6,249 research outputs found
Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits
Fault tolerant quantum computing relies on the ability to detect and correct
errors, which in quantum error correction codes is typically achieved by
projectively measuring multi-qubit parity operators and by conditioning
operations on the observed error syndromes. Here, we experimentally demonstrate
the use of an ancillary qubit to repeatedly measure the and parity
operators of two data qubits and to thereby project their joint state into the
respective parity subspaces. By applying feedback operations conditioned on the
outcomes of individual parity measurements, we demonstrate the real-time
stabilization of a Bell state with a fidelity of in up to 12
cycles of the feedback loop. We also perform the protocol using Pauli frame
updating and, in contrast to the case of real-time stabilization, observe a
steady decrease in fidelity from cycle to cycle. The ability to stabilize
parity over multiple feedback rounds with no reduction in fidelity provides
strong evidence for the feasibility of executing stabilizer codes on timescales
much longer than the intrinsic coherence times of the constituent qubits.Comment: 12 pages, 10 figures. Update: Fig. 5 correcte
Hybrid quantum information processing
The development of quantum information processing has traditionally followed
two separate and not immediately connected lines of study. The main line has
focused on the implementation of quantum bit (qubit) based protocols whereas
the other line has been devoted to implementations based on high-dimensional
Gaussian states (such as coherent and squeezed states). The separation has been
driven by the experimental difficulty in interconnecting the standard
technologies of the two lines. However, in recent years, there has been a
significant experimental progress in refining and connecting the technologies
of the two fields which has resulted in the development and experimental
realization of numerous new hybrid protocols. In this Review, we summarize
these recent efforts on hybridizing the two types of schemes based on discrete
and continuous variables.Comment: 13 pages, 6 figure
Heralded Storage of a Photonic Quantum Bit in a Single Atom
Combining techniques of cavity quantum electrodynamics, quantum measurement,
and quantum feedback, we have realized the heralded transfer of a polarization
qubit from a photon onto a single atom with 39% efficiency and 86% fidelity.
The reverse process, namely, qubit transfer from the atom onto a given photon,
is demonstrated with 88% fidelity and an estimated efficiency of up to 69%. In
contrast to previous work based on two-photon interference, our scheme is
robust against photon arrival-time jitter and achieves much higher
efficiencies. Thus, it constitutes a key step toward the implementation of a
long-distance quantum network.Comment: 6 pages, 4 figure
Comparing and combining measurement-based and driven-dissipative entanglement stabilization
We demonstrate and contrast two approaches to the stabilization of qubit
entanglement by feedback. Our demonstration is built on a feedback platform
consisting of two superconducting qubits coupled to a cavity which are measured
by a nearly-quantum-limited measurement chain and controlled by high-speed
classical logic circuits. This platform is used to stabilize entanglement by
two nominally distinct schemes: a "passive" reservoir engineering method and an
"active" correction based on conditional parity measurements. In view of the
instrumental roles that these two feedback paradigms play in quantum
error-correction and quantum control, we directly compare them on the same
experimental setup. Further, we show that a second layer of feedback can be
added to each of these schemes, which heralds the presence of a high-fidelity
entangled state in realtime. This "nested" feedback brings about a marked
entanglement fidelity improvement without sacrificing success probability.Comment: 40 pages, 12 figure
Quantum-noise--randomized data-encryption for WDM fiber-optic networks
We demonstrate high-rate randomized data-encryption through optical fibers
using the inherent quantum-measurement noise of coherent states of light.
Specifically, we demonstrate 650Mbps data encryption through a 10Gbps
data-bearing, in-line amplified 200km-long line. In our protocol, legitimate
users (who share a short secret-key) communicate using an M-ry signal set while
an attacker (who does not share the secret key) is forced to contend with the
fundamental and irreducible quantum-measurement noise of coherent states.
Implementations of our protocol using both polarization-encoded signal sets as
well as polarization-insensitive phase-keyed signal sets are experimentally and
theoretically evaluated. Different from the performance criteria for the
cryptographic objective of key generation (quantum key-generation), one
possible set of performance criteria for the cryptographic objective of data
encryption is established and carefully considered.Comment: Version 2: Some errors have been corrected and arguments refined. To
appear in Physical Review A. Version 3: Minor corrections to version
Entanglement swapping with photons generated on-demand by a quantum dot
Photonic entanglement swapping, the procedure of entangling photons without
any direct interaction, is a fundamental test of quantum mechanics and an
essential resource to the realization of quantum networks. Probabilistic
sources of non-classical light can be used for entanglement swapping, but
quantum communication technologies with device-independent functionalities
demand for push-button operation that, in principle, can be implemented using
single quantum emitters. This, however, turned out to be an extraordinary
challenge due to the stringent requirements on the efficiency and purity of
generation of entangled states. Here we tackle this challenge and show that
pairs of polarization-entangled photons generated on-demand by a GaAs quantum
dot can be used to successfully demonstrate all-photonic entanglement swapping.
Moreover, we develop a theoretical model that provides quantitative insight on
the critical figures of merit for the performance of the swapping procedure.
This work shows that solid-state quantum emitters are mature for quantum
networking and indicates a path for scaling up.Comment: The first four authors contributed equally to this work. 17 pages, 3
figure
Deterministic quantum teleportation between distant atomic objects
Quantum teleportation is a key ingredient of quantum networks and a building
block for quantum computation. Teleportation between distant material objects
using light as the quantum information carrier has been a particularly exciting
goal. Here we demonstrate a new element of the quantum teleportation landscape,
the deterministic continuous variable (cv) teleportation between distant
material objects. The objects are macroscopic atomic ensembles at room
temperature. Entanglement required for teleportation is distributed by light
propagating from one ensemble to the other. Quantum states encoded in a
collective spin state of one ensemble are teleported onto another ensemble
using this entanglement and homodyne measurements on light. By implementing
process tomography, we demonstrate that the experimental fidelity of the
quantum teleportation is higher than that achievable by any classical process.
Furthermore, we demonstrate the benefits of deterministic teleportation by
teleporting a dynamically changing sequence of spin states from one distant
object onto another
Entangled resource for interfacing single- and dual-rail optical qubits
Today's most widely used method of encoding quantum information in optical
qubits is the dual-rail basis, often carried out through the polarisation of a
single photon. On the other hand, many stationary carriers of quantum
information - such as atoms - couple to light via the single-rail encoding in
which the qubit is encoded in the number of photons. As such, interconversion
between the two encodings is paramount in order to achieve cohesive quantum
networks. In this paper, we demonstrate this by generating an entangled
resource between the two encodings and using it to teleport a dual-rail qubit
onto its single-rail counterpart. This work completes the set of tools
necessary for the interconversion between the three primary encodings of the
qubit in the optical field: single-rail, dual-rail and continuous-variable.Comment: Published in Quantu
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