Unconventional Magnetism via Optical Pumping of Interacting Spin Systems

T.E. Lee, S. Gopalakrishnan, M.D. Lukin

Phys. Rev. Lett. 110, 257204 (2013).

     Magnetism has been an important topic in physics for more than a century. A magnet is made of spins, each of which point up or down. The spins interact with each other to form interesting collective phases. People usually study magnetism in equilibrium, which means that the magnet exchanges energy with the environment until an equilibrium is reached. In this work, we study magnetism in non-equilibrium by adding dissipative processes that prevent equilibration. It turns out that the non-equilibrium nature leads to a variety of exotic features. In addition to ferromagnetic and antiferromagnetic phases, we find spin-density-wave and staggered-XY phases. We find that adding dissipation qualitatively changes the magnetic behavior and even enriches the phase diagram. Our results can be observed in experiments with atoms in optical lattices or trapped ions.

 

Nanometer-scale Thermometry in a Living Cell

G. Kucsko, P.C. Maurer, N.Y. Yao, M. Kubo, H.J. Noh, P.K. Lo, H. Park, M.D. Lukin

Nature 500: 54-58 (2013).

     The ability to monitor sub-kelvin variations over a large range of temperatures can provide insight into both organic and inorganic systems, shedding light on questions ranging from tumor metabolism to heat dissipation in integrated circuits. Moreover, by combining local light-induced heat sources with sensitive nanoscale thermometry, it may be possible to engineer biological processes at the sub-cellular level. Many promising approaches are currently being explored for this purpose, including scanning probe microscopy, Raman spectroscopy, and fluorescence-based measurements using nanoparticles and organic dyes. These methods, however, are often limited by a combination of low sensitivity, bio-incompatibility, or systematic errors owing to changes in the local chemical environment. Our new approach to nanoscale thermometry utilizes the quantum mechanical spin associated with nitrogen vacancy (NV) color centers in diamond. The operational principle of NV-based thermometry relies upon the temperature dependent lattice strain of diamond; changes in the lattice are directly reflected as changes in the spin properties of the NV, which are then optically detected with high spatial resolution.

 

Coupling of a Single Trapped Atom to a Nanoscale Optical Cavity

J.D. Thompson, T.G. Tiecke, N.P. de Leon, J. Feist, A.V. Akimov, M. Gullans, A.S. Zibrov, V. Vuletic, M.D. Lukin

Science 340, 6137: 1202-1205 (2013).

     Atoms coupled to nanoscale optical cavities are a promising system for a variety of quantum information tasks. The tight confinement of photons provided by the nanoscale cavity results in strong interactions with the atoms, which allows for rapid exchange of information before the onset of dephasing. The main challenge associated with this approach is confining atoms at sub-wavelength distances from the surface of the optical cavities, where the attractive atom-surface forces are significant. We have developed a new technique to address this problem, by using an optical tweezer to transport atoms to the surface. We are currently working on extensions of our initial work (Science, 2013) to higher quality factor optical cavities and multiple atoms.

 

Optical Magnetic Imaging of Living Cells

D. LeSage, K. Arai, D.R. Glenn, S.J. DeVience, L.M. Pham, L. Rahn-Lee, M.D. Lukin, A. Yacoby, A. Komeili, R.L. Walsworth

Nature 496: 486-489 (2013).

