Squeezed light from single atoms

Squeezed light from single atoms

  9:11:00 PM    Science Lover

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Max Planck Institute of Quantum Optics scientists generate amplitude-squeezed light fields using single atoms trapped inside optical cavities.

A single rubidium atom in a cavity squeezes the quantum
fluctuations of a weak laser beam, decreasing the fluctuations
of the amplitude at the expense of the phase.
The effect is exaggerated for clarity.

In classical optics light is usually described as a wave, but at the most fundamental quantum level this wave consists of discrete particles called photons. Over the time, physicists developed many tools to manipulate both the wave-like and the particle-like quantum properties of the light. For instance, they created single photon sources with single atoms, using their ability to absorb and emit photons one by one. A team around Professor Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics (Garching near Munich) and head of the Quantum Dynamics Division, has now observed that the light emitted by a single atom may exhibit much richer dynamics (Nature 474, 623, June 30, 2011). Strongly interacting with light inside a cavity, the atom modifies the wave-like properties of the light field, reducing its amplitude or phase fluctuations below the level allowed for classical electromagnetic radiation. This is the very first observation of “squeezed” light produced by a single atom.

The “graininess” of the photons in a light wave causes small fluctuations of the wave’s amplitude and phase. For classical beams, the minimal amount of amplitude and phase fluctuations is equal. However, by creating interactions between the photons, one can “squeeze” the fluctuations of the amplitude below this so-called “shot noise” level at the expense of increasing the fluctuations of the phase, and vice-versa. Unfortunately, the photonic interactions inside standard optical media are very weak, and require bright light beams to be observed. Single atoms are promising candidates to enable such interactions at a few-photon level. Their ability to generate squeezed light has been predicted 30 years ago, but the amount of light they emit is very tiny and so far all attempts to set this idea into realization have failed. In the Quantum Dynamics Division at MPQ sophisticated methods for cooling, isolating and manipulating single atoms have been developed over many years, and made this observation possible.



A single rubidium atom is trapped inside a cavity made of two very reflective mirrors in a distance of about a tenth of a millimetre from each other. When weak laser light is injected into this cavity, the atom can interact with one photon many times, and forms a kind of artificial molecule with the photons of the light field. As a consequence, two photons can enter the system at the same time and become correlated. “According to the model of Bohr, a single atom emits exactly one single energy quantum, i.e., one photon. That means that the number of photons is exactly known, but the phase of the light is not defined”, Professor Gerhard Rempe explains. “But the two photons that are emitted by this strongly coupled atom are indistinguishable and oscillate together. Therefore this time the wave-like properties of the propagating light field are modified.”

When the physicists use a laser beam which is resonant with the excitation frequency of the atom, the measurements show a suppression of the phase fluctuations. If the laser light is resonant with the cavity, they observe a squeezing of the amplitude instead.

The latter situation is illustrated in the figure: The atom in the cavity turns a laser beam into light which has less amplitude and more phase fluctuations than the shot-noise limit. “Our experiment shows that the light emitted by single atoms is much more complex than in the simple view of Albert Einstein concerning photo-emission”, Dr. Karim Murr emphasizes. “The squeezing that we observe is due to the coherent interaction between the two photons emitted from the system. Our measurement is in excellent agreement with the predictions of quantum electrodynamics in the strong-coupling regime.” And Dr. Alexei Ourjoumtsev, who has been working on the experiment as a post doc, adds: “Usually single quantum objects are used to manipulate the particle-like properties of light. It is interesting to see that they can also modify its wave-like properties, and create observable squeezing with excitations beams containing only two photons on average”.

So far squeezed light has only been generated with systems containing many atoms, such as crystals, using very high intensity beams, i.e. many photons. For the first time now physicists have succeeded in generating this kind of non-classical radiation with single atoms and extremely weak light fields. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

More information: A. Ourjoumtsev, A. Kubanek, M. Koch, C. Sames, P. W. H. Pinkse, G. Rempe, & K. Murr

Observation of squeezed light from one atom excited with two photons , Nature 474, 623, 30 June 2011. Provided by Max-Planck-Gesellschaft

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Quantum Pen for Single Atoms Is a Big Step Toward Large-Scale Quantum Computing

Quantum Pen for Single Atoms Is a Big Step Toward Large-Scale Quantum Computing

  9:38:00 AM    Science Lover

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Physicists at the Max Planck Institute of Quantum Optics succeeded in manipulating atoms individually in a lattice of light and in arranging them in arbitrary patterns. These results are an important step towards large scale quantum computing and for the simulation of condensed matter systems.

