Running quantum programs is fraught with challenges
on on today’s noisy intermediate scale quantum (NISQ)
devices. Many of these challenges originate from the error
characteristics that stem from rapid decoherence and noise
during measurement, qubit connections, crosstalk, the qubits
themselves, and transformations of qubit state via gates. Not only
are qubits not “created equal”, but their noise level also changes
over time. IBM is said to calibrate their quantum systems once
per day and reports noise levels (errors) at the time of such
calibration. This information is subsequently used to map circuits
to higher quality qubits and connections up to the next calibration
point.
This work provides evidence that there is room for improvement
over this daily calibration cycle. It contributes a technique
to measure noise levels (errors) related to qubits immediately
before executing one or more sensitive circuits and shows that
just-in-time noise measurements can benefit late physical qubit
mappings. With this just-in-time recalibrated transpilation, the
fidelity of results is improved over IBM’s default mappings, which
only uses their daily calibrations. The framework assess two major
sources of noise, namely readout errors (measurement errors)
and two-qubit gate/connection errors. Experiments indicate that
the accuracy of circuit results improves by 3-304% on average
and up to 400% with on-the-fly circuit mappings based on error
measurements just prior to application execution.
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This content will become publicly available on May 4, 2024
Repetitive readout and real-time control of nuclear spin qubits in 171Yb atoms
We demonstrate high fidelity repetitive projective measurements of nuclear spin qubits in an array of neutral ytterbium-171 (171Yb) atoms. We show that the qubit state can be measured with a fidelity of 0.995(4) under a condition that leaves it in the state corresponding to the measurement outcome with a probability of 0.993(6) for a single tweezer and 0.981(4) averaged over the array. This is accomplished by near-perfect cyclicity of one of the nuclear spin qubit states with an optically excited state under a magnetic field of B=58 G, resulting in a bright/dark contrast of ≈105 during fluorescence readout. The performance improves further as ∼1/B2. The state-averaged readout survival of 0.98(1) is limited by off-resonant scattering to dark states and can be addressed via post-selection by measuring the atom number at the end of the circuit, or during the circuit by performing a measurement of both qubit states. We combine projective measurements with high-fidelity rotations of the nuclear spin qubit via an AC magnetic field to explore several paradigmatic scenarios, including the non-commutivity of measurements in orthogonal bases, and the quantum Zeno mechanism in which measurements "freeze" coherent evolution. Finally, we employ real-time feedforward to repetitively deterministically prepare the qubit in the +z or −z direction after initializing it in an orthogonal basis and performing a projective measurement in the z-basis. These capabilities constitute an important step towards adaptive quantum circuits with atom arrays, such as in measurement-based quantum computation, fast many-body state preparation, holographic dynamics simulations, and quantum error correction.
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- NSF-PAR ID:
- 10432481
- Date Published:
- Journal Name:
- arXivorg
- Volume:
- 2305.02926
- ISSN:
- 2331-8422
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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