79 research outputs found
Performance of polar codes for quantum and private classical communication
We analyze the practical performance of quantum polar codes, by computing
rigorous bounds on block error probability and by numerically simulating them.
We evaluate our bounds for quantum erasure channels with coding block lengths
between 2^10 and 2^20, and we report the results of simulations for quantum
erasure channels, quantum depolarizing channels, and "BB84" channels with
coding block lengths up to N = 1024. For quantum erasure channels, we observe
that high quantum data rates can be achieved for block error rates less than
10^(-4) and that somewhat lower quantum data rates can be achieved for quantum
depolarizing and BB84 channels. Our results here also serve as bounds for and
simulations of private classical data transmission over these channels,
essentially due to Renes' duality bounds for privacy amplification and
classical data transmission of complementary observables. Future work might be
able to improve upon our numerical results for quantum depolarizing and BB84
channels by employing a polar coding rule other than the heuristic used here.Comment: 8 pages, 6 figures, submission to the 50th Annual Allerton Conference
on Communication, Control, and Computing 201
Channelization architecture for wide-band slow light in atomic vapors
We propose a ``channelization'' architecture to achieve wide-band
electromagnetically induced transparency (EIT) and ultra-slow light propagation
in atomic Rb-87 vapors. EIT and slow light are achieved by shining a strong,
resonant ``pump'' laser on the atomic medium, which allows slow and
unattenuated propagation of a weaker ``signal'' beam, but only when a
two-photon resonance condition is satisfied. Our wideband architecture is
accomplished by dispersing a wideband signal spatially, transverse to the
propagation direction, prior to entering the atomic cell. When particular
Zeeman sub-levels are used in the EIT system, then one can introduce a magnetic
field with a linear gradient such that the two-photon resonance condition is
satisfied for each individual frequency component. Because slow light is a
group velocity effect, utilizing differential phase shifts across the spectrum
of a light pulse, one must then introduce a slight mismatch from perfect
resonance to induce a delay. We present a model which accounts for diffusion of
the atoms in the varying magnetic field as well as interaction with levels
outside the ideal three-level system on which EIT is based. We find the maximum
delay-bandwidth product decreases with bandwidth, and that delay-bandwidth
product ~1 should be achievable with bandwidth ~50 MHz (~5 ns delay). This is a
large improvement over the ~1 MHz bandwidths in conventional slow light systems
and could be of use in signal processing applications.Comment: Published in SPIE Proceedings, Photonics West 2005 (San Jose, CA,
Jan. 22-27, 2005
Observation of Quantum Shock Waves Created with Ultra Compressed Slow Light Pulses in a Bose-Einstein Condensate
We have used an extension of our slow light technique to provide a method for
inducing small density defects in a Bose-Einstein condensate. These
sub-resolution, micron-sized defects evolve into large amplitude sound waves.
We present an experimental observation and theoretical investigation of the
resulting breakdown of superfluidity. We observe directly the decay of the
narrow density defects into solitons, the onset of the `snake' instability, and
the subsequent nucleation of vortices.Comment: 15 pages, 5 figure
Effective one-component description of two-component Bose-Einstein condensate dynamics
We investigate dynamics in two-component Bose-Einstein condensates in the
context of coupled Gross-Pitaevskii equations and derive results for the
evolution of the total density fluctuations. Using these results, we show how,
in many cases of interest, the dynamics can be accurately described with an
effective one-component Gross-Pitaevskii equation for one of the components,
with the trap and interaction coefficients determined by the relative
differences in the scattering lengths. We discuss the model in various regimes,
where it predicts breathing excitations, and the formation of vector solitons.
An effective nonlinear evolution is predicted for some cases of current
experimental interest. We then apply the model to construct quasi-stationary
states of two-component condensates.Comment: 8 pages, 4 figure
First-order sidebands in circuit QED using qubit frequency modulation
Sideband transitions have been shown to generate controllable interaction
between superconducting qubits and microwave resonators. Up to now, these
transitions have been implemented with voltage drives on the qubit or the
resonator, with the significant disadvantage that such implementations only
lead to second-order sideband transitions. Here we propose an approach to
achieve first-order sideband transitions by relying on controlled oscillations
of the qubit frequency using a flux-bias line. Not only can first-order
transitions be significantly faster, but the same technique can be employed to
implement other tunable qubit-resonator and qubit-qubit interactions. We
discuss in detail how such first-order sideband transitions can be used to
implement a high fidelity controlled-NOT operation between two transmons
coupled to the same resonator.Comment: 15 pages, 5 figure
On quantum limit of optical communications: concatenated codes and joint-detection receivers
When classical information is sent over a channel with quantum-state
modulation alphabet, such as the free-space optical (FSO) channel, attaining
the ultimate (Holevo) limit to channel capacity requires the receiver to make
joint measurements over long codeword blocks. In recent work, we showed a
receiver for a pure-state channel that can attain the ultimate capacity by
applying a single-shot optical (unitary) transformation on the received
codeword state followed by simultaneous (but separable) projective measurements
on the single-modulation-symbol state spaces. In this paper, we study the
ultimate tradeoff between photon efficiency and spectral efficiency for the FSO
channel. Based on our general results for the pure-state quantum channel, we
show some of the first concrete examples of codes and laboratory-realizable
joint-detection optical receivers that can achieve fundamentally higher
(superadditive) channel capacity than receivers that physically detect each
modulation symbol one at a time, as is done by all conventional (coherent or
direct-detection) optical receivers.Comment: 5 pages, 7 figures, submitted to IEEE International Symposium on
Information Theory (ISIT), 201
Detecting changes to sub-diffraction objects with quantum-optimal speed and accuracy
Detecting if and when objects change is difficult in passive sub-diffraction
imaging of dynamic scenes. We consider the best possible tradeoff between
responsivity and accuracy for detecting a change from one arbitrary object
model to another in the context of sub-diffraction incoherent imaging. We
analytically evaluate the best possible average latency, for a fixed false
alarm rate, optimizing over all physically allowed measurements of the optical
field collected by a finite 2D aperture. We find that direct focal-plane
detection of the incident optical intensity achieves sub-optimal detection
latencies compared to the best possible average latency, but that a three-mode
spatial-mode demultiplexing measurement in concert with on-line statistical
processing using the well-known CUSUM algorithm achieves this quantum limit for
sub-diffraction objects. We verify these results via Monte Carlo simulation of
the change detection procedure and quantify a growing gap between the
conventional and quantum-optimal receivers as the objects are more and more
diffraction-limited.Comment: 4 pages, 3 figure
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