With the help of a laser beam, the scientists could address single atoms in the lattice of light and change their spin state. In this way they succeeded in having total control over the single atoms and in "writing" arbitrary two-dimensional patterns. (Credit: Image courtesy of Max Planck Institute of Quantum Optics)

Physicists around the world are searching for the best way to realize a quantum computer. Now scientists of the team around Stefan Kuhr and Immanuel Bloch at the Max Planck Institute of Quantum Optics (Garching/Munich) took a decisive step in this direction. They can now address and change the spin of single atoms with laser light and arrange them in arbitrary patterns. In this way, the physicists strung the atoms along a line and could directly observe their tunneling dynamics in a “racing duel” of the atoms. A register of hundreds of addressable quantum particles could serve for storing and processing of quantum information in a quantum computer.

In the present experiment, the scientists loaded laser-cooled rubidium atoms into an artificial crystal of light. These so-called optical lattices are generated by superimposing several laser beams. The atoms are kept in the lattice of light in a way similar to marbles being contained in the hollows of an egg carton.

A few months ago, the team of Stefan Kuhr and Immanuel Bloch showed that each site of the optical lattice can be filled with exactly one atom. With the help of a microscope, the scientists visualized the array atom by atom and thereby verified the shell-like structure of this “Mott insulator.” Now the scientists succeeded in individually addressing the atoms in the lattice and in changing their respective energy state. Using the microscope, they focused a laser beam down to a diameter of about 600 nanometers, which is just above the lattice spacing, and directed it at individual atoms with high precision.

The laser beam slightly deforms the electron shell of the addressed (targeted) atom and thereby changes the energy difference between its two spin states. Atoms with a spin – i.e. an intrinsic angular momentum – behave like little magnetic needles that can align in two opposite directions. If the atoms are irradiated with microwaves that are in resonance with the modified spin transition, only the addressed atoms absorb a microwave photon, which causes their spin to flip. All other atoms in the lattice remain unaffected by the microwave field.

The scientists demonstrated the high fidelity of this addressing scheme in a series of experiments. For this purpose, the spins of all atoms along a line were flipped one after the other, by moving the addressing laser from lattice site to lattice site. After removing all atoms with a flipped spin from the trap, the addressed atoms are visible as holes, which can easily be counted. In this way, the physicists deduced that the addressing worked in 95% of the cases. Atoms at the neighboring sites are not influenced by the addressing laser. The method provides the possibility to generate arbitrary distributions of atoms in the lattice.

Starting from an arrangement of 16 atoms that were strung together on neighboring lattice sites like a necklace of beads, the scientists studied what happens when the height of the lattice is ramped down so far that the particles are allowed to “tunnel” according to the rules of quantum mechanics. They move from one lattice site to the other, even if their energy is not sufficient to cross the barrier between the lattice wells. “As soon as the height of the lattice has reached the point where tunneling is possible, the particles start running as if they took part in a horse-race”, doctoral candidate Christof Weitenberg describes. “By taking snapshots of the atoms in the lattice at different times after the "starting signal", we could directly observe the quantum mechanical tunneling-effect of single massive particles in an optical lattice for the first time.”

The new addressing technique allows many interesting studies of the dynamics of collective quantum states, as they appear in solid state systems. It also opens new perspectives in quantum information processing. “A Mott isolator with exactly one atom per lattice site acts as a natural quantum register with a few hundred quantum bits, the ideal starting point for scalable quantum information processing,” as Stefan Kuhr explains. “We have shown that we can individually address single atoms. In order for the atom to suit as a quantum bit, we need to generate coherent superpositions of its two spin states. A further step is to realize elementary logical operations between two selected atoms in the lattice, so-called quantum gates.”

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Microscope 'Sees' Atoms for First Time

Microscope 'Sees' Atoms for First Time

  9:16:00 PM    Science Lover

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UCLA researchers report in the April 30 edition of the journal Cell that they have imaged a virus structure at a resolution high enough to effectively "see" atoms, the first published instance of imaging biological complexes at such a resolution.

Cover of the journal Cell showing an artistic 
representation of a Cryo-EM 3-D reconstruction 
of an aquareovirus, the colors are added to show 
contrast between various structures in the virus. 
(Credit: Image courtesy of UCLA)

The research team, led by Hong Zhou, UCLA professor of microbiology, immunology and molecular genetics, used cryo-electron microscopy to image the structure at 3.3 angstroms. An angstrom is the smallest recognized division of a chemical element and is about the distance between the two hydrogen atoms in a water molecule.

The study, the researchers say, demonstrates the great potential of cryo-electron microscopy, or Cryo-EM, for producing extremely high-resolution images of biological samples in their native environment.

