We consider some implications of the mass defect on the frequency of atomic transitions. We have found that some well-known frequency shifts (such as gravitational and quadratic Doppler shifts) can be interpreted as consequences of the mass defect, i.e., without the need for the concept of time dilation used in special and general relativity theories. Moreover, we show that the inclusion of the mass defect leads to previously unknown shifts for clocks based on trapped ions.

We introduce and experimentally characterize a superconducting single-sideband modulator compatible with cryogenic microwave circuits, and propose its use for frequency domain multiplexing of superconducting qubit readout. The monolithic single-quadrature modulators that comprise the device are formed with purely reactive elements (capacitors and Josephson junction inductors) and require no microwave-frequency control tones. Microwave signals in the 4 to 8 GHz band, with power up to -85 dBm, are converted up or down in frequency by as much as 120 MHz. Spurious harmonics in the device can be suppressed by up to 25 dB for select probe and modulation frequencies.

Bragg-reflection waveguides emitting broadband parametric down-conversion (PDC) have been proven to be well suited for the on-chip generation of polarization entanglement in a straightforward fashion [R. T. Horn et al., Sci. Rep. 3, 2314 (2013)]. Here, we investigate how the properties of the created states can be modified by controlling the relative temporal delay between the pair of photons created via PDC. Our results offer an easily accessible approach for changing the coherence of the polarization entanglement, in other words, to tune the phase of the off-diagonal elements of the density matrix. Furthermore, we provide valuable insight in the engineering of these states directly at the source.

The Salecker-Wigner-Peres (SWP) clock is often used to determine the duration a quantum particle is supposed to spend is a specified region of space $\Om$. By construction, the result is a real positive number, and the method seems to avoid the difficulty of introducing complex time parameters, which arises in the Feynman paths approach. However, it tells little about about the particle's motion. We investigate this matter further, and show that the SWP clock, like any other Larmor clock, correlates the rotation of its angular momentum with the durations, $\tau$, which the Feynman paths spend in $\Om$, thereby destroying interference between different durations. An inaccurate weakly coupled clock leaves the interference almost intact, and the need to resolve the resulting "which way?" problem is one of the main difficulties at the centre of the "tunnelling time" controversy. In the absence of a probability distribution for the values of $\tau$, the SWP results are expressed in terms of moduli of the "complex times", given by the weighted sums of the corresponding probability amplitudes. It is shown that over-interpretation of these results, by treating the SWP times as physical time intervals, leads to paradoxes and should be avoided. We also analyse various settings of the SWP clock, different calibration procedures, and the relation between the SWP results and the quantum dwell time. The cases of stationary tunnelling and tunnel ionisation are considered in some detail. Although our detailed analysis addresses only one particular definition of the duration of a tunnelling process, it also points towards the impossibility of uniting various time parameters, which may occur in quantum theory, within the concept of a single "tunnelling time".

A renormalization group flow of Hamiltonians for two-dimensional classical partition functions is constructed using tensor networks. Similar to tensor network renormalization ([G. Evenbly and G. Vidal, Phys. Rev. Lett. 115, 180405 (2015)], [S. Yang, Z.-C. Gu, and X.-G Wen, Phys. Rev. Lett. 118, 110504 (2017)]) we obtain approximate fixed point tensor networks at criticality. Our formalism however preserves positivity of the tensors at every step and hence yields an interpretation in terms of Hamiltonian flows. We emphasize that the key difference between tensor network approaches and Kadanoff's spin blocking method can be understood in terms of a change of local basis at every decimation step, a property which is crucial to overcome the area law of mutual information. We derive algebraic relations for fixed point tensors, calculate critical exponents, and benchmark our method on the Ising model and the six-vertex model.

