43 research outputs found
Tomography of a displacement photon counter for discrimination of single-rail optical qubits
We investigate the performance of a Kennedy receiver, which is known as a
beneficial tool in optical coherent communications, to the quantum state
discrimination of the two superpositions of vacuum and single photon states
corresponding to the eigenstates in the single-rail encoding of
photonic qubits. We experimentally characterize the Kennedy receiver in
vacuum-single photon two-dimensional space using quantum detector tomography
and evaluate the achievable discrimination error probability from the
reconstructed measurement operators. We furthermore derive the minimum error
rate obtainable with Gaussian transformations and homodyne detection. Our proof
of principle experiment shows that the Kennedy receiver can achieve a
discrimination error surpassing homodyne detection
Fundamental precision limit of a Mach-Zehnder interferometric sensor when one of the inputs is the vacuum
In the lore of quantum metrology, one often hears (or reads) the following
no-go theorem: If you put vacuum into one input port of a balanced Mach-Zehnder
Interferometer, then no matter what you put into the other input port, and no
matter what your detection scheme, the sensitivity can never be better than the
shot noise limit (SNL). Often the proof of this theorem is cited to be in Ref.
[C. Caves, Phys. Rev. D 23, 1693 (1981)], but upon further inspection, no such
claim is made there. A quantum-Fisher-information-based argument suggestive of
this no-go theorem appears in Ref. [M. Lang and C. Caves, Phys. Rev. Lett. 111,
173601 (2013)], but is not stated in its full generality. Here we thoroughly
explore this no-go theorem and give the rigorous statement: the no-go theorem
holds whenever the unknown phase shift is split between both arms of the
interferometer, but remarkably does not hold when only one arm has the unknown
phase shift. In the latter scenario, we provide an explicit measurement
strategy that beats the SNL. We also point out that these two scenarios are
physically different and correspond to different types of sensing applications.Comment: 9 pages, 2 figure
Experimental demonstration of a quantum receiver beating the standard quantum limit at the telecom wavelength
Discrimination of coherent states beyond the standard quantum limit (SQL) is
an important tasknot only for quantum information processing but also for
optical coherent communication. In orderto optimize long distance optical fiber
networks, it is of practical importance to develop a quantumreceiver beating
the SQL and approaching the quantum bound at telecom wavelength. In this
paper,we experimentally demonstrate a receiver beating the conventional SQL at
telecom wavelength. Ourreceiver is composed of a displacement operation, a
single photon counter and a real time adaptivefeedback operation. By using a
high performance single photon detector operating at the telecomwavelength, we
achieve a discrimination error beyond the SQL. The demonstration in the
telecomband provides the first step important towards quantum and classical
communication beyond theSQL using a coherent state alphabet, and we envision
that the technology can be used for long-distance quantum key distribution,
effective quantum state preparation and quantum estimation
Distributed quantum sensing in a continuous variable entangled network
Networking plays a ubiquitous role in quantum technology. It is an integral
part of quantum communication and has significant potential for upscaling
quantum computer technologies that are otherwise not scalable. Recently, it was
realized that sensing of multiple spatially distributed parameters may also
benefit from an entangled quantum network. Here we experimentally demonstrate
how sensing of an averaged phase shift among four distributed nodes benefits
from an entangled quantum network. Using a four-mode entangled continuous
variable (CV) state, we demonstrate deterministic quantum phase sensing with a
precision beyond what is attainable with separable probes. The techniques
behind this result can have direct applications in a number of primitives
ranging from biological imaging to quantum networks of atomic clocks
Limit of Gaussian operations and measurements for Gaussian state discrimination, and its application to state comparison
We determine the optimal method of discriminating and comparing quantum
states from a certain class of multimode Gaussian states and their mixtures
when arbitrary global Gaussian operations and general Gaussian measurements are
allowed. We consider the so-called constant- displaced states which
include mixtures of multimode coherent states arbitrarily displaced along a
common axis. We first show that no global or local Gaussian transformations or
generalized Gaussian measurements can lead to a better discrimination method
than simple homodyne measurements applied to each mode separately and classical
postprocessing of the results. This result is applied to binary state
comparison problems. We show that homodyne measurements, separately performed
on each mode, are the best Gaussian measurement for binary state comparison. We
further compare the performance of the optimal Gaussian strategy for binary
coherent states comparison with these of non-Gaussian strategies using photon
detections.Comment: 8 pages, 3 figure