     Magnetic imaging is a powerful tool for probing biological and physical systems. However, existing techniques either have poor spatial resolution compared to optical microscopy and are hence not generally applicable to imaging of sub-cellular structure (for example, magnetic resonance imaging), or entail operating conditions that preclude application to living biological samples while providing submicrometre resolution (for example, scanning superconducting quantum interference device microscopy, electron holography and magnetic resonance force microscopy). Here we demonstratemagnetic imaging of living cells (magnetotactic bacteria) under ambient laboratory conditions and with sub-cellular spatial resolution (400 nanometres), using an optically detected magnetic field imaging array consisting of a nanometre-scale layer of nitrogen– vacancy colour centres implanted at the surface of a diamond chip. With the bacteria placed on the diamond surface, we optically probe the nitrogen–vacancy quantum spin states and rapidly reconstruct images of the vector components of themagnetic field created by chains of magnetic nanoparticles (magnetosomes) produced in the bacteria. We also spatially correlate these magnetic field maps with optical images acquired in the same apparatus. Wide-field microscopy allows parallel optical and magnetic imaging of multiple cells in a population with submicrometre resolution and a field of view in excess of 100 micrometres. Scanning electron microscope images of the bacteria confirm that the correlated optical and magnetic images can be used to locate and characterize the magnetosomes in each bacterium. Our results provide a new capability for imaging bio-magnetic structures in living cells under ambient conditions with high spatial resolution, and will enable the mapping of a wide range of magnetic signals within cells and cellular networks.

 

Phonon-induced Spin-spin Interactions in Diamond Nanostructures

S.D. Bennett, N.Y. Yao, J. Otterbach, P. Rabl, M.D. Lukin

Phys. Rev. Lett. 110, 156402 (2013).

     NV centers in diamond provide outstanding solid state quantum bits, capable of retaining an encoded bit of quantum information for a relatively long time. This is in part because they couple very weakly to their environement; on the other hand, this same quality also makes it challenging to connect NV centers together. One possible solution is to wire NV centers together by having two or more coupled to the vibrational motion of an all-diamond mechanical nanostructure. In this paper we consider a handful of NV centers embedded in a nanoscale diamond beam, and analyze whether it is possible to engineer quantum entangled states between the NVs mediated by the motion of the beam. Surprisingly, despite thermal noise in the mechanical mode and decoherence of the NV centers, we find that it should be experimentally feasible to prepare entangled states at realistic temperatures. In addition to providing a new route toward coupling NV centers together, our approach may find practical application in collectively-enhanced magnetometry using a vibration-coupled NV ensemble.

 

Coherence and Raman Sideband Cooling of a Single Atom in an Optical Tweezer

J.D. Thompson, T.G. Tiecke, A.S. Zibrov, V. Vuletic, M.D. Lukin

Phys. Rev. Lett. 110, 133001 (2013).

     Tightly focused optical dipole traps ("optical tweezers") are used in many fields of research, from atomic physics to biophysics and biology. They allow single particles, or single atoms, to be tightly confined to the focus of a laser beam, which can then be steered with a scanning mirror to control the position of the particle with high precision. In this work, we extended this positioning precision for single atoms by cooling them to their quantum ground state of motion in the trap, using a technique called Raman sideband cooling. In this way, we "freeze out" the atom's thermal position fluctuations, so that for many applications it can be described as a point particle sitting exactly at the center of the laser focus. This is an enabling step for future experiments where we would like to position an atom at very small distances from a surface.

 

Realizing Fractional Chern Insulators in Dipolar Spin Systems

N.Y. Yao, A.V. Gorshkov, C.R. Laumann, A.M. Lauchli, J. Ye, M.D. Lukin

Phys. Rev. Lett. 110, 185302 (2013).

     The quantum Hall effect represents a paradigm of modern condensed matter physics and is associated with electron liquids in the presence of a strong magnetic field. Such systems exhibit a rich variety of physical phenomena ranging from topologically protected edge transport to the splitting of the electron into fractional pieces. The question remains however: what ingredients are essential to realize these exotic features? Is it the fermionic nature of the electrons? The presence of a strong magnetic field? To answer these questions, the search is on for realizations of these exotic phenomena in non-electronic systems. This has led to the recent proposal of so-called fractional Chern insulators – novel states of matter, which include lattice-based, bosonic, non-magnetic-field analogs of conventional quantum Hall liquids. Despite tremendous interest, there has yet to be a proposal for any experimental system that can realize such Chern insulators. In this paper, the authors propose that the rotational excitations in dipolar systems of spins or molecules may provide just such a platform. At parameters consistent with near-term experimental capabilities, the authors reveal a rich phase diagram composed of competing topological (FCI) and non-topological phases.

