9.4 Josephson junctions

Josephson junctions are crucial components in superconducting electronics and quantum computing. These devices consist of two superconductors separated by a thin insulating layer, allowing Cooper pairs to tunnel between them.

The junctions exhibit unique quantum phenomena like DC and AC Josephson effects, flux quantization, and Shapiro steps. These properties make them invaluable for applications in voltage standards, SQUIDs, and superconducting qubits for quantum computing.

Josephson junctions

• Josephson junctions are a fundamental building block in superconducting electronics and quantum computing
• Consist of two superconductors separated by a thin insulating layer, allowing quantum tunneling of Cooper pairs
• Exhibit unique quantum phenomena such as the DC and AC Josephson effects, flux quantization, and Shapiro steps

Superconductor-insulator-superconductor structure

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• Josephson junctions have a sandwich-like structure: two superconducting electrodes separated by a thin insulating barrier
• The insulating layer is typically a few nanometers thick (aluminum oxide or magnesium oxide)
• The superconducting electrodes are often made of low-temperature superconductors such as aluminum, niobium, or lead
• The insulating barrier allows quantum tunneling of Cooper pairs between the superconductors

Tunneling of Cooper pairs

• In superconductors, electrons form Cooper pairs due to electron-phonon interactions
• Cooper pairs can tunnel through the insulating barrier in a Josephson junction without any applied voltage
• The tunneling of Cooper pairs is a macroscopic quantum effect, demonstrating quantum behavior at the macroscopic scale
• The tunneling current depends on the phase difference between the two superconducting electrodes

DC Josephson effect

• The DC Josephson effect describes the flow of a supercurrent through the Josephson junction without any applied voltage
• The supercurrent is given by: $I = I_c \sin(\delta)$, where $I_c$ is the critical current and $\delta$ is the phase difference between the superconductors
• The critical current $I_c$ depends on the properties of the junction (barrier thickness, area, and the superconducting gap)
• The DC Josephson effect demonstrates the coherence and phase-locking of the superconducting wavefunctions across the junction

AC Josephson effect

• When a DC voltage $V$ is applied across the Josephson junction, an AC supercurrent oscillates with a frequency $f = (2e/h)V$
• This is known as the AC Josephson effect, and the frequency is proportional to the applied voltage
• The AC Josephson effect provides a precise relationship between frequency and voltage, making it useful for voltage standards and high-frequency applications
• The Josephson frequency-voltage relation is given by: $f = (483.6 \text{ GHz/mV}) \times V$

Josephson current vs voltage

• The current-voltage (I-V) characteristic of a Josephson junction is highly nonlinear
• For currents below the critical current $I_c$, the junction exhibits a supercurrent with zero voltage drop (DC Josephson effect)
• When the current exceeds $I_c$, a voltage develops across the junction, and the junction enters the resistive state
• In the resistive state, the junction exhibits the AC Josephson effect, with an oscillating supercurrent and a DC voltage

Shapiro steps

• When an AC current is applied to a Josephson junction in addition to a DC bias, the I-V curve displays voltage steps known as Shapiro steps
• Shapiro steps occur at voltages $V_n = nhf/2e$, where $n$ is an integer, $h$ is Planck's constant, $f$ is the frequency of the AC current, and $e$ is the electron charge
• The height of the Shapiro steps is proportional to the amplitude of the applied AC current
• Shapiro steps are used in voltage standards and for studying the dynamics of Josephson junctions

Josephson penetration depth

• The Josephson penetration depth $\lambda_J$ characterizes the length scale over which magnetic fields penetrate the Josephson junction
• It is given by: $\lambda_J = \sqrt{\hbar/2e\mu_0 J_c d}$, where $\hbar$ is the reduced Planck's constant, $\mu_0$ is the vacuum permeability, $J_c$ is the critical current density, and $d$ is the effective magnetic thickness of the junction
• The Josephson penetration depth determines the spatial variation of the phase difference and the current density along the junction
• Junctions with dimensions smaller than $\lambda_J$ are considered "short" junctions, while those larger than $\lambda_J$ are "long" junctions

Flux quantization in Josephson junctions

• In a superconducting loop containing a Josephson junction, the magnetic flux threading the loop is quantized in units of the flux quantum $\Phi_0 = h/2e$
• The quantization of flux leads to periodic modulation of the critical current as a function of the applied magnetic field
• This effect is used in superconducting quantum interference devices (SQUIDs) for sensitive magnetic field measurements
• The flux quantization condition is given by: $\oint \nabla \varphi \cdot dl = 2\pi n - (2\pi/\Phi_0) \Phi$, where $\varphi$ is the phase of the superconducting wavefunction, $n$ is an integer, and $\Phi$ is the enclosed magnetic flux

SQUID: Superconducting quantum interference device

• A SQUID consists of a superconducting loop interrupted by one (RF SQUID) or two (DC SQUID) Josephson junctions
• SQUIDs are highly sensitive magnetometers that can measure extremely small magnetic fields (down to $10^{-15}$ T)
• The critical current of a SQUID is modulated by the applied magnetic flux due to interference effects
• DC SQUIDs are operated with a constant bias current, and the voltage across the SQUID is measured as a function of the applied magnetic flux
• RF SQUIDs are operated with an AC bias current, and the changes in the resonant frequency are detected

RCSJ model of Josephson junctions

• The resistively and capacitively shunted junction (RCSJ) model describes the dynamics of a Josephson junction
• In the RCSJ model, the Josephson junction is represented by an ideal junction (governed by the Josephson equations) in parallel with a resistor and a capacitor
• The resistor represents the quasiparticle tunneling and dissipation in the junction, while the capacitor represents the junction's geometric capacitance
• The RCSJ model leads to the following equation of motion for the phase difference: $\hbar C (\ddot{\delta}) + \hbar/R (\dot{\delta}) + I_c \sin(\delta) = I$, where $C$ is the capacitance, $R$ is the resistance, and $I$ is the bias current
• The RCSJ model is used to study the dynamics and switching behavior of Josephson junctions

Josephson junction applications

• Josephson junctions have numerous applications in superconducting electronics, metrology, and quantum computing
• Voltage standards: The AC Josephson effect provides a precise relationship between frequency and voltage, enabling the realization of high-precision voltage standards
• SQUIDs: Superconducting quantum interference devices are used for ultra-sensitive magnetic field measurements in various fields (geophysics, biomagnetism, and materials characterization)
• Superconducting qubits: Josephson junctions are the key building blocks for superconducting qubits, such as flux qubits, charge qubits, and transmon qubits
• Superconducting digital electronics: Josephson junctions can be used to create high-speed, low-power digital circuits, such as rapid single flux quantum (RSFQ) logic

Superconducting qubits for quantum computing

• Superconducting qubits are a leading platform for quantum computing, relying on Josephson junctions as the nonlinear circuit element
• Flux qubits: Consist of a superconducting loop interrupted by one or more Josephson junctions, with the qubit states defined by the direction of the circulating current
• Charge qubits: Consist of a superconducting island connected to a reservoir through a Josephson junction, with the qubit states defined by the number of excess Cooper pairs on the island
• Transmon qubits: A variant of charge qubits with reduced sensitivity to charge noise, achieved by operating in the regime where the Josephson energy dominates the charging energy
• Phase qubits: Exploit the different energy levels in a current-biased Josephson junction, with the qubit states defined by the phase difference across the junction
• Josephson junctions enable the strong nonlinearity required for qubit operations and the tunability of the qubit parameters