We propose a novel type of composite light-matter interferometer based on a supersolid-like phase of a driven Bose-Einstein condensate coupled to a pair of degenerate counterpropagating modes of an optical ring cavity. The supersolid-like condensate under the influence of the gravity drags the cavity optical potential with itself, thereby changing the relative phase of the two cavity modes. Monitoring the phase evolution of the cavity output fields thus allows for a nondestructive measurement of the gravitational acceleration. We show that the sensitivity of the proposed gravimeter exhibits Heisenberg-like scaling with respect to the atom number. As the relative phase of the cavity modes is insensitive to photon losses, the gravimeter is robust against these deleterious effects. For state-of-the-art experimental parameters, the sensitivity of such a gravimeter could be of the order of $10^{-10}$--$10^{-8}$ for a condensate of a half a million atoms with long measurement times.

Symmetry plays an important role in the field of quantum mechanics. In this paper, we consider a subclass of symmetric quantum states in the multipartite system $N^{\otimes d}$, namely, the completely symmetric states, which are invariant under any index permutation. It was conjectured by L. Qian and D. Chu [arXiv:1810.03125 [quant-ph]] that the completely symmetric states are separable if and only if it is a convex combination of symmetric pure product states. In this paper, we proved that this conjecture is true for both bipartite and multipartite cases. And we proved the completely symmetric state $\rho$ is separable if its rank is at most $5$ or $N+1$. For the states of rank

$6$ or $N+2$, they are separable if and only if their range contains a product vector. We apply our results to a few widely useful states in quantum information, such as symmetric states, edge states, extreme states, and nonnegative states. We also study the relation of CS states to Hankel and Toeplitz matrices.

Great efforts are currently devoted to the engineering of topological Bloch bands in ultracold atomic gases. Recent achievements in this direction, together with the possibility of tuning inter-particle interactions, suggest that strongly-correlated states reminiscent of fractional quantum Hall (FQH) liquids could soon be generated in these systems. In this experimental framework, where transport measurements are limited, identifying unambiguous signatures of FQH-type states constitutes a challenge on its own. Here, we demonstrate that the fractional nature of the quantized Hall conductance, a fundamental characteristic of FQH states, could be detected in ultracold gases through a circular-dichroic measurement, namely, by monitoring the energy absorbed by the atomic cloud upon a circular drive. We validate this approach by comparing the circular-dichroic signal to the many-body Chern number, and discuss how such measurements could be performed to distinguish FQH-type states from competing states. Our scheme offers a practical tool for the detection of topologically-ordered states in quantum-engineered systems, with potential applications in solid state.

In this contribution we determine the exact solution for the ground-state wave function of a two-particle correlated model atom with harmonic interactions. From that wave function, the nonidempotent one-particle reduced density matrix is deduced. Its diagonal gives the exact probability density, the basic variable of Density-Functional Theory. The one-matrix is directly decomposed, in a point-wise manner, in terms of natural orbitals and their occupation numbers, i.e., in terms of its eigenvalues and normalized eigenfunctions. The exact informations are used to fix three, approximate, independent-particle models. Next, a time-dependent external field of finite duration is added to the exact and approximate Hamiltonians and the resulting Cauchy problem is solved. The impact of the external field is investigated by calculating the energy shift generated by that time-dependent field. It is found that the nonperturbative energy shift reflects the sign of the driving field. The exact probability density and current are used, as inputs, to investigate the capability of a formally exact independent-particle modeling in time-dependent DFT as well. The results for the observable energy shift are analyzed by using realistic estimations for the parameters of the two-particle target and the external field. A comparison with the experimental prediction on the sign-dependent energy loss of swift protons and antiprotons in a gaseous He target is made.

This is an analysis of the recently published article `Quantum theory cannot consistently describe the use of itself' by D. Frauchiger and R. Renner~\cite{1}. Here I decipher the paradox and analyze it from the point of view of de Broglie-Bohm hidden variable theory (i.e., Bohmian mechanics). I also analyze the problem from the perspective obtained by the Copenhagen interpretation (i.e., the Bohrian interpretation) and show that both views are self consistent and do not lead to any contradiction with a `single-world' description of quantum theory.

Author(s): Wei Zhang, Ming-Xin Dong, Dong-Sheng Ding, Shuai Shi, Kai Wang, Zhi-Yuan Zhou, Guang-Can Guo, and Bao-Sen Shi

Multiphoton entangled states play a crucial role in quantum information applications such as secure quantum communication, scalable computation, and high-precision quantum metrology. Quantum memory for entangled states is a key component of quantum repeaters, which are indispensable in realizing qua...