 

Dissipative Preparation of Spin Squeezed Atomic Ensembles in a Steady State

E.G. Dalla Torre, J. Otterbach, E. Demler, V. Vuletic, M.D. Lukin

Phys. Rev. Lett. 110, 120402 (2013).

     Modern technology often requires the precise measurement of times and frequencies. One possible way to realize very precise clocks utilizes ensembles of atoms whose collective spin is “squeezed” beyond the standard limit. Unfortunately, spin-squeezed states require a high degree of quantum entanglement and are easily destroyed by the dissipative coupling to the surrounding environment. For example, the spontaneous emission of photons from the atoms reduces the amount of entanglement and, consequently, limits the precision of the clock.

     In this publication the authors propose a new method to create spin-squeezed states by collectively coupling two internal states of the atoms to a driven photonic resonator (“cavity”). When a photon escapes the cavity, the atomic states re-arrange themselves and, at long times, the system tends towards a strongly-correlated ``dark state’’. For specific values of the atom-cavity coupling, the dark state holds a high degree of spin squeezing (see Figure). Counter-intuitively, the dissipative process of cavity decay becomes a resource for entanglement.

     In practice, however, the cavity decay is not the only source of dissipation. The atoms can emit photons that are not aligned with the resonator as well. This process involves the flip of a single spin and reduces the atomic entanglement, leading to a reduction of the clock precision. The competition between cavity decay and spontaneous emission drives the atoms to a mixed state with reduced, but finite, amount of spin squeezing. For realistic situations, the proposed method should allow to improve the precision of the current state-of-the-art atomic clocks by one order of magnitude.

 

Topologically Protected Quantum State Transfer in a Chiral Spin Liquid

N.Y. Yao, C.R. Laumann, A.V. Gorshkov, H. Weimer, L. Jiang, J.I. Cirac, P. Zoller, M.D. Lukin

Nature Communications 4, 1585 (2013).

     Topology plays a central role in ensuring the robustness of a wide variety of physical phenomena. Notable examples range from the current-carrying edge states associated with the quantum Hall and the quantum spin Hall effects to topologically protected quantum memory and quantum logic operations. In this work, we propose a novel topologically protected channel for the transfer of quantum states between remote quantum nodes. In our approach, state transfer is mediated by the edge mode of a chiral spin liquid. We demonstrate that the proposed method is intrinsically robust to realistic imperfections associated with disorder and decoherence.

 

Collectively Enhanced Interactions in Solid-state Spin Qubits

H. Weimer, N.Y Yao, M.D. Lukin

Phys. Rev. Lett. 110, 067601 (2013).

     Harnessing collective phenomena by utilizing ensembles of identical particles is a powerful tool, which has been exploited in effects ranging from superradiance to scattering suppression. The coherent dynamics resulting from interactions with individual constituents of an ensemble are often too weak to be observed directly; however, as evidenced by experiments in systems such as Rydberg atoms, cavity QED, atomic ensembles, and solid state qubits, collective enhancement provides a natural route to overcoming this challenge. In this work, we demonstrate that, for electronic spin quantum registers, such collective effects enable an extended coherent coupling over large distances — an essential prerequisite for quantum information processing.

 

Stretchable Photonic Crystal Cavity with Wide Frequency Tunability

C.L. Yu, H. Kim, N. de Leon, I.W. Frank, J.T. Robinson, M. McCutcheon, M.Z. Liu, M.D. Lukin, M. Loncar, H. Park

Nanoletters 13, 1: 248-525 (2013).

