|November 21, 2014|
Poster AbstractsThe poster boards are 6 feet wide by 3 feet high, and they mount on poles that are 6 feet tall. Posters should be no more than 3 feet wide but can be as high as presenters wish. 33x44 portrait orientation is preferred.
EXPLORATIONS IN ENTANGLEMENT SUDDEN DEATH
We present the results of our studies [1,2] of the dynamics of entanglement in both qubit and continuous variable quantum systems (CVQS) undergoing decoherence. In particular, we consider the decay of quantum entanglement, quantified by the concurrence, of a pair of qubits, each of which is interacting with a reservoir at finite temperature T. For a broad class of initially entangled states, we demonstrate that the system always becomes disentangled in a finite time i.e. entanglement sudden death (ESD) occurs. Our class of states includes all states which previously had been found to have long-lived entanglement in zero temperature reservoirs. Our general result is illustrated by an example.
 A. Al-Qasimi and D. F. V. James, Nonexistence of Entanglement
Sudden Death in High NOON States, Optics Letters 34, 268-270
Optimal entanglement generation in hybrid quantum repeaters
We propose a realistic protocol to generate entanglement between quantum memories at neighboring nodes in hybrid quantum repeaters. Generated entanglement includes only one type of error, which enables efficient entanglement distillation. In contrast to the known protocols with such a property, our protocol with ideal detectors achieves the theoretical limit of the success probability and the fidelity to a Bell state, promising higher efficiencies in the repeaters. We also show that the advantage of our protocol remains even with realistic threshold detector
Paper reference: arXiv:0811.3100
An infinite sequence of additive channels
We introduce a new (infinite) class of channels for which the additivity of the Holevo capacity holds. The additivity of the simplest channel of the class induces the additivity of another one resulting in the domino effect. Moreover, for some of the channels we prove the existence of a single-letter formula for the quantum capacity and conjecture it holds for all of the channels. Finally, we prove the additivity of the classical capacity for an infinite-dimensional channel for which a single-letter formula of the quantum capacity is already known and which appears in the context of quantum field theory in curved spacetime.
HETEROMETALLIC RINGS FOR QUANTUM INFORMATION PROCESSING.
Molecular nanomagnets have been recently proposed as a novel route to a spin-based implementation of quantum-information processing. In this perspective, Cr7Ni antiferromagnetic (AF) rings  possess a number of appealing features: a level scheme suited for the qubit encoding and manipulation , stability of the magnetic core under chemical manipulation, and relatively long and chemically engineerable decoherence times .
By analyzing EPR, INS, magnetization and specific heat data, we show that Cr7Ni rings can be chemically linked to each other without altering their internal magnetic structure. We demonstrate that the inter-ring magnetic coupling can be tuned by choosing the linker, and we present calculations showing how maximally entangled states could be generated in a tripartite system comprising two Cr7Ni rings and a Cu ion by realistic microwave pulse sequences . In addition, pulse sequences allowing to perform single and two-qubit gates with these systems will be discussed.
 S. Carretta, P. Santini, G. Amoretti, T. Guidi, J. R. D. Copley, Y. Qiu, R. Caciuffo, G. Timco, and R. E. P. Winpenny, Phys. Rev. Lett. 98 (2007) 167401.
 F. Troiani, M. Affronte, S. Carretta, P. Santini, G. Amoretti, Phys. Rev. Lett. 94 (2005) 190501.
 M. Affronte, S. Carretta, G. Timco, R. Winpenny, Chem. Commun., 2007, 1789.
 G. A. Timco, S. Carretta, F. Troiani, F. Tuna, R. J. Pritchard, E. J. L. McInnes, A. Ghirri, A. Candini, P. Santini, G. Amoretti, M. Affronte and R. E. P. Winpenny, Nature Nanotechnology, 4 (2009) 173.
Paper reference: Nature Nanotechnology 4, 173 (2009).
Scattering: a viable resource for control-limited entanglement
 A. T. Costa, S. Bose, and Y. Omar, Phys. Rev. Lett. 96, 230501 (2006).
 F. Ciccarello, G. M.Palma, M. Zarcone, Y Omar & V. R. Vieira, New J. Phys 8,214 (2006); J. Phys. A 40, 7993 (2007), Las. Phys. 17, 889 (2007); F. Ciccarello, G. M.Palma & M. Zarcone, Phys. Rev. B 75, 205415 (2007) F.Ciccarello, G. M.Palma, M.Paternostro, M. Zarcone and Y Omar, to appear on Sol. Stat. Sci. (2008).
 F. Ciccarello, M.Paternostro, M. S. Kim, and G. M. Palma, Phys. Rev. Lett. 100, 150501 (2008); Int. J. Quant. Inf. 6,759 (2008).
