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arXiv:1010.0975 · Applied Physics Letters

Remote sensing and control of phase qubits

Dale Li, Fabio C. S. da Silva, Danielle Braje, Raymond W. Simmonds, David P. Pappas

Published Sep 6, 2010• 11 citations
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Abstract

We demonstrate a remote sensing design of phase qubits by separating the control and readout circuits from the qubit loop. This design improves measurement reliability because the control readout chip can be fabricated using more robust materials and can be reused to test different qubit chips. Typical qubit measurements such as Rabi oscillations, spectroscopy, and excited-state energy relaxation are presented.

DOI
10.1063/1.3488804

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We demonstrate a remote sensing design of phase qubits by separating the control and readout circuits from the qubit loop. This design improves measurement reliability because the control readout chip can be fabricated using more robust materials and can be reused to test different qubit chips. Typical qubit measurements such as Rabi oscillations, spectroscopy, and excited-state energy relaxation are presented.
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We have designed and operated a circuit based on a large-area current-biased Josephson junction whose two lowest energy quantum levels are used to implement a solid-state qubit. The circuit allows measurement of the qubit states with a fidelity of 85% while providing sufficient decoupling from external sources of relaxation and decoherence to allow coherent manipulation of the qubit state, as demonstrated by the observation of Rabi oscillations. This qubit circuit is the basis of a scalable quantum computer.
Fast on-chip control of superconducting qubits has engaged complex and power consuming RSFQ circuits that currently pose more of an experimental burden than an asset. Measurements of quantum coherent oscillations of qubits require dilution refrigerator temperatures. The motivation of this design is to minimize the necessary bias leads and power dissipation for an SFQ based control circuit. Elimination of redundant circuit elements by innovative use of fundamental elements allows small-scale control circuitry.
We report on development of a set-up for measuring intrawell dynamics in a Nb-based rf-SQUID qubit described by a double well potential, by rapidly tilting the potential, allowing escape to the adjacent well with high probability for an excited state but low probability for the ground state. The rapid tilt of the double well potential is accomplished via a readout flux pulse inductively coupled to the qubit from a microstrip transmission line on a separate chip suspended above the qubit chip. The readout pulse is analogous to the current bias pulse used to readout phase qubits and hysteretic dc-SQUID magnetometers. The coupling between the transmission line and the qubit is carefully controlled via a window in the ground plane between the signal conductor of the microstrip and the qubit loop. Since the high frequency transmission lines are on a separate chip, they can be independently characterized and reused for different qubit samples. Clean flux pulses as short as 5 ns with rise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than 10 8 s −1 , the lifetime of the excited state, and coherent oscillations between the ground and excited states within the same well.
Quantum computers have gained widespread interest as they can potentially solve problems that are intractable even for today’s supercomputers, such as the simulation of quantum systems as initially proposed by Feynman. To achieve such tremendous progress, a quantum computer relies on processing the information stored in quantum bits (qubits), the fundamental units of quantum computation, similar to the bits in a classical (i.e., non-quantum) processor. In general, a quantum processor comprising several qubits is connected through a quantum-to-classical interface consisting of classical electrical circuits for manipulating and reading out their quantum state. The accuracy of an operation on a qubit is characterized using the fidelity, which is 100 % in case of a perfect operation. In practice, a fidelity between 99 % and 99.9 % is typically achieved in today’s experiments with less than 100 qubits, limited by non-idealities in the control signals and by the implementation of the physical qubit itself. Since such qubit fidelities are too low to execute relevant quantum algorithms, quantum error correction schemes have been developed which rely on a much larger number of qubits to correct for possible errors, resulting in the need for quantum processors with thousands or millions of qubits. Solid-state qubits promise the largescale integration required to scale to millions of qubits, as they exploit the lithographic fabrication techniques borrowed from the semiconductor industry. However, state-of-theart solid-state qubits, such as transmons and single-electron spin qubits, must be typically cooled to cryogenic temperatures in a dilution refrigerator to exhibit quantum behavior. Because of the limited number of qubits (< 100) in state-of-theart solid-state quantum processors, the classical controller is currently implemented by general-purpose instruments or by tailor-made controllers operating at room temperature connected via several wires to the qubits in the cryogenic chamber. However, these systems must scale to support millions of qubits. While scaling up the underlying quantum processor is already exceptionally challenging, building the electronics required to interface such a large-scale processor is just as relevant and arduous.
The superconducting Josephson phase qubit device was fabricated by a NbN/AlN-based technology with the goal to carry out the spectroscopy of parasitic quantum two-level systems (TLSs) in the AlN tunnel barrier by combining qubit spectroscopy with mechanical strain tuning. So far all existing results on TLS spectroscopy have been obtained on Al-based Josephson qubits and thus only TLSs in amorphous aluminum oxide barriers were studied. Understanding the physics of TLS and TLS-induced decoherence requires a study of individual TLSs in tunnel barriers made from different materials. The layout of the NbN/AlN device was optimized to improve the magnetic flux coupling between the flux bias coil and the qubit loop and to minimize the unwanted coupling to the dc-readout SQUID. The area of the SiO insulation under wiring layer was made as small as possible in order to decrease the number of TLSs which can contribute to the suppression of the coherence time. The NbN/AlN Josephson phase qubit was tested in a 3He/4He dilution refrigerator at a temperature of about 60 mK by measuring Rabi oscillations and the energy relaxation time TD. Rabi oscillations had a mean decay time of about of 3-5 ns. We performed systematic studies of TD by varying the power and duration of a resonant microwave pulse. We observe a large enhancement of the qubit's relaxation time for longer microwave pulses. We explain this effect by microwave induced suppression of spontaneous qubit emission.