Does Microsoft Dream of Majorana? --- The Controversy over the Observation of the Majorana Particle in Solids ---
[Introduction]
Life is really hard.
So, have you ever thought that you want the great power[1] that solves every your worry?
Actually, there is.
That is a quantum computer.
This article summarizes the recent controversy over the discovery of the Majorana particle in solids for the realization of a topological quantum computer by Microsoft Corporation.
[Background]
Quantum computers are currently being actively researched and developed around the world [2]. Various methods have been proposed, including superconducting [3], ion trap [4], and optical [5] methods, but no practical device has yet been created. One reason for this is that quantum computers are susceptible to noise and error correction techniques have not yet been established [6].
 |
| Advances in Quantum Computing [3]. |
A topological quantum computer scheme that has attracted attention as an error-tolerant quantum computer scheme is the one proposed by Kitaev [7], which utilizes quasiparticles that follow non-Abelian statistics called Anyons, which are neither bosons nor fermions. In this scheme, errors due to local perturbations are suppressed by its topological nature, and it is expected that an error-tolerant quantum computer can be constructed based on that [8].
A typical example of an enion is the Majorana particle, in which the particle and antiparticle are identical, and its discovery and control is the first step toward the realization of a topological quantum computer [9]. In the field of elementary particles, neutrinos may correspond to the Majorana particle, and research is being conducted on them [10]. On the other hand, the possibility of their realization as quasiparticle excitations in solids, Majorana excitations, has attracted much attention [11]. In fact, in the 20 years around the year 2000, the number of publications in the field of condensed matter including "Majorana" has increased more than 10-fold [12]. Specifically, although its emergence has been reported in iron-based superconductors [13] and quantum spin liquids [14], it is superconductor-semiconductor nanowire heterostructures that are thought to be close to device applications [15].
The superconductor-semiconductor nanowire heterostructure is a structure in which semiconductor nanowires are bonded to a superconductor, and it has been theoretically proposed that topological superconductivity with Majorana particles (excitation) may be realized [16]. It has been studied as a promising method, partly because existing semiconductor manufacturing technology can be applied to the device fabrication.
 |
| Figure, Schematic of superconductor-semiconductor nanowire heterostructure [15]. |
[First report]
This heterostructure attracted further attention in 2012 when L. P. KOUWENHOVEN et al. of Delft University of Technology reported in Science that they observed a zero-bias conductance peak (one of the signs of Majorana excitation) in the InSb/NbTiN structure [17]. In addition, KOUWENHOVEN and colleagues, with the help of Microsoft, conducted further studies and reported strong evidence of Majorana excitations in the form of quantized zero-bias conductance peaks in InSb covered with Al structures in Nature, conclusively confirming the discovery of Majorana excitations [18]. With this report, Microsoft announced its goal of realizing a topological quantum computer within five years, and entered the race to realize a quantum computer using a different method than IBM and Google which focous the superconducting method.
 |
Figure, quantized zero-bias conductance peak reported in Nature [18].
However, in 2020, Frolov et al. who were co-authors of the Science paper questioned the data in the Nature paper [20], leading to the Nature paper being retracted [18 18]. A follow-up study and investigation revealed that the original data had been "unnecessarily modified" and that they could no longer provide experimental evidence for the existence of Majorana excitation. In fact, further theoretical considerations by S.D. Sarma et al. (also one of the co-authors of the Nature paper) have shown that a far less disordered system would have to be fabricated to observe Majorana excitations in this structure, and that it is likely that they would have seen another experimentally similar phenomenon (zero-bias Andreev tunneling peaks) [12, 21].
Thus, the dream of a topological quantum computer using the Majorana excitation seemed to have vanished into thin air.
[Report again]
In July 2022, a Microsoft team led by Chetan Nayak reported that they had once again "discovered the Majorana excitation," bringing the dream closer to reality once again[22]. The paper, submitted to Arxiv, claims to have observed topological superconductivity and associated Majorana excitations with a high probability (98%) with a refined protocol for device improvement and simulation-based Majorana excitation verification. Specifically, various phenomena necessary to identify the Majorana excitations, such as the control of the quantum phase transition of topological superconductivity by gate voltage and magnetic field, the associated opening and closing of the bulk band gap, and the simultaneous observation of zero-bias peaks at both ends of the device, have been reported.
 |
| Figure, reported Majorana excitation with improved device [22]. |
In response to this report, Anton Akhmerov of Delft University of Technology raised a comment article in the Journal Club for Condensed Matter Physics [23]. In his comment, he considers Microsoft's attempt positive, but points out that there are several problems with the paper, and that Microsoft's paper alone may not be enough to claim the observation of Majorana excitation. Specifically, they raise the following three issues;
- The paper does not indicate the strength of the residual coupling of zero-energy resonances at both ends of the nanowire. The paper estimates the localization length of the Majorana excitation to be about 1um based on numerical calculations, which is quite long compared to the device length of 3um. Akhmerov speculates that the experimental data could not be shown due to manufacturing yield issues, although it is necessary to verify the phenomenon in devices longer than 10um to take advantage of the topological protection property.
