| 1. |
- Gustavsson, S., et al.
(författare)
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Suppressing relaxation in superconducting qubits by quasiparticle pumping
- 2016
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 354:6319, s. 1573-1577
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Tidskriftsartikel (refereegranskat)abstract
- Copyright 2016 by the American Association for the Advancement of Science; all rights reserved.Dynamical error suppression techniques are commonly used to improve coherence in quantum systems.They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation.We investigate a complementary, stochastic approach to reducing errors: Instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. A 70%reduction in the quasiparticle density results in a threefold enhancement in qubit relaxation times and a comparable reduction in coherence variability.
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| 2. |
- Pinzani, P., et al.
(författare)
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Prostate-specific antigen mRNA and protein levels in laser microdissected cells of human prostate measured by real-time reverse transcriptase-quantitative polymerase chain reaction and immuno-quantitative polymerase chain reaction
- 2008
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Ingår i: Human Pathology. - : Elsevier BV. - 0046-8177. ; 39:10, s. 1474-1482
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Tidskriftsartikel (refereegranskat)abstract
- Laser-assisted microdissection has mainly been used in cancer studies to excise pure cell populations from heterogeneous tissues. Cancer and normal cells selected by laser-assisted microdissection have frequently been used for mRNA expression studies usually by reverse transcriptase-quantitative polymerase chain reaction (qPCR). Recently, real time immuno-qPCR was developed as a new tool for highly sensitive measurements of proteins. Using reverse transcriptase-qPCR and immuno-qPCR, we measured the amounts of prostate-specific antigen mRNA and its corresponding protein in homogeneous and comparable cell populations, collected from normal and cancer prostates by laser-assisted microdissection. With these techniques, prostate-specific antigen mRNA and protein were quantified over a wide range of concentrations with a sensitivity sufficient to analyze single prostate cells (LNCaP). We did not find significant differences in prostate-specific antigen protein and mRNA between normal and cancer cells. The expression of prostate-specific antigen protein and mRNA was highly correlated in both normal and pathological cells. In microdissected peritubular stromas areas of prostate cancers, the concentration of prostate-specific antigen protein was about 100 times higher than in normal prostate, indicating an increased transit of secreted prostate-specific antigen. In the same samples, prostate-specific antigen mRNA was not detectable. Our data demonstrate, for the first time, the feasibility of simultaneous application of reverse transcriptase-qPCR and immuno-qPCR in studies of homogeneous cell populations, collected by laser-assisted microdissection. The approach is expected to become a very powerful tool for expression studies in human cancers at both mRNA and protein levels.
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| 3. |
- Yan, F., et al.
(författare)
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Rotating-frame relaxation as a noise spectrum analyser of a superconducting qubit undergoing driven evolution
- 2013
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 4, s. 2337-
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Tidskriftsartikel (refereegranskat)abstract
- Gate operations in a quantum information processor are generally realized by tailoring specific periods of free and driven evolution of a quantum system. Unwanted environmental noise, which may in principle be distinct during these two periods, acts to decohere the system and increase the gate error rate. Although there has been significant progress characterizing noise processes during free evolution, the corresponding driven-evolution case is more challenging as the noise being probed is also extant during the characterization protocol. Here we demonstrate the noise spectroscopy (0.1-200 MHz) of a superconducting flux qubit during driven evolution by using a robust spin-locking pulse sequence to measure relaxation (T1ρ) in the rotating frame. In the case of flux noise, we resolve spectral features due to coherent fluctuators, and further identify a signature of the 1 MHz defect in a time-domain spin-echo experiment. The driven-evolution noise spectroscopy complements free-evolution methods, enabling the means to characterize and distinguish various noise processes relevant for universal quantum control. © 2013 Macmillan Publishers Limited. All rights reserved.
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| 4. |
- Yan, F., et al.
(författare)
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Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit
- 2012
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 85:17, s. 174521-
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Tidskriftsartikel (refereegranskat)abstract
- We report a direct measurement of the low-frequency noise spectrum in a superconducting flux qubit. Our method uses the noise sensitivity of a free-induction Ramsey interference experiment, comprising free evolution in the presence of noise for a fixed period of time followed by single-shot qubit-state measurement. Repeating this procedure enables Fourier-transform noise spectroscopy with access to frequencies up to the achievable repetition rate, a regime relevant to dephasing in ensemble-averaged time-domain measurements such as Ramsey interferometry. Rotating the qubit's quantization axis allows us to measure two types of noise: effective flux noise and effective critical-current or charge noise. For both noise sources, we observe that the very same 1/f-type power laws measured at considerably higher frequencies (0.2-20 MHz) are consistent with the noise in the 0.01-100-Hz range measured here. We find no evidence of temperature dependence of the noises over 65-200 mK, and also no evidence of time-domain correlations between the two noises. These methods and results are pertinent to the dephasing of all superconducting qubits. © 2012 American Physical Society.
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| 5. |
- Kjaergaard, M., et al.
(författare)
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Demonstration of Density Matrix Exponentiation Using a Superconducting Quantum Processor
- 2022
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Ingår i: Physical Review X. - 2160-3308. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Quantum computers hold the potential to outperform classical supercomputers at certain tasks. To implement algorithms on a quantum computer, programmers use conventional computers and hardware to create a set of classical control signals that implement a desired quantum algorithm. However, feeding the quantum information forward requires an inefficient conversion: extraction of quantum information, conversion to classical control signals, and reinjection of those signals into the system to implement quantum operations. Here, we demonstrate a more natively quantum strategy to programming quantum computers. Our approach uses the density matrix exponentiation (DME) protocol, a general technique for using a quantum state to enact a quantum operation. It can be thought of as a subroutine with which programmers can turn multiple copies of a quantum state into instructions for next steps in a quantum algorithm.We implement DME using two qubits in a superconducting quantum processor. Our implementation relies on a high-fidelity two-qubit gate and a novel technique called quantum measurement emulation to approximately reset a known quantum state. These developments enable us to demonstrate the DME protocol for the first time on a small-scale quantum processor and benchmark its performance.While DME was originally proposed in the context of a specific quantum machine-learning algorithm, it may also represent a fundamentally different approach to quantum programming. It allows the possibility of encoding quantum algorithms directly into quantum states and executing those algorithms on other quantum states, enabling a new class of efficient quantum algorithms.
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