Invited
Speaker Abstracts

Scott Aaronson, MIT
Quantum Computing as 21stCentury Bell Inequality
Violation
I'll advocate a perspective on quantum computing that regards
it as almost exactly a 21stcentury analogue of Bell inequality
violation: that is, an attempt to prove that Nature can't
have a classical simulation of a certain kind, with any
practical applications coming as a "bonus." I'll
argue that taking this perspective leads us to consider
quantum computing proposals that could be easier to realize
than universal quantum computing (or, say, Shor's factoring
algorithm), but for which we still have complexitytheoretic
evidence for a quantum advantage. I'll discuss the example
of BosonSampling (proposed by Alex Arkhipov and myself in
2011), and survey the progress toward realizing it.
Robert Alicki, University of
Gdansk
Thermodynamical cost of accuracy and stability of information
processing
A model of a spin1/2 strongly coupled to a quantum harmonic
oscillator and weakly interacting with a heat bath is used
to study the minimal thermodynamical cost of a binary quantum
measurement. It is shown that this cost is equivalent to
a cost of elementary gate performed on the protected single
bit. The irreversible dynamics of this system is described
by a quantum dynamical semigroup derived from the underlying
Hamiltonian model. The formula which gives the relation
between stability of encoded bit with respect to combined
thermal and quantum noise, accuracy of its readout, and
work needed to perform an elementary gate is presented.
It differs from the standard Landauer principle and allows
to estimate the minimal work needed to perform long computations.
Finally, these results illustrate the fundamental conflict
between stability and irreversibility of information processing
what makes the difference between feasibility of scalable
classical and quantum computations.
References
R. Alicki, arXiv:1305.4910; see also video by Lidia del
Rio and
Philipp Kammerlander http://www.youtube.com/watch?v=gtcPp7FY0gU
Mohammad Amin, DWave Systems Inc.
Entanglement in a quantum annealing processor
Entanglement is believed to be essential for any quantum
algorithm that is designed to solve intractable problems.
In this talk, I present experimental evidence for existence
of entanglement within DWave's 512qubit quantum annealing
processor. I provide evidence of 2 and 8qubit thermal
entanglement at three levels: First, using quantum tunneling
spectroscopy we directly observe an anticrossing between
two ferromagnetically ordered states. The observed ground
and first excited states at the center of the anticrossing
are close to the maximally entangled GHZ states. The energy
spectrum, especially the minimum gap, agrees well with the
predictions of the Hamiltonian obtained from independently
measured parameters (with no free parameters). Second, we
calculate the density matrix using the Hamiltonian and the
eigenstate populations that are directly measured. The density
matrix allows characterization of the mixed state entanglement
via known entanglement measures. Third, we introduce a new
susceptibilitybased witness for the groundstate entanglement
that does not require detailed knowledge of the Hamiltonian.
Using independently measured linear crosssusceptibilities
we demonstrate ground state entanglement through nonzero
values of the witness. All three levels of evidence indicate
that the processor has access to the robust thermal entanglement
during quantum annealing.
Andrew Cross, IBM T. J. Watson
Research Center
Entangling gates for superconducting qubits and robustness
of randomized benchmarking
Superconducting qubits show considerable potential for
realizing solid state quantum processors. I will describe
the architecture that IBM is investigating for a logical
qubit based on the surface code. Realizing this architecture
requires further investigation of two qubit gates and characterization
of residual errors. In this direction, I will present a
new experimentally demonstrated two qubit gate that requires
only microwave control, and I will discuss numerical results
on the performance of randomized benchmarking in the presence
of realistic noise, including systematic errors, leakage,
and correlated noise.
Joseph Emerson, University of
Waterloo
Negative quasiprobability, contextuality and magic are
equivalent resources for quantum computation
I will present results identifying necessary resources
for universal quantum computation using qudit systems (powers
of odd prime). First, I show that negative quasiprobability
in a distinguished representation is a necessary resource
for universal quantum computation with stabilizer codes
via magicstate distillation. This condition defines a natural
boundary in the space of quantum states which includes the
stabilizer polytope as a strict subset, and hence identifies
a large class of "bound magic states". I then
show that this negativity boundary coincides with a boundary
for contextuality in the graphtheoretic framework recently
proposed by Cabello, Severini and Winter. Timepermitting,
I will discuss a resourcetheory of magic and introduce
the concept of "mana", which is a computable magicness
monotone that can bound the overhead cost of magicstate
distillation.
Joint work with: Victor Veitch, Mark Howard, Dan Gottesman,
and Ali Hamed.
Andrew N. Jordan, University
of Rochester
Action Principle for Continuous Quantum Measurement
The process of continuous quantum measurement will be formulated
in terms of a stochastic path integral encoding every possible
quantum trajectory, the probability density of those trajectories,
the continuous measurement results, and the state disturbance.
This approach gives a new way to calculate any expectation
value or correlation function of the measurement result
or state. As an application of this approach, we provide
an answer to the question of what is the most likely path
the quantum state takes in its state space between a preselected
and a postselected state, separated by a fixed time. We
show how this answer may be found from a least action principle
and illustrate how it is important for the theory of quantum
jumping of a qubit in the Zeno measurement limit.
Reference:
Action principle for continuous quantum measurement.
A. Chantasri, J. Dressel, A. N. Jordan
arXiv:1305.5201
Paul Kwiat, University of Illinois
at UrbanaChampaign
The Death of Nonlocality
Ever since John Bell showed that quantum mechanics could
give different predictions from a local realistic model
in an appropriate experiment, there has been wide interest
in carrying out such a test. Unfortunately, to date no truly
unambiguous test has ever been completed, due to the existence
of two experimental loopholes: the “locality loophole”
and the “detection loophole”. Each of these have
been closed individually in different experimental systems
–photons, atoms, ions, and superconductors – but
no system has simultaneously closed each of the loopholes.
In fact, here we argue that until now, no system has closed
both of the loopholes, even independently. Here we present
a photonbased experiment that violates the Bell inequality,
free of the detection loophole for the first time. This
experiment has enough “efficiency overhead" to
eventually perform a fully loopholefree test of local realism,
and the high entanglement quality of the source allows us
to perform precise tests of the upper limit of quantum correlations.
Finally, we have used this source to generate secure quantum
random numbers at rates several orders of magnitude beyond
previous experiments.
Shunlong Luo, Chinese Academy of
Sciences
Discord versus Entanglement
Correlations in quantum world are basic, subtle, and complex,
and play important roles in quantum foundations and quantum
information processing. In this talk, we present a historic
review of some aspects of quantum correlations from the
perspectives of quantum measurements and information transferring,
with emphasis on quantum discord and its various features.
We further highlight the fundamental and intrinsic relationships
between quantum discord and entanglement.
Marco Merkli, Memorial University
Dynamics of open quantum systems via resonances
We consider the dynamics of a quantum system consisting
of a `system of interest' coupled to an `environment'. We
explain how the evolution of the full system plus
environment can be expressed in terms of exponentials of
complex energies (resonances). When reduced to the
open system, they give its irreversible dynamics. This approach
is mathematically rigorous and valid for all times. We present
some applications of this method. In particular, we compare
the two regimes of isolated and of overlapping resonances,
in which the typical system energy gaps are much larger
and much smaller than the systemreservoir coupling, respectively.
Christopher Monroe, JQI and
University of Maryland
Quantum Magnetism from the Bottom Up
Crystals of lasercooled atomic ions are standards for
quantum information science, with psuedospins within each
atom representing qubits that have unsurpassed levels of
quantum coherence and can be measured with nearperfect
efficiency. When spindependent optical dipole forces are
applied to a collection of atomic ions, their Coulomb interaction
is modulated in a way that allows the tailoring of spinspin
interactions that are found in theories of quantum magnetism.
Recent experiments have implemented variablerange Ising
interactions with up to 16 trapped ion spins, the largest
system of interacting qubits assembled to date. Direct measurements
of spinspin correlations has shown the emergence of antiferromagnetic
order in this highly frustrated system as well as coherent
nonequilibrium dynamics following a quench. Soon the number
of spins will be high enough where no classical computer
can predict the behavior of such a fullyconnected quantum
magnet, allowing a direct quantum simulation of the murky
behavior of quantum spin liquids and spin glasses, the measurement
of entanglement near a quantum phase transition, and investigations
in the thermalization of a closed quantum system.
William J. Munro, NTT Basic Research
Labs
Hybrid Quantum Systems: a route forward for distributed
information processing
The development of moderate scale quantum devices requires
stable quantum bits in which we can process, store and transport
quantum information. A single physical system is unlikely
to achieve all of these tasks efficiently and so a natural
solution is to examine hybrid system approaches, where we
can utilize the best aspects of the individual parts. We
demonstrate a hybrid system composed of a superconducting
flux qubit (a processor) directly but selectively coupled
to an ensemble of nitrogenvacancy centers (a memory) and
show how information can be transferred from the flux qubit
to the memory, stored and subsequently retrieved. With NV
centers possessing a natural optical transition one can
envisage the transform of information between the optical
and microwave regimes (an optical interface). With information
transport between superconducting qubits already realized,
a simple scalable integrated quantum hybrid device seems
achievable.
Joshua Nunn, Oxford University
Generating entanglement in solids, and detecting entanglement
for QKD.
The talk is in two parts. First we describe a recent demonstration
of entanglement between the vibrational modes of a pair
of remote diamond crystals, by means of Raman scattering.
In the second part we introduce a new protocol for quantum
key distribution with spectrally entangled photon pairs,
based on timetofrequency conversion, and discuss the security
of the protocol.
Kenji Ohmori, Institute for Molecular
Science
Ultrafast Coherent Control of an Ultracold Rydberg Gas
We have developed spatiotemporal coherentcontrol in which
the ultrafast wavepacket interference in a molecule is
visualized and controlled with precisions on the picometer
spatial and attosecond temporal scales [15]. Here we apply
this highprecision coherent control to quantum manybody
systems of ultracold Rydberg atoms.
A picosecond laser pulse produces Rydberg electronic wavepackets
in lasercooled Rb atoms in an optical dipole trap. We have
measured the temporal evolution of those Rydberg wavepackets
and their Ramsey interference as a function of the atom
density in the Rydberg gas. We have observed that the temporal
evolution changes, and the interferogram is phaseshifted
when we change the atom density. These results indicate
that the interatomic interactions have successfully been
triggered by a Rydberg wavepacket generated in each Rb
atom. This approach could lead to the development of a novel
simulator of quantum manybody dynamics.
References
[1] H. Katsuki and K. Ohmori et. al., Science 311, 1589
(2006).
[2] H. Katsuki and K. Ohmori et. al., Phys. Rev. Lett. 102,
103602 (2009).
[3] K. Hosaka and K. Ohmori et al., Phys. Rev. Lett. 104,
180501 (2010)
(Highlighted by Nature 465, 138 (2010); Physics 3, 38 (2010)).
[4] H. Goto and K. Ohmori et al., Nature Physics 7, 383
(2011).
(Highlighted by Nature Physics 7, 373 (2011); Nature Photonics
5, 382 (2011)).
[5] H. Katsuki, Y. Kayanuma, and K. Ohmori, Phys. Rev. B
88, 014507 (2013).
Martin B. Plenio, Ulm University
Diamond Quantum Devices and Biology
The detection of smallest magnetic fields emanating for
example from small number of nuclear and electronic spins
holds the promise for a wide variety of applications. The
presence of environmental noise presents a key obstacle
on the path towards this goal. Considering a single colour
center in diamond I will explain how resonance conditions
can be exploited to achieve the dual of goal of sensing
minute fields while protecting against environmental noise.
I will proceed to explain how such a sensor may lead to
the design of a novel room temperature quantum simulator
on the basis of nuclear or electron spin arrays in diamond.
Finally, I will present a novel approach towards nanoscale
quantum devices in which regular arrays of nanodiamonds
created by exploiting the selfassembly capabilities of
biological systems.
[1] J.M. Cai, F. Jelezko, N. Katz, A. Retzker and M.B. Plenio.
New J. Phys. 14, 093030 (2012)
[2] J. Cai, B. Naydenov, R. Pfeiffer, L. McGuiness, K.D.
Jahnke, F. Jelezko, M.B. Plenio and A. Retzker. New J. Phys.
14, 113023 (2012)
[3] J.M. Cai, F. Jelezko, M.B. Plenio, A. Retzker. New J.
Phys. 15, 013020
(2013)
[4] J.M. Cai, A. Retzker, F. Jelezko and M.B. Plenio. Nature
Physics 9, 168 173 (2013)
[5] A. Ermakova, G. Pramanik, J.M. Cai, G. AlgaraSiller,
U. Kaiser, T. Weil, H.C. Chang, L.P. McGuinness, M.B. Plenio,
B. Naydenov and F. Jelezko. Nano Letters (2013)
[6] A. Albrecht, A. Retzker, G. Koplovitz, F. Jelezko, S.