     Photonic crystals enable confinement and control of optical fields, providing a promising platform for on-chip photonic and optoelectronic devices such as lasers, filters, sensors, and quantum optical devices. One major technical goal for a variety of applications is to tune photonic crystals over a large frequency range. Previous demonstrations of photonic crystal tuning, such as heating/cooling, free carrier injection, phase change, and gas condensation, typically suffer from limited tuning ranges, and tuning beyond a small range often induces irreversible damage to the device. In this work, we demonstrate a new type of photonic crystal device, consisting of silicon nanowires embedded in a polymer substrate in a two-dimensional array. The high index of refraction of silicon enables tight light confinement, while the flexibility and stretchability of the polymer enables reversible tuning by stretching and compressing the structure. Using this platform, we have demonstrated a total reversible tuning range of 67 nm, or 130 times the resonance linewidth of the photonic crystal cavity.

 

Topological Flat Bands from Dipolar Spin Systems

N. Yao, C. Laumann, A. Gorshkov, S. Bennett, E. Demler, P. Zoller, M.D Lukin

Phys. Rev. Lett. 109, 266804 (2012).

     The discovery of the quantum Hall effect has revealed a rich variety of a physical phenomena associated with electron liquids in the presence of a strong magnetic field. These effects range from topologically protected edge transport to the splitting of the electron into fractional pieces. Now, the search is on for realizations of these exotic phenomena in non-electronic systems. In this paper, we propose that the rotational excitations in dipolar systems of spins or molecules may provide just such a platform. We theoretically predict that, in current experiments, controlled optical radiation can produces topologically non-trivial dispersion for the rotational excitations. This constitutes a roadmap for the realization of protected edge transport of rotational excitations, and the primary ingredient for the construction of more exotic fractionalized phases.

 

Free-standing Mechanical and Photonic Nanostructures in Single-crystal Diamond

M.J. Burek, N.P. de Leon, B.J. Shields, B.J.M Hausmann, Y.W. Chu, Q.M. Quan, A.S. Zibrov, H. Park, M.D. Lukin, M. Loncar

Nanoletters 12, 12: 6084-6089 (2012).

     A current area of interest is in the fabrication of novel nanophotonic structures in diamond to control interactions between NV centers and photons. In collaboration with the Loncar group, we have developed a technique for carving suspended diamond waveguides from a diamond slab by etching at an oblique angle to the substrate plane. By placing the substrate in a Faraday cage, the etching angle can be controlled and various device geometries can be achieved. This robust approach offers the possibility for highly scalable and uniform fabrication across an entire diamond chip and facilitates processing in demanding diamond substrates where, for example, the NV density may be highly localized, or additional post-processing may be required.

 

Nanoplasmonic Lattices for Ultra-cold Atoms

M. Gullans, T. Tiecke, D. Chang, J. Feist, J. Thompson, J. Cirac, P. Zoller, M. Lukin

Phys. Rev. Lett. 109, 235309 (2012).

     Optical lattices formed by interfering laser beams provide a highly controllable test bench for trapping many atoms at once and studying their interactions. Because of diffraction, however, the wavelength of the trapping light limits how close the atoms can get to one another, and thus the achievable atomic density. We propose a way to overcome this limitation with subwavelength confinement of light produced by nanoplasmonic structures.

     In the near field of a nanoparticle (i.e., at the distance of less than about one wavelength of a propagating wave), collective electron oscillations called plasmons can concentrate the electromagnetic field in regions much smaller than its wavelength. We show theoretically that the interference of an incident wave with the dipolar field it induces in a metallic particle creates a trapping region along the polarization direction of the field. In the case of silver nanospheres, slightly tuning the incident light to the blue side of an atomic resonance in rubidium should cause an atom to cling to the sphere. Arrays of nanoparticles could therefore act as anchors for an ordered lattice of ultracold atoms.

     With this method, lattice spacings could be reduced by about an order of magnitude, from 500 nanometers down to less than 60 nanometers. Should experimentalists realize such nanoplasmonic lattices in the lab, the increased atomic density would allow the exploration of new regimes of dense, ultracold quantum matter. [Adapted from Physics Synopsis by David Voss.]

