June 2008
QAP researcher wins prize for outstanding research!
Congratulations to Dr. Christine Silberhorn for winning the prestigious Heinz Maier-Leibnitz Prize 2008. Dr. Silberhorn is one of 6 young researchers to be recognised by German Research Foundation for outstanding contributions to science. The Prize has been awarded to young researchers annually since 1977 to promote the further development of outstanding independent profiles. The prize of €16,000 is simultaneously a form of recognition of past achievements and an incentive to continue climbing the scientific career ladder. As such, it is held in high regard by the whole scientific community. Dr. Silberhorn's citation for the award is included below.
Christine Silberhorn's research to date is characterised by an extremely broad range of interests and research topics as well as a wide variety of international experience, and she has attained a very good reputation in the field of experimental quantum optics within the period of just a few years. During her studies she was already interested in highly abstract topics relating to topology, before moving on to the entirely different area of quantum cryptography for her PhD thesis. She has been able to establish herself very well in the rapidly progressing and thus highly competitive research field of quantum information processing. Her main area of interest was how to process and transmit quantum information using light, an issue that is of key importance for any quantum computers that may be built in the future. Instead of the discrete variables normally employed, Silberhorn used so-called "continuous variables", thus developing a widely accepted alternative. Silberhorn is currently continuing this work as the leader of an independent Max Planck junior research group, in which she, herself still a young scientist, is combining her successful research work with the opportunity to train other young researchers and scientists.
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Figure 1: Dr. Christine Silberhorn (Front, second left) pictured with other winners of the Heinz Maier-Leibnitz Prize 2008. |
Superconducting Nanowire Photon Number Resolving Detector at Telecom Wavelength
Nature Photonics 2, 302 - 306 (2008)
Francesco Marsili, David Bitauld, Aleksander Divochiy, Alessandro Gaggero, Roberto Leoni, Francesco Mattioli, Alexander Korneev, Vitaliy Seleznev, Nataliya Kaurova, Olga Minaeva, Gregory Gol’tsman, Konstantinos G. Lagoudakis, Moushab Benkhaoul, Francis Lévy, Andrea Fiore
Overview
The characterisation of photon number states is an essential tool in photonic quantum information processing, however existing photon-number resolving (PNR) detectors are either too noisy or too slow for practical applications. QAP researchers recently demonstrated a PNR detector, the Parallel Nanowire Detector (PND) [1], which uses spatial multiplexing on a subwavelength scale to provide a single electrical output proportional to the photon number. The basic structure of the PND is the parallel connection of superconducting nanowires (N-PND). The detecting element is a few nm-thick, ~100 nm-wide NbN wire folded in a meander pattern (fig. 2a), which can be integrated in series with a bias resistors R0 (N-PND-R) (fig. 2b). Each branch acts as a superconducting single photon detector (SSPD) [2]. If a superconducting nanowire is biased close to its critical current, the absorption of a photon causes the formation of a normal barrier across its cross section and the bias current is pushed to the external circuit. In the parallel configuration proposed here, the currents from different wires can sum up on the external load, producing an output voltage pulse proportional to the number of photons. The integration of the resistance R0 improves the performance of the device in terms of speed and stability of operation [3].
PNDs were fabricated on ultrathin NbN films (4nm) on MgO [4] and R-plane
sapphire [5] using electron beam lithography (EBL) and reactive ion etching. The
photoresponse pulse is as short as 660ps (full width at half maximum). Counting
performance was observed up to 80 MHz repetition rate. Building the histograms
of the photoresponse peak (fig. 3), no multiplication noise is observable and
the one photon quantum efficiency can be estimated to be 3% (at 700 nm
wavelength and 4.2 K temperature). The PND thus significantly outperforms
existing PNR detectors in terms of simplicity, sensitivity, speed, and
multiplication noise.