Yochelis, Y. Nevo, O. Shoseyov, Y. Paltiel and M.B. Plenio.
Eprint arXiv:1301.1871 [quantph]
[7] P. London, J. Scheuer, J.M. Cai, I. Schwarz, A. Retzker,
M.B. Plenio, M. Katagiri, T. Teraji, S. Koizumi, J. Isoya,
R. Fischer, L.P. McGuinness, B. Naydenov and F. Jelezko.
To appear in Phys. Rev. Lett. 2013 and Eprint arXiv:1304.4709
Eugene Polzik, University of
Copenhagen
Deterministic teleportation of spin dynamics and other
nonlocal protocols with atomic ensembles
Continuous in phase space and in time interaction of spin
ensembles with light combined with continuous measurement
has allowed for realization of a broad variety of novel
quantum information protocols. Generation of steady state
entanglement between distant atomic objects [1], measurement
of magnetic fields beyond the standard quantum limit [2]
and stroboscopic teleportation of atomic dynamics [3] are
the most recent examples. In the experiments quantum variables
of interest are encoded in the collective spin of a macroscopic
atomic ensemble. Entanglement required for the protocols
is distributed by light propagating from one ensemble to
the other. Homodyne measurements on light ensure the deterministic
and continuous character of the protocols which succeed
in every attempt. In a recent proposal [4] this platform
has been shown to be capable of teleportation of continuous
atomic dynamics and simulation of nonlocal atomic interactions.
Hybrid protocols demonstrating continuous variable approach
to a nonGaussian atomic excitation are in progress. The
protocols demonstrated for atomic ensembles are applicable
to other important systems, such as mechanical oscillators
coupled to light or spin ensembles coupled to microwaves.
References
1.Entanglement generated by dissipation and steady state
entanglement of two macroscopic objects. H. Krauter, C.
Muschik, K. Jensen, W. Wasilewski, J. M. Petersen, J. I.
Cirac, and E. S. Polzik. Phys. Rev. Lett. 107, 080503 (2011).
2.Quantum noise limited and entanglementassisted magnetometry.
W. Wasilewski, K. Jensen, H. Krauter, J. Renema, M. Balabas,
and E.S. Polzik. Phys. Rev. Lett., 104, 133601 (2010).
3.Deterministic quantum teleportation between distant atomic
objects. H. Krauter, D. Salart, C. A. Muschik, J. M. Petersen,
Heng Shen, T. Fernholz, and E. S. Polzik. Nature Phys.,
(July 2013).
4.Quantum teleportation of dynamics and effective interactions
between remote systems. C. A. Muschik, K. Hammerer, E. S.
Polzik, and I. J. Cirac. Phys. Rev. Lett. 111, 020501 (2013).
Michael G. Raymer, University
of Oregon
Quantum Frequency Conversion of States of Light
Fourwave mixing in thirdorder nonlinear optical media
can exchange (or swap) the quantum states between two narrow
spectral bands of the optical spectrum. [1] When one spectral
band is occupied by a singlephoton wavepacket state, and
the other band is occupied by vacuum, this process can achieve
quantum frequency conversion (QFC)  changing the carrier
frequency of the photon, as demonstrated experimentally
in [2]. When both spectral bands are occupied by singlephoton
wavepacket states, twophoton (HongOuMandel) interference
is predicted to create the state 20> + 02>, that
is, the two photons end up with the same color, but that
color is indeterminate. [3] By tailoring the phasematching
conditions, one can achieve selectivity for different temporally
orthogonal wave packets, creating add/drop functionality
for a quantum internet. These operations can also be accomplished
using threewave mixing in secondorder nonlinear optical
media, with constraints on the smallness of the frequency
shifts achievable. [4, 5]
1. "Translation of quantum states by fourwave mixing
in fibers," C. J. McKinstrie, J. D. Harvey, S. Radic
and M. G. Raymer, Opt. Express 13, 9131 (2005).
2. "Quantum frequency translation of singlephoton
states in photonic crystal fiber,"
H.J. McGuinness, M.G. Raymer, C.J. McKinstrie, and S. Radic,
Phys. Rev. Lett. 105, 093604 (2010).
3. "Interference of two photons of different color,"
M. G. Raymer, S. J. van Enk, C. J. McKinstrie, and H. J.
McGuinness, Opt. Commun. 238, 747 (2010).
