Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum computation. Dynamical gates have been proposed in the context of trapped ions; however, geometric phase gates (which change only the phase of the physical qubits) offer potential practical advantages because they have higher intrinsic resistance to certain small errors and might enable faster gate implementation. Here we demonstrate a universal geometric pi-phase gate between two beryllium ion-qubits, based on coherent displacements induced by an optical dipole force. The displacements depend on the internal atomic states; the motional state of the ions is unimportant provided that they remain in the regime in which the force can be considered constant over the extent of each ion's wave packet. By combining the gate with single-qubit rotations, we have prepared ions in an entangled Bell state with 97% fidelity-about six times better than in a previous experiment demonstrating a universal gate between two ion-qubits. The particular properties of the gate make it attractive for a multiplexed trap architecture that would enable scaling to large numbers of ion-qubits.

Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate*

The present paper describes the synthesis and characterization of nanocomposite materials built from the assembly of organic polymers and two-dimensional host materials, particularly reviewing those composed of layered double hydroxide (LDH) inorganic frameworks. When the meaning commonly adopted for nanocomposites is narrowed, the system is constituted of sheets lying on top of each other in which covalent forces maintain the chemical integrity and define an interlamellar gap filled up with the polymer guest. The situation is different from an inorganic filler dispersed into a polymeric matrix. The incorporation of polymer between the galleries proceeds via different pathways such as coprecipitation, exchange, in situ polymerization, surfactant-mediated incorporation, hydrothermal treatment, reconstruction, or restacking. The latter method, recently effective via the exfoliation of the LDH layers, appears to be more favorable, in terms of crystallinity, to capture monomer entities than the whole polymer....

Polymer Interleaved Layered Double Hydroxide: A New Emerging Class of Nanocomposites

Quantum adiabatic processes--that keep constant the populations in the instantaneous eigenbasis of a time-dependent Hamiltonian--are very useful to prepare and manipulate states, but take typically a long time. This is often problematic because decoherence and noise may spoil the desired final state, or because some applications require many repetitions. "Shortcuts to adiabaticity" are alternative fast processes which reproduce the same final populations, or even the same final state, as the adiabatic process in a finite, shorter time. Since adiabatic processes are ubiquitous, the shortcuts span a broad range of applications in atomic, molecular, and optical physics, such as fast transport of ions or neutral atoms, internal population control, and state preparation (for nuclear magnetic resonance or quantum information), cold atom expansions and other manipulations, cooling cycles, wavepacket splitting, and many-body state engineering or correlations microscopy. Shortcuts are also relevant to clarify fundamental questions such as a precise quantification of the third principle of thermodynamics and quantum speed limits. We review different theoretical techniques proposed to engineer the shortcuts, the experimental results, and the prospects.

http://www.spindynamics.org/documents/conf/QUAINT2013/QUAINT2013_muga.pdf

Shortcuts to Adiabaticity

In recent years, surface codes have become a leading method for quantum error correction in theoretical large-scale computational and communications architecture designs. Their comparatively high fault-tolerant thresholds and their natural two-dimensional nearest-neighbour (2DNN) structure make them an obvious choice for large scale designs in experimentally realistic systems. While fundamentally based on the toric code of Kitaev, there are many variants, two of which are the planar- and defect-based codes. Planar codes require fewer qubits to implement (for the same strength of error correction), but are restricted to encoding a single qubit of information. Interactions between encoded qubits are achieved via transversal operations, thus destroying the inherent 2DNN nature of the code. In this paper we introduce a new technique enabling the coupling of two planar codes without transversal operations, maintaining the 2DNN of the encoded computer. Our lattice surgery technique comprises splitting and merging planar code surfaces, and enables us to perform universal quantum computation (including magic state injection) while removing the need for braided logic in a strictly 2DNN design, and hence reduces the overall qubit resources for logic operations. Those resources are further reduced by the use of a rotated lattice for the planar encoding. We show how lattice surgery allows us to distribute encoded GHZ states in a more direct (and overhead friendly) manner, and how a demonstration of an encoded CNOT between two distance-3 logical states is possible with 53 physical qubits, half of that required in any other known construction in 2D.