 F. Ciccarello, M.Paternostro, G. M. Palma, M. Zarcone, arXiv:0812.0755v1
Decoherence effects in interacting qubits under the influence
of correlated envrionments
It is now well understood that one needs entanglement for both quantum logic operations and computations. Numerous methods of producing qubit-qubit entanglement have been investigated during the past decade. A method, which is of particular interest in the context of quantum logic gate operations with systems like ion-traps and semiconductor nanostructures, relies on the coherent interactions among the qubits. However generation of entanglement does not alone serve the purpose. One also needs sustained entanglement among the qubits as a computation progress. Note that sustained entanglement can only be achieved if the quantum mechanical system under evolution is completely isolated from its surrounding. In practice though the system interacts with its environment and looses its coherence, thereby degrading entanglement. Thus the study of dynamical evolution of two entangled qubits coupled to environmental degrees of freedom is of fundamental importance in quantum information sciences. In recent years numerous studies have been done in this respect. One study in particular predicted a remarkable new behavior in the entanglement dynamics of a bi-partite system. It reported that a mixed state of an initially entangled two qubit system, under the influence of a pure dissipative environment becomes completely disentangled in a finite time . This was termed as Entanglement Sudden Death (ESD) and was recently observed . Even though numerous investigations on ESD in a variety of systems have been done so far, the question of ESD in interacting qubits remains more or less open.
In this paper we investigate the entanglement dynamics for a system of interacting qubits in contact with various correlated models of the environment. Note that correlated environments are known to be less harmful to entanglement and can be tailored to be decoherence free. Thus it is imperative to investigate the effect of coherent interaction among the qubits on the entanglement in correlated environments. We present explicit analytical results for the time dependence of concurrence by considering an initial mixed state of the qubits  with phase dependent coherence for correlated dissipative (T1 effect) and dephasing (T2 effect) models of the environment.
For the case of non-interacting qubits and correlated dissipative environment, we find no ESD. The entanglement can show a substantial slower decay. However in presence of inter-qubit interactions entanglement exhibits dark and bright periodic features but no ESD. The dark periods signify the time interval during which the qubits remain disentangled. We attribute this feature to the competition between coherent and dissipative interactions.
For the case of non-interacting qubits and correlated dephasing environment we find delayed ESD. The delay depends on the dephasing rates. However in presence of interaction among the qubits, we find that entanglement exhibits dark and bright periodic behavior which eventually leads to ESD. In addition we find that the onset of dark and bright periods is sensitive to the initial coherence.
For both models we find that the frequency of dark and bright periods depends on the strength of interaction between the qubits as well as on the correlated decay and dephasing rates.
1. T. Yu, and J. H. Eberly, Phys. Rev. Lett. 93, 140404 (2004); J. H. Eberly, and T. Yu, Science 316, 579 (2007).
2. M. P. Almeida et. al., Science 316, 579 (2007); J. Laurat, et. al. Phys. Rev. Lett. 99, 180504 (2007).
Highly efficient energy transfer in photosynthetic complexes
and quantum anti-Zeno effect
Recently it has been confirmed experimentally that long-lived electronic quantum coherence plays an importation role in the highly efficient energy transfer process in photosynthesis . Then several investigations were conducted to understand the underlying mechanism leading to this high efficiency . They revealed that environment induced decoherence can collaborate with coherence to enhance the energy transfer efficiency. Here we consider a simplified model of photosynthetic complexes and investigate the mechanism underlying the enhancement of the efficiency of energy transfer from a view point of quantum anti-Zeno effect. We also propose and analyze an experimentally feasible setup, which demonstrates the enhancement of energy transfer in an artificial physical system. We believe that understanding the mechanism underlying the enhancement of energy transfer in photosynthetic complexes gives a new way to optimize a control of quantum systems based on the sophisticated natural system, for example, to design optimized solar cells.
 G. S. Engel, et al., Nature 446, 782 (2007).
 M. B. Plenio, et al., New J Phys. 10, 113019 (2008);
P. Rebentrost, et al., New J Phys. 11, 033003 (2009).
Measurement-induced entanglement localization on a three-photon
Quantum entanglement a key resource in quantum information tasks willingly interacts with surrounding systems. Due to this interaction the entangled system under interest couples to surrounding systems and the amount of entanglement is reduced or even lost and cannot serve to its application purpose. If the entanglement is completely destroyed by the coupling, distillation protocol does not work and other correcting protocols were suggested such as unlocking of hidden entanglement or entanglement localization. Entanglement localization can concentrate back redistributed entanglement at least partially from the surroundings just by measurement on the surrounding system and proper feed-forward quantum correction.
We deal with the situation when the input state is maximally entangled state of two qubits and another qubit serves as a surrounding system. Because the surrounding qubit is inaccessible before the coupling it is in an unknown state. The qubits in our case are represented by polarization states of single photons. In this presentation we extensively study the influence of coherence between the surrounding photon and one photon from the entangled pair on the localization protocol which is parametrized by the probability p that the surrounding photon is indistinguishable. After the coupling between photons, represented by transmissivity T of the beamsplitter, the entanglement of the input state is reduced and for some T entanglement is completely redirected to the surrounding photon. We theoretically prove that for any linear coupling it is possible to localize non-zero entanglement back to the pair just by proper polarization sensitive detection of photon in surrounding photon (after coupling the surrounding photon is accessible). After measurement on the surrounding photon we may use additional single-copy filtration on both photons from the pair to further raise up the concurrence. Single-copy filtration probabilistically attenuates one polarization relatively to an orthogonal one. Qualitatively this localization is independent on the level of coherence between coupling photons. Further we show that single-copy filtration produces the state violating the Bell inequalities for any p and T (except point [0, 0]). The theoretical results were experimentally tested using polarization entangled photons created in SPDC process. An extension of the localization procedure was calculated for multiple consecutive couplings to the independent surrounding photons.
