超伝導キュービットによる量子コンピューティング | PennyLaneデモ

Quantum computing with superconducting qubits | PennyLane Demos Downloads Download Python script Download Notebook View on GitHub Copy BibTeX citation Demos / Quantum Hardware / Quantum computing with superconducting qubits Quantum computing with superconducting qubits Alvaro Ballon Published: March 21, 2022 . Last updated: April 16, 2026. Superconducting qubits are among the most promising approaches to building quantum computers. It is no surprise that this technology is being used by well-known tech companies in their quest to pioneer the quantum era. Google’s Sycamore claimed quantum advantage back in 2019 [ 1 ] and, in 2021, IBM built its Eagle quantum computer with 127 qubits [ 2 ] ! The central insight that allows for these quantum computers is that superconductivity is a quantum phenomenon, so we can use superconducting circuits as quantum systems that we can control at will. We can actually bring the quantum world to a larger scale and manipulate it more freely! By the end of this demo, you will learn how superconductors are used to create, prepare, control, and measure the state of a qubit. Moreover, you will identify the strengths and weaknesses of this technology in terms of Di Vincenzo’s criteria, as introduced in the box below. You will be armed with the basic concepts to understand the main scientific papers on the topic and keep up-to-date with the newest developments. Di Vincenzo’s criteria : In the year 2000, David DiVincenzo proposed a wishlist for the experimental characteristics of a quantum computer [ 3 ] . DiVincenzo’s criteria have since become the main guideline for physicists and engineers building quantum computers: 1. Well-characterized and scalable qubits . Many of the quantum systems that we find in nature are not qubits, so we must find a way to make them behave as such. Moreover, we need to put many of these systems together. 2. Qubit initialization . We must be able to prepare the same state repeatedly within an acceptable margin of error. 3. Long coherence times . Qubits will lose their quantum properties after interacting with their environment for a while. We would like them to last long enough so that we can perform quantum operations. 4. Universal set of gates . We need to perform arbitrary operations on the qubits. To do this, we require both single-qubit gates and two-qubit gates. 5. Measurement of individual qubits . To read the result of a quantum algorithm, we must accurately measure the final state of a pre-chosen set of qubits. Superconductivity Superconducting chip with 4 qubits To understand how superconducting qubits work, we first need to explain why some materials are superconductors. Let’s begin by addressing a simpler question: why do conductors allow for the easy passage of electrons and insulating materials don’t? Solid-state physics tells us that when an electric current travels through a material, the electrons therein come in two types. Conduction electrons flow freely through the material, while valence electrons are attached to the atoms that form the material itself. A material is a good conductor of electricity if the valence electrons require no energy to be stripped from the atoms to become conduction electrons. Similarly, the material is a semi-conductor if the energy needed is small, and it’s an insulator if the energy is large. But, if conduction electrons can be obtained for free in conducting materials, then why don’t all conductors have infinite conductivity? Even the tiniest of stimuli should create a very large current! To address this valid concern, let us recall the exclusion principle in atomic physics: the atom’s discrete energy levels have a population limit, so only a limited number of electrons can have the same energy. However, the exclusion principle is not limited to electrons in atomic orbitals. In fact, it applies to all electrons that are organized in discrete energy levels. Since conduction electrons also occupy discrete conduction energy levels , they must also abide by this law! The conductivity is then limited because, when the lower conduction energy levels are occupied, the energy required to promote valence to conduction electrons is no longer zero. This energy will keep increasing as the population of conduction electrons grows. Valence and conduction energy levels However, superconductors do have infinite conductivity. How is this even possible? It’s not a phenomenon that we see in our daily lives. For some materials, at extremely low temperatures, the conduction electrons attract the positive nuclei to form regions of high positive charge density, alternating with regions of low charge density. This charge distribution oscillates in an organized manner, creating waves in the material known as phonons . The conduction electrons are pushed together by these phonons, forming Cooper pairs . Most importantly, these coupled electrons need not obey the exclusion principle. We no longer have an electron