45 research outputs found
Observing quantum state diffusion by heterodyne detection of fluorescence
A qubit can relax by fluorescence, which prompts the release of a photon into
its electromagnetic environment. By counting the emitted photons, discrete
quantum jumps of the qubit state can be observed. The succession of states
occupied by the qubit in a single experiment, its quantum trajectory, depends
in fact on the kind of detector. How are the quantum trajectories modified if
one measures continuously the amplitude of the fluorescence field instead?
Using a superconducting parametric amplifier, we have performed heterodyne
detection of the fluorescence of a superconducting qubit. For each realization
of the measurement record, we can reconstruct a different quantum trajectory
for the qubit. The observed evolution obeys quantum state diffusion, which is
characteristic of quantum measurements subject to zero point fluctuations.
Independent projective measurements of the qubit at various times provide a
quantitative validation of the reconstructed trajectories. By exploring the
statistics of quantum trajectories, we demonstrate that the qubit states span a
deterministic surface in the Bloch sphere at each time in the evolution.
Additionally, we show that when monitoring fluorescence, coherent
superpositions are generated during the decay from excited to ground state.
Counterintuitively, measuring light emitted during relaxation can give rise to
trajectories with increased excitation probability.Comment: Supplementary material can be found in the ancillary sectio
Using Spontaneous Emission of a Qubit as a Resource for Feedback Control
Persistent control of a transmon qubit is performed by a feedback protocol
based on continuous heterodyne measurement of its fluorescence. By driving the
qubit and cavity with microwave signals whose amplitudes depend linearly on the
instantaneous values of the quadratures of the measured fluorescence field, we
show that it is possible to stabilize permanently the qubit in any targeted
state. Using a Josephson mixer as a phase-preserving amplifier, it was possible
to reach a total measurement efficiency =35%, leading to a maximum of 59%
of excitation and 44% of coherence for the stabilized states. The experiment
demonstrates multiple-input multiple-output analog Markovian feedback in the
quantum regime.Comment: Supplementary material can be found as an ancillary objec
Widely tunable, non-degenerate three-wave mixing microwave device operating near the quantum limit
We present the first experimental realization of a widely frequency tunable,
non-degenerate three-wave mixing device for quantum signals at GHz frequency.
It is based on a new superconducting building-block consisting of a ring of
four Josephson junctions shunted by a cross of four linear inductances. The
phase configuration of the ring remains unique over a wide range of magnetic
fluxes threading the loop. It is thus possible to vary the inductance of the
ring with flux while retaining a strong, dissipation-free, and noiseless
non-linearity. The device has been operated in amplifier mode and its noise
performance has been evaluated by using the noise spectrum emitted by a voltage
biased tunnel junction at finite frequency as a test signal. The unprecedented
accuracy with which the crossover between zero-point-fluctuations and shot
noise has been measured provides an upper-bound for the noise and dissipation
intrinsic to the device.Comment: Accepted for Physical Review Letters. Supplementary material can be
found in the source packag
Dynamically enhancing qubit-oscillator interactions with anti-squeezing
The interaction strength of an oscillator to a qubit grows with the
oscillator's vacuum field fluctuations. The well known degenerate parametric
oscillator has revived interest in the regime of strongly detuned squeezing,
where its eigenstates are squeezed Fock states. Owing to these amplified field
fluctuations, it was recently proposed that squeezing this oscillator would
dynamically boost its coupling to a qubit. In a superconducting circuit
experiment, we observe a two-fold increase in the dispersive interaction
between a qubit and an oscillator at 5.5 dB of squeezing, demonstrating in-situ
dynamical control of qubit-oscillator interactions. This work initiates the
experimental coupling of oscillators of squeezed photons to qubits, and
cautiously motivates their dissemination in experimental platforms seeking
enhanced interactions.Comment: 21 pages, 15 figure
Determining the position of a single spin relative to a metallic nanowire
The nanoscale localization of individual paramagnetic defects near an electrical circuit is an important step for realizing hybrid quantum devices with strong spin-microwave photon coupling. Here, we fabricate an array of individual nitrogen vacancy (NV) centers in diamond near a metallic nanowire deposited on top of the substrate. We determine the relative position of each NV center with ∼10 nm accuracy, using it as a vector magnetometer to measure the field generated by passing a DC through the wire
