24 research outputs found
Pair-cat codes: autonomous error-correction with low-order nonlinearity
We introduce a driven-dissipative two-mode bosonic system whose reservoir
causes simultaneous loss of two photons in each mode and whose steady states
are superpositions of pair-coherent/Barut-Girardello coherent states. We show
how quantum information encoded in a steady-state subspace of this system is
exponentially immune to phase drifts (cavity dephasing) in both modes.
Additionally, it is possible to protect information from arbitrary photon loss
in either (but not simultaneously both) of the modes by continuously monitoring
the difference between the expected photon numbers of the logical states.
Despite employing more resources, the two-mode scheme enjoys two advantages
over its one-mode cat-qubit counterpart with regards to implementation using
current circuit QED technology. First, monitoring the photon number difference
can be done without turning off the currently implementable dissipative
stabilizing process. Second, a lower average photon number per mode is required
to enjoy a level of protection at least as good as that of the cat-codes. We
discuss circuit QED proposals to stabilize the code states, perform gates, and
protect against photon loss via either active syndrome measurement or an
autonomous procedure. We introduce quasiprobability distributions allowing us
to represent two-mode states of fixed photon number difference in a
two-dimensional complex plane, instead of the full four-dimensional two-mode
phase space. The two-mode codes are generalized to multiple modes in an
extension of the stabilizer formalism to non-diagonalizable stabilizers. The
-mode codes can protect against either arbitrary photon losses in up to
modes or arbitrary losses and gains in any one mode.Comment: 29 pages, 9 figures, 2 tables; added a numerical compariso
Robust concurrent remote entanglement between two superconducting qubits
Entangling two remote quantum systems which never interact directly is an
essential primitive in quantum information science and forms the basis for the
modular architecture of quantum computing. When protocols to generate these
remote entangled pairs rely on using traveling single photon states as carriers
of quantum information, they can be made robust to photon losses, unlike
schemes that rely on continuous variable states. However, efficiently detecting
single photons is challenging in the domain of superconducting quantum circuits
because of the low energy of microwave quanta. Here, we report the realization
of a robust form of concurrent remote entanglement based on a novel microwave
photon detector implemented in the superconducting circuit quantum
electrodynamics (cQED) platform of quantum information. Remote entangled pairs
with a fidelity of are generated at Hz. Our experiment
opens the way for the implementation of the modular architecture of quantum
computation with superconducting qubits.Comment: Main paper: 7 pages, 4 figures; Appendices: 14 pages, 9 figure
Demonstrating a superconducting dual-rail cavity qubit with erasure-detected logical measurements
A critical challenge in developing scalable error-corrected quantum systems
is the accumulation of errors while performing operations and measurements. One
promising approach is to design a system where errors can be detected and
converted into erasures. A recent proposal aims to do this using a dual-rail
encoding with superconducting cavities. In this work, we implement such a
dual-rail cavity qubit and use it to demonstrate a projective logical
measurement with erasure detection. We measure logical state preparation and
measurement errors at the -level and detect over of cavity decay
events as erasures. We use the precision of this new measurement protocol to
distinguish different types of errors in this system, finding that while decay
errors occur with probability per microsecond, phase errors occur
6 times less frequently and bit flips occur at least 170 times less frequently.
