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Conference on Quantum Information and Quantum Control IIIContributed TalksPoster AbstractsBell Prize talkQuantum Nonlocality: How does Nature perform the trick ?!?by Nicolas Gisin Group of Applied Physics, University of Geneva, Switzerland Nature is able to produce correlations that can’t be described by any theory using only local variables. Since the early intuitions of EPR and Schrödinger in 1935, and the first quantitative predictions by John Bell in 1964 followed by their experimental confirmation by Clauser, Aspect and co-workers in the1970’s and 1980’s, this absolutely astonishing fact has become routine in many labs around the world. Moreover, during the last 20 years it has been discovered that this sort of nonlocality is a useful cryptographic resource that has also been mastered outside the lab, over standard optical fiber telecom networks. Hence, one could think that quantum nonlocality has become banal.
But a clear understanding of how these nonlocal correlations happen remains a huge open problem. There is of course no question of a mechanical explanation of the type: one localized physical system “pushes” (interacts with) its direct neighbours, as we have in classical (Newtonian mechanics and general relatively). The impossibility of such a mechanical explanation is precisely the message of Bell’s inequalities.
But then: How come the correlations? How does Nature manage to produce random events that somehow manifest themselves at several locations? In brief: How does Nature perform the trick?
More or less everyone who tries to go beyond Hilbert space calculus uses pictures like: a first event influences a second one (e.g. Einstein’s famous spooky action at a distance). We present experiments that cast serious doubts on the viability of such pictures. Moreover, taking such pictures seriously raises deep concerns about the alleged “peaceful co-existence of quantum nonlocality and relativity”.
What are the alternatives to the picture sketches above? Some vague paths will be presented. I am afraid I have nothing better to offer. Importantly, if someone tells you he has a straightforward solution, beware! The problem is a serious one, central to today’s physics. Abstracts - Invited TalksHow Much Quantum Noise is Detrimental
to Entanglement I examine the effect of quantum noise on entanglement. The source
of noise could be either an attenuator or even an amplifier which
one would presumably use in quantum communication protocols. I present
quantitative results on the survival of entanglement 1. G.S. Agarwal, S. Chaturvedi, arXiv:0906.2743 ------------------------------------------------------------------------------- The Design and Function of Quantum Information
Processors Quantum information theory provides a new framework for the development
of sensors and actuators that rely on quantum dynamics to obtain
efficiencies beyond their classical counterparts. Today we can build
laboratory examples of small quantum devices from spin systems,
optics, superconducting systems and even neutron beams. I will introduce
some of the concepts underlying these devices. In particular showing
simple schemes for improving device performance via quantum engineering.
I will also explore some near term examples of practical quantum
sensors. Solid-state quantum memories for quantum
repeaters ------------------------------------------------------------------------------- Operational Computation with Quantum Stuff We imagine that we have a supply of stuff that, we suppose, has
quantum properties that are not well known. We consider a computational
scheme in which we attach inputs and outputs to this stuff to assist
a computation (which can be classical). We consider how we might
go about operationally characterizing the stuff to enable effective
quantum computation. -------------------------------------------------------------------------------- Theory of coherent resonance energy transfer
for coherent initial condition A theory of coherent resonance energy transfer developed recently
[Journal of Chemical Physics 129, 101104 (2008)] is extended for
coherent initial condition. For the general situation where the
initial excitation can be arbitrary linear combination of donor
and acceptor excitations, a second order time local and polaron
transformed quantum master equation is derived. The inhomogeneous
terms in the resulting equation have contributions not only from
the initial donor and acceptor populations but also from their coherence
terms. Numerical tests are performed for general super Ohmic spectral
densities where the bath coupled to the donor excitation can be
correlated with that coupled to the acceptor excitation. The results
show the sensitivity of the early nonstationary population dynamics
on the relative phase of initial donor and acceptor excitations.