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Figure 2. Scanning electron microscope (SEM) images of a 14-PND (a) and a 8-PND-R (b) fabricated on a 4 nm thick NbN film on MgO. The nanowire width is w=100 nm, the meander fill factor is f=40%. In fig. (b) the active nanowires (in color) are connected in series with Au-Pd bias resistors (in blue). |
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Figure 3. Photoresponse transients of a 10x10 μm2 6-PND-R (□) probed at 5 K under illumination with 1.3 μm pulses from a laser diode, at a repetition rate of 26MHz. The red solid curves are guides to the eyes. The histogram of the photoresponse voltage peak is shown on the left hand side of the picture in turquoise. |
References
[1] A. Divochiy et al., Nature Photon. 2, 302 (2008).
[2] G. N. Gol'tsman et al., Appl. Phys. Lett. 79, 705 (2001).
[3] F. Marsili et al., J. Mod. Opt. to be published.
[4] F. Marsili et al., Opt. Express 16, 3191 (2008).
[5] G. N. Gol'tsman et al., IEEE Trans. Appl. Supercond. 13, 192 (2003).
May 2008
Migration of bosonic particles across a Mott insulator to superfluid phase interface
Physical Review Letters 100, 070602 (2008)
Michael J. Hartmann and Martin B. Plenio
Overview
Quantum many body systems are, in the forms in which they occur in nature,
difficult to study experimentally. These difficulties lead to the development of
quantum simulators with which many body effects can be emulated in the
laboratory. Recent approaches to quantum simulators now give rise to
possibilities for engineering deliberate inhomogeneities in quantum many body
systems. This development could allow for the observation of dynamics at the
boundary between two areas of a many body system which are in different
condensed matter phases.
Recent work by QAP researches studies this dynamics theoretically and finds rather surprising effects. The scientists consider a boundary between a Mott insulator in which mutual particle repulsion strongly suppresses particle movement and an area in a superfluid phase, where particles do only interact very little an move almost freely and frictionless. The work by the QAP researchers shows that, in such a situation, all particles will leave the Mott insulator part and migrate to the superfluid region, leaving the Mott insulator part completely empty. Understanding and hence being able to control and make use of such effects is an important step towards the objectives of QAP.
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Figure 1: Migration of particles from a Mott insulator to a superfluid. The plot shows particle densities (nj) for a chain of 40 sites as a function of time (t). Initially the particle densities are 1 in both parts of the chain. They drop to zero in the Mott insulator part (sites 1 - 20) and grow in the superfluid part (sites 21 - 40) |
Exact Relaxation in a Class of Nonequilibrium Quantum Lattice Systems
Physical Review Letters 100, 030602 (2008)
M. Cramer, C. M. Dawson, J. Eisert, and T. J. Osborne
Open systems dynamics in closed quantum systems: Exact relaxation in quenched quantum many-body systems
Why do systems dynamically relax to a
statistical equilibrium state? This intriguing but old question is enjoying a
renaissance recently. With new experimental techniques becoming available, the
non-equilibrium dynamics of atoms in optical lattices can be experimentally
observed. Specifically, following a quench - that is, a rapid change of the
system's parameters - the many-body system undergoes complicated dynamics. So
what happens? There is no environment, so how could it possibly relax?
Recent work by QAP researchers published in the Physical Review Letters answers
this question rigorously for a class of models that are idealized instances of
the Bose-Hubbard model that takes centre stage in this discussion of atoms in
optical lattices. The authors demonstrate that while the information on the
initial condition is of course stored in the system at all times, it becomes
diluted with time. Locally, for any subsystem, one obtains a maximally entropy
state compatible with the constants of motion. Remarkably, this is true without
a time average: The system just smoothly and nicely relaxes. So when locally
looking at the system, we think that the system has reached its equilibrium. But
only apparently so, as one day, arbitrarily far in the future, a recurrence will
show that all the time, the initial condition was not forgotten.
