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Amplifying quantum signals with Josephson tunnel junc- tion circuits

Michel Devoret, Benjamin Huard, N. Bergeal, Robert J. Schoelkopf, S. M. Girvin

Published Jan 1, 2008• 0 citations
Publisher

Abstract

Amplifiers are crucial in every experiment carrying out a very sensitive measurement. However, they always degrade the information by adding noise. Quantum mechanics puts a limit on how small this degradation can be. Theoretically, the minimum noise energy added by a phase-preserving amplifier to the signal it processes amounts at least to half a photon at the signal frequency. In this article, we show that we can build a practical microwave amplifying circuit that fulfills the minimal requirements to reach this quantum limit. The readout of solid-state qubits, and more generally, the measurement of very weak signals in various areas of science, can benefit from this new superconducting device. We also discuss how our circuit can be the basic buiding block for a variety of practical applications such as frequency conversion with and without photon number gain, dynamic cooling and production of entangled signal pairs.

DOI
10.3254/978-1-60750-039-1-151

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Amplifiers are crucial in every experiment carrying out a very sensitive measurement. However, they always degrade the information by adding noise. Quantum mechanics puts a limit on how small this degradation can be. Theoretically, the minimum noise energy added by a phase-preserving amplifier to the signal it processes amounts at least to half a photon at the signal frequency. In this article, we show that we can build a practical microwave amplifying circuit that fulfills the minimal requirements to reach this quantum limit. The readout of solid-state qubits, and more generally, the measurement of very weak signals in various areas of science, can benefit from this new superconducting device. We also discuss how our circuit can be the basic buiding block for a variety of practical applications such as frequency conversion with and without photon number gain, dynamic cooling and production of entangled signal pairs.
Three-wave mixing devices, i.e. non-linear circuits converting power among three microwave signals, are key elements of analog information processing in the microwave domain. However, they are based on dissipative components such as semiconductor diodes, or SIS tunnel junctions biased near the superconducting gap. The loss of signal limits their operation and also introduces noise above the minimum required by quantum mechanics. By using Josephson junctions and superconducting resonators, we've been able to build such a quantum device. In its amplification mode, this device can be used as a preamplifier in many experiments where a few photon wide microwave signals must be measured by commercial instruments, which are sensitive to powers several orders of magnitude larger. The price to pay for this amplification is the necessary degradation of information. Indeed, an extension of the no-cloning theorem imposes that a linear phase preserving amplifier adds at least half a quantum of noise to the signal. By amplifying the finite frequency noise emitted by an NIN tunnel junction, we demonstrated that the total measurement chain adds less than 2.3 quanta to this limit thanks to our device. The operating principle of this device is based on Josephson parametric amplification. I will explain this mechanism and present results which illustrate that it can generate quantum correlated pairs of spatially separated photons with different wavelengths known as two-mode squeezed states. This experiment exploits the tunability of our device and the ease-of-use inherent to micro-wave circuits to perform a quantum optics experiment demonstrating this entanglement.
Three-wave mixing devices, i.e. non-linear circuits converting power among three microwave signals, are key elements of analog information processing in the microwave domain. However, they are based on dissipative components such as semiconductor diodes, or SIS tunnel junctions biased near the superconducting gap. The loss of signal limits their operation and also introduces noise above the minimum required by quantum mechanics. By using Josephson junctions and superconducting resonators, we've been able to build such a quantum device. In its amplification mode, this device can be used as a preamplifier in many experiments where a few photon wide microwave signals must be measured by commercial instruments, which are sensitive to powers several orders of magnitude larger. The price to pay for this amplification is the necessary degradation of information. Indeed, an extension of the no-cloning theorem imposes that a linear phase preserving amplifier adds at least half a quantum of noise to the signal. By amplifying the finite frequency noise emitted by an NIN tunnel junction, we demonstrated that the total measurement chain adds less than 2.3 quanta to this limit thanks to our device. The operating principle of this device is based on Josephson parametric amplification. I will explain this mechanism and show preliminary results which illustrate that it can generate quantum correlated pairs of spatially separated photons with different wavelengths known as two-mode squeezed states. This experiment exploits the tunability of our device and the ease-of-use inherent to micro-wave circuits to perform a quantum optics experiment demonstrating this entanglement.
Nowadays, it is possible to control and measure the quantum state of systems with a few degrees of freedom, whether they are microscopic objects like cold atoms and single photons, or macroscopic objects like superconducting circuits. Such delicate experiments require an interface which bridges the gap of orders of magnitudes in energy between the quantum object and the data acquisition system. This problem is solved by an active device: the amplifier. Although the amplifier is necessary to overcome the noise at the stage of data acquisition, it eventually alters the signal. We have developed an amplifier for microwave signals which adds only the minimum of noise allowed by quantum laws: the equivalent of half a photon of noise referred to the input. Our amplifier is based on a superconducting circuit of Josephson junctions in superconducting resonators. We found a way to make it practical to use by tuning its center frequency over 500 MHz. We are now trying to increase its bandwidth for a given gain using more compact resonators. In this talk, I will show how the amplifier works and also its applications to quantum information processing. This circuit can be used to generate and combine twin microwave photons within an integrated circuit. Finally, I will show that such an amplifier allows real time measurement of the state of a qubit and quantum feedback protocols.
The ability to perform high-fidelity quantum nondemolition qubit readout is pivotal for the realization of large and powerful quantum computers. Such readout of superconducting qubits is generally enabled by amplifying the weak dispersive measurement signals using phase-preserving quantum-limited Josephson amplifiers with sufficient gain to dilute the contribution of the added noise by the output chain. Here, we further enhance the qubit readout fidelity by (1) simultaneously measuring the two-mode squeezed states of the amplified readout signals at the signal and idler frequencies of the nondegenerate amplifier and (2) coherently combining them at the classical processing stage following a relative rotation that maximizes the signal to noise ratio of the qubit-encoded readout quadrature. Such readout scheme exhibits enhancement in the readout fidelity for all practical values of amplifier gain and noise added by the output chain and is fully compatible with frequency multiplexed setups used in large quantum processors.
Coherent circuits based on superconducting elements hold tremendous promise for the practical implementation of quantum information technology: advanced computation, cryptography, and hardware simulation of complex materials systems are envisioned applications. Such circuits are essentially engineered atoms with level transitions in the 4-8 GHz regime; as such “read” and “write” operations are performed with the aid of fast microwave pulses and low-noise amplifiers, respectively. Recent advances in cavity-based superconducting parametric amplifiers have enabled real-time measurement and feedback in one and two qubits. We demonstrate a novel Josephson junction transmission-line based traveling-wave amplifier which employs sub-wavelength phase matching resonators to achieve high gain, several GHz of bandwidth, and near quantum-noise limited performance. Such devices will enable multiplexed qubit readout in integrated architectures comprised of a variety of microwave frequency analog and digital superconducting technologies: signal sources, multiplexers, ADCs, DACs, mixers, and amplifiers.
Detecting single-photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics.