We demonstrate and contrast two approaches to the stabilization of qubit entanglement by feedback. Our demonstration is built on a feedback platform consisting of two superconductingqubits coupled to a cavity which are measured by a nearly-quantum-limited measurement chain and controlled by high-speed classical logic circuits. This platform is used to stabilize entanglement by two nominally distinct schemes: a „passive“ reservoir engineering method and an „active“ correction based on conditional parity measurements. In view of the instrumental roles that these two feedback paradigms play in quantum error-correction and quantum control, we directly compare them on the same experimental setup. Further, we show that a second layer of feedback can be added to each of these schemes, which heralds the presence of a high-fidelity entangled state in realtime. This „nested“ feedback brings about a marked entanglement fidelity improvement without sacrificing success probability.
Significant advances in coherence have made superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherentquantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by qubits, while maintaining superior coherence. We demonstrate a novel superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for near-millisecond storage of quantum states in a resonator while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. The observed coherence times constitute an improvement of almost an order of magnitude over those of the best available superconducting qubits. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing with Josephson junction-based quantum circuits.
The `Schr“odinger’s cat‘ thought experiment highlights the counterintuitive facet of quantum theory that entanglement can exist between microscopic and macroscopicsystems, producing a superposition of distinguishable states like the fictitious cat that is both alive and dead. The hallmark of entanglement is the detection of strong correlations between systems, for example by the violation of Bell’s inequality. Using the CHSH variant of the Bell test, this violation has been observed with photons, atoms, solid state spins, and artificial atoms in superconducting circuits. For larger, more distinguishable states, the conflict between quantum predictions and our classical expectations is typically resolved due to the rapid onset of decoherence. To investigate this reconciliation, one can employ a superposition of coherent states in an oscillator, known as a cat state. In contrast to discrete systems, one can continuously vary the size of the prepared cat state and therefore its dependence on decoherence. Here we demonstrate and quantify entanglement between an artificial atom and a cat state in a cavity, which we call a `Bell-cat‘ state. We use a circuit QED architecture, high-fidelity measurements, and real-time feedback control to violate Bell’s inequality without post-selection or corrections for measurement inefficiencies. Furthermore, we investigate the influence of decoherence by continuously varying the size of created Bell-cat states and characterize the entangled system by joint Wigner tomography. These techniques provide a toolset for quantum information processing with entangled qubits and resonators. While recent results have demonstrated a high level of control of such systems, this experiment demonstrates that information can be extracted efficiently and with high fidelity, a crucial requirement for quantum computing with resonators.
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevantlimit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.
We have realized a microwave quantum-limited amplifier that is directional and can therefore function without the front circulator needed in many quantum measurements. The amplificationtakes place in only one direction between the input and output ports. Directionality is achieved by multi-pump parametric amplification combined with wave interference. We have verified the device noise performances by using it to readout a superconducting qubit and observed quantum jumps. With an improved version of this device, qubit and preamplifer could be integrated on the same chip.
Non-reciprocal devices, which have different transmission coefficients for
propagating waves in opposite directions, are crucial components in many low
noise quantum measurements. Inmost schemes, magneto-optical effects provide
the necessary non-reciprocity. In contrast, the proof-of-principle device
presented here, consists of two on-chip coupled Josephson parametric converters
(JPCs), which achieves directionality by exploiting the non-reciprocal phase
response of the JPC in the trans-gain mode. The non-reciprocity of the device
is controlled in-situ by varying the amplitude and phase difference of two
independent microwave pump tones feeding the system. At the desired working
point and for a signal frequency of 8.453 GHz, the device achieves a forward
power gain of 15 dB within a dynamical bandwidth of 9 MHz, a reverse gain of -6
dB and suppression of the reflected signal by 8 dB. We also find that the
amplifier adds a noise equivalent to less than one and a half photons at the
signal frequency (referred to the input). It can process up to 3 photons at the
signal frequency per inverse dynamical bandwidth. With a directional amplifier
operating along the principles of this device, qubit and readout preamplifier
could be integrated on the same chip.
We demonstrate full frequency conversion in the microwave domain using a
Josephson three-wave mixing device pumped at the difference between the
frequencies of its fundamental eigenmodes.By measuring the signal output as a
function of the intensity and phase of the three input signal, idler and pump
tones, we show that the device functions as a controllable three-wave
beam-splitter/combiner for propagating microwave modes, in accordance with
theory. Losses at the full conversion point are found to be less than 10^-2.
Potential applications of the device include quantum information transduction
and realization of an ultra-sensitive interferometer with controllable
feedback.