Paper reference: Phys. Rev. A 79 060304 (2009)
High Quality Source of Entangled Photons based on a Polarizing
Sagnac Interferometer Configuration
We have built a compact source of entangled photon pairs based
on a polarizing Sagnac interferometer. This type of source can incorporate
very long nonlinear crystals, in our case a 25mm PPKTP crystal,
which was pumped using a 404 nm laser diode. The spectral brightness
has been measured to be approximately 50 000 pairs/(somWonm). Using
quantum state tomography, we have found the tangle of the two photon
state to be of T = 0.968. This kind of entangled photon source finds
applications in several key optical quantum technologies. One example
is quantum key distribution, as the key generation rates are currently
limited by the brightness and entanglement quality of available
entangled photon sources.
Probabilities for Tripartite SLOCC Entanglement Transformations
For a tripartite pure state of three qubits, it is well known that there are two inequivalent classes of genuine tripartite entanglement, namely the GHZ-class and the W-class. Any two states within the same class can be transformed into each other with stochastic local operations and classical communication (SLOCC) with a non-zero probability. The optimal conversion probability, however, is only known for special cases. Here, we derive new lower and upper bounds for the optimal probability of transformation from a GHZ-state to other states of the GHZ-class. A key idea in the derivation of the upper bounds is to consider the action of the LOCC protocol on a different input state, namely 1/v2 [\ket000 - \ket111], and demand that the probability of an outcome remains bounded by 1. Moreover, we generalize some of our results to the case where each party holds a higher-dimensional system. In particular, we found that the GHZ state generalized to three qutrits, i.e., \ketGHZ3 = 1/v3 [ \ket000 + \ket111 + \ket222 ] , shared among three parties can be transformed to any state of the W-class with probability 1 via LOCC.
Acknowledgments: This work has been supported by CIFAR, CIPI, Connaught, CRC, MITACS, NSERC, QuantumWorks and the University of Toronto. Their support is gratefully acknowledged.
 W. Dür et al., Phys. Rev. A 62, 062314 (2000)
 E. Chitamber et al., Phys. Rev. Lett. 101, 140502 (2008)
Machine Learning for Adaptive Quantum Measurement
Classical measurement strategies are bounded in precision by the standard quantum limit. For instance, the estimation of an interferometric phase with a classical measurement strategy can only yield a precision ~ 1/vN at a cost of N photons. In contrast, quantum measurements could dramatically improve the scaling to 1/N, known as the Heisenberg limit. Employing feedback can enable Heisenberg limited measurements, yet devising such feedback protocols is complicated and often involves clever guesswork. We sidestep this challenge by bringing machine learning techniques to autonomously devise quantum measurement protocols.
Specifically, our approach is based on a self-learning particle swarm algorithm that can be trained on a real world experiment. Our method does not require any knowledge about the physical system, and our algorithm can learn to account for all experimental imperfections. By accounting for imperfections, our algorithm makes time-consuming error modelling and extensive calibration avoidable.
I explain our technique for the case of interferometric phase estimation,
which has applications to atomic clocks and gravitational wave detection.
In particular, I show how our machine learning algorithm can design
adaptive measurement protocols that estimate an interferometric
phase with statistical errors close to the Heisenberg limit and
better than any other protocol to date. Furthermore, I outline how
our method is applicable for devising quantum control protocols.
A fibre-coupled waveguide source of entangled photon pairs at
telecom wavelengths used for an entanglement distribution network
Many quantum communication protocols rely on the distribution of entangled photon pairs. We present a high-brightness fibre-coupled source of polarisation entanglement based on spontaneous parametric down-conversion (SPDC) in waveguides at telecom wavelengths (1550nm). Using fibre-pigtailed ppLN crystals, each containing an inscribed waveguide, we can profit from the clean and efficient c2 SPDC process in waveguides, and also from a stable and efficient coupling to single mode fibres. The crystals are arranged in an Mach-Zehnder interferometric setup and are pumped by a fiber-coupled cw laser, operating at 775nm. The whole system - from the pump laser up to the receivers - is fibres and waveguides based, resulting in maximal stability, minimal losses and the advantage of readily integrable telecom components in the 1550nm range.
Since the phase-matching condition for degenerate type-I SPDC is very broad, the source yields entangled pairs over a wavelength range of 30nm. Due to the tight energy restriction from the narrowband pump photons, entangled pairs are symmetrically located in wavelength around 1550nm. This property was used to fan out the pairs to different fibers by means of a standard 8-channel DWDM multiplexer, resulting in four pairs of entangled channels. Coincidences were detected with two InGaAs detectors, the first operated in a quasi free running mode and the other triggered from the detection event of the first. Although the quasi free running detector had a duty cycle of only 10%, coincidence rates of up to 450Hz were measured for each channel pair.
Entangled Bell-states of fidelities up to 93% are created without subtraction of the background counts. With subtraction, the fidelities rise to 99%, indicating the proper function of our source. We furthermore implemented a 4-user network with a single central source distributing entanglement to any two pairs of users using optical switches in the fibres from the source. With those incorporated switches, we achieved a two-photon entanglement distribution between any two users of the network on demand, opening the path to more complex quantum network architectures.