 

Quantum Nonlinear Optics with Single Photons Enabled by Strongly Interacting Atoms

T. Peyronel, O. Firstenberg, Q.Y. Liang, S. Hofferberth, A.V. Gorshkov, T. Pohl, M.D. Lukin, V. Vuletic

Nature 488: 57-60 (2012).

     Realizing and engineering optical non-linearity at the level of single photons is a goal of scientific and technological significance, relevant to non-classical light sources, all-optical switches and phase gates, and correlated many-photon states. We obtain strong, long-range, interaction between propagating photons by coupling them to high-laying Rydberg levels in an atomic gas. The resulting ‘Rydberg polaritons’ possess a large electric dipole-moment and interact via the Van-der-Waals forces, while slowly traversing the medium. When the interaction is set to be dissipative, it results in a ‘photon blockade,’ suppressing the transmission of photon pairs. When the interaction is dispersive, we measure large conditional phase-shifts, pertaining to all-optical quantum phase-gates.

 

Sensing Distant Nuclear Spins with a Single Electron Spin

S. Kolkowitz, Q.P. Unterreithmeier, S.D. Bennett, M.D. Lukin

Phys. Rev. Lett. 109, 137601 (2012).

     The detection and control of magnetic signals from single nuclear spins is an important outstanding problem in science and technology, with applications ranging from quantum information to biology and chemistry. In this work we use the electronic spins of single nitrogen vacancy (NV) centers in diamond to identify and interact with distant individual carbon-13 (13C) nuclear spins in the surrounding diamond lattice. By tailoring our control sequences to the unique nuclear spin environment of each NV center, we are able to resolve and control mutliple distinct nuclear spins coupled to single NVs at 13C-NV distances approaching 1 nanometer. This technique offers the potential to considerably extend the size of nuclear spin quantum registers for quantum information storage, and is a promising step in the direction of magnetic resonance imaging (MRI) with single nuclear spins.

 

Room-temperature Quantum Bit Memory Exceeding One Second

P.C. Maurer, G. Kucsko, C. Latta, L. Jiang, N.Y. Yao, S.D. Bennett, F. Pastawski, D. Hunger, N. Chisholm, M. Markham, D.J. Twitchen, J.I. Cirac, M.D. Lukin

Science 336, 6086: 1283-1286 (2012).

     Many applications in quantum communication and quantum computation rely on the ability to maintain qubit coherence for extended periods of time. Furthermore, integrating such quantum-mechanical systems in compact mobile devices remains an outstanding experimental task. Although trapped ions and atoms can exhibit coherence times as long as minutes, they typically require a complex infrastructure involving laser cooling and ultra-high vacuum. Other systems, most notably ensembles of electronic and nuclear spins, have also achieved long coherence times in bulk electron spin resonance and nuclear magnetic resonance experiments; however, owing to their exceptional isolation, individual preparation, addressing, and high-fidelity measurement remain challenging, even at cryogenic temperatures.

     Our approach is based on an individual nuclear spin in a room-temperature solid. A nearby electronic spin is used to initialize the nuclear spin in a well-defined state and to read it out in a single shot with high fidelity. A combination of laser illumination and radio-frequency decoupling pulse sequences enables the extension of our qubit memory lifetime by nearly three orders of magnitude. This approach decouples the nuclear qubit from both the nearby electronic spin and other nuclear spins, demonstrating that dissipative decoupling can be a robust and effective tool for protecting coherence in various quantum information systems.

 

A Robust Scanning Diamond Sensor for Nanoscale Imaging with Single Nitrogen-Vacancy Centers

P. Maletinsky, S. Hong, M. S. Grinolds, B. Hausmann, M. D. Lukin, R. L. Walsworth, M. Loncar, A. Yacoby

Nature Nanotechnology 10: 320-324 (2012).