This work discusses a scenario in which the dynamics of quantum phase
transitions in quantum many-body systems can be studied in experiments, in an
instance of a quantum simulation of a complex non-equilibrium process. The ideas
put forth on the simulation and quantum control of such a quantum man-body
system are very much inline with the objectives of QAP.
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Figure 2: Intuitive
picture of the relaxation process in the quenched Bose-Hubbard model:
For any lattice site i (or any block |
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Figure 3: Local density of particles as a function of time, for a periodic initial condition of atoms in an optical lattice, including an harmonic trap. This plot shows signatures of local relaxation. |
March 2008
Heralded Generation of Ultrafast Single Photons in Pure Quantum States
Physical Review Letters, 100, 133601 (2008)
Peter J. Mosley, Jeff S. Lundeen, Brian J. Smith, Piotr Wasylczyk, Alfred B. U'Ren, Christine Silberhorn, Ian A. Walmsley
Overview
Single photons (discrete wavepackets of light) are not only interesting in terms of fundamental physics, but also from the point of view of applications in the emerging field of quantum information processing - a field that has the potential to revolutionize computing by harnessing the data processing power inherent in quantum mechanics. In this paper, researchers at the University of Oxford present the results of a new technique, based on photon pair generation, that for the first time allows the preparation of single photons of exceptionally high quality, conditioned on the detection of their twin. Furthermore, these photons have a temporal duration of as little as 65 femtoseconds (65 millionths of a billionth of a second), believed to be the shortest single photons ever generated. The precise timing and consistent attributes of these photons make them ideal for implementing photonic quantum logic gates and conducting experiments requiring large numbers of single photons, as required by quantum computing algorithms.
The source is based on the process of parametric downconversion in nonlinear optical crystals. In a single downconversion event, one "parent" photon is destroyed, and two "daughter" photons of lower energy are created (see Figure 1). Downconversion is a commonly used method for generating photon pairs, with the detection of one daughter photon heralding the arrival of the second daughter photon. This simple detection arrangement makes downconversion an attractive prospect for generating the single photons required by many quantum information processing protocols. However, the detection of the herald photon usually ruins the quality (or purity) of the second daughter photon. Energy and momentum conservation in the generation process generally result in strong quantum correlations within each pair (known as entanglement). Upon detection of one photon, the possibility of obtaining distinguishing information about the other photon ruins the purity of its quantum state. Some experiments have countered this problem by spectrally filtering the photons, effectively throwing away the unwanted photons. Unfortunately this process greatly reduces the efficiency of the source, and the purity of the daughter photons only becomes 100% in the limit that all photons are filtered out!
In the work highlighted here, QAP researchers at Oxford have built a source which requires no filtering of the daughter photons but still delivers photons of excellent purity and with a high efficiency. By carefully controlling the parameters which dictate the momentum conservation in the downconversion process, Mosely et al. were able to engineer a source in which detection of one daughter photon does not automatically destroy the purity of the second daughter photon. The purity of the daughter photons was tested by constructing two identical downconversion sources and then combining the heralded daughter photons on a 50:50 beamsplitter (see Figure 1). If the photons from each source are of high quality and are therefore indistinguishable, they will bunch together at the beamsplitter and exit in the same direction: this is the Hong-Ou-Mandel effect. The plot in Figure 1 shows that when the heralded photons from each source reach the beamsplitter at the same time, the number of coincidences between the four detectors drops off dramatically due to this bunching effect. The dip measured in this work indicates that the photons have a purity of over 95%, generated with a high efficiency. This work will be a substantial benefit for future quantum information processing applications requiring high purity photons as it has eliminated the need for lossy spectral filters.
The Oxford group are now using such sources for quantum enhanced precision metrology, heralded entanglement generation and linear optical quantum computation schemes.