It is also shown that the contribution of inhomogeneous terms is
more significant for coherent initial excitations than for initial
excitation localized in the donor only. The overall numerical results
demonstrate the importance of including all the competing effects
such as nonequilibrium, nonMarkovian, and quantum coherence for
quantitative modeling of population dynamics of resonance energy
transfer. Quantum Communication and Information
Processing with Photons - Experiments and Outlook Photons are good candidates for quantum information processing,
and they have been widely used in implementations of quantum communication
and quantum information processing. I will outline recent experimental
results on long distance quantum communication with entangled photons,
and the ongoing activities towards performing satellite based quantum
networks. Furthermore, I will present the results of theoretical
studies on the actual requirements for photon sources and detectors
in order to perform photonic quantum computing outside the coincidence
basis, which will be an important next experimental step towards
scalable quantum information processing with photons. -------------------------------------------------------------------------------- Complementarity and security of quantum
key distribution In the seminal paper by Einstein, Podolsky and Rosen, one finds
a pair of complementary tasks that can never be completed at the
same time, since otherwise it would violate the uncertainty relation.
By suitably defining such a pair of tasks, we are able to establish
simple quantitative relations between the feasibility of the complementary
tasks and distillable entanglement or distillable key. This approach
is also useful for proving the security of practical quantum key
distribution protocols, since the security is established through
an operationally defined quantity, namely, the success probability
of the complementary task. In some occasions, this probability can
be estimated even when little is known about the apparatuses used
in the protocol. ---------------------------------------------------------------------------------- Synthesizing arbitrary photon states
in a superconducting resonator: The quantum digital to analog converter Two-level systems, or qubits, can be prepared in arbitrary quantum
states with exquisite control, just using classical electrical signals.
Achieving the same degree of control over harmonic resonators has
remained elusive, due to their infinite number of equally spaced
energy levels. Here we exploit the good control over a superconducting
phase qubit by using it to pump photons into a high-Q coplanar wave
guide resonator and, subsequently, to read out the resonator state.
This scheme has previously allowed us to prepare and detect photon
number states (Fock states) in the resonator and to measure their
decay. Using a generalization of this scheme by Law and Eberly,
we can now create arbitrary quantum states of the photon field with
up to approximately 10 photons. We analyze the prepared states by
directly mapping out the corresponding Wigner function, which is
the phase-space equivalent to the density matrix and provides a
complete description of the quantum state. ------------------------------------------------------------------------------ Electronic excited states in optically
active biomolecules: functional quantum systems with a tuneable
environment interaction Optically active molecules (chromophores) are crucial to the function of wide range of biomolecules. Examples, include the green flourescent protein, porphyrins associated with photosynthesis, and retinal associated with vision. The electronic states of the chromophores can be viewed as discrete quantum systems which are interacting with an environment composed of the surrounding protein and water. The interaction of the chromophore with its environment may be modelled quantum mechanically by an independent boson model which describes a two-level quantum system interacting with a bath of harmonic oscillators. Femtosecond laser spectroscopy experiments give a parametrisation of the spectral density describing the system-environment interaction for a wide range of chromophores and proteins. This spectral density completely determines the quantum dynamics and decoherence of electronic excited states. We have recently proposed and analysed several continuum dielectric models of the environment[1]. Our results provide a framework to understand experimental measurements and molecular dynamics simulations, including the relative importance of the contributions of the protein, the water bound to the surface of the protein, and the bulk water to decoherence. Our results show that because biomolecules function in a ``hot and wet'' environment, quantum coherence will generally not be significant for processes occuring slower than a picosend, the timescale for the dielectric relaxation of water. The ``collapse'' of the quantum state of the chromophore due to continuous measurement of its state by the environment occurs on the timescale of 10's femtoseconds. [1] J. Gilmore and R.H. McKenzie, J. Phys. Chem. A 112, 2162 (2008). ------------------------------------------------------------------------------- Quantum Networks with Ions, Phonons, and
Photons ------------------------------------------------------------------------------ Spectroscopy of biological molecules using
coherent control Coherent control as a field of current research has expanded significantly
in recent years. In chemistry the core competence remains the steering
of photo-induced processes into a desired channel while suppressing
unwanted pathways. Also in biology it has been experimentally demonstrated
in natural occurring complexes [1,2] as well as in artificial dyads
[3] that the ratio between reaction pathways can be influenced by
phase and amplitude shaped laser pulses. [1] J. L. Herek et al., Nature 417, 533 (2002). -------------------------------------------------------------------------- A photonic cluster state machine gun Cluster states are multi-qubit entangled states which have the
remarkable property that, once prepared, they can be used to perform
quantum computation by making only single qubit measurements. -------------------------------------------------------------------------------- Going beyond Gaussian limits on continuous
variable processing and measurement The amplitude and phase quadratures of optical field, the so called
continuous variables (CVs), play major roles both in photonic- and
quantum-information and communications technology (P- and Q-ICT,
respectively). Gaussian states, typically coherent states, and Gaussian
operations on CVs serve as a complete basis for P-ICT. They are,
however, only a part of the full potential of optical fields. Q-ICT
provides a new paradigm to overcome the limits of P-ICT and to realize
the quantum-limited measurement and the ultimate capacity of optical
channels. Recent theories revealed that higher order operations
beyond a fully Gaussian setting, namely non-Gaussian operations
on CVs, are essentially required to realize the Q-ICT paradigm.