[Phys. Rev. A 98, 063820] Published Thu Dec 13, 2018

Author(s): Andrey B. Matsko and Sergey P. Vyatchanin

We find that the measurement sensitivity of an optical integrating gyroscope is fundamentally limited due to ponderomotive action of the light leading to the standard quantum limit of the rotation angle detection. The uncorrelated quantum fluctuations of power of clockwise and counterclockwise elect...

[Phys. Rev. A 98, 063821] Published Thu Dec 13, 2018

Author(s): Wen-Hao Zhang, Geng Chen, Xing-Xiang Peng, Xiang-Jun Ye, Peng Yin, Ya Xiao, Zhi-Bo Hou, Ze-Di Cheng, Yu-Chun Wu, Jin-Shi Xu, Chuan-Feng Li, and Guang-Can Guo

Self-testing is a method with which a classical user can certify the state and measurements of quantum systems in a device-independent way. In particular, self-testing of entangled states is of great importance in quantum information processing. An understandable example is that the maximal violatio...

[Phys. Rev. Lett. 121, 240402] Published Thu Dec 13, 2018

Author(s): Sushant Saryal, Juliane U. Klamser, Tridib Sadhu, and Deepak Dhar

There is a misconception, widely shared among physicists, that the equilibrium free energy of a one-dimensional classical model with strictly finite-ranged interactions, and at nonzero temperatures, cannot show any singularities as a function of the coupling constants. In this Letter, we discuss an ...

[Phys. Rev. Lett. 121, 240601] Published Thu Dec 13, 2018

Researchers can change the shape of a liquid drop by placing it between two stretched elastic films, allowing the drop to be used as a tiny adjustable lens.

[Physics] Published Thu Dec 13, 2018

Categories: Physics

The implementation of minimum length in quantum mechanics (QM) can be done either by modification of position and momentum operators or by restriction of their domains. In the former case the resulting classical dynamics is drastically different from the usual one. Starting with the latter possibility, we propose a non-local modification of QM. It has close ties to the band-limited QM, but in contrast to it one can easily work out the corrections to various processes and discuss further the semi-classical limit of the theory. Surprisingly enough, the classical limit proves again to be unacceptably altered. In the last section a further modification is suggested to alleviate this problem.

Exploiting novel aspects of the quantum geometry of charged particles in a magnetic field via gauge-invariant variables, we provide tangible connections between the response of quantum Hall fluids to non-uniform electric fields and the characteristic geometry of electronic motion in the presence of magnetic and electric fields. The geometric picture we provide motivates the following conjecture: non-uniform electric fields mimic the presence of spatial curvature. Consequently, the gravitational coupling constant also appears in the charge response to non-uniform electric fields.

Tensor network states and methods have erupted in recent years. Originally developed in the context of condensed matter physics and based on renormalization group ideas, tensor networks lived a revival thanks to quantum information theory and the understanding of entanglement in quantum many-body systems. Moreover, it has been not-so-long realized that tensor network states play a key role in other scientific disciplines, such as quantum gravity and artificial intelligence. In this context, here we provide an overview of basic concepts and key developments in the field. In particular, we briefly discuss the most important tensor network structures and algorithms, together with a sketch on advances related to global and gauge symmetries, fermions, topological order, classification of phases, entanglement Hamiltonians, AdS/CFT, artificial intelligence, the 2d Hubbard model, 2d quantum antiferromagnets, conformal field theory, quantum chemistry, disordered systems, and many-body localization.

Quantum coherence is the most distinguished signature of quantum mechanics, also recognized to be an essential resource for many promising quantum technologies, playing a central role in phenomena related to quantum information science, quantum thermodynamics and quantum biology, just to mention few examples. However, the resource theory of coherence is in the preliminary stage of its development, being still limited to the study of the manipulation and quantification of coherence, mostly from a theoretical viewpoint. Here, we propose an experimental method to directly measure the relative entropy of coherence, which according to the resource theory is one of the main quantifiers of coherence. To achieve this aim, we show how to measure the von Neumann entropy of a generic quantum state directly in terms of the Shannon entropy of the probability distribution of outcomes in a defined measurement basis. In both cases, by direct we mean that tomographic methods are not required. Two quantum-optical applications of our method are discussed in order to give support to our predictions.