The SPDC pair-production, in a gain regime where higher order terms
can no longer be neglected, leads not only to high count rates but
also the emergence of multi-photon quantum states beyond the usual
EPR pairs. While these are mainly of interest for fundamental investigations,
they can also be used to increase the security and efficiency of
adapted QKD schemes.
Tight Noise Thresholds for Quantum Computation with Perfect
An important question in quantum computing is how much noise can be tolerated by a universal gate set before its quantum-computational power is lost. There have been a number of recent results concerning the power of a computational model with perfect stabilizer operations in addition to some imperfect non-stabilizer resource. This resource could be access to a non-Clifford gate or a supply of non-stabilizer states. Here we show a tight threshold exists for all non-Clifford single-qubit gates undergoing depolarizing noise. This is in contrast to the situation wherein non-stabilizer qubit states are used; the thresholds in that case are not currently known to be tight.
Photon bunching in parametric down-conversion with continuous
Light sources based on spontaneous parametric down-conversion (SPDC) have become an essential component in the toolbox of quantum optics laboratories in the last decade, allowing state-of-the-art experiments on entanglement and quantum information. The topic of multi-pair emission of such sources is both relevant for applied quantum information, and to more fundamental quantum optics experiments, since multi-pair emission affects the purity of the investigated state.
We present here the first direct observation that the light field produced in SPDC via a continuous excitation (cw-laser) is bunched, i.e. it has a thermal statistics. Previous results with continuous pumped SPDC have so far been interpreted as poissonian statistics. We give an explanation why this has been the case. The thermality of the cw-pumped field should come to no surprise, since investigations with pulsed pumping demonstrated full photon bunching as early as 1998. We give experimental evidence that the statistics of a continuous wave pumped SPDC field are thermal by looking at photon emission from a single, 532nm cw-laser pumped, nonlinear crystal (periodically-poled KTP of 30mm) for type-I down-conversion, in a single output arm only. A direct characterisation of the photon number statistics is provided by the second-order temporal correlation function g(2), using a Hanbury Brown-Twiss setup, in which only the signal photons of a SPDC source are registered to determine the marginal photon statistics. The data is then compared to the g(2) of a fully thermal field of the same coherence length, but smeared due to our insufficient timing resolution. The agreement with our measured data is excellent, and hence confirm the thermal statistics of cw-pumped SPDC.
Our approach shows the feasibility of investigating photon number statistics with compact cw-pumped sources and might prove fruitful in the development of SPDC sources using cw-pumping in a time domain not accessible to experiments with pulsed pumping, which are limited by the pulse repetition rate. In particular, in the rapid progress of entangled-photon sources based on SPDC, achieving higher photon-pair rates will immediately lead to higher double-pair production. The work presented here will be extended in the future to cover cw-SPDC sources used for production of entanglement.
Paper reference: arXiv:0810.4785
Accidental cloning of a single photon qubit in two-channel continuous-variable
A single-photon input is automatically cloned in a continuous-variable (CV) quantum teleportation because a photon number is not preserved. In a two-mode polarization case CV teleportation randomly creates clones in each polarization mode. Cloning fidelity of the input polarization is limited because of no-cloning theorem. We analyze how close the output is to optimal cloning and show that nearly optimal cloning is achieved at experimentally feasible squeezing levels.
Weak measurements of photon polarization by two-path-interference
We present the experimental realization of a weak measurement of
photon polarization based on the two-path interference between the
horizontally (H) and vertically (V) polarized components of the
input light. If the polarization is unchanged, no interference is
observed and the output is independent of input polarization. To
obtain a non-zero measurement strength, the polarizations in the
arms of the interferometer are rotated towards each other, resulting
in an interference that distingishes between the diagonal polarizations.
Effectively, we control the measurement back-action which eliminates
the distingishability of H and V polarization. When the initial
separation of H and V is reversed at a beam splitter, the interference
effects automatically produce the measurement corresponding to this
coherent back-action effect. In optical systems, such an interference
based approach to weak measurements may be easier to set up and
control than a weak interaction of polarization and spatial propagation
in birefringent materials. In particular, it may be a convenient
method for the performance of large numbers of weak measurements
in optical networks.
Local expansion of W states using linear optics
Recently, a number of proposals have been made on local expansion of multipartite entangled states, which give a systematic way of producing entanglement over many qubits. In the case of W states, deterministic local expansion is impossible even in principle. In recent studies, probabilistic optical gates for locally expanding an N-photon W state to an (N+1)- or (N+2)-photon W state were proposed. These gates use polarization-dependent beamsplitters(PDBSs) or polarization-independent beamsplitters(BSs), and use 1- or 2-photon Fock state as an ancilla.
In this study, we discuss the maximum success probability of such
gates composed of linear optics and an ancilla mode in a Fock state.
We consider the case of mixing 1-photon from an N-photon W state
with an ancillary n-photon Fock state by a PDBS, and then applying
arbitrary linear optical operations to each output port to produce
an (N+n)-photon W state. We have derived an upper bound on the success
probability, which is achieved by a PDBS and (n-1) BSs. In the case
of n=2, the optimal success probability was found to be higher than
that of the expanding gate proposed before.