population limit in the lower conduction energy levels, allowing for infinite conductivity! [ 4 ] Cooper pairs are formed by alternating regions of high and low density of positive charge (phonons) represented by the density of red dots. PennyLane plugins: You can run your quantum algorithms on actual superconducting qubit quantum computers on the Cloud. The Qiskit, Amazon Braket, Cirq, and Rigetti PennyLane plugins give you access to some of the most powerful superconducting quantum hardware. Building an artificial atom When we code in PennyLane, we deal with the abstraction of a qubit. But how is a qubit actually implemented physically? Some of the most widely used real-life qubits are built from individual atoms. But atoms are given to us by nature, and we cannot easily alter their properties. So although they are reliable qubits, they are not very versatile. We may adapt our technology to the atoms, but they seldom adapt to our technology. Could there be a way to build a device with the same properties that make atoms suitable qubits? Let’s see if we can build an artificial atom! Our first task is to isolate the features that an ideal qubit should have. First and foremost, we must not forget that a qubit is a physical system with two distinguishable configurations that correspond to the computational basis states. In the case of an atom, these are usually the ground and excited states, \(\left\lvert g \right\rangle\) and \(\left\lvert e \right\rangle,\) of a valence electron. In atoms, we can distinguish these states reliably because the ground and excited states have two distinct values of energy that can be resolved by our measuring devices. If we measure the energy of the valence electron that is in either \(\left\lvert g \right\rangle\) or \(\left\lvert e \right\rangle,\) we will measure two — and only two — possible values \(E_0\) and \(E_1,\) associated to \(\left\lvert g \right\rangle\) and \(\left\lvert e \right\rangle\) respectively. Most importantly, the physical system under consideration must exhibit quantum properties . The presence of discrete energy levels is indeed one such property, so if we do build a device that stores energy in discrete values, we can suspect that it obeys the laws of quantum mechanics. Usually, one thinks of a quantum system as being at least as small as a molecule, but building something so small is technologically impossible. It turns out that we don’t need to go to such small scales. If we build a somewhat small electric circuit using superconducting wires and bring it to temperatures of about 10 mK, it becomes a quantum system with discrete energy levels. Finally, we must account for the fact that electrons in atoms have more states available than just \(\left\lvert g \right\rangle\) and \(\left\lvert e \right\rangle.\) In fact, the energy levels in an atom are infinitely many. How do we guarantee that an electron does not escape to another state that is neither of our hand-picked states? The transition between the ground and the excited state only happens when the electron absorbs a photon (a particle of light) with energy \(\Delta E = E_1 - E_0.\) To get to another state with energy \(E_2,\) the electron would need to absorb a photon with energy \(E_2 - E_1\) or \(E_2-E_0.\) In an atom, these energy differences are always different: there is a non-uniform spacing between the energy levels . Therefore, if we limit ourselves to interacting with the atom using photons with energy \(\Delta E,\) we will not go beyond the states that define our qubit. Photons with a particular energy excite electrons Let’s then build the simplest superconducting circuit. We do not want the circuit to warm up, or it will lose its quantum properties. Of all the elements that an ordinary circuit may have, only two do not produce heat when they’re superconducting: capacitors and inductors . Capacitors are two parallel metallic plates that store electric charge. They are characterized by their capacitance \(C,\) which measures how much charge they can store when connected to a given power source. Inductors are wires shaped as a coil and store magnetic fields when a current passes through. These magnetic fields, in turn, slow down changing currents that pass through the inductor. They are described by an inductance \(L,\) which measures the strength of the magnetic field stored in the inductor at a fixed current. The simplest superconducting circuit is, therefore, a capacitor connected to an inductor, also known as an LC circuit, as shown below: Superconducting LC circuit Sadly, this simple circuit has a problem: the spacing between energy levels is constant, which means identical photons will cause energy transitions between many neighbouring pairs of states. This makes it impossible to isolate just two specific states for our qubit. Non-uniform energy levels in an atom vs. uniform energy levels in a superconducting LC circuit But there turns out to be a fix for the even spacing. Enter the Josephson juncti