We consider a class of holographic quantum error-correcting codes, built from perfect tensors in network configurations dual to Bruhat-Tits trees and their quotients by Schottky groups corresponding to BTZ black holes. The resulting holographic states can be constructed in the limit of infinite network size. We obtain a $p$-adic version of entropy which obeys a Ryu-Takayanagi like formula for bipartite entanglement of connected or disconnected regions, in both genus-zero and genus-one $p$-adic backgrounds, along with a Bekenstein-Hawking-type formula for black hole entropy. We prove entropy inequalities obeyed by such tensor networks, such as subadditivity, strong subadditivity, and monogamy of mutual information (which is always saturated). In addition, we construct infinite classes of perfect tensors directly from semiclassical states in phase spaces over finite fields, generalizing the CRSS algorithm, and give Hamiltonians exhibiting these as vacua.

The ability to completely characterize the state of a quantum system is an essential element for the emerging quantum technologies. Here, we present a compressed-sensing inspired method to ascertain any rank-deficient qudit state, which we experimentally encode in photonic orbital angular momentum. We efficiently reconstruct these qudit states from a few scans with an intensified CCD camera. Since it requires only a few intensity measurements, our technique would provide an easy and accurate way to identify quantum sources, channels, and systems.

Semi-quantum key distribution protocols are designed to allow two parties to establish a shared secret key, secure against an all-powerful adversary, even when one of the users is severely restricted in their quantum capabilities. While interesting from a theoretical standpoint, these protocols have the disadvantage that a two-way quantum communication channel is necessary which generally limits their theoretical efficiency and noise tolerance. In this paper, we construct a new semi-quantum key distribution (SQKD) protocol which actually takes advantage of this necessary two-way channel, and we show it is able to tolerate a channel noise level higher than any prior SQKD protocol to-date. We also compare the noise tolerance of our protocol to other two-way fully quantum protocols, along with BB84 with Classical Advantage Distillation (CAD). Here we discover that our new protocol's noise tolerance is higher than other two-way fully quantum protocols; it also compares favorably to BB84 with CAD (and we discover some interesting quantum-level differences between the two). Along the way, we develop techniques that can be applied to the security analysis of other (S)QKD protocols reliant on a two-way quantum communication channel.

Recent development of mixed-state encoding (MSE) allows pure-state logical information to be encoded by a bosonic (continuous-variable) system in mixed physical state. Despite interest due to its counter-intuitiveness, the utility of the current MSE scheme is limited due to several operational drawbacks, namely redundant information carrier, probabilistic initialisation, and requirement of discrete-variable measurement. In this work, we present a simplified MSE that does not suffer from any of these drawbacks. Specifically, our protocol encodes each qubit by only one mixed-state bosonic mode, and the logical basis can be deterministically initialised from thermal equilibrium. All logical operations of this encoding can be performed with continuous-variable interaction and measurement only. Without the necessity of ground state cooling, our proposal could broaden the set of candidate systems for implementing quantum computers, and reduce the reliance on demanding refrigerating facility for current quantum computing architectures. Additionally, our protocol can enhance the noise tolerance of logical qubit even if the system can be efficiently cooled.

The Quantum Approximate Optimization Algorithm, QAOA, uses a shallow depth quantum circuit to produce a parameter dependent state. For a given combinatorial optimization problem instance, the quantum expectation of the associated cost function is the parameter dependent objective function of the QAOA. We demonstrate that if the parameters are fixed and the instance comes from a reasonable distribution then the objective function value is concentrated in the sense that typical instances have (nearly) the same value of the objective function. This applies not just for optimal parameters as the whole landscape is instance independent. We can prove this is true for low depth quantum circuits for instances of MaxCut on large 3-regular graphs. Our results generalize beyond this example. We support the arguments with numerical examples that show remarkable concentration. For higher depth circuits the numerics also show concentration and we argue for this using the Law of Large Numbers. We also observe by simulation that if we find parameters which result in good performance at say 10 bits these same parameters result in good performance at say 24 bits. These findings suggest ways to run the QAOA that reduce or eliminate the use of the outer loop optimization and may allow us to find good solutions with fewer calls to the quantum computer.

A quantum emitter decays due to vacuum fluctuations at its transition frequency. By virtue of the entwined nature of dissipation and fluctuations, this process can be controlled by engineering the impedance of the environment. We study how the structured vacuum environment of a microwave photonic crystal can be used for bath engineering of a transmon qubit. The photonic crystal is realized by a step-impedance transmission line which suppresses and enhances the quantum spectral density of states akin to a Purcell filter. We demonstrate a bath engineering protocol upon driving an emitter near the photonic band edge that allows dissipation to produce non-trivial steady-states.