Superconducting quantum computing is a branch of quantum computing and solid-state physics that implements superconducting electronic circuits as qubits in a quantum processor. These devices are typically microwave-frequency electronic circuits containing Josephson junctions, which are fabricated on solid state chips.
Superconducting circuits are one of many possible physical implementations of qubits, the quantum computer's equivalent of a traditional bit in a classic computer. Qubits refer to a two-state quantum mechanical system, and have two logic states, the ground state and the excited state, often denoted (for ground and excited), or . Superconducting quantum computing implementations are categorized as "solid state" quantum computers, where qubits are intrinsically integrated in a solid-state device. Solid state quantum computers also borrow fabrication techniques developed for solid state classical computation.
Superconducting architecture is the dominant method in the industry for developing quantum processing units, or QPUs. Research in superconducting quantum computing is conducted by companies such as Google, IBM, IMEC, BBN Technologies, Rigetti, and Intel. Alternatives to superconducting qubits include trapped ions, and neutral atoms.
Ongoing research in superconducting quantum computing includes device-level improvement, developing error correction methods, and demonstrating quantum advantage by comparing a quantum processor's performance to a classical computer.
History Quantum computers were first proposed by Richard Feynman, who in 1982 proposed using such a computer to simulate and understand the properties of other quantum systems. In the 1990s, two quantum algorithms were published, which further stirred interest in realizing quantum computers. Peter Shor proposed Shor's algorithm, a quantum algorithm for finding the prime factors of an integer, which could in theory break RSA encryption. Similarly, Lov Grover proposed the Grover search algorithm, which provides an alternative to binary search that can be done with quadratic speedup.
At the time, superconducting quantum circuits were already being used to construct highly sensitive SQUID devices, and had also been used to demonstrate macroscopic quantum phenomena, such as quantized energy levels. It became apparent that these superconducting qubits could be used to achieve quantum computation. This was especially true because such "solid state" approaches to quantum computing were seen as far more viable than other approaches at the time, including NMR (nuclear magnetic resonance) quantum computing, due in part to the fact that existing fabrication techniques would apply.
In 1999, a paper was published by Yasunobu Nakamura, Yu. A. Pashkin, and Jaw-Shen Tsai, demonstrating the first superconducting qubit. It is a form of Cooper pair box, now known as the "charge qubit". Although the design had been proposed in 1997 by the Saclay team (including Devoret), this paper was the first to show coherent control and readout, in the form of Rabi oscillations between the ground and excited states of the qubit. However, even after this first result, it was unclear if superconducting qubits would be viable, and some argued that the system was not truly capable of containing quantum information. Part of the problem was that this initial design maintained coherence for less than a nanosecond, not long enough to do any calculations.
In the following years, several other superconducting qubits were invented, including the phase qubit, flux qubit, quatronium, the transmon qubit, and the fluxonium. Successive advances in qubit design, materials science, and fabrication techniques improved coherence by roughly five orders of magnitude over two decades. The transmon design, introduced in 2007, reduced sensitivity to charge noise and has since become the dominant architecture in superconducting quantum computing processors deployed by IBM, Google, and others. Transmon coherence times were brought into the range of , though further gains on that architecture plateaued for nearly a decade. In 2021, a fluxonium qubit achieved a coherence time exceeding 1 ms, an improvement by an order of magnitude. In 2025, developments in readout and design allowed superconducting transmon qubits to reach millisecond coherence times as well.
Google in 2016, implemented 16 qubits to convey a demonstration of the Fermi-Hubbard Model. In another experiment, Google used 17 qubits to optimize the Sherrington-Kirkpatrick model. In 2019, Google produced the Sycamore quantum computer which performed a task in 200 seconds that Google claimed would have taken 10,000 years on a classical computer. The task was random circuit sampling, a common benchmark for claims of "quantum supremacy" or quantum advantage.
As of 2025, superconducting quantum processors have exceeded 1,000 qubits, the largest being IBM Condor, a 1,121-qubit quantum processor. In 2025, Google announced one of the first independently verifiable quantum advantages on hardware using the Willow processor.
Background Quantum computing
Classical computation models rely on physical implementations consistent with the laws of classical mechanics. Some very small systems, or certain systems under extreme conditions, are instead described by the quantum mechanics, which obey different sets of physical rules.
Quantum computation is a method of constructing a quantum system for the purpose of encoding information. Applications of a quantum computer would include simulating quantum phenomena beyond the scope of classical approximation, and speeding up certain calculations, particularly those that involve an "oracle". Certain algorithms designed for quantum computers, such as Grover Search or Shor's algorithm, are believed to be able to do some calculations better than their classical counterparts.
