Periodic Reporting for period 2 - SuperProtected (SUPERinductance for hardware-PROTECTED superconducting qubits)
Okres sprawozdawczy: 2023-03-01 do 2024-08-31
To preserve the quantum coherence necessary to any computational advantage, most of the existing approaches rely on Quantum Error Correction, encoding one logical qubit into a very large number of physical qubits. SuperProtected follows a different route, which aims at creating an intrinsically protected physical qubit. This protection should lead to striking improvements. It relies on a new physical phenomenon (tunneling of pairs of Cooper pairs or tunneling of pairs of fluxons, which allows the encoding of quantum information in the parity of the number of Cooper pairs or of the number of fluxons respectively) and on technical innovations to harness it (new type of superinductances made from amorphous superconductors fabricated on suspended membranes or etched substrates).
Longer coherence qubits would enable previously unthinkable experiments in quantum sensing or quantum mechanics foundations. Importantly, the novel superinductances proposed here will unlock many possibilities beyond intrinsically protected qubits. Microwave characterization of InOx at milliKelvin temperature offers a unique experimental possibility: measuring the effective London penetration depth of this material close to the SIT. This will undoubtedly have a strong impact on the mesoscopic superconductivity community.
In the field of mesoscopic physics, there have been forty years of effort to observe the effect of Bloch Oscillations, which would allow the closure of a metrological triangle relating the ampere, the volt, and the second, with an obvious impact on the definition of international units. Superinductances in the μH range have been identified as the missing component to observe this effect.
Superinductances can also be turned into very sensitive detectors of millimeter-waves for astrophysics or into time-resolved photo-detectors. Finally, the large inductance/low capacitance platform envisioned here will have direct applications in the field of electro-mechanics or for coherent optics-to-microwave conversion.
The objectives of SuperProtected are as follows:
- Our project aims to develop the next generation of superinductances with unprecedented figures of merit (L = 10μH). In this project, we will use Indium Oxide, which has been recently characterized by our group, to boost the inductive part of the device at microwave frequencies and milliKelvin temperatures. To minimize stray capacitance, the devices will be fabricated on a thin, suspended silicon membrane or onto an etched substrate. Our goal is to design, fabricate, and characterize a novel type of superinductances that can be used for various applications.
- Our team plans to develop hardware-protected qubits based on superinductances that use Cooper-pair or fluxon pairing. This latest technology is expected to outperform the existing superconducting qubits by several orders of magnitude.
- We will showcase protected gates and develop new control procedures to ensure that the envisioned qubits are not only the most coherent but also have the highest gate fidelity. These techniques have strong similarities with "braiding" in the context of Majorana fermions.
Longer coherence qubits would enable previously unthinkable experiments in quantum sensing or quantum mechanics foundations. Importantly, the novel superinductances proposed here will unlock many possibilities beyond intrinsically protected qubits. Microwave characterization of InOx at milliKelvin temperature offers a unique experimental possibility: measuring the effective London penetration depth of this material close to the SIT. This will undoubtedly have a strong impact on the mesoscopic superconductivity community.
In the field of mesoscopic physics, there have been forty years of effort to observe the effect of Bloch Oscillations, which would allow the closure of a metrological triangle relating the ampere, the volt, and the second, with an obvious impact on the definition of international units. Superinductances in the μH range have been identified as the missing component to observe this effect.
Superinductances can also be turned into very sensitive detectors of millimeter-waves for astrophysics or into time-resolved photo-detectors. Finally, the large inductance/low capacitance platform envisioned here will have direct applications in the field of electro-mechanics or for coherent optics-to-microwave conversion.
The objectives of SuperProtected are as follows:
- Our project aims to develop the next generation of superinductances with unprecedented figures of merit (L = 10μH). In this project, we will use Indium Oxide, which has been recently characterized by our group, to boost the inductive part of the device at microwave frequencies and milliKelvin temperatures. To minimize stray capacitance, the devices will be fabricated on a thin, suspended silicon membrane or onto an etched substrate. Our goal is to design, fabricate, and characterize a novel type of superinductances that can be used for various applications.
- Our team plans to develop hardware-protected qubits based on superinductances that use Cooper-pair or fluxon pairing. This latest technology is expected to outperform the existing superconducting qubits by several orders of magnitude.
- We will showcase protected gates and develop new control procedures to ensure that the envisioned qubits are not only the most coherent but also have the highest gate fidelity. These techniques have strong similarities with "braiding" in the context of Majorana fermions.
Despite the rapid progress in the field, achieving coherence times that are long enough to be practically useful for a quantum computer remains a paramount challenge. This is primarily due to the susceptibility of quantum electrical circuits to noise. One solution to this issue is to encode the 0 and 1 of the qubit into states with differing parities. This approach has been proposed in seminal theoretical works and is expected to provide potentially infinitely long relaxation times. However, despite several attempts, reaching such a protection regime turned out to request stringent circuit parameters that were beyond the limits of experimental reach.