Signatures of "Quantum Interference" with a Classical
Chirped-pulse interferometry (CPI) captures the metrological advantages of quantum Hong-Ou-Mandel (HOM) interferometry in a completely classical system. Modified HOM interferometers are the basis for a number of seminal quantum-interference effects. Here, the corresponding modifications to CPI allow for the first observation of classical analogues to the HOM peak and quantum beating. They also allow a new classical technique for generating phase super-resolution exhibiting a coherence length dramatically longer than that of the laser light, analogous to increased two-photon coherence lengths in entangled states.
Paper reference: Phys. Rev. Lett. 102, 243601 (2009)
Spiral Zone Plates
The main goal of this paper is to study optical properties of Spiral Zone Plates. SZPs are apertures focusing light in a way similar to Fresnel Zone Plates which have been investigated for a few dozen years. SZPs can be used to generate optical vortices or construct optical tweezers.
First, I produced SZPs in a simple and inexpensive way. In my research I took photographs of foci. I prepared an experimental setup, which included home-made components such as a guide rail with a camera and a electronic control system. I discovered and investigated many dependencies between the shape of SZP, the intensity of focused light and the appearance of foci. I proposed and experimentally verified formulae describing a position of foci and a number of bright lines observed in a picture of a focus. I wrote computer programs processing the experimental data. Experimental results were compared with my numerical simulations and I noticed qualitative agreement between the two.
I hope that it will be possible to apply results of our research e.g. in adaptive optics.
Decoherence Supression via environment preparation
Decoherence provides an elegant framework to explain why an open quantum system coupled to its environment will exhibit a set of preferred states, usually ruling out a coherent superposition of arbitrary states. This framework relies essentially on the interaction between the system and its environment. In the simplest model of decoherence, it was readily realized that there exist initial state of the environment that allow for decoherence-free unitary evolution of the quantum system.
We investigate the conditions under which such special initial
states do exist in a framework where the quantum system interacts
with its environment and the environment also evolves by itself.
The results obtained underline the crucial role of the environment's
self-evolution. The ability to identify those special initial states
and to prepare them might be used in order to store quantum states.
Indeed, even if the environment cannot be controlled, it might be
possible to prepare it in a specific initial state. However, our
results restrict what can be expected from such a technique. More
precisely, we obtain a mathematical characterization for the existence
of an initial state allowing decoherence-free evolution in presence
of an interaction hamiltonian and a self-evolution of the environment.
This result is stated in terms of the structure of the two hamiltonians.
We also present topological evidences that indicate that pairs of
hamiltonians allowing for decoherence-free evolution are rare among
pairs of hamiltonians.
Decoherence of a quantum gyroscope
A quantum reference frame can be used as a valuable physical ressource by allowing the measurement of relational observables on other systems. In a theory restricted by a superselection rule, e.g. lacking a reference direction in space, those measurements would otherwise be impossible to perform. The study of the dynamics of a quantum reference frame is of great importance for the design of quantum information processors since components of quantum hardware can be subject to quantum fluctuations and decoherence effects. One important question is to quantify the longevity of the reference, i.e. how many times it can be used before producing unreliable measurements.
Poulin and Yard have shown that a quantum gyroscope prepared in
a coherent state would evolve semi-classically and be useful to
measure spin-½ particles along a direction. Here, we push
analysis further by focusing on the decoherence of the reference.
We demonstrate that their model is equivalent to another physically
motivated model where the quantum reference interacts with several
spin-½ particle, one after the other, through a Heisenberg
interaction. Techniques used to establish this result could be used
for interaction with spins of higher-order. We also show that a
superposition of two coherent states will decohere into a statistical
mixture of those two states. Preliminary results indicate that these
coherent states minimize purity loss. The study of the decoherence
of such a quantum reference exhibits an interesting transition between
quantum and semi-classical behaviour.
An experimental test of Svetlichny's inequality
It is well known that quantum mechanics is incompatible with local
realistic theories. Svetlichny showed, through the development of
a Bell-like inequality, that quantum mechanics is also incompatible
with a restricted class of nonlocal realistic theories for three
particles where any two-body nonlocal correlations are allowed.
In the present work, we experimentally generate three-photon GHZ
states to test Svetlichny's inequality. Our states are fully characterized
by quantum state tomography using an overcomplete set of measurements
and have a fidelity of (84±1)% with the target state. We
measure a convincing, 3.6 standard deviations, violation of Svetlichny's
inequality and rule out this class of restricted nonlocal realistic
Fast Error Correction of Codeword-Stabilized Quantum Codes
Codeword stabilized (CWS) codes is a general class of quantum codes
that also includes stabilizer codes. The main goal of this paper
is to simplify error correction of CWS codes. Our analysis shows
that these codes may require complicated nonlocal error-correcting
measurements that have exponential complexity in code length instead
of the polynomial complexity known for stabilizer codes. To simplify
error correction, quasilinear CWS codes (with a partial group word
operator set) are first introduced in this paper. It is then shown
that for these codes nonlocal measurements can be decomposed into
simple local measurements. Secondly, a new error correction scheme
is proposed, which exponentially reduces the total number of decoding
measurements, and also converts a generic CWS code into a quasilinear
CWS code. As a result, both the number and the complexity of nonlocal
measurements are reduced exponentially for general CWS codes.
