|
Alipasha Vaziri |
|
Current Research
New approaches to structural and functional imaging Howard Hughes Medical Institute, Janelia Farm Research Campus
My recent work at Janelia Farm is directed on bringing the body of knowledge encompassed in modern optics to the imaging problem in neuroscience. This includes nonlinear optics, quantum optics and ultra fast optics. The techniques under development provide among others high axial sectioning, minimal biological damage and high resolution high contrast functional imaging of neural activity. One of my long term goals is to utilize the techniques used in quantum optics and ultra fast spectroscopy to gain insight into the dynamics of the neural network on the molecular level.
Past Research
Quantum phenomena in Bose Einstein Condensates (BEC) Experiments at the Laser Cooling and Trapping Group at NIST
Macroscopic rotational quantum superposition states of a BEC Using superpositions of different orbital angular momentum states of photons we could experimentally demonstrate the coherent transfer of these states to a Sodium Bose Einstein Condensate (BEC). In this experiment a standing wave of light carrying orbital angular momentum is exposed to the atoms of a BEC held in a magnetic trap. Each atom in the BEC absorbs one photon from one laser beam and emitted one photon in the path of the other laser beam, picking up the difference in orbital angular momentum between the two photons. The interference of the two counter propagating beams carrying orbital angular momentum creates a corkscrew-like interference pattern, inducing the BEC to rotate. By using two beams in which each photon is in a superposition state of carrying one unit of orbital angular momentum and carrying no orbital angular momentum the atoms in the condensate were put in a superposition state of rotation and non-rotation. Such an experiment is the first step toward the creation of macroscopic quantum states or “cat states” http://www.nist.gov/public_affairs/techbeat/tb2006_1109.htm#tornado M. F. Andersen, C. Ryu, P. Claude, V. Natarajan, A. Vaziri, K. Helmerson and W. D. Phillips, Quantized Rotation of Atoms From Photons with Orbital Angular Momentum, Phys. Rev. Lett. 97 170406 (2006)
Higher order quantum resonances in a BEC The δ-kicked rotor is a paradigm in classical chaos theory, in which a pulsed external torque is applied to a rotor. The quantum analogue of this system exhibits non-classical phenomena such as dynamical localization and quantum resonances, in which the energy transferred to the rotor is less (localization) or greater (resonances) than in the classical case. We experimentally observe quantum resonances by applying kicks, using short pulses of a standing wave of laser light, to a Bose-Einstein condensate of sodium atoms; each pulse diffracts the atoms and populates several momentum states. The system’s dynamics depend on the atoms’ initial quasimomentum, and the time interval between successive kicks. These dynamics can be characterized by the evolution of the atomic momentum distribution with time, and by the dependence of atoms’ mean kinetic energy on the number and time period of the δ-kicks. We observe the phenomena of dynamical localization and quantum resonances and investigate their parametric dependence. Depending on the quasimomentum and kicking period, low- and high-order quantum resonances and anti-resonances may be observed. Click here for a more detail presentation C. Ryu, M. F. Anderson, A. Vaziri, M. B. d'Arcy, J. M. Grossmann, K. Helmerson and W.D. Phillips, Higher-Order Quantum Resonances Observed in a Periodically Kicked Bose-Einstein Condensate, Phys. Rev. Lett. Vol 96, 160403 (2006)
Higher dimensional quantum entanglement
Quantum entanglement of orbital angular momentum of light Entangled quantum states are not separable, regardless of the spatial separation of their components. This is a manifestation of an aspect of quantum mechanics known as quantum nonlocality. An important consequence of this is that the measurement of the state of one particle in a two-particle entangled state defines the state of the second particle instantaneously, whereas neither particle possesses its own well-defined state before the measurement. So far, experimental realizations of entanglement have hitherto been restricted to two-state quantum systems involving, for example, the two orthogonal polarization states of photons. We have been able to extend this concept to multilevel systems and thereby create higher dimensional entangled states. Higher dimensional entangled states allow the realization of certain quantum communication protocols which are fundamentally not possible to realize by using two dimensional entanglement. Moreover these states are more robust to environmental noise which is the main source of decoherence in practical quantum communication. As a source for higher dimensional entanglement we have used the orbital angular momentum of light which is distinct from its spin angular momentum associated with its polarization. The wave front of light carrying orbital angular momentum has a helical phase structure with a phase singularity in the center of axis of propagation. As a result the amplitude of the electromagnetic field has to vanish which creates the typical “donut” shape on these modes. In a series of experiments we have first demonstrated the entanglement of the orbital angular momentum of light and then demonstrated several quantum protocols using this type of entanglement. |
|
A. Mair, A. Vaziri, G. Weihs and A. Zeilinger, Entanglement of the orbital angular momentum states of photons, Nature. 412, 313 (2001)
Quantum cryptography with qutrits We produce two identical keys using, for the first time, entangled trinary quantum systems (qutrits) for quantum key distribution. The advantage of qutrits over the normally used binary quantum systems is an increased coding density and a higher security margin. The orbital angular momentum is controlled with phase holograms. In an Ekert-type protocol the violation of a three-dimensional Bell inequality verifies the security of the generated keys. A key is obtained with a qutrit error rate of approximately 10%. S. Gröblacher, T. Jennewein, A. Vaziri, G. Weihs, A. Zeilinger, Experimental Quantum Cryptography with Qutrits, NJP Vol. 8, 75, (2006)
Experimental quantum coin tossing This protocol belongs to a class of “two-party” cryptographic problems, where the communication partners distrust each other. As with a number of such two-party protocols, the best implementation of the quantum coin tossing requires qutrits, resulting in a higher security than using qubits. In this way, we have also performed the first complete quantum communication protocol with qutrits. In our experiment the two partners succeeded to remotely toss a row of coins using photons entangled in the orbital angular momentum. Click here for a more detail presentation G. Molina-Terriza, A. Vaziri, R. Ursin, A. Zeilinger, Experimental Quantum Coin Tossing, Phys. Rev. Lett. Vol. 94, 040501 (2005)
Concatenation of higher dimensional entanglement Enhancement of entanglement is necessary for most quantum communication protocols many of which are defined in Hilbert spaces larger than two. We have developed an experimental technique for entanglement concentration of orbital angular momentum entangled photons. It’s based on selective local filtering where we have investigated the specific case of three dimensions and the possibility of generating different entangled states out of an initial state. Our results are applicable to pure states as well as to mixed states.
|
|
A. Vaziri, J. W. Pan, T. Jennewein, G. Weihs and A. Zeilinger, Concentration of Higher Dimensional Entanglement , Phys. Rev. Lett. Vol. 91, 227902 (2003)
Violation of Bell’s inequality using two-photon three-dimensional quantum entanglement Orbital angular momentum entangled photons emitted by a down-conversion source are in higher dimensional entangled states. We have experimentally confirmed the violation of a generalized Clauser-Horne-Shimony-Holt–type Bell inequality in three dimensions by more than 18 standard deviations. Higher dimensional entangled states allow the realization of new types of quantum communication protocols. They also provide a more secure quantum cryptography scheme. Therefore our experimental results are likely to have applications in future quantum communication technology. |
|
A. Vaziri, G. Weihs and A. Zeilinger, Experimental Two-Photon Three-Dimensional Quantum Entanglement , Phys. Rev. Lett. Vol. 89 Nr. 24 240401-1 (2002) |