CUA - Center for Ultracold Atoms

Research Highlights

MIT-Harvard Center for Ultracold Atoms, A National Science Foundation Physics Frontier Center

Magnetic resonance detection of individual proton spins using quantum reporters
Published 4.21.2016

We demonstrate a method of magnetic resonance imaging with single nuclear-spin sensitivity under ambient conditions. Our method employs isolated electronic-spin quantum bits (qubits) as magnetic resonance “reporters” on the surface of high purity diamond. These spin qubits are localized with nanometer-scale uncertainty, and their quantum state is coherently manipulated and measured optically via a proximal nitrogen-vacancy color center located a few nanometers below the diamond surface. This system is then used for sensing, coherent coupling, and imaging of individual proton spins on the diamond surface with angstrom resolution. Our approach may enable direct structural imaging of complex molecules that cannot be accessed from bulk studies. It realizes a new platform for probing novel materials, monitoring chemical reactions, and manipulation of complex systems on surfaces at a quantum level.



Coherent feedback control of a single qubit in diamond
Published 4.7.2016

Engineering desired operations on qubits subjected to the deleterious effects of their environment is a critical task in quantum information processing, quantum simulation and sensing. The most common approach relies on open-loop quantum control techniques, including optimal-control algorithms based on analytical or numerical solutions, Lyapunov design and Hamiltonian engineering. An alternative strategy, inspired by the success of classical control, is feedback control. Because of the complications introduced by quantum measurement, closed-loop control is less pervasive in the quantum setting and, with exceptions its experimental implementations have been mainly limited to quantum optics experiments. Here we implement a feedback-control algorithm using a solid-state spin qubit system associated with the nitrogen vacancy centre in diamond, using coherent feedback to overcome the limitations of measurement-based feedback, and show that it can protect the qubit against intrinsic dephasing noise for milliseconds. In coherent feedback, the quantum system is connected to an auxiliary quantum controller (ancilla) that acquires information about the output state of the system (by an entangling operation) and performs an appropriate feedback action (by a conditional gate). In contrast to open-loop dynamical decoupling techniques, feedback control can protect the qubit even against Markovian noise and for an arbitrary period of time (limited only by the coherence time of the ancilla), while allowing gate operations. It is thus more closely related to quantum error-correction schemes although these require larger and increasing qubit overheads. Increasing the number of fresh ancillas enables protection beyond their coherence time. We further evaluate the robustness of the feedback protocol, which could be applied to quantum computation and sensing, by exploring a trade-off between information gain and decoherence protection, as measurement of the ancilla–qubit correlation after the feedback algorithm voids the protection, even if the rest of the dynamics is unchanged.



A Microscopic View of Fermionic Mott-Insulators
Published 2.26.2016

Conventional band theory predicts an insulating behavior of a solid if the freely moving electrons occupy every possible quantum state in the highest energy band, whereas the state is conducing otherwise. This simple picture is altered in the presence of strong interactions, which can lead to an insulating behavior even in the presence of a half-filled energy band. These Mott-insulators, named after the British physicist Sir Nevill F. Mott, are one of the conceptually simplest examples of many-body systems, where strong correlations lead to surprising phenomena.

Researchers at Harvard have now synthesized and studied such Mott-insulators with high-resolution microscopy by cooling Li-6 atoms to extremely low temperatures below 50 nK, deep in the quantum degenerate regime, and trapping them into a crystalline structure formed by intersecting laser beams. This synthetic version of a solid by means of an optical lattice gives the researchers an exceptional tunability and control over the system, for example the strength of the interactions. By combining this approach with a novel imaging technique developed for neutral fermionic atoms, the team around Prof. Markus Greiner at Harvard University was able to image the atomic distribution of the Mott-insulating state with single site and single atom resolution.

The researchers’ results demonstrate that combining ultracold fermions in optical lattices and quantum gas microscopy could allow shedding new light into complex states of quantum matter in the future, where even the most sophisticated theoretical methods cannot provide a reliable prediction. Examples range from high-temperature superconductors, which is believed to occur in the proximity of an antiferromagnetic Mott-insulator, to frustrated quantum magnets, which show an intriguing interplay between the lattice geometry and the magnetic ordering. In addition, quantum gas microscopy even allows creating arbitrary potential landscapes for the ultracold atoms. This opens entirely new perspectives for studying and probing quantum many-body systems at the site-resolved level.



