71 research outputs found
Measurements of noise in Josephson-effect mixers
We present new heterodyne receiver results obtained at 100 GHz using resistively-shunted Nb and NbN tunnel junctions. In addition, we have carried out accurate measurements of the available noise power of these devices at the L-band (1.5 GHz) IF frequency. Both the heterodyne and the output noise measurements show that the noise of these devices can be a factor of five or more higher than that predicted by the simple current-biased RSJ model. The noise approaches the appropriate thermal or thermal and shot noise limits for bias voltages where the nonlinearity is not strong (i.e., V>ICRN), but as expected from the RSJ model, can be significantly higher at the low voltages where the mixers are typically biased. The bias voltage dependence of the noise shows structure which is associated with resonances in the RF embedding circuit. Surprisingly, we find that changes in the high-frequency (100 GHz) impedance presented to the junction can dramatically affect the magnitude and voltage dependence of the low-frequency (1.5 GHz) noise. This emphasizes the necessity of very closely matching the junction to free space over a wide frequency range
Hardware-efficient autonomous quantum error correction
We propose a new method to autonomously correct for errors of a logical qubit
induced by energy relaxation. This scheme encodes the logical qubit as a
multi-component superposition of coherent states in a harmonic oscillator, more
specifically a cavity mode. The sequences of encoding, decoding and correction
operations employ the non-linearity provided by a single physical qubit coupled
to the cavity. We layout in detail how to implement these operations in a
practical system. This proposal directly addresses the task of building a
hardware-efficient and technically realizable quantum memory.Comment: 12 pages,6 figure
Deterministic protocol for mapping a qubit to coherent state superpositions in a cavity
We introduce a new gate that transfers an arbitrary state of a qubit into a
superposition of two quasi-orthogonal coherent states of a cavity mode, with
opposite phases. This qcMAP gate is based on conditional qubit and cavity
operations exploiting the energy level dispersive shifts, in the regime where
they are much stronger than the cavity and qubit linewidths. The generation of
multi-component superpositions of quasi-orthogonal coherent states, non-local
entangled states of two resonators and multi-qubit GHZ states can be
efficiently achieved by this gate
Dynamically protected cat-qubits: a new paradigm for universal quantum computation
We present a new hardware-efficient paradigm for universal quantum
computation which is based on encoding, protecting and manipulating quantum
information in a quantum harmonic oscillator. This proposal exploits
multi-photon driven dissipative processes to encode quantum information in
logical bases composed of Schr\"odinger cat states. More precisely, we consider
two schemes. In a first scheme, a two-photon driven dissipative process is used
to stabilize a logical qubit basis of two-component Schr\"odinger cat states.
While such a scheme ensures a protection of the logical qubit against the
photon dephasing errors, the prominent error channel of single-photon loss
induces bit-flip type errors that cannot be corrected. Therefore, we consider a
second scheme based on a four-photon driven dissipative process which leads to
the choice of four-component Schr\"odinger cat states as the logical qubit.
Such a logical qubit can be protected against single-photon loss by continuous
photon number parity measurements. Next, applying some specific Hamiltonians,
we provide a set of universal quantum gates on the encoded qubits of each of
the two schemes. In particular, we illustrate how these operations can be
rendered fault-tolerant with respect to various decoherence channels of
participating quantum systems. Finally, we also propose experimental schemes
based on quantum superconducting circuits and inspired by methods used in
Josephson parametric amplification, which should allow to achieve these driven
dissipative processes along with the Hamiltonians ensuring the universal
operations in an efficient manner.Comment: 28 pages, 11 figure
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