We present recent advances of non-Gaussian operations and measurements
to overcome the Gaussian limits. We first present latest results
on non-Gaussian state generation and manipulation using photon counting
and squeezed states, and then present an implementation of quantum
receiver with a photon number resolving detector. Coherently wired light-harvesting in a
photosynthetic marine alga at ambient temperature The photosynthetic machinery of plants, algae, and bacteria has diversified and evolved over billions of years. The initial reactions involve absorption of light by molecules in specialized light-harvesting antenna proteins followed by remarkably efficient funneling of that electronic excitation energy within and between proteins to a reaction center. Isolated antenna proteins have proven to be important model systems enabling researchers to learn how excitation energy is transmitted by resonance energy transfer, and recent work has discovered a role of quantum-coherence in energy funneling for some antenna proteins at temperatures as high as 180K. Quantum-coherence means that light-absorbing molecules in the protein capture and funnel energy according to quantum-mechanical probability laws instead of classical laws. The subject has stimulated cross-disciplinary interest because it was previously thought that long-range quantum-coherence between molecules could not be sustained in complex biological systems, even at low temperature. Here we report observations of quantum coherence at ambient temperature in the energy funnel of the phycocyanin 645 (PC645) antenna protein isolated from the marine cryprophyte alga Chroomonas CCMP270. Electronic excitations interfere in a way to 'wire' distant molecules together in the PC645 photosynthetic antenna protein. We find that quantum-coherence is maintained for long enough that it may feasibly be employed within live cryptophyte marine algae to increase the spatial cross-section for light-harvesting. This work leads to new, more demanding, questions such as This opens up questions such as 'do quantum effects offer an evolutionary advantage in biology, and if so, how'? ---------------------------------------------------------------------------------- Photon-Echo Quantum Memory and Controlled
State Manipulation Quantum memories, as a part of a quantum repeater, are key elements to extend quantum communication beyond its current distance limit of around 100 km. In addition to memories, quantum repeaters also require the distribution of entangled photons as well as state manipulation, which is generally accomplished by means of interferometric optical setups. We experimentally investigate a novel approach based on photon-echo type atom light-interaction that allows combining storage with controlled transformation of quantum states [1,2]. As an example, we perform a proof-of-principle demonstration of unambiguous state discrimination in an Er:LiNbO3 waveguides cooled to 3K using states encoded into pulses of light in superposition of different temporal modes. Our approach can easily be extended to any unitary transformation. The high robustness and flexibility compared to current optical setups for state manipulation makes it promising for quantum communication and computation protocols that require storage and manipulation of photons, in particular quantum repeaters.
[2] A. Delfan, C. La Mela and W. Tittel, in Proceedings of SPIE 6903: 690308 (6 pp.), San Jose, United States of America (SPIE , Bellingham, United States of America, 2008). ------------------------------------------------------------------------------------------- Quantum dots in photonic crystals: from
quantum information processing to optical switching at a single
photon level Quantum dots in photonic crystals are interesting both as a testbed
for fundamental cavity quantum electrodynamics (QED) experiments,
as well as a platform for quantum and classical information processing.
These demonstrations lie at the core of a number of proposals for quantum information processing, and could also be employed to build novel devices, such as optical switches controlled at a single photon level. [1]. Dirk Englund, Andrei Faraon, Ilya Fushman, Nick Stoltz, Pierre
Petroff, and Jelena Vuckovic, "Controlling cavity reflectivity
with a single quantum dot," Nature, vol. 450, No. 7171, pp.