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Figure1: Scheme of the high purity photon pair
generation by parametric downconversion. |
Nonclassical Interference and Entanglement Generation Using a Photonic Crystal Fiber Pair Photon Source
Physical Review Letters, 99, 120501 (2007)
Jérémie Fulconis, Olivier Alibart, Jeremy L. O'Brien, William J. Wadsworth, and John G. Rarity
The scaling of linear optical networks to many qubits is currently limited by
the lack of bright single photon sources. Researchers at the University of
Bristol, in collaboration with the University of Bath have developed a versatile
solution based on photonic crystal fibres. Their work reports on the suitability
of these new fibre sources for optical quantum information processing.
Standard pair photon sources usually relied on spontaneous parametric
downconversion where the practical limit to multiphoton experiment is five or
six photons due to a high pump power requirement (typically a few Watts). Using
photonic crystal fibres, a pair of pump photons produces a correlated pair of
photons at widely spaced wavelengths, via a χ(3)
four-wave mixing process. The confined geometry of the fibre enhances the
non-linear interaction and the required average photon number per pulse is
reached at milliWatt pump powers.
Two key experiments are reported. The demonstration of high visibility
nonclassical interference between heralded photons from separate fibre sources
and high fidelity entangled photon pair generation by configuring the fibre in a
Sagnac loop interferometer. The photons are created within a single fibre mode
which can be efficiently coupled into linear optics circuits and on to
detectors. This source promises much higher rates of multiphoton generation than
conventional sources. This will open the way to a variety of novel experiments
including cascaded linear gates, or building large entangled cluster states,
which is inline with the objectives of the QAP project.
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Figure1: Scheme of the non-classical interference
setup (left) and entanglement generation (right). |
October 2007
Entangling independent photons by time measurement
Nature Physics, 3, 692 (2007)
Matthaus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon and Hugo Zbinden
Overview
Researchers in the Group of Applied Physics (GAP) at Geneva University have demonstrated an entanglement swapping experiment with autonomous sources. Normally, entanglement between two photons is produced by emitting them simultaneously from the same source. In this experiment the researchers show that entanglement can be transferred (or "swapped") on two photons, originating from autonomous sources and hence formerly completely independent. This is the first time that two autonomous photons have been entangled.
The scheme of Entanglement Swapping allows entangling photons, which have
been emitted by independent processes in remote sources (A and B, Fig. 1). Each
source emits a pair of entangled photons (A1-A2 and B1-B2, respectively). By
taking one photon from each pair (A1 and B1) and performing a joint measurement
(Bell state measurement, BSM), they are detected in an entangled state, one of
the four Bell states. For a successful outcome of the BSM, A2 and B2 are
projected onto an entangled state as well, even though they never interacted or
share any common past.
To enable a BSM, the photons A1 and B1 must be indistinguishable, which means in
particular, arrive at the same time at the beam splitter BS. Until now, this was
achieved by the use of pulsed sources with synchronized emission times and
matched fiber lengths. This is the first demonstration that precise timing of
photons at telecom wavelength can be achieved by detection and postselection. To
make this possible, the photons’ coherence length has to exceed the detectors
temporal resolution. Single photon detectors based on superconductors and
semiconductors, featuring a timing resolution of 70-100ps. By narrowly filtering
(10pm) the photons, their coherence length is increased to the order of 300ps.
Thus no synchronization between the sources is required anymore which makes them
truly autonomous.
This setup is compatible with some of the quantum memories currently under development in project QAP, because the source and memory bandwidths are within the same order of magnitude. The work is thus an important step towards quantum repeaters, one of the main QAP objectives.
For a more detailed summary, see the News and Views section of Nature Physics, October 2007 on the UNIGE website.
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Figure1: Scheme of the Entanglement Swapping setup |
Enhancement of the recombination rate of InAs quantum dots coupled to micro-cavities emitting at the telecom wavelengths
Applied Physics Letters 91, 123115 (2007)
L. Balet, M. Francardi, A. Gerardino, N. Chauvin, B. Alloing, C. Zinoni, C.
Monat, L. H. Li, N. Le Thomas, R. Houdré and A. Fiore.