A SIC-POVM via Weak Measurements
Realization of a qubit with three p-Wave Superfluid Vortices
We show that Majorana fermions trapped in three vortices in a p-wave
superfluid form a qubit in a topological quantum computing (TQC).
It has been already proposed that a qubit may be implemented with
two or four Majorana fermions, where a qubit operation is performed
by exchanging the positions of vortices. The set of quantum gates
thus obtained is, however, a discrete subset of the relevant unitary
group. Here, we propose a new scheme, where three Majorana fermions
form a qubit. We show that continuous qubit operations are possible
by exchange of the positions of the Majorana fermions complemented
with dynamical phase change.
We report on our experimental progress in implementing quantum
memory in Pr-doped crystals, based on the original proposal of Duan,
Lukin, Cirak and Zoller (DLCZ). We optimized state preparation sequence
theoretically, and have an experimental evidence of successful selection
and initialization of an ensemble of ions. Preliminary experiments
with Raman scattering suggest the need to employ an extremely narrow
(1MHz FWHM) filter to separate non-classical light from the ensemble
from a strong pump beam. We have developed and tested such filter
via hole-burning in a separate Pr-doped crystal.
This work on the generation of entanglement and its survival in coupled waveguides is motivated by the possibility of using coupled waveguides as basic units of quantum circuits. We study entanglement in terms of quantitative measures and examine the robustness of waveguide structures in retaining the entanglement.
We first study the dynamics of entanglement for a non-Gaussian state. We assume that single photons are coupled to each of the two waveguides. In this case the initially separable state evolves into an entangled state. We quantify the entanglement of the state at time t using the log negativity E which is related to the negative eigen-values of the transposed density matrix of the system. The log negativity E is a non-negative quantity and a non-zero value of would mean that the state is entangled. We found that the entanglement quantified by the log negativity shows an oscillatory behavior and the system gets entangled and disentangled periodically.
Further we investigate the dynamics of entanglement for Gaussian states. For this purpose we assume that squeezed light is injected in each of the two waveguides. We evaluate the logarithmic negativity to study the time evolution of entanglement. We find that log negativity oscillates between non-zero and zero value. This suggests that during its propagation the separable input state evolves into an entangled state and vice versa due to the coupling between the waveguide modes.
We also address the question of de-coherence in coupled waveguides by considering the leakage of the modes in case of an initial separable Gaussian states. We find that substantial amount of entanglement persists between the waveguide modes even for considerable decay rates.
1. J. C. F. Matthews, A. Politi, A. Stefanov, and J. L. O'Brien, Nature Photonics 3, 346 (2009).
2. D. W. Berry and H. M. Wiseman, Nature Photonics 3, 317 (2009).
3. A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien,
Science 320, 646 (2008).
Measuring Bohmian Trajectories of a Photon Using Weak Measurement
In the article "Grounding Bohmian Mechanics in Weak Measurements
and Bayesianism", Howard Wiseman proposed an operational definition
for a measurement of what he calls the "naively observable
velocity" of a particle. We use this proposal to experimentally
reconstruct the trajectories of a single photon in a double-slit
interferometer. These trajectories have been theoretically derived
in "Bohmian Trajectories for Photons".
Spatial Super-Resolution with Triphoton N00N States
Robust entanglement generation in a system based on a nitrogen-
vacancy centre via numerically-optimised control pulses
We discuss schemes for the generation of an entangling gate between the electronic and nuclear spins in the system of a single nitrogen- vacancy centre and an adjacent Carbon-13 atom in diamond, which is robust against two types of systematic errors: pulse-length and off- resonance errors. The errors occur when the apparatus controlling the dynamics of the system operates in an imprecise but reproducible manner.
We investigate effects when these systematic errors are present in various pulse sequences which, if perfectly executed, yield the desired entangling gate. We examine their effect in a basic sequential application of rectangular pulses of microwave and radio- frequency radiation on the composite electron nuclear spin system. Furthermore, when exposed to such systematic errors we compare the performance of this basic sequential pulse with more robust pulses: composite pulses, and numerically-optimised pulses produced from a modified algorithm derived from gradient ascent pulse engineering (GRAPE). The sequential pulses have previously been used to generate two-qubit entanglement , while composite pulses are known to be capable of correcting systematic errors in nuclear magnetic resonance (NMR) experiments . The GRAPE algorithm was also initially developed in NMR experiments to produce pulses that minimise the time required to implement a target unitary operator  and some quantum algorithmic elements . GRAPE pulses have been experimentally demonstrated in a single qubit trapped ion system . They have also been used to implement high-fidelity single qubit operations in a noisy environment due to random telegraph noise in superconducting solid-state qubits .
We adapt and develop the methods of composite pulses and the GRAPE algorithm to be able operate in the the combined electron and nuclear spin system when exposed to these types of systematic errors, to produce a highly robust entangling gate. We have found that the gate created by the GRAPE numerically-optimised pulses is more robust against systematic errors and has faster implementation time than that required by the corresponding composite pulses.