We investigate relationships between two forms of Hilbert-Schmidt two-re[al]bit and two-qubit "separability functions"--those recently advanced by Lovas and Andai (arXiv:1610.01410), and those earlier presented by Slater (J. Phys. A 40 [2007] 14279). In the Lovas-Andai framework, the independent variable $\varepsilon \in [0,1]$ is the ratio $\sigma(V)$ of the singular values of the $2 \times 2$ matrix $V=D_2^{1/2} D_1^{-1/2}$ formed from the two $2 \times 2$ diagonal blocks ($D_1, D_2$) of a randomly generated $4 \times 4$ density matrix $D$. In the Slater setting, the independent variable $\mu$ is the diagonal-entry ratio $\sqrt{\frac{d_ {11} d_ {44}}{d_ {22} d_ {33}}}$--with, importantly, $\mu=\varepsilon$ or $\mu=\frac{1}{\varepsilon}$ when both $D_1$ and $D_2$ are themselves diagonal. Lovas and Andai established that their two-rebit function $\tilde{\chi}_1 (\varepsilon )$ ($\approx \varepsilon$) yields the previously conjectured Hilbert-Schmidt separability probability of $\frac{29}{64}$. We are able, in the Slater framework (using cylindrical algebraic decompositions [CAD] to enforce positivity constraints), to reproduce this result. Further, we similarly obtain its new (much simpler) two-qubit counterpart, $\tilde{\chi}_2(\varepsilon) =\frac{1}{3} \varepsilon ^2 \left(4-\varepsilon ^2\right)$. Verification of the companion conjecture of a Hilbert-Schmidt separability probability of $\frac{8}{33}$ immediately follows in the Lovas-Andai framework. We obtain the formulas for $\tilde{\chi}_1(\varepsilon)$ and $\tilde{\chi}_2(\varepsilon)$ by taking $D_1$ and $D_2$ to be diagonal, allowing us to proceed in lower (7 and 11), rather than the full (9 and 15) dimensions occupied by the convex sets of two-rebit and two-qubit states. We also investigate extensions of these analyses to rebit-retrit and qubit-qutrit ($6 \times 6$) settings.

Achieving the Heisenberg limit (HL) in an experiment with very large number of atoms N is a challenging task. One mechanism for doing so is to make use of the experimentally achievable one axis twist spin squeezing in combination with unsqueezing which results in the generation of a Schr\"odinger cat state corresponding to an equal superposition of the extremal Dicke collective states. However, the protocol for achieving this result critically requires the knowledge of whether the total number of atoms is even or odd. Here, we describe a protocol which employs null detection of one of the collective states that circumvents this problem. Specifically, we show that this detection process produces fringes that are narrowed by a factor of N with unit visibility when N is even, and yields zero signal when N is odd. Thus, over repeated measurements under which the probability of N being even or odd is equal, the signal from the odd cases get filtered out, and HL sensitivity is achieved for the $\sim N/2$ atoms corresponding to the even cases. For all N atoms, the sensitivity is below the HL by a factor of $\sqrt{2}$. We also show that a degree of sensitivity enhancement very close to this value can also be achieved for a much lower degree of squeezing than what is required for reaching the cat states. We show that the Schr\"odinger cat case corresponds to interference between collective states with Compton frequencies $\sim 10^{31}$ Hz for $^{87}$Rb atoms with $N = 10^6$. Aside from conventional application to precision metrology, such a Schr\"odinger cat atom interferometer may serve as a test-bed for various aspects of fundamental physics, such as the effect of gravitational interaction on macroscopic decoherence. Finally, we note that the proposed scheme can also be used to realize an HL Schr\"odinger cat atomic clock, for which the base frequency is effectively enhanced by a factor of N.

Uncertainty relation lies at the heart of quantum mechanics, characterizing the incompatibility of non-commuting observables in the preparation of quantum states. An important question is how to improve the lower bound of uncertainty relation. Here we present a variance-based sum uncertainty relation for $N$ incompatible observables stronger than the simple generalization of the uncertainty relation for two observables derived by Maccone and Pati [Phys. Rev. Lett. {\bf113}, 260401 (2014)]. Further comparisons of our uncertainty relation with other related ones for spin-$\frac{1}{2}$ and spin-$1$ particles indicate that the obtained uncertainty relation gives a better lower bound.

