation of Rydberg-atom ensembles beyond the superatom model.

In an ensemble of laser-driven atoms involving strongly interacting Rydberg states, the steady-state excitation probability is usually substantially suppressed. In contrast, here we identify a regime in which the Rydberg excited fraction is enhanced by the interaction. This effect is associated with the buildup of many-body coherences induced by coherent multiphoton excitations between collective states. The excitation enhancement should be observable under currently existing experimental conditions and may serve as a direct probe for the presence of coherent multiphoton dynamics involving collective quantum states.

Coexistence and competition of multiple charge-density-wave orders in rare-earth tritellurides

The occurrences of collective quantum states, such as superconductivity (SC) and charge- or spin-densitywaves (CDWs or SDWs), are among the most fascinating phenomena in solids. To date much effort has been made to explore the interplay between different orders, yet little is known about the relationship of multiple orders of the same type. Here we report optical spectroscopy study on CDWs in the rare-earth tri-telluride compounds RTe3 (R = rare earth elements). Besides the prior reported two CDW orders, the study reveals unexpectedly the presence of a third CDW order in the series which evolves systematically with the size of R element. With increased chemical pressure, the first and third CDW orders are both substantially suppressed and compete with the second one by depleting the low energy spectral weight. A complete phase diagram for the multiple CDW orders in this series is established.

Creation of collective many-body states and single photons from two-dimensional Rydberg lattice gases

The creation of collective many-body quantum states from a two-dimensional lattice gas of atoms is studied. Our approach relies on the van der Waals interaction that is present between alkali metal atoms which are laser excited to high-lying Rydberg s-states. We focus on a regime in which the laser driving is strong compared to the interaction between Rydberg atoms. Here energetically low-lying many-particle states can be calculated approximately from a quadratic Hamiltonian. The potential usefulness of these states as a resource for the creation of deterministic single-photon sources is illustrated. The properties of these photon states are determined from the interplay between the particular geometry of the lattice and the interatomic spacing.

Quasiclassical asymptotics and coherent states for bounded discrete spectra

We consider discrete spectra of bound states for nonrelativistic motion in attractive potentials \documentclass[12pt]{minimal}\begin{document}$V_{\sigma }(x) = -|V_{0}|\, |x|^{-\sigma },\, 0<\sigma \le 2$\end{document}Vσ(x)=−|V0||x|−σ,0<σ≤2. For these potentials the quasiclassical approximation for n → ∞ predicts quantized energy levels \documentclass[12pt]{minimal}\begin{document}$e_{\sigma }(n)$\end{document}eσ(n) of a bounded spectrum varying as \documentclass[12pt]{minimal}\begin{document}$e_{\sigma }(n) \sim -n^{-2\sigma /(2-\sigma )}$\end{document}eσ(n)∼−n−2σ/(2−σ). We construct collective quantum states using the set of wavefunctions of the discrete spectrum assuming this asymptotic behavior. We give examples of states that are normalizable and satisfy the resolution of unity, using explicit positive functions. These are coherent states in the sense of Klauder and their completeness is achieved via exact solutions of Hausdorff moment problems, obtained by combining Laplace and Mellin transform methods. For σ in the range 0 < σ ⩽ 2/3 we present exact implementations of such states for the parametrization σ = 2(k − l)/(3k − l) with k and l positive integers satisfying \documentclass[12pt]{minimal}\begin{document}$k>l$\end{document}k>l.

Generation of coherent states of photon-added type via pathway of eigenfunctions

We obtain and investigate the regular eigenfunctions of simple differential operators xr dr + 1/dxr + 1, r = 1, 2, …, with the eigenvalues equal to 1. With the help of these eigenfunctions, we construct a non-unitary analogue of a boson displacement operator which will be acting on the vacuum. In this way, we generate collective quantum states of the Fock space which are normalized and equipped with the resolution of unity with the positive weight functions that we obtain explicitly. These states are thus coherent states in the sense of Klauder. They span the truncated Fock space without first r lowest-lying basis states: |0⟩, |1⟩, …, |r − 1⟩. These states are squeezed, sub-Poissonian in nature and reminiscent of photon-added states in Agarwal and Tara (1991 Phys. Rev. A 43 492).

Bose-Einstein Condensation in Microgravity

Albert Einstein's insight that it is impossible to distinguish a local experiment in a "freely falling elevator" from one in free space led to the development of the theory of general relativity. The wave nature of matter manifests itself in a striking way in Bose-Einstein condensates, where millions of atoms lose their identity and can be described by a single macroscopic wave function. We combine these two topics and report the preparation and observation of a Bose-Einstein condensate during free fall in a 146-meter-tall evacuated drop tower. During the expansion over 1 second, the atoms form a giant coherent matter wave that is delocalized on a millimeter scale, which represents a promising source for matter-wave interferometry to test the universality of free fall with quantum matter.

Electrons Acquire a Split Personality in Bismuth

Unusual collective quantum states seen only in two-dimensional layers of electrons may have now been observed in a three-dimensional metal.

