A near-quantum-limited Josephson traveling-wave parametric amplifier.

Detecting single-photon level signals­carriers of both classical and quantum information­is 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.

Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits.

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

Observing single quantum trajectories of a superconducting quantum bit.

The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture--a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a 'quantum trajectory' determined by the measurement outcome. Here we use weak measurements to monitor a microwave cavity containing a superconducting quantum bit (qubit), and track the individual quantum trajectories of the system. In this set-up, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or the amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring, and validate the foundation of quantum feedback approaches based on Bayesian statistics. Moreover, our experiments suggest a new means of implementing 'quantum steering'--the harnessing of action at a distance to manipulate quantum states through measurement.

Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback.

The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this 'instantaneous' process can be stretched using weak measurements, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements. Other applications include quantum state stabilization, entanglement generation using measurement, state purification and adaptive measurements.

Heralded state preparation in a superconducting qubit.

We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a superconducting flux qubit in which the pointer state distributions can be resolved to below one part in 1000. In the weak excitation regime, continuous measurement permits the use of heralding to ensure initialization to a fiducial state, such as the ground state. This procedure boosts readout fidelity to 93.9% by suppressing errors due to spurious thermal population. Furthermore, heralding potentially enables a simple, fast qubit reset protocol without changing the system parameters to induce Purcell relaxation.

Preparation and detection of a mechanical resonator near the ground state of motion.

Cold, macroscopic mechanical systems are expected to behave contrary to our usual classical understanding of reality; the most striking and counterintuitive predictions involve the existence of states in which the mechanical system is located in two places simultaneously. Various schemes have been proposed to generate and detect such states, and all require starting from mechanical states that are close to the lowest energy eigenstate, the mechanical ground state. Here we report the cooling of the motion of a radio-frequency nanomechanical resonator by parametric coupling to a driven, microwave-frequency superconducting resonator. Starting from a thermal occupation of 480 quanta, we have observed occupation factors as low as 3.8 +/- 1.3 and expect the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion. Further cooling is limited by random excitation of the microwave resonator and heating of the dissipative mechanical bath. This level of cooling is expected to make possible a series of fundamental quantum mechanical observations including direct measurement of the Heisenberg uncertainty principle and quantum entanglement with qubits.

Complete clinical response to neoadjuvant chemoradiotherapy in patients with rectal cancer: opinions of British and Irish specialists.

INTRODUCTION: Advances in neoadjuvant treatment have highlighted the phenomenon of complete clinical response (CCR) in a proportion of patients with rectal cancer. Radical surgery may be associated with a poor functional outcome and quality of life and has a small but significant risk of mortality. This study aimed to assess opinion of colorectal surgeons on issues surrounding the question of nonoperative management in patients who demonstrate complete response after neoadjuvant therapy. METHOD: A questionnaire was sent to members of the Association of Coloproctology of Great Britain and Ireland regarding investigations, clinical management, pathological assessment and oncological outcome in rectal cancer patients with a complete response to neoadjuvant chemoradiotherapy. RESULTS: A total of 122 consultants responded (26% response rate). Most surgeons (58%) would not consider conservative management of patients with a complete response and even more (69%) expressed that they would never discuss nonoperative management in patients with rectal cancer who are fit for curative surgery. Over 70 different combinations of investigations and imaging modalities were suggested to define a CCR. Eighty-six per cent of consultants felt that a pathology report stating no evidence of residual adenocarcinoma did not rule out the presence of tumour cells and all respondents estimated the percentage of patients with pathological complete response as < 20%. CONCLUSIONS: No consensus exists as to what defines a complete response and at present there is resistance to offering nonoperative management in selected patients. With improvements in neoadjuvant treatment modalities, it will be increasingly important to consider nonoperative management in the future.

The 'modified triple staple' technique: a variant stapling technique for anastomosis after low anterior resection.

INTRODUCTION: Stapled techniques of coloanal anastomosis in anterior resection have gained widespread acceptance over hand anastomosis. We believe a modification of the 'triple staple technique' has ergonomic advantages over existing stapling methods and present our technique and experience here. METHODS: Fifty consecutive patients underwent anterior resection with a concomitant defunctioning ileostomy in 44 (86%) patients. A modified triple staple technique of side to end coloanal anastomosis was performed without the need of a purse string suture on the proximal and the distal segments. RESULTS: There were no major intra-operative complications. 2/50 (4%) clinical leaks and 2/37 (5.4%) radiological leaks were noted. A combined leak rate of 4/50 (8%) was reported. The incidence of anastomotic stricture encountered was 1/50 (2%). CONCLUSION: The modified triple staple technique for side to end anastomosis in anterior resection has ergonomic advantages and comparable safety to the existing techniques of stapling coloanal anastomosis. We believe this technique can be widely adopted as an added alternative to the current techniques of stapled anastomosis after anterior resection.