857-861, December 2007 [3]. Ilya Fushman, Dirk Englund, Andrei Faraon, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Controlled phase shift with a single quantum dot," Science, vol. 320, number 5877, pp. 769-772 ( 2008) -------------------------------------------------------------------------------------------- We use a photonic quantum computer to simulate the hydrogen molecule. This is the first experimental demonstration of efficient quantum chemistry, which promises to be a powerful new tool in biology, chemistry, and materials science. In principle, it is possible to model any physical system exactly using quantum mechanics; in practice, it quickly becomes infeasible. Recognising this, Richard Feynman suggested that quantum systems be used to model quantum problems. For example, the fundamental problem faced in quantum chemistry is the calculation of molecular properties, which are of practical importance in fields ranging from materials science to biochemistry. Within chemical precision, the total energy of a molecule as well as most other properties, can be calculated by solving the Schrodinger equation. However, the computational resources required to obtain exact solutions on a conventional computer generally increase exponentially with the number of atoms involved. In the late 1990's an efficient algorithm was proposed to enable a quantum processor to calculate molecular energies using resources that increase only polynomially in the molecular size. Despite the many different physical architectures that have been explored experimentally since that time---including ions, atoms, superconducting circuits, and photons---this appealing algorithm has not been demonstrated to date. Here we take advantage of recent advances in photonic quantum computing
to present an optical implementation of the smallest quantum chemistry
problem: obtaining the energies of H_2, the hydrogen molecule, in
a minimal basis. We perform a key algorithmic step---the iterative
phase estimation algorithm---in full, achieving a high level of
precision and robustness to error. We implement other algorithmic
steps with assistance from a classical computer, and explain how
this non-scalable approach could be avoided. Finally we provide
new theoretical results which lay the foundations for the next generation
of simulation experiments using quantum computers. We have made
early experimental progress towards the long term goal of exploiting
quantum information to speed up quantum chemistry calculations. Contributed TalksQuantum computers: A new state of matter?by Stephen Bartlett School of Physics, The University of Sydney Coauthors: Andrew Doherty, Sean Barrett, Terry Rudolph, David Jennings A recent breakthrough in quantum computing has been the realization that quantum computation can proceed solely through single-qubit measurements on an appropriate quantum state. One exciting prospect is that the ground or low-temperature thermal state of an interacting quantum many-body system can serve as such a resource state for quantum computation. The system would simply need to be cooled sufficiently and then subjected to local measurements.
Paper reference: arXiv:0802.4314 (accepted to PRL); arXiv:0807.4797 Scalable quantum computation via local
control of only two qubits We apply quantum control techniques to control a large spin chain by only acting on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on the latter and swap operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging. Paper reference: arXiv:0905.3373 Flipping quantum coins Coin flipping is a cryptographic primitive in which two distrustful parties wish to generate a random bit in order to choose between two alternatives. This task is impossible to realize when it relies solely on the asynchronous exchange of classical bits: one dishonest player has complete control over the final outcome. It is only when coin flipping is supplemented with quantum communication that this problem can be alleviated although partial bias remains. Unfortunately, practical systems are subject to loss of quantum data, which allows a cheater to force a bias that is complete or arbitrarily close to complete in all previous protocols. We report herein on the first implementation of a quantum coin-flipping protocol that is impervious to loss. Moreover, in the presence of unavoidable experimental noise, we propose to use this protocol sequentially to implement many coin flips, which guarantees that a cheater unwillingly reveals asymptotically, through an increased error rate, how many outcomes have been fixed. Hence, we demonstrate for the first time the possibility of flipping coins in a realistic setting. Paper reference: http://arxiv.org/abs/0904.3946 Multipartite entanglement for one photon
shared among four optical modes Access to genuine multipartite entanglement of quantum states enables advances in quantum information science and also contributes to the understanding of strongly correlated quantum systems. A critical requirement for realizing these extraordinary promises, however, is an efficient and unambiguous method to detect and characterize the purported entanglement. We report the detection and characterization of heralded entanglement in a multipartite quantum state composed of four optical modes that share one photon, a so-called W state [1]. By reducing the relative phase coherence between bipartite components of the W state, we observe the transitions from four- to three- to two-mode entanglement. These observations are possible for our system because our entanglement verification protocol makes use of quantum uncertainty relations to simultaneously detect the entangled states that span the Hilbert space of interest [2]. Paper reference: [1] Science 324, 764 (2009); [2] New J. Phys.
