108 research outputs found
One-way Quantum Key Distribution System based on Planar Lightwave Circuits
We developed a one-way quantum key distribution (QKD) system based upon a
planar lightwave circuit (PLC) interferometer. This interferometer is expected
to be free from the backscattering inherent in commercially available two-way
QKD systems and phase drift without active compensation. A key distribution
experiment with spools of standard telecom fiber showed that the bit error rate
was as low as 6% for a 100-km key distribution using an attenuated laser pulse
with a mean photon number of 0.1 and was determined solely by the detector
noise. This clearly demonstrates the advantages of our PLC-based one-way QKD
system over two-way QKD systems for long distance key distribution.Comment: 23 pages, 5 figure
Classical reconstruction of interference patterns of position-wavevector-entangled photon pairs by time-reversal method
The quantum interference of entangled photons forms a key phenomenon
underlying various quantum-optical technologies. It is known that the quantum
interference patterns of entangled photon pairs can be reconstructed
classically by the time-reversal method; however, the time-reversal method has
been applied only to time-frequency-entangled two-photon systems in previous
experiments. Here, for the first time, we apply the time-reversal method to the
position-wavevector-entangled two-photon systems: the two-photon Young
interferometer and the two-photon beam focusing system. We experimentally
demonstrate that the time-reversed systems classically reconstruct the same
interference patterns as the position-wavevector-entangled two-photon systems.Comment: 10 pages, 8 figure
Tracking Quantum Error Correction
To implement fault-tolerant quantum computation with continuous variables,
the Gottesman--Kitaev--Preskill (GKP) qubit has been recognized as an important
technological element. We have proposed a method to reduce the required
squeezing level to realize large scale quantum computation with the GKP qubit
[Phys. Rev. X. {\bf 8}, 021054 (2018)], harnessing the virtue of analog
information in the GKP qubits. In the present work, to reduce the number of
qubits required for large scale quantum computation, we propose the tracking
quantum error correction, where the logical-qubit level quantum error
correction is partially substituted by the single-qubit level quantum error
correction. In the proposed method, the analog quantum error correction is
utilized to make the performances of the single-qubit level quantum error
correction almost identical to those of the logical-qubit level quantum error
correction in a practical noise level. The numerical results show that the
proposed tracking quantum error correction reduces the number of qubits during
a quantum error correction process by the reduction rate
for -cycles
of the quantum error correction process using the Knill's code
with the concatenation level . Hence, the proposed tracking quantum error
correction has great advantage in reducing the required number of physical
qubits, and will open a new way to bring up advantage of the GKP qubits in
practical quantum computation
Analog quantum error correction with encoding a qubit into an oscillator
To implement fault-tolerant quantum computation with continuous variables,
Gottesman-Kitaev-Preskill (GKP) qubits have been recognized as an important
technological element. However, the analog outcome of GKP qubits, which
includes beneficial information to improve the error tolerance, has been
wasted, because the GKP qubits have been treated as only discrete variables. In
this paper, we propose a hybrid quantum error correction approach that combines
digital information with the analog information of the GKP qubits using the
maximum-likelihood method. As an example, we demonstrate that the three-qubit
bit-flip code can correct double errors, whereas the conventional method based
on majority voting on the binary measurement outcome can correct only a single
error. As another example, a concatenated code known as Knill's C4/C6 code can
achieve the hashing bound for the quantum capacity of the Gaussian quantum
channel. To the best of our knowledge, this approach is the first attempt to
draw both digital and analog information from a single quantum state to improve
quantum error correction performance
High-threshold fault-tolerant quantum computation with analog quantum error correction
To implement fault-tolerant quantum computation with continuous variables,
the Gottesman-Kitaev-Preskill (GKP) qubit has been recognized as an important
technological element. However,it is still challenging to experimentally
generate the GKP qubit with the required squeezing level, 14.8 dB, of the
existing fault-tolerant quantum computation. To reduce this requirement, we
propose a high-threshold fault-tolerant quantum computation with GKP qubits
using topologically protected measurement-based quantum computation with the
surface code. By harnessing analog information contained in the GKP qubits, we
apply analog quantum error correction to the surface code.Furthermore, we
develop a method to prevent the squeezing level from decreasing during the
construction of the large scale cluster states for the topologically protected
measurement based quantum computation. We numerically show that the required
squeezing level can be relaxed to less than 10 dB, which is within the reach of
the current experimental technology. Hence, this work can considerably
alleviate this experimental requirement and take a step closer to the
realization of large scale quantum computation.Comment: 14 pages, 7 figure
- …