https://iopscience.iop.org/article/10.1088/1367-2630/14/12/123011/pdf

Surface code quantum computing by lattice surgery

Elementary Theory of Angular Momentum. By M. E. Rose. Pp. x, 248. 80s. 1957. (Wiley)

The layered double hydroxide (LDH) of Mg with Al decomposes at 450°C to yield a mixed metal oxide. The oxide reconstructs back to the parent LDH either on cooling in air or soaking in water. This reversible thermal behaviour was attributed to the formation of an unstable defect rocksalt phase (comprising Al3+ ions partially substituting for Mg2+ in MgO) on decomposition of the LDH. However a simple oxide mixture of MgO and Al2O3 taken in the 6∶1 molar ratio is also found to yield a LDH-like phase when soaked in an aqueous solution of a suitable anion, suggesting that the reconstruction occurs through a simple dissolution–reprecipitation mechanism. The formation of the defect rocksalt phase is not a necessary precondition for reconstruction. However, the kinetics of the reconstruction reaction depends upon the nature of the anion and the thermal history of the oxides. The presence of carbonate ions and unsintered oxides hastens the reconstruction reaction.

Reversible thermal behavior of the layered double hydroxide of Mg with Al: Mechanistic studies

We have measured the storage instabilities of electrons in a Penning trap at low magnetic fields. These measurements are carried out as a function of the trapping voltage, for different magnetic fields. It is seen that these instabilities occur at the same positions when the trapping voltage is expressed as a percentage of the maximum voltage, given by the stability limit. The characteristic frequencies at which these instabilities occur, obey a relation that is given by n
zω
z + n
+ω
+ + n
-ω
- = 0, where ω
z, ω
+ and ω
- are the axial, perturbed cyclotron and the magnetron frequencies of the trapped electrons respectively, and the n's are integers. The reason for these instabilities are attributed to higher order static perturbations in the trapping potential.

Instabilities of an electron cloud in a Penning trap

We report on the design and testing of an array of Penning ion traps made from printed circuit board. The system enables fast shuttling of ions from one trapping zone to another, which could be of use in quantum information processing. We describe simulations carried out to determine the optimal potentials to be applied to the trap electrodes for enabling this movement. The results of a preliminary experiment with a cloud of laser cooled calcium ions demonstrate a round-trip shuttling efficiency of up to 75%.

Fast shuttling of ions in a scalable Penning trap array

We report here on studies of reorientation of human red blood cells (RBCs) in an optical trap. We have measured the time required, t re , for the plane of the RBC entering the optical trap to undergo a 90-deg rotation to acquire an edge on orientation with respect to the beam direction. This has been studied as a function of laser power, P , at the trap center. The variation of t re with increasing P shows an initial sharp decrease followed by a much smaller rate of further decrease. We find that this experimentally measured variation is not in complete agreement with the variation predicted by a theoretical model where the RBC is treated as a perfectly rigid circular disk-like body. We argue that this deviation arises due to deformation of the RBC. We further reason that this feature is dominated by the elastic behavior of the RBC membrane. We compare the studies carried out on normal RBCs with RBCs where varying conditions of membrane stiffness are expected. We propose that the value of energy used for maximum deformation possible during a reorientation process is an indicator of the membrane elasticity of the system under study.

Orientational dynamics of human red blood cells in an optical trap.

A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell’s dimple region and the slow axis is along a diameter. We report an average retardation of 3.5±1.5  nm for linearly polarized green light (λ=546  nm). We also estimate a retardation of 1.87±0.09  nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-19/issue-11/115004/Birefringence-of-a-normal-human-red-blood-cell-and-related/10.1117/1.JBO.19.11.115004.pdf

Sharath Ananthamurthy

Papers

Reversible thermal behavior of the layered double hydroxide of Mg with Al: Mechanistic studies

Instabilities of an electron cloud in a Penning trap

Fast shuttling of ions in a scalable Penning trap array

Orientational dynamics of human red blood cells in an optical trap.

Birefringence of a normal human red blood cell and related optomechanics in an optical trap.