Paper reference: arXiv:0903.3827
Switchable effective interactions in molecule-based quantum
Magnetic molecules containing strongly exchange-coupled transition-metal ions have been recently proposed as promising candidates for qubit encoding and manipulation [1,2]. A major obstacle of this approach is the need to vary the value of molecule-molecule exchange couplings during each two-qubit operation. This can be hardly done directly in a controlled way.
We show that molecules made of isosceles antiferromagnetic triangles constitute advantageous units to solve this problem . In fact, the peculiar spin structure of their low-energy wavefunctions enables switchable effective intermolecular couplings in presence of permanent microscopic interactions.
Another promising scheme is a magnetic link between neighboring nanomagnets made by a magnetic complex (e.g., a dimer) with a singlet ground-state. Effective interactions can be switched on by exciting the linking complex to a magnetic state. System of this type have just begun to be synthesized.
 M.N. Leuenberger and D. Loss, Nature 410, 789 (2001).
 F. Troiani, A. Ghirri, M. Affronte, S. Carretta, P. Santini, G. Amoretti, S. Piligkos, G. Timco and R. E. P. Winpenny, Phys. Rev. Lett. 94, 207208 (2005).
 S. Carretta, P. Santini, G. Amoretti, F. Troiani, and M. Affronte,
Phys. Rev. B 76, 024408 (2007).
Inhomogeneous broadening of energy levels is one of the principal limiting factors for achieving ßlow" or ßtationary" light in solid state media by means of electromagnetically induced transparency (EIT), a quantum version of stimulated Raman adiabatic passage (STIRAP). Stark-shift-chirped rapid-adiabatic-passage (SCRAP) has been shown to be far less sensitive to inhomogeneous broadening than STIRAP, a population transfer technique to which it is closely related. We further optimise the pulses used in SCRAP to be even less sensitive to inhomogeneous broadening in a lambda-type three-level system. The optimised pulses perform at a higher fidelity than the standard gaussian pulses for a wide range of detunings (i.e. large inhomogeneous broadening).
Paper reference: arXiv:0905.0052v1
Noise poses a challenge for any real-world implementation in quantum
information science. The theory of error correction deals with this
problem via methods to encode and recover quantum information in
a way that is resilient against that noise. Unitarily correctable
codes are an error correction technique wherein a single unitary
recovery operation is applied without the need for an ancilla Hilbert
space. Here, we present the ?rst optical implementation of a non-trivial
unitarily correctable code for a noisy quantum channel with no decoherence-free
subspaces or noiseless subsystems. We show that recovery of our
initials states is achieved with high ?delity (= 0.97), quantitatively
proving the efficacy of this unitarily correctable code.
Quantum Systems and Control Theory: Constraints by Symmetry
and by Relaxation
We investigate the universality of multi-qubit controlled systems in architectures of various symmetries in coupling type and topology. Explicit reachability sets under symmetry constraints are provided. Thus for a given experimental coupling architecture practical decision problems can be solved in a unified way: (i) can a target Hamiltonian be simulated? (ii) can a target gate be synthesised? (iii) to which extent is the system observable by a given set of detection operators? -- Constraints by relaxation are sketched in the framework of Lie semigroups. Its practical implications reach from Markovianity of quantum channels to average Liouvillians.
In turn, lack of symmetry is a convenient necessary condition for full controllability. Though much easier to assess than the well-established Lie-algebra rank condition, is not yet sufficient. We present simple further conditions that add to lack of symmetry ensuring full controllability and universality of the controlled hardware set-up.
Paper reference: arXiv:0904.4654, arXiv:0811.3906
Stability and Utility of Chiral Spin Currents for Quantum Computing
in Quantum Dots
Within degenerate subspaces, states are commonly classified according to symmetries present in the underlying Hamiltonian. However, alternate classifications may be preferable with respect to stability and coherence, particularly if they exhibit many-body correlations. We have surveyed the low-lying degenerate eigenstates of two-dimensional quantum dots containing both spatial [O(1)] and spin [O(3)] rotational symmetry. The three-particle system in particular is four-fold degenerate with S = 1/2 whose states are conventionally described by the quantum numbers (Lz, Sz) = (±1, ±1/2). Transitions between these states are essentially single-particle transitions. However, states with a definite spin chirality may be alternatively created from these states, each carrying a well-defined chiral spin current. These are classified by quantum numbers (C, Q) = (±1, ±1), where C is the chirality and Q is a topological charge associated with the spin winding. Within a degenerate subspace, C and Q are conserved quantities. These are many-body correlated states and can be expected to yield longer lifetimes in the presence of single-particle perturbations compared to states classified by Lz and Sz. Our calculations employ configuration-interaction techniques for confined particles with long-range Coulomb repulsion. We directly compute the correlated many-body eigenstates of the system containing thousands of Slater determinants. We present conclusive evidence of statistical and Coulomb induced chiral spin textures within energetically degenerate subspaces, and present results examining their stability with respect to single-particle perturbations.
Quantum phase estimation in the presence of phase noise in qubit
We address quantum estimation protocols designed to estimate physical parameters of qubit gates in the presence of phase noise. We derive analytical formulae for the precision of phase estimation obtainable with qubit probe and show the optimality of equatorial qubit probes. We explicitly evaluate quantum Fisher information and show that ultimate quantum limit to precision may be achieved by an observable measurable with current technology. An experimental setup for the implementation of the suggested measurement is discussed in some details.
