Figure 1
(a) The transition between even and odd parity states; both initial and final states are tuned at a fixed charge gate n. (b) The energy profile at a junction between a large reservoir (lead) and a small island. The even to odd transition rate Γ eo and the opposite one are depicted by arrows from the corresponding initial to final states. Large transition energy makes the even to odd transitions much more frequent than that in the opposite direction. (c) Temperature dependence of NQP relaxation time for the superconducting energy gap ∆ = 2.1K, R = 10KΩ, the nonequilibrium quasiparticle density 0.1/µm 3 , and the density of states 10 6 /µm 3 K. From top to bottom the junction phase fluctuations is taken from ρ c = 0.08 to 0.8 with equal steps. Upper Insets: temperature dependence of relaxation rate in nonequilibrium (solid) and equilibrium (dashed), where the upper curve is for ρ c = 0.04 and the lower one for 1.6; Lower Inset: the slope of NQP relaxation rate as a function of temperature and ρ. Minima in the transition rate appears for all ρ c > ρ * c at finite temperature.
Source publication
In superconducting qubits the lifetime of quantum states cannot be prolonged arbitrarily by decreasing temperature. At low temperature quasiparticles tunneling between the electromagnetic environment and superconducting islands takes the condensate state out of equilibrium due to charge imbalance. We obtain the tunneling rate from a phenomenologica...
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Citations
... There is a clear knowledge gap in the literature on enabling lower qubit error rates. While careful control of material noise sources, such as trap states [14,15] and nonequilibrium quasiparticle tunnelling [16][17][18], improves qubit coherence time and leads to a lower gate error rate, however, gates should be protected against non-material sources of errors, such as leakage to or from non-computational levels, and crosstalk due to stray couplings between interacting qubits. ...
In two-qubit gates activated by microwave pulses, by turning pulse on or off, the state of qubits are swapped between entangled or idle modes. In either mode, the presence of stray couplings makes qubits accumulate coherent phase error. However, the error rates in the two modes differ because qubits carry different stray coupling strengths in each mode; therefore, eliminating stray coupling from one mode does not remove it from the other. We propose to combine such a gate with a tunable coupler and show that both idle and entangled qubits can become free from stray couplings. This significantly increases the operational switch fidelity in quantum algorithms. We further propose a weakly-tunable qubit as an optimum coupler to bring the two modes parametrically near each other. This remarkably enhances the tuning process by reducing its leakage.
... Such an inter-action generates a conditional phase error and therefore reduces the state fidelity during a typical 2-qubit gate length ∼ 0.1 µs. This phase error rate is yet much slower than charge and flux error rates, which indicates that well-shielding qubits against electromagnetic fields [12][13][14][15] and nonequilibrium quasiparticle tunnelling [16][17][18][19][20] do not change it. ...
We propose a protected 2-qubit gate composed of two sub-gates that switches between idling and entangling modes. These sub-gates are separated by a large energy barrier. In superconducting circuits, this gate is derived by harnessing microwave driving and coupling modulation. The idle mode decouples qubits and leaves them with highly localized wave functions. The entangled mode, however, makes them interact in absence of the adversarial $ZZ$ interactions. Our theoretical results show that this gate is a promising way to switch on and off a fast and high fidelity 2-qubit interaction.
... Tunnelling is a phenomenon peculiar to the quantum world that has been widely applied in physics, chemistry, and on which is based the functioning of many technological devices, such as diodes, superconducting quantum interference devices, quantum antennas and superconducting qubits for quantum computers [1][2][3][4][5][6][7][8]. The quantum tunnelling problem can be addressed using two distinct approaches. ...
In this paper, the tunnelling of a particle through a potential barrier is investigated in the presence of a time-dependent perturbation. The latter is attributed to the process of the energy measurement of the scattered particle. The time-dependent Schrödinger equation of the model is exactly solved. The probability density inside the barrier is calculated from the obtained wave function, proving that the tunnelling dynamics is determined not only by the transmitted and reflected waves but also by their interference. Furthermore, the interference term is time-dependent and contribute to the scattering process duration. The tunnelling time is calculated as the time needed to get the probability density inside the barrier to zero. This is the minimum duration of the measurement process before detecting the particle beyond the barrier. Based on this, a new method of estimating the tunnelling time by energy experimental measuring is proposed.
... These milestones can be achieved with the advent of long coherence qubits that can quickly go from a perfect isolated state to a strongly interacting state and vice versa, using programmable electronics [5][6][7]. In the last decade many advances have taken place to eliminate or suppress degrading interactions between qubits and environmental noise and quasiparticle tunnelling processes [8][9][10][11][12][13]. However there are still other means of imperfections in state-of-the-art quantum systems [14]. ...