- The validation protocol contains statistical biases that overestimate the discovery of Majorana excitation. In particular, the method for selecting the parameters that pass through the protocol is not specified, so if loose parameters are chosen, false positives will be observed.
- The authors' experiments are described only vaguely, making it impossible for the scientific community to follow up on them. For example, the details of the methods used in the numerical calculations are not described, making the calculations unreproducible. Also, the materials used to construct the devices, their sizes, and the selection criteria are not described, making experimental verification impossible.
The aforementioned Frolov also posted skepticism about the Microsoft report on Twitter [24]. Frolov claims that the following five criteria must be met to report a finding of Majorana excitation, and the current paper does not appear to meet those criteria.
- Accurate zero bias peak
- Reflecting the anisotropy of spin-orbit interactions
- That a phase diagram has been established showing the presence of zero bias peaks for voltage and magnetic field
- Zero bias peak oscillating with respect to the electric field
- Simultaneous observation of zero bias peaks at both ends of the nanowire
[Summary]
This article summarizes the controversy over Microsoft's report of the Majorana particle discovery. My impression is that we are not yet in a situation where the Majorana excitation has actually been observed. Even if the Majorana excitations are discovered, there are still many things that need to realize a quantum computer.
However, there is a lot to do, I am personally optimistic because if it is theoretically possible, I believe it will eventually be found.
The dream of a topological quantum computer never ends...
[References]
[2] Farzan Jazaeri et al., A Review on Quantum Computing: Qubits, Cryogenic Electronics and Cryogenic MOSFET Physics, arXiv:1908.02656
[3] Morten Kjaergaard et al., Superconducting Qubits: Current State of Play, Annual Review of Condensed Matter Physics, Vol. 11:369-395 (Volume publication date March 2020)
[4] Colin D. Bruzewicz et al., Trapped-ion quantum computing: Progress and challenges, Applied Physics Reviews 6, 021314 (2019)
[5] Warit Asavanant and Akira Furusawa, Optical Quantum Computers: A Route to Practical Continuous Variable Quantum Information Processing [AIP Publishing (online), Melville, New York, 2022]
[6] Joschka Roffe, Quantum Error Correction: An Introductory Guide, arXiv:1907.11157
[7] A.Yu.Kitaev, Fault-tolerant quantum computation by anyons, Annals of Physics
[8] Chetan Nayak et al., Non-Abelian anyons and topological quantum computation, Rev. Mod. Phys. 80, 1083 – Published 12 September 2008
[9] Sarma, S., Freedman, M. & Nayak, C. Majorana zero modes and topological quantum computation. npj Quantum Inf 1, 15001 (2015).
[10] A. Baha Balantekin and Boris Kayse, On the Properties of Neutrinos, Annual Review of Nuclear and Particle Science, Vol. 68:313-338 (Volume publication date 19 October 2018)
[11] Search for Majorana Fermions in Superconductors, Annual Review of Condensed Matter Physics, Vol. 4:113-136 (Volume publication date April 2013)
[13] Lina Sang et al., Majorana zero modes in iron-based superconductors, Matter
[14] Takagi, H., Takayama, T., Jackeli, G. et al. Concept and realization of Kitaev quantum spin liquids. Nat Rev Phys 1, 264–280 (2019).
[15] Pasquale Marra, Majorana nanowires for topological quantum computation: A tutorial, arXiv:2206.14828
[16] Roman M. Lutchyn, Jay D. Sau, and S. Das Sarma, Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures, Phys. Rev. Lett. 105, 077001 – Published 13 August 2010
[17] V. MOURIK et al., Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices, SCIENCE 12 Apr 2012 Vol 336, Issue 6084 pp. 1003-1007
[18] Zhang, H., Liu, CX., Gazibegovic, S. et al. RETRACTED ARTICLE: Quantized Majorana conductance. Nature 556, 74–79 (2018).
[20] Sergey Frolov, So, You Think You Discovered a New State of Matter?, May 12, 2021• Physics 14, 68. Sergey Frolov, Vincent Mourik, We cannot believe we overlooked these Majorana discoveries, arXiv:2203.17060
[21] Sankar Das Sarma, Haining Pan, Disorder-induced zero-bias peaks in Majorana nanowires, Phys. Rev. B 103, 195158 – Published 28 May 2021
[22] Microsoft Quantumy, InAs-Al Hybrid Devices Passing the Topological Gap Protocol, arXiv:2207.02472
[23] Anton Akhmerov, What can we learn from the reported discovery of Majorana states?, DOI: 10.36471/JCCM_July_2022_01
[24] Tweet by S.Frolov
[25] Sergey Frolov, Quantum computing’s reproducibility crisis: Majorana fermions, Nature 592, 350-352 (2021)
[Other References] (In Japanese)
コメント
コメントを投稿