51 research outputs found
Superconducting quantum node for entanglement and storage of microwave radiation
Superconducting circuits and microwave signals are good candidates to realize
quantum networks, which are the backbone of quantum computers. We have realized
a quantum node based on a 3D microwave superconducting cavity parametrically
coupled to a transmission line by a Josephson ring modulator. We first
demonstrate the time-controlled capture, storage and retrieval of an optimally
shaped propagating microwave field, with an efficiency as high as 80%. We then
demonstrate a second essential ability, which is the timed-controlled
generation of an entangled state distributed between the node and a microwave
channel.Comment: 6 pages, 4 figures. Supplementary information can be downloaded as
the ancillary file her
Persistent control of a superconducting qubit by stroboscopic measurement feedback
Making a system state follow a prescribed trajectory despite fluctuations and
errors commonly consists in monitoring an observable (temperature,
blood-glucose level...) and reacting on its controllers (heater power, insulin
amount ...). In the quantum domain, there is a change of paradigm in feedback
since measurements modify the state of the system, most dramatically when the
trajectory goes through superpositions of measurement eigenstates. Here, we
demonstrate the stabilization of an arbitrary trajectory of a superconducting
qubit by measurement based feedback. The protocol benefits from the long
coherence time (s) of the 3D transmon qubit, the high efficiency
(82%) of the phase preserving Josephson amplifier, and fast electronics
ensuring less than 500 ns delay. At discrete time intervals, the state of the
qubit is measured and corrected in case an error is detected. For Rabi
oscillations, where the discrete measurements occur when the qubit is supposed
to be in the measurement pointer states, we demonstrate an average fidelity of
85% to the targeted trajectory. For Ramsey oscillations, which does not go
through pointer states, the average fidelity reaches 75%. Incidentally, we
demonstrate a fast reset protocol allowing to cool a 3D transmon qubit down to
0.6% in the excited state.Comment: 7 pages, 3 figures and 1 table. Supplementary information available
as an ancilla fil
Experimental Demonstration of Continuous Quantum Error Correction
The storage and processing of quantum information are susceptible to external noise, resulting in computational errors. A powerful method to suppress these effects is quantum error correction. Typically, quantum error correction is executed in discrete rounds, using entangling gates and projective measurement on ancillary qubits to complete each round of error correction. Here we use direct parity measurements to implement a continuous quantum bit-flip correction code in a resource-efficient manner, eliminating entangling gates, ancillary qubits, and their associated errors. An FPGA controller actively corrects errors as they are detected, achieving an average bit-flip detection efficiency of up to 91%. Furthermore, the protocol increases the relaxation time of the protected logical qubit by a factor of 2.7 over the relaxation times of the bare comprising qubits. Our results showcase resource-efficient stabilizer measurements in a multi-qubit architecture and demonstrate how continuous error correction codes can address challenges in realizing a fault-tolerant system
Experimental demonstration of continuous quantum error correction
The storage and processing of quantum information are susceptible to external
noise, resulting in computational errors that are inherently continuous A
powerful method to suppress these effects is to use quantum error correction.
Typically, quantum error correction is executed in discrete rounds where errors
are digitized and detected by projective multi-qubit parity measurements. These
stabilizer measurements are traditionally realized with entangling gates and
projective measurement on ancillary qubits to complete a round of error
correction. However, their gate structure makes them vulnerable to errors
occurring at specific times in the code and errors on the ancilla qubits. Here
we use direct parity measurements to implement a continuous quantum bit-flip
correction code in a resource-efficient manner, eliminating entangling gates,
ancilla qubits, and their associated errors. The continuous measurements are
monitored by an FPGA controller that actively corrects errors as they are
detected. Using this method, we achieve an average bit-flip detection
efficiency of up to 91%. Furthermore, we use the protocol to increase the
relaxation time of the protected logical qubit by a factor of 2.7 over the
relaxation times of the bare comprising qubits. Our results showcase
resource-efficient stabilizer measurements in a multi-qubit architecture and
demonstrate how continuous error correction codes can address challenges in
realizing a fault-tolerant system.Comment: 12 pages, 7 figure
A Multi-Qubit Quantum Gate Using the Zeno Effect
The Zeno effect, in which repeated observation freezes the dynamics of a
quantum system, stands as an iconic oddity of quantum mechanics. When a
measurement is unable to distinguish between states in a subspace, the dynamics
within that subspace can be profoundly altered, leading to non-trivial
behavior. Here we show that such a measurement can turn a non-interacting
system with only single-qubit control into a two- or multi-qubit entangling
gate, which we call a Zeno gate. The gate works by imparting a geometric phase
on the system, conditioned on it lying within a particular nonlocal subspace.
We derive simple closed-form expressions for the gate fidelity under a number
of non-idealities and show that the gate is viable for implementation in
circuit and cavity QED systems. More specifically, we illustrate the
functioning of the gate via dispersive readout in both the Markovian and
non-Markovian readout regimes, and derive conditions for longitudinal readout
to ideally realize the gate.Comment: 20+12 pages. 13 figure
Practical Single Microwave Photon Counter with sensitivity
Single photon detection played an important role in the development of
quantum optics. Its implementation in the microwave domain is challenging
because the photon energy is 5 orders of magnitude smaller. In recent years,
significant progress has been made in developing single microwave photon
detectors (SMPDs) based on superconducting quantum bits or bolometers. In this
paper we present a new practical SMPD based on the irreversible transfer of an
incoming photon to the excited state of a transmon qubit by a four-wave mixing
process. This device achieves a detection efficiency and an
operational dark count rate , mainly due to the
out-of-equilibrium microwave photons in the input line. The corresponding power
sensitivity is , one order of
magnitude lower than the state of the art. The detector operates continuously
over hour timescales with a duty cycle , and offers
frequency tunability of MHz around 7 GHz
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