Achieving high fidelity two qubit gates requires elimination of unwanted interactions among qubits. Weakly anharmonic superconducting qubits in the absence of external driving exhibit an always-on phase error mainly due to a sub-MHz repulsion between computational and non-computational energy levels, the so-called static ZZ interaction. We study how static ZZ interaction can be avoided by combining qubits with opposite sign anharmonicity. As soon as external driving turns on a new component is added on top of the static ZZ interaction that changes its strength. We show that driving pulse can be tuned so that it cancels the static part, resulting in ZZ-free qubits. This can be universally realized by combining qubits with any anharmonicity signs. Scaling up the number of such qubits can mitigate high fidelity gate operation.
... Knowing how entropy flows as the result of interactions between the resonator, cavity and other parts of the circuit can help to obtain important information about the possibility of leakage or dephasing in the system and ultimately give rise to modifications of quantum circuits [4]. A good understanding of cavities/resonators is beneficial to search for the nature of non-equilibrium quasiparticles in quantum circuits [42,43]. This can help with detecting light particles like muons whose tunnelling in a quantum circuit can signal a sudden jump in the entropy flow [44][45][46]. ...
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... Knowing how entropy flows as the result of interactions between the resonator, cavity and other parts of the circuit can help to obtain important information about the possibility of leakage or dephasing in the system and ultimately give rise to modifications of quantum circuits [4]. A good understanding of cavities/resonators is beneficial to search for the nature of non-equilibrium quasiparticles in quantum circuits [42,43]. This can help with detecting light particles like muons whose tunnelling in a quantum circuit can signal a sudden jump in the entropy flow [44][45][46]. ...
Currently `time' does not play any essential role in quantum information theory. In this sense quantum information theory is underdeveloped similar to how quantum physics was before Erwin Schrodinger introduces his equation for the evolution of a quantum wave function. In this review article, we cope with the problem of time for one of the central quantities in quantum information theory, entropy. Recently a replica trick formalism, the so-called `multiple parallel world', has been proposed that revolutionizes entropy evaluation for quantum systems. This formalism in one of the first attempts to introduce `time' in quantum information theory. With the total entropy being conserved in a closed system, entropy can flow internally between subsystems, however we show that this flow is not limited only to physical correlations as the literature suggest. The nonlinear dependence on density matrix introduces new types of correlations with no analogue in physical quantities. Evolving a number of replicas simultaneously makes it possible that they exchange particles between different replicas. We will summarize some of the recent news about entropy in some example quantum devices. Moreover, we take a quick look at a new correspondence that was recently proposed that provides an interesting link between quantum information theory and quantum physics. The mere existence of such a correspondence allows exploring new physical phenomena as the result of controlling entanglement in a quantum device.
... Consider that the charge undergoes a fluctuation around its macroscopic value q so that the stochastic charge is q = q + δq. Following the analogue discussion for a Josephson junction phase, (see [29][30][31] and references therein) the charge fluctuation corresponds to the effective current fluctuations across the wire ...
We study the rate of quantum phase slips in an ultranarrow superconducting
nanowire exposed to weak electromagnetic radiations. The superconductor is in
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of quantum phase slips. This can help to outline a new type of detector for
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phenomenon.
... However if a non-equilibrium situation is imposed, e.g. by applying a finite bias voltage, the average number of quasiparticles may be increased. Recently such non-equilibrium quasiparticle effects have been investigated in superconducting qubits [9][10][11][12][13][14] and single electron transistors (SETs) [15][16][17][18][19] . In this context the interplay of charge transport, the excitation of non-equilibrium quasiparticles and the observation of the parity effect has been the subject of recent experiments with SETs 18 . ...
We study the parity effect and transport due to quasiparticles in circuits
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in their more limited regimes of validity. As an example we study transport
through linear arrays of Josephson junctions in the limit of negligible
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In two-qubit gates activated by microwave pulses, by turning the pulse on or off, the state of qubits is swapped between entangled or idle modes. In either mode, the presence of stray couplings makes qubits accumulate coherent phase error. However, the error rates in the two modes differ because qubits carry different stray coupling strengths in each mode; therefore, eliminating stray coupling from one mode does not remove it from the other. We propose to combine such a gate with a tunable coupler and show that both idle and entangled qubits can become free from stray couplings. This significantly increases the operational switch fidelity in quantum algorithms. We further propose a weakly tunable qubit as an optimum coupler to bring the two modes parametrically near each other. This remarkably enhances the tuning process by reducing its leakage.
Superconducting qubits on a circuit exhibit an always-on state-dependent phase error. This error is due to sub-MHz parasitic interaction that repels computational levels from noncomputational ones. We study a general theory to evaluate the “static” repulsion between seemingly idle qubits as well as the “dynamical” repulsion between entangled qubits under microwave driving gate. By combining qubits of either the same or opposite anharmonicity signs we find the characteristics of static and dynamical ZZ freedoms. The latter universally eliminate the parasitic repulsion, leading us to mitigate high-fidelity gate operation. Our theory introduces the opportunities for making perfect entangled and unentangled states, which is extremely useful for quantum technology.
