91 research outputs found
Tomography by noise
We present an efficient and robust method for the reconstruction of photon
number distributions by using solely thermal noise as a probe. The method uses
a minimal number of pre-calibrated quantum devices, only one on/off
single-photon detector is sufficient. Feasibility of the method is demonstrated
by the experimental inference of single-photon, thermal and two-photon states.
The method is stable to experimental imperfections and provides a direct,
user-friendly quantum diagnostics tool
Quantum techniques using continuous variables of light
We present schemes for the generation and evaluation of continuous variable
entanglement of bright optical beams and give a brief overview of the variety
of optical techniques and quantum communication applications on this basis. A
new entanglement-based quantum interferometry scheme with bright beams is
suggested. The performance of the presented schemes is independent of the
relative interference phase which is advantageous for quantum communication
applications.Comment: 11 pages, 5 figures; minor correction, accepted versio
Continuous Variable Quantum Cryptography - beating the 3 dB loss limit
We demonstrate that secure quantum key distribution systems based on
continuous variables implementations can operate beyond the apparent 3 dB loss
limit that is implied by the beam splitting attack . The loss limit was
established for standard minimum uncertainty states such as coherent states. We
show that by an appropriate postselection mechanism we can enter a region where
Eve's knowledge falls behind the information shared between Alice and Bob even
in the presence of substantial losses.Comment: 4 pages, 2 figure
Sub shot noise phase quadrature measurement of intense light beams
We present a setup to perform sub shot noise measurements of the phase
quadrature for intense pulsed light without the use of a separate local
oscillator. A Mach--Zehnder interferometer with an unbalanced arm length is
used to detect the fluctuations of the phase quadrature at a single side band
frequency. Using this setup, the non--separability of a pair of quadrature
entangled beams is demonstrated experimentally.Comment: 9 pages, 2 figures, accepted for publication in Optics Letter
Direct probing of the Wigner function by time-multiplexed detection of photon statistics
We investigate the capabilities of loss-tolerant quantum state
characterization using a photon-number resolving, time-multiplexed detector
(TMD). We employ the idea of probing the Wigner function point-by-point in
phase space via photon parity measurements and displacement operations,
replacing the conventional homodyne tomography. Our emphasis lies on
reconstructing the Wigner function of non-Gaussian Fock states with highly
negative values in a scheme that is based on a realistic experimental setup. In
order to establish the concept of loss-tolerance for state characterization we
show how losses can be decoupled from the impact of other experimental
imperfections, i.e. the non-unity transmittance of the displacement
beamsplitter and non-ideal mode overlap. We relate the experimentally
accessible parameters to effective ones that are needed for an optimised state
reconstruction. The feasibility of our approach is tested by Monte Carlo
simulations, which provide bounds resulting from statistical errors that are
due to limited data sets. Our results clearly show that high losses can be
accepted for a defined parameter range, and moreover, that (in contrast to
homodyne detection) mode mismatch results in a distinct signature, which can be
evaluated by analysing the photon number oscillations of the displaced Fock
states.Comment: 22 pages, 13 figures, published versio
Quantum key distribution using gaussian-modulated coherent states
Quantum continuous variables are being explored as an alternative means to
implement quantum key distribution, which is usually based on single photon
counting. The former approach is potentially advantageous because it should
enable higher key distribution rates. Here we propose and experimentally
demonstrate a quantum key distribution protocol based on the transmission of
gaussian-modulated coherent states (consisting of laser pulses containing a few
hundred photons) and shot-noise-limited homodyne detection; squeezed or
entangled beams are not required. Complete secret key extraction is achieved
using a reverse reconciliation technique followed by privacy amplification. The
reverse reconciliation technique is in principle secure for any value of the
line transmission, against gaussian individual attacks based on entanglement
and quantum memories. Our table-top experiment yields a net key transmission
rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per
second for a line with losses of 3.1 dB. We anticipate that the scheme should
remain effective for lines with higher losses, particularly because the present
limitations are essentially technical, so that significant margin for
improvement is available on both the hardware and software.Comment: 8 pages, 4 figure
Equivalent efficiency of a simulated photon-number detector
Homodyne detection is considered as a way to improve the efficiency of
communication near the single-photon level. The current lack of commercially
available {\it infrared} photon-number detectors significantly reduces the
mutual information accessible in such a communication channel. We consider
simulating direct detection via homodyne detection. We find that our particular
simulated direct detection strategy could provide limited improvement in the
classical information transfer. However, we argue that homodyne detectors (and
a polynomial number of linear optical elements) cannot simulate photocounters
arbitrarily well, since otherwise the exponential gap between quantum and
classical computers would vanish.Comment: 4 pages, 4 figure
Quantum state and mode profile tomography by the overlap
VS acknowledge support from the National Council for Scientific and Technological Development (CNPq) of Brazil, grant 304129/2015-1, and by the São Paulo Research Foundation (FAPESP), grant 2015/23296-8. DM acknowledge support from the EUproject Horizon-2020 SUPERTWIN id.686731, the National Academy of Sciences of Belarus program ‘Convergence’ and FAPESP grant 2014/21188-0. NK acknowledges the support from the Scottish Universities Physics Alliance (SUPA) and from the International Max Planck Partnership (IMPP) with Scottish Universities. JT and CS acknowledge support from European Union Grant No. 665148 (QCUMbER). TB acknowledges support from theDFG under TRR 142.Any measurement scheme involving interference of quantum states of the electromagnetic field necessarily mixes information about the spatiotemporal structure of these fields and quantum states in the recorded data. Weshow that in this case, a trade-off is possible between extracting information about the quantum states and the structure of the underlying fields, with the modal overlap being either a goal or a convenient tool of the reconstruction. Weshow that varying quantum states in a controlled way allows one to infer temporal profiles of modes. Vice versa, for the known quantum state of the probe and controlled variable overlap, one can infer the quantum state of the signal. We demonstrate this trade-off. by performing an experiment using the simplest on-off detection in an unbalanced weak homodyning scheme. For the single-mode case, we demonstrate experimentally inference of the overlap and a few-photon signal state. Moreover, we show theoretically that the same single-detector scheme is sufficient even for arbitrary multi-mode fields.Publisher PDFPeer reviewe
Entangled light in transition through the generation threshold
We investigate continuous variable entangling resources on the base of
two-mode squeezing for all operational regimes of a nondegenerate optical
parametric oscillator with allowance for quantum noise of arbitrary level. The
results for the quadrature variances of a pair of generated modes are obtained
by using the exact steady-state solution of Fokker-Planck equation for the
complex P-quasiprobability distribution function. We find a simple expression
for the squeezed variances in the near-threshold range and conclude that the
maximal two-mode squeezing reaches 50% relative to the level of vacuum
fluctuations and is achieved at the pump field intensity close to the
generation threshold. The distinction between the degree of two-mode squeezing
for monostable and bistable operational regimes is cleared up.Comment: 7 pages, 4 figures; Content changed: more details added to the
discussion. To be published in Phys. Rev.
Experimentally realizable characterizations of continuous variable Gaussian states
Measures of entanglement, fidelity and purity are basic yardsticks in quantum
information processing. We propose how to implement these measures using linear
devices and homodyne detectors for continuous variable Gaussian states. In
particular, the test of entanglement becomes simple with some prior knowledge
which is relevant to current experiments.Comment: 4 pages, This paper supersedes quant-ph/020315
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