Visible-frequency hyperbolic metasurface
Published 1.15.2016

Metamaterials are artificial optical media composed of sub-wavelength metallic and dielectric building blocks that feature optical phenomena not present in naturally occurring materials. Although they can serve as the basis for unique optical devices that mould the flow of light in unconventional ways, three-dimensional metamaterials suffer from extreme propagation losses. Two-dimensional metamaterials (metasurfaces) such as hyperbolic metasurfaces for propagating surface plasmon polaritons have the potential to alleviate this problem. Because the surface plasmon polaritons are guided at a metal–dielectric interface (rather than passing through metallic components), these hyperbolic metasurfaces have been predicted to suffer much lower propagation loss while still exhibiting optical phenomena akin to those in three-dimensional metamaterials. Moreover, because of their planar nature, these devices enable the construction of integrated metamaterial circuits as well as easy coupling with other optoelectronic elements. Here we report the experimental realization of a visible-frequency hyperbolic metasurface using single-crystal silver nanostructures defined by lithographic and etching techniques. The resulting devices display the characteristic properties of metamaterials, such as negative refraction, and diffraction-free propagation, with device performance greatly exceeding those of previous demonstrations. Moreover, hyperbolic metasurfaces exhibit strong, dispersion-dependent spin–orbit coupling, enabling polarization- and wavelength-dependent routeing of surface plasmon polaritons and two-dimensional chiral optical components. These results open the door to realizing integrated optical meta-circuits, with wide-ranging applications in areas from imaging and sensing to quantum optics and quantum information science.



A new look at superfluidity
Published 8.10.2015

MIT physicists have created a superfluid gas, the so-called Bose-Einstein condensate, for the first time in an extremely high magnetic field. The magnetic field is a synthetic magnetic field, generated using laser beams, and is 100 times stronger than that of the world’s strongest magnets. Within this magnetic field, the researchers could keep a gas superfluid for a tenth of a second — just long enough for the team to observe it.



MIT team creates ultracold molecules at near absolute zero
Published 6.10.2015

The air around us is a chaotic superhighway of molecules whizzing through space and constantly colliding with each other at speeds of hundreds of miles per hour. Such erratic molecular behavior is normal at ambient temperatures.

But scientists have long suspected that if temperatures were to plunge to near absolute zero, molecules would come to a screeching halt, ceasing their individual chaotic motion and behaving as one collective body. This more orderly molecular behavior would begin to form very strange, exotic states of matter — states that have never been observed in the physical world.



Vanishing friction
Published 6.4.2015

Friction is all around us, working against the motion of tires on pavement, the scrawl of a pen across paper, and even the flow of proteins through the bloodstream. Whenever two surfaces come in contact, there is friction, except in very special cases where friction essentially vanishes — a phenomenon, known as “superlubricity,” in which surfaces simply slide over each other without resistance.

Now physicists at MIT have developed an experimental technique to simulate friction at the nanoscale. Using their technique, the researchers are able to directly observe individual atoms at the interface of two surfaces and manipulate their arrangement, tuning the amount of friction between the surfaces. By changing the spacing of atoms on one surface, they observed a point at which friction disappears.



Researchers build new fermion microscope
Published 5.13.2015

Fermions are the building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Without fermions, the physical world would not exist.

Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particles. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers are trying to use ultracold gases of fermionic atoms as stand-ins for other fermions.

But atoms are extremely sensitive to light: When a single photon hits an atom, it can knock the particle out of place — an effect that has made imaging individual fermionic atoms devilishly hard.



Thousands of atoms entangled with a single photon
Published 3.25.2015

Physicists from MIT and the University of Belgrade have developed a new technique that can successfully entangle 3,000 atoms using only a single photon.



New ‘switch’ could power quantum computing
Published 4.9.2014

A light lattice that traps atoms may help scientists build networks of quantum information transmitters.



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