Light-shift imbalance induced blockade of collective excitations beyond the lowest order

Current proposals focusing on neutral atoms for quantum computing are mostly based on using single atoms as quantum bits (qubits), while using cavity induced coupling or dipole–dipole interaction for two-qubit operations. An alternative approach is to use atomic ensembles as quantum bits. However, when an atomic ensemble is excited, by a laser beam matched to a two-level transition (or a Raman transition) for example, it leads to a cascade of many states as more and more photons are absorbed [R.H. Dicke, Phys. Rev. 93 (1954) 99]. In order to make use of an ensemble as a qubit, it is necessary to disrupt this cascade, and restrict the excitation to the absorption (and emission) of a single photon only. Here, we show how this can be achieved by using a new type of blockade mechanism, based on the light-shift imbalance (LSI) in a Raman transition. We describe first a simple example illustrating the concept of light-shift imbalance induced blockade (LSIIB) using a multi-level structure in a single atom, and show verifications of the analytic prediction using numerical simulations. We then extend this model to show how a blockade can be realized by using LSI in the excitation of an ensemble. Specifically, we show how the LSIIB process enables one to treat the ensemble as a two-level atom that undergoes fully deterministic Rabi oscillations between two collective quantum states, while suppressing excitations of higher order collective states.

Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping

Bose-Einstein condensation is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve Bose-Einstein condensation, one can either decrease the temperature or increase the density of bosons. It has been predicted that a quasi-equilibrium system of bosons could undergo Bose-Einstein condensation even at relatively high temperatures, if the flow rate of energy pumped into the system exceeds a critical value. Here we report the observation of Bose-Einstein condensation in a gas of magnons at room temperature. Magnons are the quanta of magnetic excitations in a magnetically ordered ensemble of magnetic moments. In thermal equilibrium, they can be described by Bose-Einstein statistics with zero chemical potential and a temperature-dependent density. In the experiments presented here, we show that by using a technique of microwave pumping it is possible to excite additional magnons and to create a gas of quasi-equilibrium magnons with a non-zero chemical potential. With increasing pumping intensity, the chemical potential reaches the energy of the lowest magnon state, and a Bose condensate of magnons is formed.

A Quantum Critical Route to Field-Induced Superconductivity

Conventional phase transitions, such as ice melting to water or ferromagnetic iron losing its strong intrinsic magnetization, are driven by thermal effects. A different class of transitions, called quantum phase transitions, can occur at absolute zero, driven by quantum fluctuations. In their Perspective, [Mackenzie and Grigera][1] discuss results reported in the same issue by [ LA©vy et al. ][2] in which quantum critical phenomena are found to play a role in a surprising new kind of superconductivity. At low magnetic fields, the material URhGe is slightly superconducting. Increasing the field strength destroys the superconductivity, which then returns in even stronger form at very high magnetic field. As a result of this and related experiments, quantum criticality is becoming recognized as providing a new framework for discovering exotic superconductivity and other collective quantum states. [1]: http://www.sciencemag.org/cgi/content/full/309/5739/1330 [2]: http://www.sciencemag.org/cgi/content/short/309/5739/1343

Extracting atoms on demand with lasers

We propose a scheme that allows us to coherently extract cold atoms from a reservoir in a deterministic way. The transfer is achieved by means of radiation pulses coupling two atomic states which are object to different trapping conditions. A particular realization is proposed, where one state has zero magnetic moment and is confined by a dipole trap, whereas the other state with nonvanishing magnetic moment is confined by a steep microtrap potential. We show that in this setup a predetermined number of atoms can be transferred from a reservoir, a Bose-Einstein condensate, into the collective quantum state of the steep trap with high efficiency in the parameter regime of present experiments.

Multimode entanglement of light and atomic ensembles via off-resonant coherent forward scattering

Quantum theoretical treatment of coherent forward scattering of light in a polarized atomic ensemble with an arbitrary angular momentum is developed. We consider coherent forward scattering of a weak radiation field interacting with a realistic multilevel atomic transition. Based on the concept of an effective Hamiltonian and on the Heisenberg formalism, we discuss the coupled dynamics of the quantum fluctuations of the polarization Stokes components of propagating light and of the collective spin fluctuations of the scattering atoms. We show that in the process of coherent forward scattering, this dynamics can be described in terms of a polariton-type spin wave created in the atomic sample. Our work presents a general example of an entangling process in the system of collective quantum states of light and atomic angular momenta, previously considered only for the case of spin-$\frac{1}{2}$ atoms. We use the developed general formalism to test the applicability of the spin-$\frac{1}{2}$ approximation for modeling the quantum nondemolishing measurement of atoms with a higher angular momentum.

Non-classical light generated by a quantum dot: multi-color photons with tunable statistics

We report the intensity and polarization correlations of two consecutively emitted photons from an optically excited single semiconductor quantum dot (SCQD). We show that a quantum dot is a source of non-classically correlated photons of various wavelengths, emitted due to the recombination of electron–hole (e–h) pairs from different collective many-carrier quantum states confined in the dot. We show that there are excitation power dependent intensity and polarization correlations among photons emitted at the same (“auto-correlations”), and at different wavelengths (“cross-correlations”). We observed a preferentially co-circularly-polarized cross-correlation at low excitation power, which is tentatively explained in terms of long electronic spin memory.