11, 063029 (2009). Ultrafast optical spin echo for electron
spins in semiconductors Spin-based quantum computing and magnetic resonance techniques
rely on the ability to measure the coherence time, T2, of a spin
system. We report on the experimental implementation of all-optical
spin echo to determine the T2 time of a semiconductor electron-spin
system. We use three ultrafast optical pulses to rotate spins an
arbitrary angle and measure an echo signal as the time between pulses
is lengthened. Unlike previous spin-echo techniques using microwaves,
ultrafast optical pulses allow clean T2 measurements of systems
with dephasing times (T2*) fast in comparison to the timescale for
microwave control. This demonstration provides a step toward ultrafast
optical dynamic decoupling of spin-based qubits. Such a scheme could
be used to extend the spin-memory time of a spin-based quantum computer
and can be integrated into quantum bus schemes for quantum computing. Ultrafast quantum state tomography Everybody hates tomography. And with good reason! Experimentalists hate it because it is inefficient and difficult. Theorists hate it because it isn't very "quantum." But because of our current lack of meso-scale quantum computers capable of convincingly performing non-classical calculations, tomography seems like a necessary evil. In this talk, I will attempt to banish quantum state tomography to the Hell of Lost Paradigms where it belongs. I hope to achieve this by introducing several heuristics for learning quantum states more efficiently, in some cases exponentially so. One such heuristic runs in polynomial time and outputs a polynomial-sized classical approximation of the state (in matrix product state form.) Another takes advantage of the fact that most interesting states are close to pure states to get a quadratic speedup using ideas from compressed sensing. Both algorithms come with rigorous error bounds.
Quantum-Bayesian Coherence In the quantum-Bayesian development of quantum mechanics, the Born Rule cannot be interpreted as a rule for setting measurement-outcome probabilities from an objective quantum state. But if not, what is the role of the rule? In this paper, we argue that it should be seen as an empirical addition to Bayesian reasoning itself. Particularly, we show how to view the Born Rule as a normative rule in addition to usual Dutch-book coherence. It is a rule that takes into account how one should assign probabilities to the consequences of various intended measurements on a physical system, but explicitly in terms of prior probabilities for and conditional probabilities consequent upon the imagined outcomes of a special counterfactual reference measurement. This interpretation is seen particularly clearly by representing quantum states in terms of probabilities for the outcomes of a fixed, fiducial symmetric informationally complete (SIC) measurement. We further explore the extent to which the general form of the new normative rule implies the full state-space structure of quantum mechanics. It seems to get quite far Paper reference: arXiv:0906.2187v1 [quant-ph] Entanglement and nonlocality in multiqubit pure states Multiqubit entanglement is a crucial ingredient for large-scale quantum information processing and can also play a role in quantum criticality phenomena in condensed matter systems. Entanglement between qubits can lead to violations of Bell-type inequalities, indicating the nonlocal nature of the correlations between qubits. We have derived relationships between genuine multiqubit entanglement and nonlocality for families of 3-qubit pure states. Our results show that these relationships are counterintuitive and can be quite different from the well-known relationship between 2-qubit entanglement and violation of the Bell-CHSH inequality. We identify tripartite entangled states that do not violate the Svetlichny inequality, which tests for genuine tripartite nonlocal correlations. On the other hand, we show that all members of a set of states called the maximal slice states violate the Svetlichny inequality and analogous to the 2-qubit case, the amount of violation increases with the amount of entanglement. The generalized GHZ states and the maximal slice states have unique tripartite entanglement and nonlocality properties in the set of all pure states. Our results can be simply generalized to analyze multiqubit entanglement and nonlocality in systems of 4 or more qubits. Paper reference: arXiv:0812.3695 Non-adiabatic quantum control of multiple
quantum dots embedded in cavities with global femtosecond optical
pulses There are several proposals utilising quantum dots embedded in
optical cavities as physical or logical qubits. The advantage of
these systems is that distant qubits can be controllably coupled
through virtual cavity modes. While there has been recent experimental
progress in coherent manipulation in such systems, further progress
is hampered by at least two limitations. One, is that transitions
are accomplished adiabatically; since the light-qubit coupling is
typically rather weak, switching times are not appreciably shorter
than the relaxation time of these systems. Second, lasers typically
need to address individual dots in the cavity, which is exceedingly
difficult. We present results of our work showing how both these
limitations can be simultaneously overcome. In the experimentally
relevant case of dots of varying size, nonadiabatic transitions
can be achieved using chirped pulses applied globally to the cavity.
