In apparent contradiction to the laws of thermodynamics, Maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath exploiting the informationabout its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. Here, we realize a Maxwell demon experiment that tracks the state of each constituent both in the classical and quantum regimes. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.
This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantumprocessor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.
We propose a Quantum Non Demolition (QND) read-out scheme for a
superconducting artificial atom coupled to a resonator in a circuit QED
architecture, for which we estimate a very highmeasurement fidelity without
Purcell effect limitations. The device consists of two transmons coupled by a
large inductance, giving rise to a diamond-shape artificial atom with a logical
qubit and an ancilla qubit interacting through a cross-Kerr like term. The
ancilla is strongly coupled to a transmission line resonator. Depending on the
qubit state, the ancilla is resonantly or dispersively coupled to the
resonator, leading to a large contrast in the transmitted microwave signal
amplitude. This original method can be implemented with state of the art
Josephson parametric amplifier, leading to QND measurements in a few tens of
nanoseconds with fidelity as large as 99.9 %.
A new method for detecting the magnetic resonance of electronic spins at low
temperature is demonstrated. It consists in measuring the signal emitted by the
spins with a superconductingqubit that acts as a single-microwave-photon
detector, resulting in an enhanced sensitivity. We implement this new type of
electron-spin resonance spectroscopy using a hybrid quantum circuit in which a
transmon qubit is coupled to a spin ensemble consisting of NV centers in
diamond. With this setup we measure the NV center absorption spectrum at 30mK
at an excitation level of thicksim15,mu_{B} out of an ensemble of 10^{11}
spins.