4. "A quantum pulse gate based on spectrally engineered
sum frequency generation," A. Eckstein, B. Brecht,
and C. Silberhorn, Opt. Express 19, 13770 (2011).
5. "Temporal mode selectivity by frequency conversion
in secondorder nonlinear optical waveguides," D. V.
Reddy, M. G. Raymer, C. J. McKinstrie, L. Mejling, and K.
Rottwitt (Opt. Express, 2013).
Moshe Shapiro, The University
of British Columbia
The quantum dynamics experienced by a single molecular
eigenstate excited by incoherent light
Contrary to conventional wisdom that all dynamics is a
result of interference (or ``dephasing'') between many (at
least 2) energy eigenstates, we show that when a continuum
of states is present, even a single molecular eigenstate
undergoes ``steadystate'' quantum dynamics. Moreover, this
type of dynamics can be initiated by incoherent (e.g., solar)
light sources. Continua are invariably involved in molecular
systems due to a variety of sources such as the ever present
bath modes; spontaneously emitted photons; the detachment
of electrons; or the dissociation of chemical bonds. Contrary
to a single bound energyeigenfunction which is a real (``standingwaves'')
function that carries no flux, hence has no dynamics, a
single (complex) continuum energyeigenfunction carries
``steadystate'' flux given by the group velocity of the
energetically narrow wave packet it represents. When this
energy eigenfunction is a multimode resonance embedded
in a continuum via a chain of intramolecular couplings,
this dynamics may be initiated by any (light) source, and
is controlled, contrary to coherent wave packet dynamics,
by the position of the resonance rather than its width.
Yaron Silberberg, The Weizmann
Institute of Science
Quantum Walks in Photonic Lattices
Photonic circuits are a natural choice for experimental
studies of quantum walks because of their excellent transmission
properties and robustness to various decoherence processes.
We have shown that the propagation of photons in waveguide
lattices are essentially an implementation of continuoustime
quantum walks. These systems enable implementation of large
scale, decoherencefree quantum walks exhibiting, in periodic
lattices, linear expansion vs time. In this talk I shall
review our studies on the emergence of unique quantum correlations
between two indistinguishable quantum walkers in these systems.
We show that two such noninteracting walkers develop unusual
correlations depending on their initial state. We discuss
experimental results with photons, but also extend these
ideas to fermions and entangled particles. We also discuss
quantum walks in random lattices and the quantum correlations
of walkers undergoing Anderson localization.
Kartik Srinivasan, National
Institute of Standards and Technology
Frequency conversion interfaces for photonic quantum systems
Manipulating the wavelength of quantum states of light
is an important resource in the development of photonic
quantum information technology, where it can be used to
interface disparate physical systems, overcome fabricationinduced
inhomogeneity, and allow for more optimal detection. In
this talk, I will outline our laboratory’s efforts
at generating and manipulating the color of single photon
states. I will begin by briefly reviewing how we generate
single photons from single semiconductor quantum dots embedded
in guided wave nanophotonic structures. I’ll then describe
experiments in which we use such a single photon source
in conjunction with threewavemixing in a nonlinear crystal
to demonstrate telecomtovisible conversion and produce
identical photons from initially spectrally distinct sources.
Finally, I will discuss efforts to develop quantum frequency
converters in a scalable, chipbased platform, using both
material nonlinearities (fourwavemixing) and engineered
nonlinearities based on radiation pressure coupling between
photons and phonons (cavity optomechanics
Gregor Weihs, University of Innsbruck
Semiconductor sources of entanglement
For fundamental tests of quantum physics as well as for
quantum communications nonclassical states of light are
an important tool. I will present two approaches towards
semiconductorbased and integrated sources of single photons
and entangled photon pairs.
In the first approach we demonstrate entangled photon pair
generation in an AlGaAs Braggreflection waveguide. Spontaneous
parametric downconversion creates photon pairs at telecommunication
wavelengths. This approach can to lead to a fully integrated
photon pair source with the pump laser, active and passive
optical devices all on a single semiconductor chip.
In our second approach we use resonant twophoton excitation
of a single InAs/GaAs quantum dot to deterministically trigger
a biexcitonexciton cascade. We demonstrate Rabi oscillations,
Ramsey interference and alloptical coherent control of
the quantum dot resulting in single and paired photons with
a high degree of indistinguishability. This indistinguishability
results in timebin entanglement, which is a useful variant
for long distance communication.