The nonadiabaticity enables switching times far quicker than either
the relaxation time or (one over) the Rabi frequency. These global
pulses further eliminate the need to address a single dot with a
single pulse. We show results showing fast entangling operations
on distant qubits even for arbitrarily closely-spaced energy levels.
This level of quantum control has not yet been been demonstrated
for multiple quantum dots embedded in cavities. Our scheme can be
implemented with present-day experimental capabilities. Designed photons from birefrigent waveguides The capability to produce photon pairs and non-classical states of light, such as squeezed states, with controlled spatial and temporal mode structure is a crucial requirement for optical quantum technologies such as photonic quantum information processing, quantum cryptography, and quantum metrology. We develop a theoretical model of photon pair generation by spontaneous four-wave mixing in birefringent waveguides, such as optical fibers. The model demonstrates that a wide variety of spectral correlations can be designed into the photon pairs. We present experimental results in photonic crystal fiber and birefringent standard fiber where we eliminate all correlations, enabling the heralding of single photons in pure quantum state, which is a requirement for high-fidelity operation of photonic quantum logic gates. Paper reference: http://dx.doi.org/10.1103/PhysRevLett.102.123603 Applications of Four-Wave Mixing in Quantum
Information One of the most important resources in quantum mechanics is entanglement, which is at the basis of applications such as quantum cryptography, quantum imaging, teleportation, etc. In order to fully take advantage of this resource, it is necessary to develop a number of tools to manipulate and control it. We show that non-degenerate four-wave mixing in rubidium vapor is a good candidate for the implementation of some of those tools. Its dispersive properties make it possible to control the propagation velocity of light. We have used this property to delay entanglement without a significant degradation, effectively implementing a short term quantum memory. In addition, its operation as an almost ideal amplifier has allowed us to clone one of the beams of an entangled state of light. Paper reference: Nature 457, 859 (2009) and PRL 103, 010501 (2009) -------------------------------------------------------------------------------- Quantum matchgate computation is as powerful as space-bounded
quantum computation Quantum matchgate computation is a family of uniform quantum circuits comprising only certain two-qubit unitary gates acting arbitrarily in each even and odd parity subspaces on any nearest neighboring pair of two qubits. This model has a tight connection to the time evolution of the free fermionic system ubiquitous in physics, for example, the dynamics of one-dimenisional spin chains via the so-called Jordan-Wigner transformation. Indeed, the matchgates constitute a strict subset of a universal set of elementary gates as a quantum circuit model, and the matchgate computation has been known to be efficiently simulatable by the classical computer, i.e., it is in P. Here we prove that the matchgate computation is equally powerful
as space-bounded quantum computation where, compared to the size
n of the input, the size of the working register is exponentially
small, i.e., logn, and thus that may be called quantum log-space
computation, QL shortly. Since it can be said that QL captures computational
capability of small-sized quantum computers which would be only
available in practice in the next decade by our current technology,
our result not only shows that the matchgate is in QL, but also
provides explicitly how any computation in QL can be simulated by
the matchgates conversely and characterizes a gap to the full-scaled
quantum computers. Some implications to quantum simulation with
a small quantum computer, such as that of one-dimensional quantum
systems, are discussed. Merging photonic crystal cavities and single quantum dots: a
practical source of entangled photon pairs This work was supported by the National Sciences and Engineering Research Council of Canada and the Canadian Foundation for Innovation. [1] N. Akopian et. al., Phys. Rev. Lett. 96, 130501 (2006). [2] R. M. Stevenson et. al., Nature (London) 439, 179 (2006). [3] R. Seguin et. al., Appl. Phys. Lett. 89, 263109 (2006); D. J. P. Ellis et. al., ibid. 90, 011907 (2007). [4] J. E. Avron et. al., Phys. Rev. Lett. 100, 120501 (2008); see also P. K. Pathak and S. Hughes, Phys. Rev. Lett., In Press (2009) [arXiv:0905.4420v1]. [5] M. E. Reimer et. al., Phys. Rev. B 78, 195301 (2008). [6] See, e.g., J. P. Reithmaier et. al., Nature (London) 432, 197 (2004); T. Yoshie et. al., ibid. 432, 200 (2004). [7] R. Johne et. al., Phys. Rev. Lett. 100, 240404 (2008). [8] P. K. Pathak and S. Hughes, Phys. Rev. B 79, 205416 (2009). [9] P. K. Pathak and S. Hughes, arXiv:0906.3035 (1999).