     The exceptional spin-coherence of nitrogen-vacancy (NV) defect centers in diamond allows for extremely sensitive optical measurements of magnetic fields. By incorporating a single NV-defect into the tip of an atomic force microscope, the Yacoby, Lukin and Loncar groups at Harvard have realized a scanning magnetometer with nm-scale spatial resolution, operating at room temperature. Applications include the measurement of the magnetic field of a single electron spin, the measurement of the domain structure of magnetic materials, investigation of the spin texture of exotic correlated-electron materials, etc. In addition, increasing the NV-spin coherence further could allow for an increase in magnetic field sensitivity to the level of a single nuclear spin, which would permit spatially-resolved, single-spin NMR. The latter would radically change the field of proteomics, which is currently limited by the time-consuming process of crystallizing proteins in order to perform x-ray diffraction.

 

Long-range Quantum Gates using Dipolar Crystals

H. Weimer, N.Y Yao, C.R. Laumann, M.D. Lukin

Phys. Rev. Lett. 108, 100501 (2012).

     The ability to carry out quantum gates between spatially remote qubits forms a crucial component of quantum information processing. Theoretical and exper- imental work addressing this challenge has largely been focused upon using photons, spin chains and other hybrid systems as quantum buses, which mediate long-range quantum information transfer. In these approaches, this transfer is achieved by either encoding the information in a traveling wavepacket, or by coupling the remote qubits to a shared spatially extended mode. In this work, we propose a novel approach to this outstanding problem and demonstrate that adi- abatic driving of a dipolar spin system across a quantum phase transition can be used to implement a robust controlled-phase (CP) gate. Our approach is particularly suitable for dipolar spin systems, composed, for example, of ultracold atoms and molecules, or solid-state spin ensembles, where natural imperfections invariably lead to disorder.

 

Scalable Architecture for a Room Temperature Soild-state Quantum Information Processor

N.Y. Yao, L. Jiang, A.V. Gorshkov, P.C. Maurer, G. Giedke, J.I. Cirac, M.D. Lukin

Nature Communications 3, 800 (2012).

     The realization of a scalable quantum information processor has emerged over the past decade as one of the central challenges at the interface of fundamental science and engineering. Much progress has been made towards this goal. Indeed, quantum operations have been demonstrated on several trapped ion qubits, and other solid-state systems are approaching similar levels of control. Extending these techniques to achieve fault-tolerant operations in larger systems with more qubits remains an extremely challenging goal, in part, due to the substantial technical complexity of current implementations. In this work, we propose and analyze an architecture for a scalable, solid-state quantum information processor capable of operating at room temperature. The architecture is applicable to realistic conditions, which include disorder and relevant decoherence mechanisms, and includes a hierarchy of control at successive length scales.

 

Quantum Interference of Single Photons from Remote Nitrogen-Vacancy Centers in Diamonds

A. Sipahigil, M.L. Goldman, E. Togan, Y. Chu, M. Markham, D.J. Twitchen, A.S. Zibrov, A. Kubanek, M.D. Lukin

Phys. Rev. Lett. 108, 143601 (2012).

     We have observed quantum interference between photons emitted by two nitrogen-vacancy centers located in distinct diamond samples separated by two meters. This result demonstrates our ability to produce photons that are indistinguishable – in frequency, polarization, and propagation direction – from solid-state single-photon emitters that are addressed independently. Satisfying the indistinguishability condition with a solid-state system is a non-trivial task because of natural variations in the local environment of each NV center. This result, combined with recent demonstrations of spin-photon entanglement and second long memory qubits, will allow quantum network experiments using NV centers in diamond.

 

Coherent Sensing of a Mechanical Resonator with a Single-Spin Orbit

S. Kolkowitz, A.C. Blezynski Jayich, Q. Unterreithmeier, S.D. Bennett, P. Rabl, J.G.E. Harris, M.D. Lukin

Science 335, 6076:1603-1606 (2012).