Optics Letters 32, 2747
(2007)
N. Chauvin, L. Balet, B. Alloing, C. Zinoni, L. Li, A. Fiore, L. Grenouillet, P.
Gilet, N. Olivier, A. Tchelnokov, M. Terrier and J.-M. Gérard.
Quantum computing and quantum communication need sources of single
indistinguishable photons emitting at telecom wavelengths. Such requirements are
reachable using single quantum dots (QDs) embedded in micro-cavities. The
coupling of a QD with the cavity mode increases the recombination rate of the QD
exciton due to the Purcell effect. Thus the impact of the decoherence processes
is reduced and an increase of the repetition rate of the single photon source is
possible. Moreover the coupling of the QD emission with the mode enables good
collection efficiency.
Micro-photoluminescence (MPL) and time-resolved experiments have been performed
on InAs QDs embedded in photonic crystal (PhC) and micro-pillar (MP) cavities.
The QD growth and cavity fabrication have been optimized for emission at 1300
nm. The cavity modes present a high quality factor: up to 15000 for the PhC
cavities and up to 2500 for the MP cavities.
Studies performed on PhC cavities have revealed an increase in the intensity of
single QD lines when tuned with the cavity mode. Time resolved experiments
performed on these single QD lines have revealed an enhancement of the
recombination rate up to eightfold.
An increase of the recombination rate by a factor 2 is also observed for an
ensemble of QDs coupled to one mode of a MP cavity.
These results are very promising for the realization of efficient single photon
sources based on single QDs in a micro-cavity.
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Figure1: Time resolved dynamics of QDs spatially
outside the PhC region (blue squares) and of a single exiton line in
resonance of the cavity mode (black triangle) |
August 2007
Entanglement percolation in quantum networks
Nature Physics, 3, 256 (2007)
Antonio Acín, J. Ignacio Cirac and Maciej Lewenstein
Overview
The standard scenario in any quantum communication application consists of
several distant nodes that can exchange information encoded on quantum particles
through quantum channels, e.g. photons sent through optical fibers. The nodes
aim at establishing long-distance perfect quantum correlations, or maximally
entangled states, by performing local operations on each node and exchanging
classical communication. Therefore, in order to optimize the operation of a
quantum network, it is required to design efficient protocols for the
establishment of maximally entangled states between different distant nodes.
Until now, the standard solution for the distribution of entanglement between
distant nodes was by means of quantum repeaters. In this scenario, two distant
nodes are connected by a chain of quantum repeaters in a one-dimensional
configuration, see also Figure 1(a), below. General quantum networks, however,
have a richer and more connected structure, see also Figure 1(b) and 1(c). Our
recent work shows that intriguing quantum phenomena appear when studying the
distribution of entanglement through these more connected networks, depending on
the way the nodes are connected and the entanglement parameters characterizing
these connections. Actually, the distribution of entanglement through these
networks defines a new type of phase transition. This entanglement phase
transition is related to classical percolation, a concept that is well known in
Statistical Mechanics, so we named this phenomenon Entanglement Percolation.
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Figure1: Diagram illustrating the distinction
between a one-dimensional quantum network (a) and the richer structure
of a more general quantum network (b,c). |
July 2007
Twisted photons
Nature Physics, 3, 305 (2007)
G. Molina-Terriza, J. P. Torres and L. Torner
Overview
Quantum optical information applications make use of only a portion of the rich structure of atoms and photons. Most of these applications make use of the polarization or spin angular momentum of photons. This corresponds to using two-dimensional systems, that is systems whose state can be described by a combination of two orthogonal states, as any polarization state can be described by weighted superpositions of two linear or circular polarization states.
But the total angular momentum can also contain an orbital contribution, which comes from a more complex combination of the phase and amplitude profiles of an optical field. Contrary to the case of polarization, the orbital angular momentum lives in an infinite dimensional Hilbert space. Indeed, the dimensions of the working space can be readily tailored. This offers the possibility to explore quantum algorithms that either inherently live in a higher dimensional Hilbert space (qudits), or exhibit enhanced efficiency in increasingly higher dimensions.