These findings represent the first confirmation of the expected error hierarchy
necessary to concatenate dual-rail erasure qubits into a highly efficient
erasure code
Generating higher order quantum dissipation from lower order parametric processes
9 pages, 5 figuresInternational audienceStabilization of quantum manifolds is at the heart of error-protected quantum information storage and manipulation. Nonlinear driven-dissipative processes achieve such stabilization in a hardware efficient manner. Josephson circuits with parametric pump drives implement these nonlinear interactions. In this article, we propose a scheme to engineer a four-photon drive and dissipation on a harmonic oscillator by cascading experimentally demonstrated two-photon processes. This would stabilize a four-dimensional degenerate manifold in a superconducting resonator. We analyze the performance of the scheme using numerical simulations of a realizable system with experimentally achievable parameters
Stabilization and operation of a Kerr-cat qubit
International audienceQuantum superpositions of macroscopically distinct classical states, so-called Schr\"{o}dinger cat states, are a resource for quantum metrology, quantum communication, and quantum computation. In particular, the superpositions of two opposite-phase coherent states in an oscillator encode a qubit protected against phase-flip errors. However, several challenges have to be overcome in order for this concept to become a practical way to encode and manipulate error-protected quantum information. The protection must be maintained by stabilizing these highly excited states and, at the same time, the system has to be compatible with fast gates on the encoded qubit and a quantum non-demolition readout of the encoded information. Here, we experimentally demonstrate a novel method for the generation and stabilization of Schr\"{o}dinger cat states based on the interplay between Kerr nonlinearity and single-mode squeezing in a superconducting microwave resonator. We show an increase in transverse relaxation time of the stabilized, error-protected qubit over the single-photon Fock-state encoding by more than one order of magnitude. We perform all single-qubit gate operations on time-scales more than sixty times faster than the shortest coherence time and demonstrate single-shot readout of the protected qubit under stabilization. Our results showcase the combination of fast quantum control with the robustness against errors intrinsic to stabilized macroscopic states and open up the possibility of using these states as resources in quantum information processing
To catch and reverse a quantum jump mid-flight
Added sections to the Supplementary Information on the concepts and signal-to-noise ratio of the experiment. Corrected a few minor typos and clarified text. Revised and expanded citationsInternational audienceA quantum system driven by a weak deterministic force while under strong continuous energy measurement exhibits quantum jumps between its energy levels. This celebrated phenomenon is emblematic of the special nature of randomness in quantum physics. The times at which the jumps occur are reputed to be fundamentally unpredictable. However, certain classical phenomena, like tsunamis, while unpredictable in the long term, may possess a degree of predictability in the short term, and in some cases it may be possible to prevent a disaster by detecting an advance warning signal. Can there be, despite the indeterminism of quantum physics, a possibility to know if a quantum jump is about to occur or not? In this paper, we answer this question affirmatively by experimentally demonstrating that the completed jump from the ground to an excited state of a superconducting artificial atom can be tracked, as it follows its predictable "flight," by monitoring the population of an auxiliary level coupled to the ground state. Furthermore, we show that the completed jump is continuous, deterministic, and coherent. Exploiting this coherence, we catch and reverse a quantum jump mid-flight, thus preventing its completion. This real-time intervention is based on a particular lull period in the population of the auxiliary level, which serves as our advance warning signal. Our results, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as early detection of error syndromes
The Kerr-Cat Qubit: Stabilization, Readout, and Gates
Quantum superpositions of macroscopically distinct classical states, so-called Schr\"{o}dinger cat states, are a resource for quantum metrology, quantum communication, and quantum computation. In particular, the superpositions of two opposite-phase coherent states in an oscillator encode a qubit protected against phase-flip errors. However, several challenges have to be overcome in order for this concept to become a practical way to encode and manipulate error-protected quantum information. The protection must be maintained by stabilizing these highly excited states and, at the same time, the system has to be compatible with fast gates on the encoded qubit and a quantum non-demolition readout of the encoded information. Here, we experimentally demonstrate a novel method for the generation and stabilization of Schr\"{o}dinger cat states based on the interplay between Kerr nonlinearity and single-mode squeezing in a superconducting microwave resonator. We show an increase in transverse relaxation time of the stabilized, error-protected qubit over the single-photon Fock-state encoding by more than one order of magnitude. We perform all single-qubit gate operations on time-scales more than sixty times faster than the shortest coherence time and demonstrate single-shot readout of the protected qubit under stabilization. Our results showcase the combination of fast quantum control with the robustness against errors intrinsic to stabilized macroscopic states and open up the possibility of using these states as resources in quantum information processing
Experimental implementation of a Raman-assisted eight-wave mixing process
13 pages, 5 figuresInternational audienceNonlinear processes in the quantum regime are essential for many applications, such as quantum-limited amplification, measurement, and control of quantum systems. In particular, the field of quantum error correction relies heavily on high-order nonlinear interactions between various modes of a quantum system. However, the required order of nonlinearity is often not directly available or weak compared to dissipation present in the system. Here, we experimentally demonstrate a route to obtain higher-order nonlinearity by combining more easily available lower-order nonlinear processes, using a generalization of the Raman transition. In particular, we show a transformation of four photons of a high-Q superconducting resonator into two excitations of a superconducting transmon mode and two pump photons, and vice versa. The resulting eight-wave mixing process is obtained by cascading two fourth-order nonlinear processes through a virtual state. We expect this type of process to become a key component of hardware-efficient quantum error correction using continuous-variable error-correction codes