     Hybrid quantum systems offer many potential applications in quantum information science and quantum metrology. One promising hybrid system is a macroscopic mechanical resonator coupled to a single quantum bit, or qubit. In such a system the coupled mechanical resonator would act as a transducer to convert the state of the qubit into an optical or electrical signal, which could be used for efficient spin readout or to engineer long range spin-spin interactions for quantum information applications. In this work we demonstrate that the coherent evolution of a single electronic spin associated with a nitrogen vacancy (NV) center in diamond can be coupled to the motion of a magnetized mechanical resonator. We use coherent manipulation of the NV spin to detect both driven motion and thermally induced fluctuations of the resonator at ambient conditions. We also show that with future improvements, this technique could be used to detect mechanical zero-point fluctuations, and to implement mechanical quantum spin transducers.

 

Integrated Diamond Networks for Quantum Nanophotonics

B.J.M. Hausmann, B. Shields, Q. Quan, P. Maletinsky, M. McCutcheon, J.T. Choy, T.M. Babinec, A. Kubanek, A. Yacoby, M.D. Lukin, M. Loncar

Nanoletters 12, 3: 1578-1582 (2012).

     We have developed ring resonators to realize the building block of an all-diamond nanophotonic network operating at room temperature. Quality factors as high as 12,600 have been observed with bare ring resonators. A ring resonator with Q-factors as high as 3200 establishes coupling of single Nitrogen Vacancy centers to the cavity modes. This network node is evanescently connected to low loss waveguides on a SiO2/Si substrate. The routing of single photons along the network is observed by free-space extraction via grating couplers with overall efficiencies of 10%. The compact architecture and low loss material make our diamond platform suitable for large scale integration where multiple devices can be connected via single photon channels, thus enabling on-chip photonic networks.

 

Optomechanical Quantum Information Processing with Photons and Phonons

K. Stannigel, P. Komar, S.J.M Habraken, S.D. Bennett, M.D. Lukin, P. Zoller, P. Rabl

Phys. Rev. Lett. 109, 013603 (2012).

     The past few years have seen rapid development in the field of optomechanics, which is the interaction between light and mechanical motion. In its simplest form, an optomechanical system consists of a pair of reflecting mirrors forming a resonant optical cavity, where one of the mirrors is free to vibrate. The intensity of light in the cavity exerts a force on the mirror, and in turn the motion of the mirror modifies the length of the cavity and the frequency of the resonant light. One of the central questions in optomechanics is what happens in the extreme quantum limit where single light quanta (photons) or single mechanical quanta (phonons) become important. However, this limit is difficult to reach in the standard optomechanical setup with one optical and one mechanical mode, because of the very large frequency mismatch by more than seven orders of magnitude. In this work we propose that this limitation can be overcome by using two optical modes whose frequency difference is equal to the mechanical frequency, resulting in a resonant interaction between all three modes. We analyze the requirements to use this approach for quantum information applications such as generating single photons, building an optomechanical single photon transistor, or performing quantum logic between stored phonons.

 

Tailoring Light-matter Interactions with a Nanoscale Plasmon Resonator

N.P. de Leon, B. Shields, C.L. Yu, D.E. England, A.V. Akimov, M.D. Lukin, H. Park

Phys. Rev. Lett. 108, 226803 (2012).

     The interaction between light and matter can be enhanced by increasing the lifetime of a confined optical excitation, as in high quality factor dielectric resonators, or by reducing the effective mode volume of the confined radiation, as is currently explored in plasmonic nanostructures capable of confining light to dimensions well below the diffraction limit. Plasmonic resonators, which combine the benefits of both strategies, have potential for engineering light-matter interaction at nanoscales and achieving large coupling between an emitter and the radiation field. However, experimental realization of such structures has remained an outstanding challenge. In this work, we propose and experimentally demonstrate a new strategy to realize a plasmonic resonator with an exceptionally small mode volume and a moderate quality factor to drastically modify the interaction between a quantum emitter and surface plasmon polaritons. In our approach, we use a patterned dielectric around a plasmonic waveguide to define distributed bragg reflectors. The plasmonic waveguide is a chemically synthesized, highly crystalline, silver nanowire, which allows for long surface plasmon propagation lengths. With these devices, we demonstrate enhancement of spontaneous emission by a factor of 75, and we also show coupling and enhancement of the emission of a single nitrogen vacancy center in diamond to the cavity mode of a plasmon resonator.