One hundred second bit-flip time in a two-photon dissipative oscillator
Current implementations of quantum bits (qubits) continue to undergo too many
errors to be scaled into useful quantum machines. An emerging strategy is to
encode quantum information in the two meta-stable pointer states of an
oscillator exchanging pairs of photons with its environment, a mechanism shown
to provide stability without inducing decoherence. Adding photons in these
states increases their separation, and macroscopic bit-flip times are expected
even for a handful of photons, a range suitable to implement a qubit. However,
previous experimental realizations have saturated in the millisecond range. In
this work, we aim for the maximum bit-flip time we could achieve in a
two-photon dissipative oscillator. To this end, we design a Josephson circuit
in a regime that circumvents all suspected dynamical instabilities, and employ
a minimally invasive fluorescence detection tool, at the cost of a two-photon
exchange rate dominated by single-photon loss. We attain bit-flip times of the
order of 100 seconds for states pinned by two-photon dissipation and containing
about 40 photons. This experiment lays a solid foundation from which the
two-photon exchange rate can be gradually increased, thus gaining access to the
preparation and measurement of quantum superposition states, and pursuing the
route towards a logical qubit with built-in bit-flip protection
Mapping the optimal route between two quantum states
A central feature of quantum mechanics is that a measurement is intrinsically
probabilistic. As a result, continuously monitoring a quantum system will
randomly perturb its natural unitary evolution. The ability to control a
quantum system in the presence of these fluctuations is of increasing
importance in quantum information processing and finds application in fields
ranging from nuclear magnetic resonance to chemical synthesis. A detailed
understanding of this stochastic evolution is essential for the development of
optimized control methods. Here we reconstruct the individual quantum
trajectories of a superconducting circuit that evolves in competition between
continuous weak measurement and driven unitary evolution. By tracking
individual trajectories that evolve between an arbitrary choice of initial and
final states we can deduce the most probable path through quantum state space.
These pre- and post-selected quantum trajectories also reveal the optimal
detector signal in the form of a smooth time-continuous function that connects
the desired boundary conditions. Our investigation reveals the rich interplay
between measurement dynamics, typically associated with wave function collapse,
and unitary evolution of the quantum state as described by the Schrodinger
equation. These results and the underlying theory, based on a principle of
least action, reveal the optimal route from initial to final states, and may
enable new quantum control methods for state steering and information
processing.Comment: 12 pages, 9 figure
Stabilizing entanglement autonomously between two superconducting qubits
Quantum error-correction codes would protect an arbitrary state of a
multi-qubit register against decoherence-induced errors, but their
implementation is an outstanding challenge for the development of large-scale
quantum computers. A first step is to stabilize a non-equilibrium state of a
simple quantum system such as a qubit or a cavity mode in the presence of
decoherence. Several groups have recently accomplished this goal using
measurement-based feedback schemes. A next step is to prepare and stabilize a
state of a composite system. Here we demonstrate the stabilization of an
entangled Bell state of a quantum register of two superconducting qubits for an
arbitrary time. Our result is achieved by an autonomous feedback scheme which
combines continuous drives along with a specifically engineered coupling
between the two-qubit register and a dissipative reservoir. Similar autonomous
feedback techniques have recently been used for qubit reset and the
stabilization of a single qubit state, as well as for creating and stabilizing
states of multipartite quantum systems. Unlike conventional, measurement-based
schemes, an autonomous approach counter-intuitively uses engineered dissipation
to fight decoherence, obviating the need for a complicated external feedback
loop to correct errors, simplifying implementation. Instead the feedback loop
is built into the Hamiltonian such that the steady state of the system in the
presence of drives and dissipation is a Bell state, an essential building-block
state for quantum information processing. Such autonomous schemes, broadly
applicable to a variety of physical systems as demonstrated by a concurrent
publication with trapped ion qubits, will be an essential tool for the
implementation of quantum-error correction.Comment: 39 pages, 7 figure