Paper reference: in preparation
Comparison of maximum-likelihood and linear reconstruction schemes
in quantum measurement tomography
The effects on quantum states caused by measurement apparatuses can be described in general by sets of completely positive maps called instruments. There exists a linear reconstruction scheme for the instrument describing a given measurement apparatus from experimental data, but the scheme has the disadvantage that it can give unphysical reconstructions. In this poster we propose a maximum-likelihood reconstruction scheme that addresses this disadvantage. We show that our scheme always gives a physical reconstruction, and that it does so more efficiently than the linear scheme.
Fock-Space Coherence in Multilevel Quantum Dots
A Fock-space coherence can occur in systems where states with different
particle numbers are simultaneously available. In such systems --
for example, a quantum dot with open transport channels -- the lifetime
and robustness of these coherent states may be long lived relative
to the more common single-particle-number coherence -- for example,
a Hilbert-space coherence between spin states. We have developed
a microscopic, non-Markovian theory to investigate real-time features
of this Fock-coherence in multilevel quantum dots far from equilibrium.
In a model where the dominant relaxation mechanism is through sequential
tunnelling transport, we observe a decoupling between the evolution
of the Fock-space coherence and that of the occupation probabilities
for the dot states. In experimentally relevant parameter regimes,
the lifetime of the Fock-space coherence is dramatically increased
even when the Hilbert-space coherence between states with same particle
number decays to zero. This is a dramatic example of how a many-body
coherence can remain robust even in the presence of rather large
Optimal control of a qubit coupled to a Non-Markovian Environment
A central challenge for implementing quantum computing in the solid state is decoupling the qubits from the intrinsic noise of the material. We investigate the implementation of quantum gates for a paradigmatic, non-Markovian model: a single-qubit coupled to a two-level system that is exposed to a heat bath. We systematically search for optimal pulses using a generalization of the novel open systems gradient ascent pulse engineering algorithm. Next to the known optimal bias point of this model, there are optimal pulses which lead to high-fidelity quantum operations for a wide range of decoherence parameters.
Paper reference: Phys. Rev. Lett. 102, 090401 (2009).
A Light-Matter Interface for Quantum Information Processing
We will present our design and preliminary data of a light-matter
interface for tasks such as two-qubit quantum gate in quantum information.
Using cavity resonance, we modify the spectrum of spontaneously
parametric down-conversion to match the atomic resonance. We use
laser cooled and trapped atoms to see the interaction between light
Deterministic Quantum Phase Gates for Two Atoms Trapped in Separate
Cavities Mediated by An Optical Fiber
The existence of quantum computing algorithms shows that a quantum
computer can solve specific problems that are intractable with classical
computers . This discovery has stimulated a flurry of research
into this mathematical concept. It has been shown that any quantum
computational operation can be decomposed into a series of quantum
logic gates, and two-qubit controlled phase gate and one-qubit gate
are universal for constructing a quantum computer . For the physical
implementation of quantum computation, a quantum system is needed.
The real quantum system should satisfy five requirements as follows
: (i) qubits can be initialized to arbitrary values; (ii) gate
operation on specific qubits can be turned on and off at will; (iii)
qubits can be read easily; (iv) quantum gates faster than decoherence
time; (v) the system should be scalable. Several physical systems
are competent, such as trapped ions , cavity QED , superconducting
circuits , semiconductor quantum dots , linear optics ,
impurity spins in solids , etc.
Direct observation of Hardy's paradox by joint weak measurement
with an entangled photon pair
We implemented a joint weak measurement of the trajectories of two photons in a photonic version of Hardy's experiment. The joint weak measurement has been performed via an entangled meter state in polarization degrees of freedom of the two photons. Unlike Hardy's original argument in which the contradiction is inferred by retrodiction, our experiment reveals its paradoxical nature as preposterous values actually read out from the meter. Such a direct observation of a paradox will give us a new insight into the spooky action of quantum mechanics.
Paper reference: arXiv:0811.1625
Coherent control of vibrational states in optical lattices via
interference between one- and two-phonon excitation
We demonstrate that the control of quantum vibrational states in
an optical lattice can be achieved by using interference between
two-phonon excitation at w and one-phonon excitation at 2w. We use
this technique to improve the ratio of coherent coupling to loss
in our system. In our experiment, 85 Rb atoms are trapped in a vertical
optical lattice, leading to a tilted-washboard potential when the
effect of gravity is considered. While neighboring vibrational states
in one well may be coherently coupled by sinusoidal drive of the
lattice displacement at the secular frequency w, this also leads
to leakage into higher excited states and eventual loss from the
lattice. We use coherent control to mitigate this problem, by adding
a simultaneous parametric drive at 2w, directly coupling states
of the same parity. The resonant drive corresponds to Raman scattering
of laser beams phase-modulated (PM) at w, while the parametric drive
corresponds to Raman scattering of laser beams amplitude-modulated
(AM) at 2w. We demonstrate experimentally that quantum interference
between the absorption of two PM quanta and one AM quantum can be
used to control the branching ratio, and specifically, to improve
the ratio of coherent coupling to loss.