16 research outputs found
Entanglement of light-shift compensated atomic spin waves with telecom light
Entanglement of a 795 nm light polarization qubit and an atomic Rb spin wave
qubit for a storage time of 0.1 s is observed by measuring the violation of
Bell's inequality (S = 2.65 \pm 0.12). Long qubit storage times are achieved by
pinning the spin wave in a 1064 nm wavelength optical lattice, with a
magic-valued magnetic field superposed to eliminate lattice-induced dephasing.
Four-wave mixing in a cold Rb gas is employed to perform light qubit conversion
between near infra red (795 nm) and telecom (1367 nm) wavelengths, and after
propagation in a telecom fiber, to invert the conversion process. Observed Bell
inequality violation (S = 2.66 \pm 0.09), at 10 ms storage, confirms
preservation of memory/light entanglement through the two stages of light qubit
frequency conversion.Comment: 5 pages, 3 figure
Implementing and characterizing precise multi-qubit measurements
There are two general requirements to harness the computational power of
quantum mechanics: the ability to manipulate the evolution of an isolated
system and the ability to faithfully extract information from it. Quantum error
correction and simulation often make a more exacting demand: the ability to
perform non-destructive measurements of specific correlations within that
system. We realize such measurements by employing a protocol adapted from [S.
Nigg and S. M. Girvin, Phys. Rev. Lett. 110, 243604 (2013)], enabling real-time
selection of arbitrary register-wide Pauli operators. Our implementation
consists of a simple circuit quantum electrodynamics (cQED) module of four
highly-coherent 3D transmon qubits, collectively coupled to a high-Q
superconducting microwave cavity. As a demonstration, we enact all seven
nontrivial subset-parity measurements on our three-qubit register. For each we
fully characterize the realized measurement by analyzing the detector
(observable operators) via quantum detector tomography and by analyzing the
quantum back-action via conditioned process tomography. No single quantity
completely encapsulates the performance of a measurement, and standard figures
of merit have not yet emerged. Accordingly, we consider several new fidelity
measures for both the detector and the complete measurement process. We measure
all of these quantities and report high fidelities, indicating that we are
measuring the desired quantities precisely and that the measurements are highly
non-demolition. We further show that both results are improved significantly by
an additional error-heralding measurement. The analyses presented here form a
useful basis for the future characterization and validation of quantum
measurements, anticipating the demands of emerging quantum technologies.Comment: 10 pages, 5 figures, plus supplemen
Universal logic with encoded spin qubits in silicon
Qubits encoded in a decoherence-free subsystem and realized in
exchange-coupled silicon quantum dots are promising candidates for
fault-tolerant quantum computing. Benefits of this approach include excellent
coherence, low control crosstalk, and configurable insensitivity to certain
error sources. Key difficulties are that encoded entangling gates require a
large number of control pulses and high-yielding quantum dot arrays. Here we
show a device made using the single-layer etch-defined gate electrode
architecture that achieves both the required functional yield needed for full
control and the coherence necessary for thousands of calibrated exchange pulses
to be applied. We measure an average two-qubit Clifford fidelity of with randomized benchmarking. We also use interleaved randomized
benchmarking to demonstrate the controlled-NOT gate with
fidelity, SWAP with fidelity, and a specialized entangling
gate that limits spreading of leakage with fidelity
Coherent Oscillations inside a Quantum Manifold Stabilized by Dissipation
Manipulating the state of a logical quantum bit (qubit) usually comes at the expense of exposing it to decoherence. Fault-tolerant quantum computing tackles this problem by manipulating quantum information within a stable manifold of a larger Hilbert space, whose symmetries restrict the number of independent errors. The remaining errors do not affect the quantum computation and are correctable after the fact. Here we implement the autonomous stabilization of an encoding manifold spanned by Schrödinger cat states in a superconducting cavity. We show Zeno-driven coherent oscillations between these states analogous to the Rabi rotation of a qubit protected against phase flips. Such gates are compatible with quantum error correction and hence are crucial for fault-tolerant logical qubits