Quantum Process Discrimination, Waveguides,
and Fault Tolerant Quantum Processes Given the emerging potential of quantum information science where
devices including photonic quantum circuits are being miniaturized,
making their identification challenging, quantum process discrimination
(QPD) has pragmatic, as well as foundational, considerations. Discrimination
between unknown quantum processes chosen from a finite set is experimentally
shown to be possible even in the case of nonorthogonal processes.
We demonstrate unambiguous deterministic QPD of nonorthogonal processes
using properties of entanglement, additional known unitaries, or
classical communication. In principle these discrimination protocols can be realised with 100% confidence, however the usual imperfect input states and imperfect processes contribute to experimental errors and our discrimination confidence of 97% is not perfect. In this spirit, we go on to discuss recent improved tests of quantum photonic devices (waveguides) and report unprecedented fidelities, which demonstrate that these devices can operate within the fault tolerant regime, by some accepted measures.
The structure of spin There are a number of experiments on particles with spin that quantum mechanics cannot explain. These include the double slit experiment and coincidence experiments using photons. It is proposed that intrinsic spin angular momentum has a two dimension structure and this leads to a new angular momentum state of magnitude 1/sqrt(2). In order to form this new spin state it is necessary to relax the hermitian postulate of quantum mechanics and admit non-hermitian states, which is manifest as a quantum phase. However for both isolated and entangled spins, hermitian states naturally result. ------------------------------------------------------------------------------- "Piecewise" vs. "Coherently controlled"
adiabatic passage. We develop a technique for executing robust and selective transfer of populations between pre-selected superpositions of energy eigenstates. Viewed in the frequency domain, our methods stem from the idea of Coherently Controlled Adiabatic Passage [1], in which several adiabatic passage pathways coherently add up to provide the desired population transfer. Viewed in the time domain, the methods work by piecewise accumulation of the wavefunction in the target wave packet, applying the Piecewise Adiabatic Passage technique [2] in the multi-state regime. The presentation will introduce the basic concepts behind the technique and will discuss its recent theoretical and experimental developments. [1] P. Kral, I. Thanopulos, M. Shapiro, Rev. Mod. Phys. 79, 53
(2007). [2] E.A. Shapiro et.al., Phys. Rev. Lett. 99, 033002 (2007). Quantum Repeaters I will briefly describe recent progress on the development of practical quantum repeater architectures, where the most immediate goal is to outperform the direct transmission of quantum states. I will focus on architectures using solid-state atomic ensembles as quantum memories, which may allow very efficient temporal multiplexing through the implementation of multimode memories. I will also discuss a promising approach based on trapped ions, which builds on the impressive experimental progress achieved with the goal of quantum computation in mind. Paper reference: PRL 98, 190503; PRA 76, 050301; PRA 77, 062301;
Nature 456, 773; PRA 79, 042340; PRA 79, 052329 Decoherence and the quantum-to-classical transition of a symmetry
breaking coherent control scenario in an optical lattice An experimentally accessible way to study the quantum-to-classical
(hbar ? 0) transition of a symmetry breaking coherent control scenario,
in both isolated systems and in the presence of tunable amounts
of decoherence, is proposed. The setup exploits the experimental
control over the depth of the potential wells in optical lattices
to define an effective hbar that, in principle, can be experimentally
manipulated. Simulations of the transition show that the symmetry
breaking effect survives in the classical limit and hence that matter
interference effects are not required for the emergence of coherent
laser control. Even when the average photoinduced momentum (i.e.
the net degree of symmetry breaking) approaches smoothly its classical
limit, the probability distribution of the observable does not,
having an extremely fine oscillatory structure superimposed on the
classical background that has little effect on the average. This
fine structure due to quantum coherences is extremely fragile to
environmental decoherence in the small hbar limit and a very small
amount of decoherence is required to ensure the classical limit.
