24 research outputs found
Coherent Operation of a Gap-tunable Flux Qubit
We replace the Josephson junction defining a three-junction flux qubit's
properties with a tunable direct current superconducting quantum interference
devices (DC-SQUID) in order to tune the qubit gap during the experiment. We
observe different gaps as a function of the external magnetic pre-biasing field
and the local magnetic field through the DC-SQUID controlled by high-bandwidth
on chip control lines. The persistent current and gap behavior correspond to
numerical simulation results. We set the sensitivity of the gap on the control
lines during the sample design stage. With a tuning range of several GHz on a
qubit dynamics timescale, we observe coherent system dynamics at the degeneracy
point.Comment: 3 pages, 4 figure
A superconducting qubit as a quantum transformer routing entanglement between a microscopic quantum memory and a macroscopic resonator
We demonstrate experimentally the creation and measurement of an entangled
state between a microscopic two level system and a macroscopic superconducting
resonator where their indirect interaction is mediated by an artificial atom, a
superconducting persistent current qubit (PCQB). We show that the microscopic
two level system, formed by a defect in an oxide layer, exhibits an order of
magnitude longer dephasing time than the PCQB, while the dephasing time of the
entangled states between the microscopic two level system and macroscopic
superconducting resonator is significantly longer than the dephasing time in
the persistent current qubits. This demonstrates the possibility that a qubit
of moderate coherence properties can be used in practice to address low
decoherence quantum memories by connecting them to macroscopic circuit QED
quantum buses, leading future important implications for quantum information
processing tasks.Comment: 4 figure
High-frequency suppression of inductive coupling between flux qubit and transmission line resonator
We perform theoretical calculations to investigate the naturally occurring
high-frequency cutoff in a circuit comprising a flux qubit coupled inductively
to a transmission line resonator (TLR). Our results agree with those of past
studies that considered somewhat similar circuit designs. In particular, a
decoupling occurs between the qubit and the high-frequency modes. As a result,
the coupling strength between the qubit and resonator modes increases with mode
frequency as at low frequencies and decreases as
at high frequencies. We derive expressions for the
multimode-resonator-induced Lamb shift in the qubit's characteristic frequency.
Because of the natural decoupling between the qubit and high-frequency modes,
the Lamb-shift-renormalized qubit frequency remains finite.Comment: 24 pages (preprint), 5 figure
Coherent control of a flux qubit by phase-shifted resonant microwave pulses
The quantum state of a flux qubit was successfully pulse-controlled by using
a resonant microwave. We observed Ramsey fringes by applying a pair of
phase-shifted pi/2 microwave pulses without introducing detuning. With this
method, the qubit state can be rotated on an arbitrary axis in the x-y plane of
the Bloch sphere in a rotating frame. We obtained a qubit signal from a
coherent oscillation with an angular velocity of up to 2pi*11.4 Grad/s. In
combination with Rabi pulses, this method enables us to achieve full control of
single qubit operation. It also offers the possibility of orders of magnitude
increases in the speed of the arbitrary unitary gate operation.Comment: 3 pages, 3 figure
Quantum Time-evolution in Qubit Readout Process with a Josephson Bifurcation Amplifier
We analyzed the Josephson bifurcation amplifier (JBA) readout process of a
superconducting qubit quantum mechanically. This was achieved by employing
numerical analyses of the dynamics of the density operator of a driven
nonlinear oscillator and a qubit coupled system during the measurement process.
In purely quantum cases, the wavefunction of the JBA is trapped in a
quasienergy-state, and bifurcation is impossible. Introducing decoherence
enables us to reproduce the bifurcation with a finite hysteresis. Moreover, we
discuss in detail the dynamics involved when a qubit is initially in a
superposition state. We have observed the qubit-probe (JBA) entangled state and
it is divided into two separable states at the moment of the JBA transition
begins. This corresponds to "projection". To readout the measurement result,
however, we must wait until the two JBA states are macroscopically well
separated. The waiting time is determined by the strength of the decoherence in
the JBA.Comment: 4 pages, 5 figure