In the May 2007 issue of Nature Physics (see Nature Physics 3, 305, 2007 and cover page) researchers Molina-Terriza, Torres and Torner, from ICFO-Institute of Photonic Sciences in Barcelona, review progress in the generation, understanding and use of the orbital angular momentum of photons. The orbital angular momentum of photons can be used to demonstrate the violation of bipartite, three dimensional Bell inequalities, to implement new protocols such as the so called quantum coin tossing, to generate quantum states in ultra-high dimensional spaces, to implement new quantum cryptography protocols, and as an powerful enabling tool to generate and to control multidimensional quantum states of matter, and therefore adds to the existing techniques for the full control of atoms.
The exploration of the orbital angular momentum of photons is an important tool of the Qubit Applications Project (QAP). It is related to the development of theory and design of experiments to exploit the rich structure of photons, which includes the development of techniques for the independent control of polarization and transverse wave-vector, and the investigation of new quantum communication protocols using the space degree of freedom of single photons.
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Figure: What is the shape of a photon which carries
orbital angular momentum? |
Trapped Ion Chain as a Neural Network: Error-Resistant Quantum Computation
Physical Review Letters, 98, 023003 (2007)
M. Pons, V. Ahufinger, C. Wunderlich, A. Sanpera, S. Braungardt, A. Sen(De), U. Sen, and M. Lewenstein
Overview
Exposing an ion - that is part of a linear array of ions confined in a Paul
trap - to a qubit-state-dependent force gives rise to a long-range interaction
between individual ionic qubits. In the January issue of Physical Review Letters
QAP researchers show that taking advantage of this long
range interaction an array of ions may be employed to realize a neural network
composed of individual atoms. This network permits storage of information
distributed over the whole atomic array. Moreover, error-resistant quantum
computation may be implemented using such a neural network.
As part of Subproject 4 of QAP the research group at the
University of Siegen
works on the implementation of ion spin molecules and will explore their
suitability, for instance, for realizing such a neural network.
June 2007
Remote preparation of an Atomic Quantum Memory
Physical Review Letters, 98, 050504 (2007)
Wenjamin Rosenfield, Stefan Berner, Jürgen Volz, Markus Weber, and Harald Weinfurter
Overview
Remote state preparation of a distant atomic quantum memory has been
demonstrated by researchers at Ludwig
Maximillians University (LMU), Germany. This is a significant step towards
the development of quantum repeaters, and quantum networks – two of the
principal research interests in the EU Project QAP.
Many of the fundamental concepts in quantum communication and quantum
information processing are predicated on the ability to faithfully map quantum
information between photons and matter-based quantum processors. Entanglement
between matter and light is crucial for this task. In this work, researchers at
LMU experimentally demonstrate the suitability of atom-photon entanglement as an
interface between a quantum memory device and a quantum communication channel.
In the demonstration at LMU, a single rubidium atom acting as the quantum memory
is prepared in a desired quantum state by a remote sender (Alice) using a remote
state preparation (RSP) protocol. The RSP protocol consists of three main steps.
Initially, entanglement is generated between the spin of a single Rubidium atom
confined in a dipole trap, and the polarization of a single spontaneously
emitted photon; the photon is then transferred to Alice via an optical fiber.
Secondly, Alice imprints the desired qubit on the photon using the spatial modes
of a polarization-independent Mach-Zehnder interferometer. Finally, the spatially
encoded qubit is transferred to the spin state of the atom by performing a
Bell-state measurement in the joint polarization - spatial-mode Hilbert space of
the photon. This is possible because of the atom-photon entanglement generated
in the first step.