 

Unforgeable Noise-tolerant Quantum Tokens

F. Pastawski, N.Y. Yao, L. Jiang, M.D. Lukin, J.I. Cirac

PNAS 109, 40: 16079-16082 (2012).

     The realization of devices that harness the laws of quantum mechanics represents an exciting challenge at the interface of modern technology and fundamental science. An exemplary paragon of the power of such quantum primitives is the concept of “quantum money”. A dishonest holder of a quantum bank-note will invariably fail in any forging attempts; indeed, under assumptions of ideal measurements and decoherence-free memories such security is guaranteed by the no-cloning theorem. In any practical situation, however, noise, decoherence and operational imperfections abound. Thus, the development of secure "quantum money"-type primitives capable of tolerating realistic infidelities is of both practical and fundamental importance. In this work, we propose a novel class of such protocols and demonstrate their tolerance to noise; moreover, we prove their rigorous security by determining tight fidelity thresholds. Our proposed protocols require only the ability to prepare, store and measure single qubit quantum memories, making their experimental realization accessible with current technologies.

 

Laser Cooling and Real-time Measurement of Nuclear Sping Environment using a Solid-state Qubit

E. Togan, Y. Chu, A. Imamoglu, M.D. Lukin

Nature 478: 497-501 (2011).

     The NV center's electronic spin interacts with a collection of nuclear spins in its environment, for example the 14N spin of the NV itself and 13C spins in the diamond lattice. This nuclear spin bath can cause decoherence for quantum information stored in the NV, but can also used as a resource if they can be controlled. In this experiment, we use laser excitation of the NV center to probe and manipulate the nuclear spin bath. For particular states of the spins bath, the NV center can become trapped in a "dark state" that becomes decoupled from the optical field. By monitoring the fluorescence of the NV center, we can not only determine the state of the nuclear spins, but also prepare particular nuclear configurations through optical pumping or measurement of the NV center's electronic state. This method could allow us to probe interesting dynamics of nuclear spins on fast time scales and improve the performance of NV spin-based magnetic field sensors.

 

Photon-photon Interactions via Rydberg Blockade

A.V. Gorshkov, J. Otterbach, M. Fleischhauer, T. Pohl, M.D. Lukin

Phys. Rev. Lett. 107, 133602 (2011).

     A crucial building block of many quantum information applications is the implementation of a photonic phase-gate with a high degree of controllability and fidelity. However photon-photon interactions based on local optical nonlinearities mediated by atoms are generally weak and their creation requires elaborate schemes and advanced experimental techniques.

    This paper discusses a promising alternative towards the generation of strong and non-local interactions between photons propagating through a medium exhibiting electromagnetically induced transparency (EIT) coupled to high-lying Rydberg states. In usual EIT media, the presence of a strong control field renders the medium transparent for a weak resonant probe field, that would otherwise be absorbed. However, due to the strong van-der-Waals interactions between Rydberg atoms it is not possible to excite two atoms into their Rydberg state, if they are closer than the so-called blockade radius. This phenomenon, known as Rydberg blockade, consequently destroys EIT for photons that are within the blockaded region. As a result photons inside this region will be absorbed and the remaining ones keep a distance that is larger than the blockade radius, leading to anti-bunched photons and implying strong photon-photon interactions (see figure).

     On the other hand, using probe photons that are far off-resonant with the opaque transition, such that they are not absorbed in absence of EIT, will experience a shift in the refractive index when passing through the blockaded region. This conditional depends on the presence of a second photon in the system and such presents a direct implementation of a photon-photon phase-gate.

     Summarized the combination of EIT with strong, long-range Rydberg interactions present a promising route towards the generation of strongly correlated photonic many-body states and implementations of photonic phase-gates.