"This is the first study to determine an atomic resolution structure through Cryo-EM alone," said Xing Zhang, a postdoctoral candidate in Zhou's group and lead author of the Cell paper. "By proving the effectiveness of this microscopy technique, we have opened the door to a wide variety of biological studies."

With traditional light microscopy, a magnified image of a sample is viewed through a lens. Some samples, however, are too small to diffract visible light (in the 500 to 800 nm range, or 5,000 to 8,000 angstroms) and therefore cannot be seen. To image objects at the sub-500 nm scale, scientists must turn to other tools, such as atomic force microscopes, which use an atomically thin tip to generate an image by probing a surface, in much the same way a blind person reads by touching Braille lettering.

With electron microscopy, another sub-500 nm technology, a beam of electrons is fired at a sample, passing through empty areas and bouncing off dense areas. A digital camera reads the path of the electrons passing through the sample to create a two-dimensional projection image of the sample. By repeating this process at hundreds of different angles, a computer can construct a three-dimensional image of the sample at a very high resolution.

Zhou is faculty director of the Electron Imaging Center for Nanomachines (EICN) at UCLA's California NanoSystems Institute, which is using cryo-electron microscopy to create 3-D reconstructions of nano-machineries, nano-devices and biological nano-structures, such as viruses.

Structurally accurate 3-D reconstructions of biological complexes are possible with cryo-electron microscopy because the samples are flash frozen, which allows them to be imaged in their native environment, and the microscope operates in a vacuum, because electrons travel better in that environment. The Cell paper focused on a structural study of the aquareovirus, a non-envelope virus that causes disease in fish and shellfish, in an effort to better understand how non-envelope viruses infect host cells.

"We are extremely excited about the recent breakthrough achieved by Hong Zhou and his team at the EICN lab," said Leonard H. Rome, senior associate dean for research at the David Geffen School of Medicine at UCLA and associate director of the California NanoSystems Institute. "The ability to understand the structure of viruses at an atomic level will open avenues for manipulating them for use in drug delivery and propel numerous innovations in treatments of diseases. UCLA is fortunate to have such specialized instrumentation and the expertise of Professor Zhou and his team to take advantage of these marvelous microscopes."

Viruses can be classed into two types: envelope and non-envelope. Envelope viruses, which include influenza and HIV, are surrounded by an envelope-like membrane which the virus uses to fuse with and infect a host cell. Non-envelope viruses lack this membrane and instead use a protein to fuse with and infect cells. This process was poorly understood until Zhou's study.

"Through better knowledge of virus structures, we hope to engineer medications in three ways," Zhou said. "If we understand how viruses work, first we can identify small molecules or drugs that block their infection; second, we can engineer ultra-stable and non-infectious virus-like particles as optimal vaccines; and third, we can alter their characteristics so that instead of delivering a disease, viruses could deliver medications.

"Indeed, we are working with UCLA physicians and engineers to engineer viruses for gene therapy and drug delivery," he said. "In essence, we hope to take advantage of millions of years of evolution that have made viruses incredibly effective delivery platforms."

From the high-resolution 3-D images produced with the cryo-electron microscopy, Zhou's group was able to determine that the aquareovirus employs a priming stage to accomplish cell infection. In its dormant state, the virus has a protective protein covering, which it sheds during priming. Once the outer shell has been shed, the virus is in a primed state and is ready to use a protein called an "insertion finger" to infect a cell.

The team's study ushers in a new era of structural biology for understanding important biological processes. The group was able to discover this functionality because of the accurate structural model produced through cryo-electron microscopy. In addition to producing a high-resolution 3-D image of samples, the technology allows samples to be imaged in their native environment, so the structural model is faithful to the original sample. From a technical point of view, this work also demonstrates the power of cryo-electron microscopy in obtaining 3-D structures of biological complexes without needing to grow a crystal.

The California NanoSystems Institute at UCLA is an integrated research center operating jointly at UCLA and UC Santa Barbara whose mission is to foster interdisciplinary collaborations for discoveries in nanosystems and nanotechnology; train the next generation of scientists, educators and technology leaders; and facilitate partnerships with industry, fueling economic development and the social well-being of California, the United States and the world. The CNSI was established in 2000 with $100 million from the state of California and an additional $250 million in federal research grants and industry funding. At the institute, scientists in the areas of biology, chemistry, biochemistry, physics, mathematics, computational science and engineering are measuring, modifying and manipulating the building blocks of our world -- atoms and molecules. These scientists benefit from an integrated laboratory culture enabling them to conduct dynamic research at the nanoscale, leading to significant breakthroughs in the areas of health, energy, the environment and information technology.

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