Author(s): M. Stammeier, S. Garcia, T. Thiele, J. Deiglmayr, J. A. Agner, H. Schmutz, F. Merkt, and A. Wallraff

In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispe…

[Phys. Rev. A 95, 053855] Published Wed May 24, 2017

Author(s): José J. Gil, Ari T. Friberg, Tero Setälä, and Ignacio San José

It has recently been demonstrated that a general three-dimensional (3D) polarization state cannot be considered an incoherent superposition of (1) a pure state, (2) a two-dimensional unpolarized state, and (3) a 3D unpolarized state [J. J. Gil, Phys. Rev. A **90**, 043858 (2014)]. This fact is intimatel…

[Phys. Rev. A 95, 053856] Published Wed May 24, 2017

Author(s): Gilad Rosenblatt and Meir Orenstein

It has recently been shown that passive lens designs can retain perfect lensing despite intrinsic loss in the comprising left-handed materials. Here we show that energy conservation is not at odds with the operation of such lossy perfect lenses: The irreversible transfer of electromagnetic power to …

[Phys. Rev. A 95, 053857] Published Wed May 24, 2017

Author(s): Ehud Amitai, Niels Lörch, Andreas Nunnenkamp, Stefan Walter, and Christoph Bruder

Optomechanical systems driven by an effective blue-detuned laser can exhibit self-sustained oscillations of the mechanical oscillator. These self-oscillations are a prerequisite for the observation of synchronization. Here, we study the synchronization of the mechanical oscillations to an external r…

[Phys. Rev. A 95, 053858] Published Wed May 24, 2017

Author(s): Isaac Nape, Bienvenu Ndagano, and Andrew Forbes

Quantum erasers with paths in the form of physical slits have been studied extensively and proven instrumental in probing wave-particle duality in quantum mechanics. Here we replace physical paths (slits) with abstract paths of orbital angular momentum (OAM). Using spin-orbit hybrid entanglement of …

[Phys. Rev. A 95, 053859] Published Wed May 24, 2017

Author(s): J. Koepsell, T. Thiele, J. Deiglmayr, A. Wallraff, and F. Merkt

When internal states of atoms are manipulated using coherent optical or radio-frequency (rf) radiation, it is essential to know the polarization of the radiation with respect to the quantization axis of the atom. We first present a measurement of the two-dimensional spatial distribution of the elect…

[Phys. Rev. A 95, 053860] Published Wed May 24, 2017

Author(s): Cyril Laplane, Pierre Jobez, Jean Etesse, Nicolas Gisin, and Mikael Afzelius

Crystals with rare-earth ions could lead to quantum repeaters that enable secure quantum communications over long distances.

[Phys. Rev. Lett. 118, 210501] Published Wed May 24, 2017

Author(s): Kutlu Kutluer, Margherita Mazzera, and Hugues de Riedmatten

Crystals with rare-earth ions could lead to quantum repeaters that enable secure quantum communications over long distances.

[Phys. Rev. Lett. 118, 210502] Published Wed May 24, 2017

Author(s): Joshua Nunn

Crystals with rare-earth ions could lead to quantum repeaters that enable secure quantum communications over long distances.

[Physics 10, 55] Published Wed May 24, 2017

Categories: Physics

Author(s): Jaehak Lee, Jiyong Park, and Hyunchul Nha

Quantum teleportation is one of the crucial protocols in quantum information processing. It is important to accomplish an efficient teleportation under practical conditions, aiming at a higher fidelity desirably using fewer resources. The continuous-variable (CV) version of quantum teleportation was…

[Phys. Rev. A 95, 052343] Published Wed May 24, 2017

Author(s): Zhi-Chao Zhang, Ke-Jia Zhang, Fei Gao, Qiao-Yan Wen, and C. H. Oh

For general bipartite quantum systems, many sets of locally indistinguishable orthogonal product states have been constructed so far. Here, we first present a general method to construct multipartite orthogonal product states in d1⊗d2⊗⋯⊗dn(d1,2,⋯,n≥3,n≥4) by using some locally indistinguishable bipa…

[Phys. Rev. A 95, 052344] Published Wed May 24, 2017

Contextuality is a fundamental feature of quantum theory and is necessary for quantum computation and communication. Serious steps have therefore been taken towards a formal framework for contextuality as an operational resource. However, the most important component for a resource theory - a concrete, explicit form for the free operations of contextuality - was still missing. Here we provide such a component by introducing noncontextual wirings: a physically-motivated class of contextuality-free operations with a friendly parametrization. We characterize them completely for the general case of black-box measurement devices with arbitrarily many inputs and outputs. As applications, we show that the relative entropy of contextuality is a contextuality monotone and that maximally contextual boxes that serve as contextuality bits exist for a broad class of scenarios. Our results complete a unified resource-theoretic framework for contextuality and Bell nonlocality.