16,814 research outputs found
Design of an RSFQ Control Circuit to Observe MQC on an rf-SQUID
We believe that the best chance to observe macroscopic quantum coherence
(MQC) in a rf-SQUID qubit is to use on-chip RSFQ digital circuits for
preparing, evolving and reading out the qubit's quantum state. This approach
allows experiments to be conducted on a very short time scale (sub-nanosecond)
without the use of large bandwidth control lines that would couple
environmental degrees of freedom to the qubit thus contributing to its
decoherence. In this paper we present our design of a RSFQ digital control
circuit for demonstrating MQC in a rf-SQUID. We assess some of the key
practical issues in the circuit design including the achievement of the
necessary flux bias stability. We present an "active" isolation structure to be
used to increase coherence times. The structure decouples the SQUID from
external degrees of freedom, and then couples it to the output measurement
circuitry when required, all under the active control of RSFQ circuits.
Research supported in part by ARO grant # DAAG55-98-1-0367.Comment: 4 pages. More information and publications at
http://www.ece.rochester.edu:8080/users/sde/research/publications/index.htm
Unified framework for quantumness -- coherence, discord, and entanglement
From an operational perspective, quantumness characterizes the exotic
behavior in a physical process which cannot be explained with Newtonian
physics. There are several widely used measures of quantumness, including
coherence, discord, and entanglement, each proven to be essential resources in
particular situations. There exists evidence of fundamental connections amongst
the three measures. However, those quantumnesses are still regarded differently
and such connections are yet to be elucidated. Here, we introduce a general
framework of defining a unified quantumness with an operational motivation
founded on the capability of interferometry. The quantumness appears
differently as coherence, discord, and entanglement in different scenarios with
local measurement, weak reference frame free measurement, and strong reference
frame free measurement, respectively. Our results also elaborate how these
three measures are related and how they can be transformed from each other.
This framework can be further extended to other scenarios and serves as a
universal quantumness measure.Comment: 9 pages, 4 figure
Stabilizing Rabi Oscillation of a Charge Qubit via Atomic Clock Technique
We propose a superconducting circuit-atom hybrid, where the Rabi oscillation
of single excess Cooper pair in the island is stabilized via the common
atomic-clock technique. The noise in the superconducting circuit is mapped onto
the voltage source which biases the Cooper-pair box via an inductor and a gate
capacitor. The fast fluctuations of the gate charge are significantly
suppressed by an inductor-capacitor resonator, leading to a
long-relaxation-time Rabi oscillation. More importantly, the residual
low-frequency fluctuations are further reduced by using the general
feedback-control method, in which the voltage bias is stabilized via
continuously measuring the dc-Stark-shift-induced atomic Ramsey signal. The
stability and coherence time of the resulting charge-qubit Rabi oscillation are
both enhanced. The principal structure of this Cooper-pair-box oscillator is
studied in detail.Comment: 4 figure
Weak and strong measurement of a qubit using a switching-based detector
We analyze the operation of a switching-based detector that probes a qubit's
observable that does not commute with the qubit's Hamiltonian, leading to a
nontrivial interplay between the measurement and free-qubit dynamics. In order
to obtain analytic results and develop intuitive understanding of the different
possible regimes of operation, we use a theoretical model where the detector is
a quantum two-level system that is constantly monitored by a macroscopic
system. We analyze how to interpret the outcome of the measurement and how the
state of the qubit evolves while it is being measured. We find that the answers
to the above questions depend on the relation between the different parameters
in the problem. In addition to the traditional strong-measurement regime, we
identify a number of regimes associated with weak qubit-detector coupling. An
incoherent detector whose switching time is measurable with high accuracy can
provide high-fidelity information, but the measurement basis is determined only
upon switching of the detector. An incoherent detector whose switching time can
be known only with low accuracy provides a measurement in the qubit's energy
eigenbasis with reduced measurement fidelity. A coherent detector measures the
qubit in its energy eigenbasis and, under certain conditions, can provide
high-fidelity information.Comment: 20 pages (two-column), 6 figure
Pulsed energy-time entangled twin-photon source for quantum communication
A pulsed source of energy-time entangled photon pairs pumped by a standard
laser diode is proposed and demonstrated. The basic states can be distinguished
by their time of arrival. This greatly simplifies the realization of 2-photon
quantum cryptography, Bell state analyzers, quantum teleportation, dense
coding, entanglement swapping, GHZ-states sources, etc. Moreover the
entanglement is well protected during photon propagation in telecom optical
fibers, opening the door to few-photon applications of quantum communication
over long distances.Comment: 8 pages, 4 figure
A Fast and Compact Quantum Random Number Generator
We present the realization of a physical quantum random number generator
based on the process of splitting a beam of photons on a beam splitter, a
quantum mechanical source of true randomness. By utilizing either a beam
splitter or a polarizing beam splitter, single photon detectors and high speed
electronics the presented devices are capable of generating a binary random
signal with an autocorrelation time of 11.8 ns and a continuous stream of
random numbers at a rate of 1 Mbit/s. The randomness of the generated signals
and numbers is shown by running a series of tests upon data samples. The
devices described in this paper are built into compact housings and are simple
to operate.Comment: 23 pages, 6 Figs. To appear in Rev. Sci. Inst
Measuring coherence of quantum measurements
The superposition of quantum states lies at the heart of physics and has been
recently found to serve as a versatile resource for quantum information
protocols, defining the notion of quantum coherence. In this contribution, we
report on the implementation of its complementary concept, coherence from
quantum measurements. By devising an accessible criterion which holds true in
any classical statistical theory, we demonstrate that noncommutative quantum
measurements violate this constraint, rendering it possible to perform an
operational assessment of the measurement-based quantum coherence. In
particular, we verify that polarization measurements of a single photonic
qubit, an essential carrier of one unit of quantum information, are already
incompatible with classical, i.e., incoherent, models of a measurement
apparatus. Thus, we realize a method that enables us to quantitatively certify
which quantum measurements follow fundamentally different statistical laws than
expected from classical theories and, at the same time, quantify their
usefulness within the modern framework of resources for quantum information
technology.Comment: close to published versio
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