Kent A. G. Fisher

Experimental investigation of the uncertainty principle in the presence of quantum memory and its application to witnessing entanglement

The uncertainty principle tells us that two associated properties of a particle cannot be simultaneously known with infinite precision. However, if the particle is entangled with a quantum memory, the uncertainty of a measurement is reduced. This concept is now observed experimentally.

Experimental nonlocal and surreal Bohmian trajectories

Researchers reconstruct trajectories of two entangled photons showing the nonlocal and “surreal” nature of the trajectories in Bohmian mechanics. Weak measurement allows one to empirically determine a set of average trajectories for an ensemble of quantum particles. However, when two particles are entangled, the trajectories of the first particle can depend nonlocally on the position of the second particle. Moreover, the theory describing these trajectories, called Bohmian mechanics, predicts trajectories that were at first deemed “surreal” when the second particle is used to probe the position of the first particle. We entangle two photons and determine a set of Bohmian trajectories for one of them using weak measurements and postselection. We show that the trajectories seem surreal only if one ignores their manifest nonlocality.

Storage and retrieval of THz-bandwidth single photons using a room-temperature diamond quantum memory.

We report the storage and retrieval of single photons, via a quantum memory, in the optical phonons of a room-temperature bulk diamond. The THz-bandwidth heralded photons are generated by spontaneous parametric down-conversion and mapped to phonons via a Raman transition, stored for a variable delay, and released on demand. The second-order correlation of the memory output is g((2))(0)=0.65±0.07, demonstrating a preservation of nonclassical photon statistics throughout storage and retrieval. The memory is low noise, high speed and broadly tunable; it therefore promises to be a versatile light-matter interface for local quantum processing applications.

Experimental three-photon quantum nonlocality under strict locality conditions

Violation of the classical bound of the three-particle Mermin inequality by nine standard deviations is experimentally demonstrated by closing both the locality and freedom-of-choice loopholes; only the fair-sampling assumption is required. To achieve this, a light source for producing entangled multiphoton states and measurement technologies for precise timing and efficient detection were developed.

Frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory.

The spectral manipulation of photons is essential for linking components in a quantum network. Large frequency shifts are needed for conversion between optical and telecommunication frequencies, while smaller shifts are useful for frequency-multiplexing quantum systems, in the same way that wavelength division multiplexing is used in classical communications. Here we demonstrate frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory. Heralded 723.5 nm photons, with 4.1 nm bandwidth, are stored as optical phonons in the diamond via a Raman transition. Upon retrieval from the diamond memory, the spectral shape of the photons is determined by a tunable read pulse through the reverse Raman transition. We report central frequency tunability over 4.2 times the input bandwidth, and bandwidth modulation between 0.5 and 1.9 times the input bandwidth. Our results demonstrate the potential for diamond, and Raman memories in general, as an integrated platform for photon storage and spectral conversion.Kent A.G. Fisher,1 Duncan G. England,2 Jean-Philippe W. MacLean,1 Philip J. Bustard,2 Kevin J. Resch,1 and Benjamin J. Sussman2, 3 Institute for Quantum Computing and Department of Physics & Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, K1A 0R6, Canada Physics Department, University of Ottawa, 150 Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canada (Dated: September 21, 2015)

Effect of pseudogap formation on the penetration depth of underdoped high- T c cuprates

The penetration depth is calculated over the entire doping range of the cuprate phase diagram with emphasis on the underdoped regime. Pseudogap formation on approaching the Mott transition, for doping below a quantum critical point, is described within a model based on the resonating valence bond spin liquid which provides an ansatz for the coherent piece of the Greens function. Fermi surface reconstruction, which is an essential element of the model, has a strong effect on the superfluid density at T=0 producing a sharp drop in magnitude, but does not change the slope of the linear low temperature variation. Comparison with recent data on Bi-based cuprates provides validation of the theory and shows that the effects of correlations, captured by Gutzwiller factors, are essential for a qualitative understanding of the data. We find that the Ferrell-Glover-Tinkham sum rule still holds and we compare our results with those for the Fermi arc and the nodal liquid models.

Optimal linear optical implementation of a single-qubit damping channel

We experimentally demonstrate a single-qubit decohering quantum channel using linear optics. We implement the channel, whose special cases include the familiar amplitude-damping channel and the bit-flip channel, using a single, static optical setup. Following a recent theoretical result (Piani et al 2011 Phys. Rev. A 84 032304), we realize the channel in an optimal way, maximizing the probability of success, i.e. the probability for the photonic qubit to remain in its encoding. Using a two-photon entangled resource, we characterize the channel using ancilla-assisted process tomography and find average process fidelities of 0.98±0.01 and 0.976±0.009 for amplitude-damping and the bit-flip case, respectively.

Pure-state tomography with the expectation value of Pauli operators

Xian Ma,1, 2 Tyler Jackson,3, 1 Hui Zhou,4, 5 Jianxin Chen,6 Dawei Lu,1, 2 Michael D. Mazurek,1, 2 Kent A. G. Fisher,1, 2 Xinhua Peng,4, 5, 1 David Kribs,3, 1 Kevin J. Resch,1, 2 Zhengfeng Ji,1 Bei Zeng,3, 1, 7 and Raymond Laflamme1, 2, 7, 8 Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Department of Mathematics & Statistics, University of Guelph, Guelph, Ontario, N1G 2W1, Canada Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230036, China Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Joint Center for Quantum Information and Computer Science, University of Maryland, College Park, Maryland, USA Canadian Institute for Advanced Research, Toronto, Ontario, M5G 1Z8, Canada Perimeter Institute for Theoretical Physics, Waterloo, Ontario, N2L 2Y5, Canada (Dated: January 21, 2016)

Certifying the Presence of a Photonic Qubit by Splitting It in Two

We present an implementation of photonic qubit precertification that performs the delicate task of detecting the presence of a flying photon without destroying its qubit state, allowing loss-sensitive quantum cryptography and tests of nonlocality even over long distance. By splitting an incoming single photon in two via parametric down-conversion, we herald the photons arrival from an independent photon source while preserving its quantum information with up to (92.3±0.6)% fidelity. With reduced detector dark counts, precertification will be immediately useful in quantum communication.

Electron pockets and pseudogap Dirac point in underdoped cuprate superconductors

We consider a model of the pseudogap specifically designed to describe the underdoped cuprates and which exhibits particle-hole asymmetry. The presence of electron pockets, besides the usual hole pockets, leads to the appearance of new vectors beyond the usual so-called octet model in the joint density of states (JDOS), which underlies the analysis of Fourier-transform scanning tunneling spectroscopy (FT-STS) data. These new vectors are associated with distinct patterns of large amplitude in the JDOS and are expected to occur primarily at positive bias. Likewise a pseudogap Dirac point occurs at positive bias and this point can be determined either through FT-STS or through extrapolation of data from the autocorrelation function of angle-resolved photoemission spectroscopy.