There exists a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension owing to its single delay element. We introduce a linear two-photon interferometer containing two independent delays spanning the two-photon state. By unlocking the fixed phase relationship between the interfering two-photon probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free two-photon reference against which the other beats, thereby resolving ambiguities in two-photon state identification typical of HOM interferometry. We discuss the operation of this phase-unlocked HOM on a variety of input states focusing on instances where this new interferometer outperforms a traditional HOM interferometer: frequency-correlated states and states produced by a pulse doublet pump. Additionally, this interferometer affords the opportunity to synchronize two-photon states in a manner analogous to an HOM interferometer; moreover, it extends that capability to the aforementioned class of states.
We describe an optical implementation of a CNOT gate in which the control qubit is the polarization of a single
photon and the target qubit is the spatial parity of the same photon. The gate is implemented with a polarizationsensitive
spatial light modulator. We characterize the operation of the gate using quantum process tomography and
the spatial parity is analyzed with a modified Mach-Zehnder interferometer. We also demonstrate the CNOT-gate
operation with arbitrary rotation of the target qubit and discuss the possibility of implementing multi-qubit CNOT
gates using the same approach.
In recent years the interest in the manipulation of quantum systems has furthered new strategies for maintaining
their coherence, continuously threatened by unwanted and uncontrollable interactions with the environment.
Photons interact weakly with the surroundings. Even so decoherence may significantly affect their polarization
state during the propagation within dispersive media because of the unavoidable presence of more than a single
frequency in the envelope of the photon pulse. Here we report on a suppression of the polarization decoherence in
a ring cavity obtained by properly retooling for the photon qubit the "bang-bang" protection technique already
employed for nuclear spins and nuclear-quadrupole qubits. Our results show that bang-bang control can be
profitably extended to all quantum information processes involving flying polarization qubits.
One-way quantum channels play a fundamental role in the security of the communication between two distant parties, in particular within the frame of "quantum key distribution". Nevertheless quite recently it has been introduced the possibility of using two-way quantum channels for the same purpose. Although the first attempts in this direction did not feature any particular advantage with respect to the one-way counterpart some recent results obtained by our group suggest that this new class of protocols provides higher thresholds of security.
We demonstrate the possibility of using periodically poled lithium
niobate crystals (PPLN) as a direct source of entangled photon
pairs. Two configurations are studied. The first enables the
generation of polarization-entangled states from a one-dimensional
PPLN structure at different frequencies; the second is dedicated
to the production of frequency-entangled states from a
bi-dimensional PPLN structure. The engineering of both of these
PPLN structures are described from a theoretical perspective.
Traditional ellipsometric measurements are limited in their accuracy because of the use of an external reference sample for calibration, and because of the noise inherent in the source at low light levels. We demonstrate that these limitations can be circumvented by using a non-classical source of light, namely, twin photons generated by the process of spontaneous parametric downconversion, in conjunction with a novel polarization interferometer and coincidence-counting detection scheme. The twin-photon nature of the source is a unique feature of our scheme. We are guaranteed, on the detection of a photon in one of the arms of the setup, that its twin will be in the other. We present experimental results showing how the technique operates.
We present results of experimental demonstration of secure Quantum Key Distribution (QKD) at Elsag spa based on the implementation of BB84 protocol using polarization entangled states produced in the nonlinear process of type-II spontaneous parametric down conversion (SPDC). This enables us to avoid the use of active polarization modulation components and increases the overall key distribution rate. The high quality of polarization-entangled state generated by parametric down conversion and the high efficiency of coupling entangled-photon pairs into a single-mode optical fiber has enabled us to perform QKD with quantum bit-error rate compatible with acceptable security levels. The complete software system architecture includes a QKD protocol implementing all phases of the key distillation process. The system runs in a server and two users configuration on three different PCs connected over a local area network (LAN). Friendly graphical user interfaces (GUI) are available to start and to monitor the whole key generation and distillation process.
We report on the construction of polarization-entangled photon sources for quantum key distribution (QKD) testbed using nonlinear
optical process of spontaneous parametric down-conversion (SPDC)
pumped by a 266-nm and 351-nm continuous wave (cw) and a 415-nm
femtosecond-pulsed laser sources. The efficient coupling of
down-converted photons at 702 nm and 830 nm into optical
single-mode fiber has enabled us to increase the rate of
entangled-photon pairs available for transmission over
communication channels with a high degree of polarization
entanglement. The detection and characterization of the
entangled-photon-state properties has been performed using
commercially available Silicon avalanche photodiodes (APD) as well
as using a novel photon-number-resolving cryogenic photodetector,
which has been developed by our colleagues at the Boulder Division
of NIST and brought to BU for tests with elements of the future
DARPA Quantum Network.
We describe the development of a quantum key distribution (QKD)
scheme based on ultrafast laser pumped sources of entangled photon
pairs and the engineering of their entanglement properties. Though
quantum entanglement has been shown to be a useful resource for
quantum key distribution, little work has been carried out in
making use of the full range of joint entanglement behavior present in hyper-entangled photon pairs. We consider the principal advantages of our QKD scheme in connection with the way it makes use of ultrafast laser pumped spontaneous parametric down-conversion and hyper-entanglement. In particular, we consider how polarization quantum interference may be modified by manipulating the spatial features of the down-converted light.