During the first half of the SuperProtected project, we have taken another look at the Fluxonium qubit, which was invented a decade ago and demonstrated that it possesses such a protection mechanism. When the circuit is connected to a large superinductance (L ~1 μH) and placed in an external magnetic flux of zero, its ground and excited state wavefunctions belong to different flux quanta parity. In our proof-of-principle experiment, the qubit relaxation time exceeded 100 μs, even though we did not use state-of-the-art nanofabrication techniques, showcasing the qubit's built-in protection against energy relaxation. Additionally, the qubit exhibits protection against charge noise and a first-order protection against flux noise. We measure a dephasing time of around 75 μs and provide a complete coherence budget showing how it can be easily improved by a better design.
Moreover our team has made an important discovery regarding the behavior of amorphous superconductors. We have found evidence of a first-order quantum phase transition, marked by a surprising discontinuity in the zero-temperature superfluid stiffness at the point where the material transitions from a superconducting state to an insulating state.
Our research has identified two primary mechanisms behind this transition. Firstly, we have found that strong disorder within the material alters the nature of the superconducting transition itself, transforming it into a phase-driven transition that is primarily governed by fluctuations in the phase of the order parameter. Secondly, we have discovered that the Cooper-pair glass insulator state, which ultimately terminates the superconductivity, competes with the superconducting state, preventing the usual continuous, second-order quantum phase transition that is typically observed. Instead, this competition leads to the emergence of a first-order breakdown of superconductivity.
These findings represent a significant step forward in our understanding of the behavior of amorphous superconductors and may have important implications for the development of new materials and technologies in the future.
During the first half of the SuperProtected project, we have taken another look at the Fluxonium qubit, which was invented a decade ago and demonstrated that it possesses such a protection mechanism. When the circuit is connected to a large superinductance (L ~1 μH) and placed in an external magnetic flux of zero, its ground and excited state wavefunctions belong to different flux quanta parity. In our proof-of-principle experiment, the qubit relaxation time exceeded 100 μs, even though we did not use state-of-the-art nanofabrication techniques, showcasing the qubit's built-in protection against energy relaxation. Additionally, the qubit exhibits protection against charge noise and a first-order protection against flux noise. We measure a dephasing time of around 75 μs and provide a complete coherence budget showing how it can be easily improved by a better design.
Moreover our team has made an important discovery regarding the behavior of amorphous superconductors. We have found evidence of a first-order quantum phase transition, marked by a surprising discontinuity in the zero-temperature superfluid stiffness at the point where the material transitions from a superconducting state to an insulating state.
Our research has identified two primary mechanisms behind this transition. Firstly, we have found that strong disorder within the material alters the nature of the superconducting transition itself, transforming it into a phase-driven transition that is primarily governed by fluctuations in the phase of the order parameter. Secondly, we have discovered that the Cooper-pair glass insulator state, which ultimately terminates the superconductivity, competes with the superconducting state, preventing the usual continuous, second-order quantum phase transition that is typically observed. Instead, this competition leads to the emergence of a first-order breakdown of superconductivity.
These findings represent a significant step forward in our understanding of the behavior of amorphous superconductors and may have important implications for the development of new materials and technologies in the future.
The two main results mentionned previouslsy significantly advanced the state of the art.
In the second part of our project, we have set two objectives that we want to achieve. The first objective is to produce a new generation of superinductances, based on either suspended or etched substrates. These superinductances will help in providing better protection to our qubits. By reducing the amount of spurious capacitance, we can create fewer spurious and uncontrolled modes, which would negatively affect our qubits.
The second objective is to create novel protected gates that take advantage of the intrinsic protection mechanism of our qubits. By developing these gates, we aim to increase the reliability of our qubits and reduce the likelihood of errors occurring during calculations. These gates are designed to provide better protection against external noise.
By focusing on these objectives, we hope to make significant strides in the development of quantum computing technology.
In the second part of our project, we have set two objectives that we want to achieve. The first objective is to produce a new generation of superinductances, based on either suspended or etched substrates. These superinductances will help in providing better protection to our qubits. By reducing the amount of spurious capacitance, we can create fewer spurious and uncontrolled modes, which would negatively affect our qubits.
The second objective is to create novel protected gates that take advantage of the intrinsic protection mechanism of our qubits. By developing these gates, we aim to increase the reliability of our qubits and reduce the likelihood of errors occurring during calculations. These gates are designed to provide better protection against external noise.
By focusing on these objectives, we hope to make significant strides in the development of quantum computing technology.