We conclude that the detrimental effects induced by interaction
with environmental degrees of freedom in this coherent laser control
scheme are due to a decay of the temporal correlations in the system's
dynamics, and not due to a decay of matter interference effects
due to decoherence. Measuring High-Order Coherences of Chaotic
and Coherent Optical States We demonstrate a new approach to measuring high-order temporal
coherences that uses a four-element superconducting nanowire single-photon
detector (SNSPD) in which four independent, single-photon-sensitive
elements are interleaved over a single spatial mode of the optical
beam. We show the power of this technique by measuring nth-order
coherences (n = 2,3,4) both of a chaotic, pseudo-thermal source
that exhibits high-order photon bunching (up to n!), and of a coherent
state source for which all coherences are ~1. Our results demonstrate
that using multiple detector elements to parse an optical beam over
dimensions smaller than the minimum diffraction-limited spot size
can be equivalent-and in some cases superior-to using multiple beamsplitters
and discrete detectors that each sample a replica of the entire
mode. --------------------------------------------------------------------------- Unconditionally secure entanglement-based quantum key distribution
experiment Entanglement-based quantum key distribution (QKD) [1,2] is expected to achieve long distance secure communication with a relatively simple setup. One promising entanglement-based QKD technique is the Bennett-Brassard-Mermin 1992 (BBM92) protocol [2] where Alice and Bob use threshold detectors with linear optics. Recently, Koashi et al. provided an unconditional security proof that can be directly applied to a practical implementation of BBM92 [3]. In this presentation, we report the first unconditionally secure BBM92 QKD experiment based on this theory.
Complete process tomography of experimental one-way quantum
computation We present full quantum process tomography of photonic cluster-state
quantum computation. We demonstrated basic gates of one-way quantum
computation and reconstructed these process matrices by using maximum
likelihood estimation. From the completely reconstructed process
matrices, we can evaluate several gate performances such as process
fidelity, purity, and entanglement capability. We also discuss the
relation between experimentally obtained process matrices and fault-tolerant
threshold theory with several error models such as independent stochastic
Pauli errors, independent stochastic completely-positive trace-preserving
errors, and general local unitary errors (e.g. slight over-rotation). Quantum Computer Simulations of Time Dependent Hamiltonians Feynman's original motivation for the quantum computer resulted
from a conjecture that quantum computers could efficiently simulate
any quantum system, whereas classical computers cannot. Since then
many quantum simulation schemes have verified his conjecture for
sparse time independent Hamiltonians. However all proposals that
have been put forward so far for simulating time dependent Hamiltonians
only address sparse Hamiltonians and have complexity that scales
as O(t^{3/2}). We address these issues by presenting a quantum computer
simulation scheme that can simulate many non-sparse time-dependent
Hamiltonians that uses arbitrary-order decomposition formulae to
achieve complexity of O(t^{1+epsilon)) for arbitrary epsilon>0,
provided the time dependence is suitably smooth. Since O(t) complexity
has been proven to be optimal, our simulation scheme demonstrates
near optimal performance for the broadest class of Hamiltonians
considered so far. Looking into the relation between quantum phase
transition and entanglment via density functional theorey Density functional theory (DFT) is shown to provide a novel conceptual
and computational framework for entanglement in interacting many-body
quantum systems. DFT can, in particular, shed light on the intriguing
relationship between quantum phase transitions and entanglement.
We use DFT concepts to express entanglement measures in terms of
the first or second derivative of the ground state energy. We illustrate
the versatility of the DFT approach via a variety of analytically
solvable models. As a further application we discuss entanglement
and quantum phase transitions in the case of mean field approximations
for realistic models of many-body systems. Disentanglement and Control of Qubit Systems Coherence dynamics of quantum systems is a generic paradigm that
has been widely discussed in research fields ranging from atomic
and optical physics to condensed matter physics and to quantum information
science. In this talk I will present highlights of our recent work
on several key issues in entanglement dynamics including evolution
of spin entanglement under phonon noise, the sudden death of entanglement,
two-body open system entanglement, probing many-body entanglement
subject to thermal noise, and entanglement control. Long-lived Quantum Memories A memory based on hyperfine atomic coherences (spin waves) which can be read out optically at the single photon level, when classical noise sources have been eliminated, is a quantum memory. The spin waves are generally sensitive to ambient magnetic fields that limit their storage time to tens of microseconds. By optical pumping of the atoms and use of the clock coherence sensitivity to magnetic fields can be greatly reduced. Even in ultra-cold atomic samples motional dephasing becomes important on a scale of hundreds of microseconds. We present results of our work which circumvent both of these difficulties to achieve an atomic memory with a lifetime of several milliseconds. We will discuss various applications of the long-lived atomic memory, including deterministic single photon sources, matter qubit rotations, and matter-light entanglement. |
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