The success of the RSP protocol described above has been verified at LMU by
conducting full quantum state tomography of the atomic qubit, after the
preparation process. The mean fidelity of quantum state transfer was found to be
82.2%. This work marks a significant step towards implementation of an atomic
quantum repeater.
Globally Controlled Quantum Wires for Perfect Qubit Transport, Mirroring, and Computing
Physical Review Letters, 97, 090502 (2006)
Joe Fitzsimmons and Jason Twamley
Overview
Recent work by researchers at Oxford and Macquarie Universities has shown
that the necessary requirements for practical quantum computing may be lower
than previously thought. In their paper, Joe Fitzsimons and Jason Twamley
outline a scheme for performing universal quantum computation within a uniform
Ising spin chain without the need to control individual spins separately.
The need to individually address qubits within a quantum computer has proved a
roadblock to achieving scalable quantum computing within many systems. While a
number of architectures exist which overcome this problem using periodic
arrangements of different types, or species, of qubits, this new scheme
substantially lowers the experimental requirements, requiring only a single
physical species of qubit together with a common physical interaction.
The paper includes a prescription for quantum state transfer, the task of moving
quantum information from one location to another, as well as a procedure for
efficiently mirroring the order of qubits stored within the spin chain. These
are both important tasks within quantum communication, and it is likely that
similar quantum wires will provide important communications buses within any
large scale solid-state quantum processor.
Additionally, the paper describes how universal computing can be achieved within
the spin chain without resorting to local control. The density with which
logical qubits can be packed (˝) within the chain is the highest yet achieved
within such a scheme, and has since been increased to the maximum possible
density of one qubit per spin (quant-ph/0606188),
offering computational power competitive with locally controlled schemes.
Shortly after publication of this paper, a successful experimental
implementation of the scheme was reported using NMR techniques (quant-ph/0606188).
This work substantially lowers the barriers to scalable solid-state quantum
computing.
May 2007
Observation of Entanglement of a Single Photon with a Trapped Atom
Physical Review Letters, 96, 030404 (2006)
J. Volz, M. Weber, D. Schlenk, W. Rosenfeld, J. Vrana, K. Saucke, C. Kurtsiefer, and H. Weinfurter
Overview
Entanglement between the polarization of a single photon and the internal
state of a single neutral atom has been observed by researchers at Ludwig
Maximillians University (LMU), Germany. This is a crucial step towards
implementation of long range quantum networks: one of the principal areas of
research in the EU Integrated Project QAP .
Entanglement is a key element for quantum communication and information
applications. Future applications such as quantum networks and the quantum
repeater are predicated on the distrubution of entanglement between separate
quantum processors. For this purpose, entanglement between different quantum
objects such as atoms and photons forms the interface between atomic quantum
processors and photonic quantum communication channels, finally allowing the
distibution of quantum information over arbitrary distances.
In this implementation a single Rubidium atom, confined within a dipole trap of
radius 3.5µm, acts as a quantum processor. The atom is trapped using a far
off-resonance laser field. The instantaneous dipole moment induced on the atom
by the field results in an intensity dependent force upon the atom. For a large
negative frequency detuning this dipole force pushes atoms towards regions of
higher laser intensity, thus allowing confinement of atoms within a region of
suitably intersecting laser beams.
Selective pumping prepares the trapped atom in an excited hyperfine state which
can decay to three possible ground states with magnetic quantum numbers MF=-1,0
or +1 by spontaneously emitting a σ+, π or σ- polarized photon, respectively.
Photons with σ-polarization are detected in the same spatial mode and analysed
to determine their polarization while those with π-polarization are ignored.
Provided the emission processes are indistinguishable in all degrees of freedom
other than polarization, the atom-photon system is in a maximally entangled
state. This entanglement has been verified at LMU using a full tomographic
analysis of the atomic state via a state selective stimulated Raman adiabatic
passage (STIRAP) technique.
The techniques employed at LMU represent an important step towards an interface
between a quantum computer and a photonic quantum communication channel.