Gate-based quantum computing is a method of quantum computing that, much like traditional computing, use qubits (analogous to bits) and quantum gates (analogous to classical gates).
Qubits
A qubit is any two-level system in quantum mechanics. Much like a classical bit, it is a system with two possible states. However, the difference lies in the fact that because a qubit obeys the laws of quantum mechanics, it is capable of occupying a quantum superposition of both states.
The primary requirement for physically constructing a qubit is the ability to be able to individually address the first two states, in this case energy levels, of the system. This is difficult, as most systems contain a near-infinite number of energy levels. In superconducting quantum computers, these qubits are constructed using superconducting resonant circuits. Each superconducting qubit is essentially a nonlinear LC circuit with a capacitor and a Josephson junction, a superconducting element with a nonlinear inductance. Because the circuit is non-linear, there is unequal spacing between its energy levels, allowing the first two states to be individually addressable.
In theory, due to its nonlinearity, the qubit is affected only by photons with the energy difference required to jump from the ground state to the excited state. In practice, however, noise in the system can still cause it to leave the computational subspace. In many cases, the higher energy levels of a superconducting qubit need to be considered. This is especially true in transmons, which have weak anharmonicity by design.
Because the circuit is superconducting, it has zero resistance, and dissipates almost no energy. However, this comes at the price of extremely low operation temperatures.
Quantum gates
A quantum gate is a generalization of a logic gate describing the transformation of qubits from their initial state to a different state, often a superposition.
In superconducting qubits, quantum gates are implemented as microscopic pulses applied to the circuit using microwave resonators. Pulses are sent through resonators capacatively coupled to the qubit, which are harmonic oscillators that are detuned from the qubit itself. By applying an external drive to the qubit, the normal unitary evolution of the system implements a single qubit gate after a certain length of time has passed.
Two qubit gates, such as the iSWAP gate, can be achieved through coherent exchange or parametric coupling between two qubits. In coherent exchange, the transition frequency of one of the qubits is tuned such that it matches the transition frequency of the second qubit. This method relies on the frequency tunability of the qubit, and does not work in fixed-frequency cases. Parametric coupling on the other hand is done by changing the coupling constant between two qubits at the sum or difference of their two frequencies.
Criteria for a viable quantum processor There are many possible physical implementations of qubits, with superconducting circuits being one of them. In order for a given implementation to be considered viable for constructing a quantum computer, one set of criterion is the DiVincenzo's criteria, a set of criteria for the physical implementation of superconducting quantum computing. The initial five criteria ensure that the quantum computer is in line with the postulates of quantum mechanics and the remaining two pertaining to the relaying of this information over a network.
Superconducting qubits already meet a large number of DiVincenzo's criteria. They are already highly scalable from a fabrication standpoint, they can be initialized by thermal relaxation, and single-qubit gates combined with two-qubit gates form a universal gate set. However, superconducting qubits still struggle with having short coherence times, making preserving quantum information a challenge.
Superconductors
Superconducting qubits are circuits made from superconducting metal material. Superconductivity is a phenomenon that occurs in some metals at low temperatures where electrical current experiences zero resistance in a material, allowing the current to flow without loss of energy, and be nearly dissipation-less.
This phenomenon occurs because the basic charge carriers are pairs of electrons (known as Cooper pairs), rather than single electrons as found in typical conductors. While single electrons are fermions (with half-integer spin), Cooper pairs of electrons are bosons (with integer spin), and as such they no longer obey the Pauli exclusion principle, meaning these Cooper pairs can occupy the same states. Under certain conditions, this allows them to form a state of matter known as a Bose–Einstein condensate, where all of the pairs of electrons in the condensate each occupy the same position in space and have equal momentum. In this way, there is nothing distinguishing the pairs from each other, and they occupy the same state. As a result, the electron pairs move coherently as a single wave, bypassing the disturbances in the lattice that usually cause resistance. Thus, superconductors possess near infinite conductivity and near zero resistance.
Superconductivity generally only occurs near absolute zero, since that is when it is more energetically favorable for electrons to pair up than repel each other. This is one of the primary reasons why superconducting qubits must be cooled to ultra cold temperatures.
Superconducting electrical circuits Superconducting electrical circuits are networks of electrical elements described by a single condensate wave function, wherein charge flow is well-defined by some complex probability amplitude. Quantization in the circuit results from complex amplitude continuity, since only discrete numbers of magnetic flux quanta can penetrate a superconducting loop. Parameters of superconducting circuits are designed by setting (classical) values to the electrical elements composing them, such as capacitance or inductance.
Josephson junctions